Electrolyzer and Usage

Abstract

To provide an electrochemical system configured to electrolytically reduce a carbon oxide, a method of electrolytically reducing a carbon oxide, and a membrane electrode assembly.SOLUTION: The electrochemical system comprises: (a) a membrane electrode assembly (MEA) including a cathode layer including a carbon oxide reduction catalyst that promotes reduction of a carbon oxide, an anode layer including a catalyst that promotes water oxidation, and a bipolar polymer electrolyte membrane (PEM) layer disposed between and in contact with the cathode layer and the anode layer; and (b) a source of anode water in contact with the anode layer, connected to MEA in a way capable of supplying MEA with a salt. The bipolar PEM layer includes an anion conductive polymer layer, a cation conductive polymer layer and a bipolar interface between the anion conductive polymer layer and the cation conductive polymer layer, with the cation conductive polymer layer disposed between the anode layer and the anion conductive polymer layer.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of U.S. Provisional Application No. 62 / 772,460, filed November 28, 2018, and U.S. Provisional Application No. 62 / 939,960, filed November 25, 2019, which are incorporated herein by reference in their entireties for all purposes.

[0002] [Statement of Government Support] This invention was made with government support under Award No. NNX17CJ02C awarded by the National Aeronautics and Space Administration and Award No. DE-AR0000819 awarded by the United States Department of Energy (ARPA-E). The government has certain rights in this invention.

[0003] FIELD OF THE DISCLOSURE The present disclosure relates generally to the field of electrolytic carbon oxide reduction, and specifically to systems and methods for the operation of electrolytic carbon oxide reactors. [Background technology]

[0004] Electrolytic carbon dioxide reactors require balancing various operating conditions, including the composition of reactants at the anode and cathode, the electrical energy supplied to the anode and cathode, and the physicochemical environment of the electrolyte, anode, and cathode. Balancing these conditions can have a strong impact on the electrolytic reactor's operating voltage, faradaic yield, and the mix of products generated at the cathode, including carbon monoxide (CO) and / or other carbon-containing products (CCPs) and hydrogen.

[0005] The background and contextual discussion contained herein is provided solely for the purpose of generally setting out the context of the present disclosure. Much of the present disclosure presents work of the inventors, and merely because such work is set forth in the background section or presented as context elsewhere herein does not mean that such work is admitted to be prior art. Summary of the Invention

[0006] One aspect of the present disclosure relates to a membrane electrode assembly (MEA) that can be characterized by the following features: (a) a cathode layer including a carbon oxide reduction catalyst that promotes the reduction of carbon oxides, (b) an anode layer including a catalyst that promotes the oxidation of water, (c) a polymer electrolyte membrane (PEM) layer disposed between and in contact with the cathode and anode layers, and (d) salt ions from a salt solution in contact with the MEA, where the salt in the salt solution has a concentration of at least about 10 μM. The MEA in contact with the salt solution can have a concentration of salt (or salt ions) that deviates from the concentration of salt in the salt solution.

[0007] In some embodiments, the concentration of salt or salt ions (consisting of multiple counterions contributed by multivalent ions) in the MEA is lower than the concentration of salt in the salt solution.

[0008] In certain embodiments, the carbon oxide is carbon dioxide and the carbon oxide reduction catalyst comprises gold, silver, copper, or a combination thereof. In certain embodiments, the carbon oxide is carbon monoxide and the carbon oxide reduction catalyst comprises gold, silver, copper, or a combination thereof.

[0009] In certain embodiments, the cathode layer comprises an anion-conducting polymer, and the anode layer comprises a cation-conducting polymer.

[0010] In certain embodiments, the MEA is bipolar and has at least one layer of a cation-conducting polymer and at least one layer of an anion-conducting polymer. In some implementations, the PEM layer includes a polymer electrolyte layer and a cathode buffer layer. As an example, the PEM layer may include a cation-conducting polymer, and the cathode buffer layer includes an anion-conducting polymer. In some cases, the PEM layer includes an anion-conducting polymer.

[0011] In certain embodiments, the salt ions comprise alkali metal ions. In some cases, the salt ions comprise an anion selected from the group consisting of phosphate, sulfate, carbonate, bicarbonate, and hydroxide.

[0012] In certain embodiments, the MEA is a bipolar MEA and the carbon oxide reduction catalyst comprises copper. In some such cases, the salt comprises (i) an alkali metal cation and (ii) a bicarbonate, sulfate, or hydroxide anion. Such salts may be present in the salt solution at a concentration of about 1 mM to about 1 M, or about 1 mM to about 50 mM.

[0013] In some cases, the bipolar MEA is configured to produce methane by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt ions are sodium ions. In some cases, the bipolar MEA is configured to produce one or more organic compounds having two or more carbon atoms by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt ions include ions of potassium, cesium, rubidium, or any combination thereof.

[0014] In certain embodiments, the MEA is a bipolar MEA, the carbon oxide reduction catalyst comprises gold, and the salt comprises (i) an alkali metal cation and (ii) a bicarbonate, hydroxide, or sulfate anion. In some embodiments, such salt is present in the salt solution at a concentration of about 10 μM to about 200 mM, or about 100 μM to about 20 mM.

[0015] In some cases, the bipolar MEA is configured to produce carbon monoxide by reducing carbon dioxide at the cathode layer, and the salt ions include alkali metal ions. In some cases, the bipolar MEA is substantially free of transition metal ions.

[0016] In certain embodiments, all of the polymers in the MEA are anion-conducting polymers, the carbon oxide reduction catalyst comprises copper, and the salt comprises (i) an alkali metal cation and (ii) a bicarbonate or hydroxide anion. In some implementations, the salt is present in the salt solution at a concentration of about 10 mM to about 15 M, or about 50 mM to about 1 M.

[0017] In certain embodiments, the MEA having the anion-conducting polymer is configured to produce methane by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt ions include sodium ions. Such an MEA may be configured to produce one or more organic compounds having two or more carbon atoms by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt ions may include ions of potassium, cesium, rubidium, or any combination thereof.

[0018] Some embodiments of the present disclosure relate to electrochemical systems configured for the electrolytic reduction of carbon oxides. Such systems may be characterized by the following features: (a) a membrane electrode assembly (MEA) including: (i) a cathode layer including a carbon oxide reduction catalyst that promotes the reduction of carbon oxides; (ii) an anode layer including a catalyst that promotes the oxidation of water; and (iii) a polymer electrolyte membrane (PEM) layer disposed between and in contact with the cathode and anode layers; and (b) an anode water source including a salt in the anode water having a concentration of at least about 10 μM, the anode water source connected to the MEA in a manner that allows the anode water to contact the anode layer and provide the salt to the MEA.

[0019] In certain embodiments, the carbon oxide reduction catalyst comprises gold, silver, copper, or a combination thereof. In certain embodiments, the cathode layer comprises an anion-conducting polymer. In certain embodiments, the anode layer comprises a cation-conducting polymer.

[0020] In some implementations, the PEM layer includes a polymer electrolyte layer and a cathode buffer layer. By way of example, the PEM layer may include a cation-conducting polymer, and the cathode buffer layer includes an anion-conducting polymer. In some implementations, the PEM layer includes an anion-conducting polymer.

[0021] In certain embodiments, the salt comprises an alkali metal ion, hi certain embodiments, the salt comprises an anion selected from the group consisting of phosphate, sulfate, carbonate, bicarbonate, and hydroxide.

[0022] In some cases, the MEA of the electrochemical system is a bipolar MEA having at least one layer of a cation-conducting polymer and at least one layer of an anion-conducting polymer.

[0023] In certain bipolar MEA embodiments, the carbon oxide reduction catalyst includes copper, and the salt includes (i) an alkali metal cation and (ii) a bicarbonate, sulfate, or hydroxide anion. As an example, the salt is present in the anode water at a concentration of about 1 mM to about 1 M, or about 1 mM to about 50 mM. In some implementations, the bipolar MEA is configured to produce methane by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt includes sodium ions. In some implementations, the bipolar MEA is configured to produce one or more organic compounds having two or more carbon atoms by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt includes ions of potassium, cesium, rubidium, or any combination thereof.

[0024] In certain bipolar MEA embodiments, the carbon oxide reduction catalyst comprises gold and the salt comprises (i) an alkali metal cation and (ii) a bicarbonate, hydroxide, or sulfate anion.

[0025] In some cases, the salt is present in the anode water at a concentration of about 10 μM to about 200 mM, or about 100 μM to about 20 mM. In some cases, the bipolar MEA is configured to produce carbon monoxide by reducing carbon dioxide in the cathode layer, and the salt includes alkali metal ions. In some implementations, the bipolar MEA configured to produce carbon monoxide is substantially free of transition metal ions.

[0026] In certain embodiments, all polymers in the MEA are anion-conducting polymers, the carbon oxide reduction catalyst includes copper, and the salt includes (i) an alkali metal cation and (ii) a bicarbonate or hydroxide anion. In some implementations, the salt is present in the anode water at a concentration of about 10 mM to about 15 M, or about 50 mM to about 1 M. In certain embodiments, the MEA having the anion-conducting polymer is configured to produce methane by reducing carbon dioxide and / or carbon monoxide in the cathode layer, and the salt includes sodium ions. In certain embodiments, the MEA having the anion-conducting polymer is configured to produce one or more organic compounds having two or more carbon atoms by reducing carbon dioxide and / or carbon monoxide in the cathode layer, and the salt includes ions of potassium, cesium, rubidium, or any combination thereof.

[0027] In certain embodiments, the electrochemical system further includes a recirculation loop connected to the MEA, the recirculation loop configured to collect anode water from the MEA, store and / or treat the collected anode water, and supply the stored or treated anode water to the MEA. In some cases, the recirculation loop includes a reservoir for storing the anode water. In some cases, the recirculation loop includes an inlet for receiving purified water. In certain embodiments, the recirculation loop includes an anode water purification element configured to remove impurities from the anode water. In some embodiments, the recirculation loop is connected to an anode water source.

[0028] In certain embodiments, the electrochemical system further includes a cathode water conduit connected to the anode water recirculation loop. The cathode water conduit may be configured to provide water recovered from the carbon oxide stream to the recirculation loop after the carbon oxide stream contacts the cathode layer of the MEA. In some cases, the electrochemical system further includes a water separator coupled to the cathode water conduit and configured to separate the cathode water from the carbon oxide stream.

[0029] Another aspect of the present disclosure relates to a method for electrolytically reducing carbon oxides, which may be characterized by the following operations (in any order): (a) providing a salt solution to a membrane electrode assembly (MEA) including: (i) a cathode layer including a carbon oxide reduction catalyst that promotes the reduction of carbon oxides; (ii) an anode layer including a catalyst that promotes the oxidation of water; and (iii) a polymer electrolyte membrane layer (PEM layer) disposed between and in contact with the cathode and anode layers, wherein the salt solution includes at least about 10 μM salt; and (b) electrolytically reducing carbon oxides at the cathode layer of the MEA while the MEA is in contact with the salt solution.

[0030] In various embodiments, the methods can use an MEA, salt, and associated system components as described above for the MEA and electrochemical system aspects of the present disclosure. Note that while some aspects described above provide salt to the MEA via the anode water, not all methods require this. For example, salt can be preloaded into the MEA by injecting it into the MEA prior to operation.

[0031] In some embodiments, the method provides a salt solution to the MEA by supplying anode water to the anode layer of the MEA. In some implementations, the method further includes (i) recovering the anode water supplied to the MEA and (ii) recycling the recovered anode water to the anode layer of the MEA. In some implementations, the method further includes storing and / or treating the recovered anode water before recycling it to the anode layer of the MEA. In some implementations, the method further includes purifying the anode water and / or the recovered anode water to remove impurities from the anode water.

[0032] In certain embodiments, the method further includes (i) recovering water from the carbon oxide stream after the carbon oxide stream contacts the cathode layer of the MEA, and (ii) providing the water recovered from the carbon oxide stream to the anode layer of the MEA.

[0033] These and other features of the present disclosure are presented in more detail below with reference to the associated drawings. [Brief explanation of the drawings]

[0034] [Figure 1A] 1 shows an example of an electrolytic carbon reduction system that can be used to control the composition and flow of water within an MEA cell. [Figure 1B] 1 shows an example of an electrolytic carbon reduction system that can be used to control the composition and flow of water within an MEA cell. [Figure 2] FIG. 1 is a schematic diagram of a membrane electrode assembly for use in COx reduction according to certain embodiments of the present disclosure. [Figure 3] A bipolar MEA is shown in which bicarbonate and / or carbonate ions can combine with hydrogen ions between the cathode and anode layers to form carbonic acid, which can decompose to form gaseous CO2. [Figure 4] Shown is an MEA in which CO2 gas is supplied to the cathode catalyst layer. [Figure 5] 1 shows an MEA having a cathode catalyst layer, an anode catalyst layer, and an anion-conducting PEM configured to promote a CO reduction reaction. [Figure 6] 1A and 1B are schematic diagrams showing examples of the morphology of cathode particles in which a catalyst is supported on catalyst-supporting particles. [Figure 7] It shows a similar MEA to that shown in Figure 3, but additionally shows information related to mass transfer and the production of CO2 and water at the bipolar interface. [Figure 8A] MEA designs are presented that include features that resist delamination and optionally provide a path for reaction products to leave the interfacial region. [Figure 8B] MEA designs are presented that include features that resist delamination and optionally provide a path for reaction products to leave the interfacial region. [Figure 8C] MEA designs are presented that include features that resist delamination and optionally provide a path for reaction products to leave the interfacial region. [Figure 8D] MEA designs are presented that include features that resist delamination and optionally provide a path for reaction products to leave the interfacial region. [Figure 9] 1 shows a partial MEA including an anion-conducting polymer layer, which may be a cathode buffer layer, and a polymer electrolyte membrane, which may be a cation-conducting polymer layer. [Figure 10] FIG. 1 is a schematic diagram showing the main components of a COx reduction reactor (CRR) according to an embodiment of the present invention. [Figure 11] FIG. 1 is a schematic diagram showing the main components of a CRR, with arrows indicating the flow of molecules, ions, and electrons according to one embodiment of the present invention. [Figure 12] FIG. 1 is a schematic diagram showing the main inputs and outputs of the CRR reactor. [Figure 13A] 1 presents performance plots for a carbon dioxide electrolyzer with no salt in the anode water. [Figure 13B] A performance plot of a carbon dioxide electrolyzer containing 2 mM NaHCO3 in the anode water is presented. [Figure 14] Plots are presented showing the performance-enhancing effect of salt on the faradaic yield and voltage efficiency of a CO electrolyzer system producing methane and ethylene. The cell used a bipolar MEA and copper catalyst (cathode). [Figure 15] Experimental data are presented showing that 6 mM NaHCO3 showed the highest faradaic yield compared to concentrations of 2 mM, 8 mM, and 10 mM. Results depend on the size of the cell's surface area. All MEA cells used bipolar MEAs and gold catalysts (cathode). [Figure 16] An example is given showing how salt concentration affects the production yield of C2 hydrocarbons (e.g., ethylene and ethanol). [Figure 17] Data are presented for an experiment in which the anode water was changed from NaHCO3 to KHCO3 during the reaction. The selectivity for methane decreased, but the selectivity for ethylene increased. [Figure 18] Data are presented showing improved selectivity to ethanol when using KHCO3 versus NaHCO3. [Figure 19A] 1 shows the improvement in selectivity and voltage after adding fresh salt solution or replacing the old solution in the anolyte reservoir. [Figure 19B] 1 is a table showing the improvement in selectivity and voltage after adding fresh salt solution or replacing old solution in the anolyte reservoir. [Figure 20] A salt concentration scan for the selectivity of the copper catalyst towards methane in the range of 1 mM to 30 mM NaHCO3 is presented. [Figure 21A] Data from testing various salts for their effect on ethylene selectivity at various concentrations are presented. [Figure 21B] Data from testing various salts for their effect on ethylene selectivity at various concentrations are presented. DETAILED DESCRIPTION OF THE INVENTION

[0035] [Introduction and Overview] Polymer electrolyte membrane electrolyzers are used to electrolyze water to produce oxygen at the anode and hydrogen at the cathode. In typical water electrolyzers, care must be taken to prepare the membrane electrode assembly so that no ions other than H+ or OH- are introduced. Also, during operation, only pure water is introduced into the anode side of the cell.

[0036] The electrolyzer system of the present disclosure can produce oxygen at the anode from the oxidation of water and one or more carbon-based compounds through the electrochemical reduction of carbon dioxide or other carbon oxides introduced at the cathode. As used herein, the term carbon oxide includes carbon dioxide and / or carbon monoxide. In some embodiments, carbon monoxide is used as the reducible reactant. In some embodiments, carbon dioxide is used as the reducible reactant. In some embodiments, a mixture of carbon dioxide and carbon monoxide is used as the reducible reactant.

[0037] In contrast to water electrolyzers, where salt ions are undesirable, the inventors have found that salt ions can have a positive impact on the performance of carbon oxide electrolyzers. Cations can be introduced into carbon oxide electrolyzers via water circulating through the anode of the electrolyzer, or by incorporation into the polymer electrolyte membrane, catalyst, or catalyst support used to fabricate the membrane electrode assembly.

[0038] The presence of salt has been observed to reduce the MEA cell voltage, improve the faradaic yield, alter product selectivity, and / or reduce the rate of decay of operating parameters (e.g., voltage efficiency) during operation of a carbon oxide reduction electrolyzer.

[0039] The introduction of salt ions can affect the electrolytic performance of carbon dioxide through any of several possible mechanisms. Without wishing to be bound by theory, the following is a list of examples of mechanisms by which salts can affect the operation of an MEA cell during electrolytic carbon dioxide reduction:

[0040] The presence of cations and / or anions from salts lowers the activation energy of one or more catalytic pathways. This can be due to any of a number of possible mechanisms. For example, salts may alter the local electrolyte structure and / or electron density on the catalyst surface. In some carbon dioxide reduction systems, salt ions have been observed to increase the faradaic yield. The presence of certain ions has also been observed to alter the selectivity of a catalyst for one reaction over another.

[0041] Cations and / or anions from the salt may aid in the hydration of the polymer electrolyte, particularly the anion exchange polymer. Ions migrate as hydrates, meaning they carry water molecules with them as they move through the polymer layer. Hydration of the MEA, particularly the portion of the MEA near the cathode catalyst, may facilitate the reduction reaction by preventing the water in the MEA from evaporating due to flowing carbon dioxide. In general, salt ions may facilitate hydration of the MEA, particularly in areas of the MEA that are prone to drying out. In various embodiments, the presence of salt in the polymer increases the hygroscopicity of the polymer.

[0042] The presence of salt and ions from the salt can increase the conductivity of one or more MEA layers. In particular, the ions can increase the conductivity of anion-exchange polymers, which tend to have relatively low conductivity compared to cation-exchange polymers. Increasing the conductivity of the polymer can reduce the overall resistance of the MEA cell.

[0043] The presence of salts can increase the pH of one or more polymer electrolyte layers. This should be contrasted with proton-donating additives, which lower the pH of the polymer.

[0044] The presence of cations and / or anions from the salt alters the water uptake and swelling of the polymer electrolyte layer. If the volume change due to swelling is not matched between the anode and cathode sides of the MEA, the mechanical stress on the MEA can degrade cell performance. In certain embodiments, the presence of a defined concentration of salt adjusts the relative amount of swelling in two or more different layers of the MEA to equalize the swelling exhibited by these layers.

[0045] The presence of cations and / or anions provided by salt can alter the conductivity at the interface between two layers of an MEA. For example, at a bipolar interface, protons may have to jump an interfacial gap to encounter anions. This jump has an associated resistance. The presence of salt can reduce the barrier for protons and anions to converge across the interface. Note that the pores of Nafion and similar polymers contain sulfonic acid groups, allowing protons to move with low resistance. At a bipolar interface, these groups are absent to facilitate continued movement. Salt can provide an uncharged depletion region at the interface, facilitating the convergence of protons and anions (e.g., protons coming from the anode side react with bicarbonate ions from the cathode side). In other words, the salt solution present at the interface can provide a conductive or ionically conductive bridge between the anion-conducting polymer and the cation-conducting polymer.

[0046] The cations and / or anions provided by the salt may provide counterions for the charged carbon-based species formed by the cathodic reduction reaction. Such charged species require available counterions to maintain charge neutrality. In some embodiments, the reduction reaction at the cathode produces a carboxylate product (e.g., formate, oxalate, or acetate). However, if relatively few cations are available, the reaction may be unfavorable. This may be the case when the cathode layer includes an anion exchange polymer, such as an anion exchange membrane (AEM), that blocks the flow of protons (potential counterions). The cations donated by the salt may provide the species necessary to drive the carboxylate-forming reaction.

[0047] A salt concentration gradient can induce osmotic pressure. For example, the salt concentration can be higher on the anode side, which draws water away from the cathode, thereby reducing the occurrence of cathode flooding. Note that the water present on the cathode side can be provided, at least in part, by the reaction of hydrogen ions with bicarbonate ions within the MEA. This water is initially free of salt ions, which contributes to the concentration gradient.

[0048] [Characteristics of salts used in MEA cells] Various types of salts can be used in MEA cells. Such salts can have inorganic or organic cations and anions. The salt composition can affect cell operating conditions such as overpotential, faradaic efficiency, and / or selectivity among multiple carbon oxide reduction reactions. Various factors that influence the selection of salt composition are described herein.

[0049] [Cation reactivity] The composition of the salt can depend on the catalyst used in the cathode. In certain embodiments, the salt does not contain cations that could poison the cathode catalyst. For example, the salt may not contain cations that could be reduced by a cathode catalyst, such as a catalyst containing gold or another precious metal. Such catalysts may be used in MEA cells configured to reduce carbon dioxide to carbon monoxide or other reduction products. It has been found that the reduction of metal ions, such as iron ions or other transition metal ions, on catalyst particles can poison the catalyst or otherwise reduce the catalytic conversion of carbon dioxide to reduction products such as carbon monoxide.

[0050] In certain embodiments, the salt used in the carbon dioxide reduction reactor contains only cations that cannot be reduced to elemental metals in an aqueous medium under operating conditions for carbon dioxide reduction at the cathode. In certain embodiments, the salt used in the reactor does not have transition metal ions. In certain embodiments, the salt used in the reactor has only alkali metal and / or alkaline earth cations.

[0051] The production of carbon monoxide from carbon dioxide can be achieved using gold or silver catalysts, while the production of hydrocarbons and / or organic oxygen-containing compounds from carbon oxides can be achieved using copper or other transition metal catalysts at the cathode. In some cases, the salts used in cells configured to produce hydrocarbons and / or organic oxygen-containing compounds have one or more cations that are not alkali metal or alkaline earth ions. For example, an MEA with a transition metal catalyst can be constructed with salts containing one or more transition metals.

[0052] The type of salt used and its concentration can vary depending on whether the carbon dioxide reduction reactor uses a bipolar MEA, an anion exchange polymer-only MEA, or some other MEA configuration. Cells configured to reduce carbon monoxide can use an anion exchange polymer-only MEA because little or no bicarbonate is formed at the cathode; therefore, the MEA does not need to contain a cation-conducting polymer to block bicarbonate transport to the anode, which could liberate carbon dioxide (which would otherwise be used in the reduction reaction at the cathode). Such cells can use salts containing cations of transition metals or other metals that could poison precious metal catalysts. In certain embodiments, carbon dioxide reduction cells with bipolar MEAs use salts that do not have transition metal ions.

[0053] In certain embodiments, the salt contains a cation that adjusts the pH at one or more locations within the carbon oxide reduction cell (e.g., the anode, the cathode, or an intermediate ion-conducting polymer layer). In some cases, during operation, the salt adjusts the pH at one or more such locations to be more acidic or more basic. In certain embodiments, the anion is ammonium, a derivatized ammonium cation such as a quaternary ammonium ion, an alkali metal ion, or an alkaline earth metal ion.

[0054] [Reactivity of anions] The composition of the salt can be affected by reactions at the anode of the carbon dioxide reduction cell. In certain embodiments, the salt contains an anion that is not readily oxidized at the anode and / or not readily reduced at the cathode under the operating conditions of the cell. In certain embodiments, the anion is not a halide. In some cases, the anion is not chloride, bromide, or iodide. Halides oxidize at the anode, which can themselves form elemental halogens. Note, however, that in certain embodiments, halides are used in carbon dioxide reduction cells in which the reduction product is a halogen compound. In certain embodiments, the salt has an anion that is not an oxidizable nitrogen-containing anion, such as a nitrite or an amine. In certain embodiments, the salt has an anion that is not an organic anion, e.g., the salt does not contain a carboxylate ion.

[0055] In certain embodiments, the salt contains an anion that adjusts the pH at one or more locations within the carbon dioxide reduction cell (e.g., the anode, the cathode, or an intermediate ion-conducting polymer layer). In some cases, during operation, the salt adjusts the pH at one or more such locations to be more acidic or more basic. In certain embodiments, the anion is hydroxide, bicarbonate, sulfite, or sulfate.

[0056] [Ion mobility] One consideration in selecting the cations and / or anions of the salt is the mobility of the ions. In certain embodiments, the ions have relatively high mobility in the polymer of the MEA. In some cases, one or more layers of the MEA in which the salt is present each have an ionic conductivity of at least about 4 mS / cm. In some implementations, ions with relatively low atomic weights are used. In some cases, the cations of the salt have an atomic weight or molecular weight of about 140 or less, or about 90 or less, or about 40 or less. In some cases, the atomic weight or molecular weight of the anions of the salt is about 100 or less.

[0057] [Solubility] In certain embodiments, the salt is relatively soluble in aqueous media. For example, the salt may have a solubility in deionized water at 25° C. of at least about 1 mol / L, or at least about 2 mol / L, or at least about 10 mol / L.

[0058] [Improved product selectivity, voltage efficiency, lifetime, and reduced decay rate] The type of salt can affect the product selectivity of the MEA cell: choosing one cation over another can change the ratio of one product to another, for example, by at least about 10%.

[0059] In certain embodiments, sodium-containing salts, such as sodium bicarbonate, when used in an MEA cell having a gold catalyst on the cathode, selectively increase the production of carbon monoxide over by-product hydrogen during carbon dioxide reduction. This increase in carbon monoxide production is observed compared to a similar gold-catalyst-containing MEA cell that does not contain the salt. For example, an MEA cell with a gold catalyst and using sodium bicarbonate may increase carbon monoxide production by at least about 100% when compared to a similar MEA cell that does not use the salt. In other words, an MEA cell using a sodium-containing salt, such as sodium bicarbonate, produces carbon monoxide in a molar amount at least about two times higher than that produced by the same MEA cell operated in the same manner but without substantially any salt. In some embodiments, an MEA cell using a sodium-containing salt produces carbon monoxide in a molar amount at least about three times higher. In some cases, an MEA cell using a potassium-containing salt, such as potassium bicarbonate, produces carbon monoxide in a molar amount at least about two times higher than that produced by the same MEA cell operated in the same manner but without substantially any salt. In some cases, MEA cells using salts with higher atomic weight alkali metals, such as cesium or rubidium, produce carbon monoxide in molar amounts at least about two times higher than that produced by the same MEA cell operating in the same manner but substantially free of salt.

[0060] In some embodiments, MEA cells configured to produce carbon monoxide from carbon dioxide use alkali metal containing salts and operate in a manner to produce, among other products, a product at the cathode having at least about 70 mol % carbon monoxide or at least about 80 mol % carbon monoxide, but no unreacted carbon dioxide. Other products that may be produced at the cathode include hydrogen, one or more hydrocarbons, and one or more carbonyl-containing products. MEA cells configured to produce carbon monoxide may include gold, silver, or other precious metals at the cathode. MEA cells configured to produce carbon monoxide may include a bipolar membrane assembly.

[0061] In certain embodiments, the concentration of the sodium-, potassium-, cesium-, or rubidium-containing salt in the water delivered to the MEA cell is about 1 mM to 20 mM. This concentration range is applicable to MEA cells configured to produce carbon monoxide from carbon dioxide. In certain embodiments, such cells include gold or other noble metals as a cathode catalyst. As used herein, a noble metal is a metal that is highly resistant to chemical attack. Examples include platinum and silver, in addition to gold.

[0062] In some cases, a relatively small surface area (e.g., approximately 10 cm2 assuming a flat surface) 2 Approximately 50cm from 2 ), biased towards a relatively low concentration range, such as about 1 mM to 5 mM, but with a relatively large surface area (e.g., about 50 cm 2 Approximately 1000cm from 2 ) is biased towards a relatively high concentration range, such as about 5 mM to 20 mM.

[0063] In certain embodiments, salts such as sodium bicarbonate, when supplied via anode water to MEA cells with a gold catalyst on the cathode, improve energy efficiency by approximately 9% to 25%. Furthermore, in certain embodiments, desirable levels of CO selectivity and cell voltage observed during initial cell operation are stabilized by more than an order of magnitude when salt solutions are used.

[0064] In some cases, MEA cells using sodium-containing salts, such as sodium bicarbonate, have a voltage efficiency for producing carbon monoxide that is at least about 5% higher than the voltage efficiency of the same MEA cell operated in the same manner but substantially free of salt. In some cases, MEA cells using sodium-containing salts, such as sodium bicarbonate, have a voltage efficiency for producing carbon monoxide that is at least about 10% higher, or at least about 20% higher, than the voltage efficiency of the same MEA cell operated in the same manner but substantially free of salt. In some cases, MEA cells using potassium-containing salts, such as potassium bicarbonate, have a voltage efficiency for producing carbon monoxide that is at least about 5% higher than the voltage efficiency of the same MEA cell operated in the same manner but substantially free of salt. In some cases, MEA cells using salts containing higher atomic weight alkali metals, such as cesium or rubidium, have a voltage efficiency for producing carbon monoxide that is at least about 5% higher than the voltage efficiency of the same MEA cell operated in the same manner but substantially free of salt. In certain embodiments, the voltage efficiency for producing carbon monoxide in a bipolar MEA with a gold or other precious metal cathode catalyst is at least about 25%.

[0065] As an example, a tested cell with no salt in the anode water had a current of 0.5 A / cm 2 The average voltage for the first hour was 3.86 V, the average CO faradaic yield was 0.53, and the current was 500 mA / cm. 2 The decay rate from 2 to 5 hours is 144 mV / hr, with a CO faradaic yield / hr of 0.018. By comparison, the same cell operating with 2 mM NaHCO3 has a current of 0.5 A / cm 2 The average voltage for the first hour was 3.52 V, the average CO faradaic yield was 0.936, and the average current was 500 mA / cm. 2 and a decay rate of 15.5 mV / hr for 2-5 hours and a CO faradaic yield / hr of 0.001. In certain embodiments, an MEA cell configured to produce carbon monoxide from carbon dioxide has an average voltage of up to about 3.6 V during the first hour of operation and a decay rate of about 16 mV / hr or less for 2-5 hours.

[0066] In certain embodiments, the voltage efficiency and / or product selectivity for producing carbon monoxide in an MEA cell using sodium-, potassium-, cesium-, or rubidium-containing salts in water is stable over a period of operation that is at least 10 times longer than the period of operation of a corresponding MEA cell that is operated in the same manner and for the same period but that is substantially free of salt. In certain embodiments, the voltage efficiency and / or product selectivity for producing carbon monoxide in an MEA cell using aqueous sodium-, potassium-, cesium-, or rubidium-containing salts is stable over a period of operation that is at least 10 times longer than the period of operation of a corresponding MEA cell that is operated in the same manner and for the same period. In certain embodiments, the voltage efficiency and / or product selectivity for producing carbon monoxide in an MEA cell that uses aqueous sodium-, potassium-, cesium-, or rubidium-containing salts is stable over a period of operation that is at least 10 times longer than the period of operation of a corresponding MEA cell that is substantially free of salt. 2 In certain embodiments, the mole fraction of carbon monoxide among all other products (excluding carbon dioxide) produced at the cathode of an MEA cell using aqueous sodium, potassium, cesium, or rubidium-containing salts increases by no more than about 16 mV, or about 0.5%, per hour at an applied current density of 600 mA / cm for more than 8 hours of operation. 2 In certain embodiments, the voltage for carbon monoxide production in an MEA cell using aqueous sodium, potassium, cesium, or rubidium-containing salts decreases by no more than about 1% per hour at applied current densities of 300 mA / cm for over 100 hours of operation. 2 In certain embodiments, the mole fraction of carbon monoxide among all other chemicals produced at the cathode of an MEA cell using aqueous sodium, potassium, cesium, or rubidium-containing salts increases by no more than about 0.05 mV or about 0.03% per hour for more than 100 hours of operation at an applied current density of 300 mA / cm. 2 At the following applied current densities, it does not decrease by more than about 0.1% per hour:

[0067] Faradaic efficiency, sometimes called the faradaic yield, coulombic efficiency, or current efficiency, is the efficiency with which charge is transferred in a system driving an electrochemical reaction. The use of the Faraday constant in faradaic efficiency correlates charge with moles of material and electrons. Faradaic losses occur when electrons or ions participate in unwanted side reactions. These losses manifest as heat and / or chemical by-products.

[0068] Voltage efficiency represents the fraction of energy lost due to overpotential or resistance to charge transfer within an MEA cell. For electrolysis cells, this is the ratio of the cell's thermodynamic potential divided by the cell's experimental cell voltage, converted to a percentile. The cell's voltage loss due to overpotential is expressed by the voltage efficiency. For a given type of electrolysis reaction, an electrolysis cell with a relatively high voltage efficiency will have a relatively low overall cell voltage loss due to resistance.

[0069] In certain embodiments, sodium-containing salts such as sodium bicarbonate when used in a bipolar MEA cell with a copper catalyst on the cathode produce methane with improved voltage efficiency proportional to the increase in salt concentration. A 6.5% increase in voltage efficiency was observed when increasing the salt concentration from 3 mM to 20 mM sodium bicarbonate.

[0070] In certain embodiments, sodium-containing salts such as sodium bicarbonate, when used in bipolar MEA cells with a copper catalyst on the cathode, produce methane at improved voltage efficiencies compared to deionized water. When sodium bicarbonate was used as the anolyte, the initial voltage efficiency was improved by at least about 30% and the voltage decay rate was improved by at least about 8 times compared to deionized water.

[0071] In certain embodiments, potassium-containing salts, such as potassium bicarbonate, used in MEA cells with a copper catalyst on the cathode selectively produce ethanol and ethylene over methane during the reduction of carbon dioxide. In contrast, sodium-containing salts, such as sodium bicarbonate, when used in MEA cells with a copper catalyst on the cathode selectively produce methane during the reduction of carbon dioxide. In MEA cells using a copper reduction catalyst, salts containing higher atomic weight cations increase the faradaic yield of multi-carbon products (e.g., ethylene).

[0072] In one example, a bipolar MEA setup with a copper catalyst at the cathode was used with an anolyte sodium bicarbonate concentration of 3 mM to give a product selectivity distribution of about 61.3 mol% methane, about 3 mol% ethylene, about 1.3 mol% carbon monoxide, and about 28.5 mol% hydrogen, demonstrating a high methane to ethylene ratio (greater than 20:1) when carbon dioxide electrolysis is carried out in the presence of sodium salts.

[0073] In one example, a cell containing a bipolar MEA with a copper catalyst at the cathode and sodium bicarbonate salt as the anolyte at a conductivity of 279 microsiemens (~3 mM concentration) was shown to produce approximately 40% methane, approximately 20 mol% ethylene, approximately 1 mol% carbon monoxide, and approximately 17 mol% hydrogen. When the salt solution in the same setup was changed to potassium bicarbonate at a similar conductivity (~2 mM), a significant change in product selectivity was observed. The total production of ethylene and liquids C2-C3 increased by approximately 170 mol%, while methane production decreased by approximately 73 mol% and hydrogen by approximately 40 mol%.

[0074] In various embodiments, potassium cation salts favor selectivity for ethylene over methane at a molar ratio of at least about 5:1. In various embodiments, sodium cation salts favor selectivity for methane over ethylene at a molar ratio of at least about 20:1. These embodiments apply to bipolar MEA cells. In some cases, the MEA cells use a copper catalyst. Cesium has a similar effect as potassium in bipolar MEA cells.

[0075] In certain embodiments, bipolar MEA cells with copper catalysts and sodium-containing salts provide a faradaic efficiency of at least about 50% (e.g., up to about 73%) for methane from carbon dioxide. In certain embodiments, bipolar MEA cells with copper catalysts and potassium-containing salts provide a faradaic efficiency of at least about 20% (e.g., up to about 33%) for ethylene from carbon dioxide. Cesium can be used with similar effectiveness as potassium in bipolar MEA cells. In certain embodiments, anion-conducting polymer-only cells with copper catalysts and potassium-containing salts provide a faradaic efficiency of at least about 30% (e.g., about 41%) for ethylene from carbon dioxide.

[0076] In some implementations, an MEA cell configured to produce methane from carbon dioxide uses a sodium-containing salt and operates to produce a product at the cathode having at least about 50 mol% methane or at least about 70 mol% methane. Other products that may be produced at the cathode include hydrogen, carbon monoxide, and one or more, two or more carbon organic molecules. An MEA cell configured to produce methane may include copper or other transition metals at the cathode. An MEA cell configured to produce methane may include a bipolar membrane assembly.

[0077] In some implementations, MEA cells configured to produce ethylene and / or other organic compounds having two or more carbon atoms from carbon dioxide use potassium, cesium, or rubidium-containing salts and operate to produce a product at the cathode having at least about 60 mol % ethylene and / or other organic compounds having two or more carbon atoms, or at least about 80 mol % ethylene and / or other organic compounds having two or more carbon atoms. Other products that may be produced at the cathode include hydrogen, methane, and carbon monoxide. MEA cells configured to produce ethylene and / or other organic compounds having two or more carbon atoms may include copper or other transition metals at the cathode. MEA cells configured to produce ethylene and / or other organic compounds having two or more carbon atoms may include a bipolar membrane assembly.

[0078] In certain embodiments, the voltage efficiency and / or product selectivity for producing methane or organic compounds in an MEA cell using sodium, potassium, cesium, or rubidium-containing salts in water does not decrease by more than about 1%, or more than about 0.3%, or more than about 0.01% at 90 A-hr.

[0079] The cathode catalysts described herein include alloys, doped materials, and other variations of the described materials. For example, an MEA cathode catalyst described as containing gold or other precious metals is understood to include alloys, doped metals, and other variations of gold or other precious metals. Similarly, an MEA cathode catalyst described as containing copper or other transition metals is understood to include alloys, doped metals, and other variations of copper or other transition metals.

[0080] [Typical example of salt] In certain embodiments, the salt used in the reactor has a cation that is not an ion of a transition metal. In certain embodiments, the salt contains a cation that is an alkali metal ion or an alkaline earth metal ion. In certain embodiments, the salt contains lithium ions, sodium ions, potassium ions, cesium ions, and / or rubidium ions. In certain embodiments, the salt contains no cations other than sodium ions and / or potassium ions. In some implementations, the salt contains only cations that are monovalent, such as alkali metal ions.

[0081] In certain embodiments, the salt contains an anion that is hydroxide, bicarbonate, carbonate, perchlorate, phosphate, or sulfate. In some cases, the salt contains an anion that is hydroxide, bicarbonate, carbonate, or sulfate. In certain embodiments, the salt does not contain a halide ion. In certain embodiments, the salt contains an anion generated from a carbon dioxide reduction reaction. Examples include carboxylates such as formate, oxalate, and acetate.

[0082] In certain embodiments, the salt is selected from the group including sodium bicarbonate, potassium bicarbonate, potassium sulfate, sodium sulfate, cesium bicarbonate, cesium sulfate, and any combination thereof.

[0083] In some cases, the MEA uses multiple salts or mixed salts. For example, the MEA may use multiple cations (e.g., sodium ions and potassium ions) but only a single anion (e.g., sulfate). In another example, the MEA uses only a single cation (e.g., sodium ions) but multiple anions (e.g., bicarbonate and sulfate). In yet another example, the MEA uses at least two cations and at least two anions. In certain embodiments, the salt includes a combination of sodium bicarbonate and potassium bicarbonate. In certain embodiments, the salt includes a combination of potassium bicarbonate and potassium phosphate.

[0084] [Salt transfer to MEA] Salt can be delivered to the cell in a variety of ways. In one example, salt is provided with the as-fabricated MEA and / or with a reconstituted MEA. In another example, salt is provided with the feedstock (reactant-containing composition) to the anode or cathode. In some implementations, water is the reactant at the anode, and salt is provided with the anode reactant. Water supplied to the anode is sometimes referred to as "anode water." Anode water can be an aqueous solution that flows to the anode during operation. In some embodiments, the anode reaction is the oxidation of water to produce oxygen. In some embodiments, liquid water containing salt is delivered to the cathode in any of a variety of ways. For example, salt can be delivered by flowing the solution through the cathode during operation. The liquid may contain dissolved carbon dioxide or dissolved carbon monoxide. In some cases, an aqueous solution of salt is delivered to the cathode as a liquid and gas mixture. For example, the salt solution can be sprayed onto the MEA.

[0085] The salt-containing solution provided to the MEA during operation, either directly or via the anode water, can be prepared in a variety of ways. In some cases, the salt-containing solution is made by dissolving salt directly in water. In other cases, the salt-containing solution is made by passing water through a resin (optionally in a column) that releases salt into the water.

[0086] [Salt concentration] In embodiments in which salt is provided to the MEA via liquid water, such as anode water, the salt may be provided at a set concentration, which may vary depending on the MEA configuration and the particular cathode catalyst used and the associated carbon oxide reduction reaction.

[0087] In some embodiments using bipolar membrane MEAs, the salt is provided in an aqueous solution at a concentration of about 1 mM to about 30 mM, or about 3 mM to about 30 mM. In some embodiments using bipolar membrane MEAs, the salt is provided at a concentration of about 2 mM to about 15 mM. In some embodiments using bipolar membrane MEAs, the salt is provided at a concentration of about 0.1 mM to about 30 mM, or about 5 mM to about 10 mM.

[0088] In some embodiments using bipolar membrane MEAs configured for hydrocarbon production from carbon dioxide, salt is provided in the anode water or other source at a concentration of about 2 mM to about 50 mM. In some MEAs used in cells configured for methane production from carbon dioxide, salt is provided at a concentration of about 10 mM to 30 mM. In various implementations, such cells use a salt and copper catalyst selected from the group including sodium bicarbonate, potassium bicarbonate, potassium sulfate, sodium sulfate, cesium bicarbonate, cesium sulfate, and any combination thereof. In various embodiments, the salt used for methane selectivity is sodium bicarbonate, which has been shown to increase the methane-to-ethylene ratio by at least about 20:1.

[0089] In certain embodiments using bipolar membrane MEAs configured to produce hydrocarbon products from carbon oxides, particularly carbon dioxide, the salt is provided at a concentration of about 2 mM to 1 M. In some implementations, the salt is potassium bicarbonate, which has been shown to increase the selectivity of C2-C3 products relative to methane by about a 5:1 ratio compared to sodium bicarbonate, and the salt is provided at a concentration of about 100 mM to about 500 mM. In certain embodiments where the MEA is configured to reduce carbon dioxide to ethylene using a copper catalyst as the cathode, the potassium bicarbonate concentration is about 1 mM to 5 mM. In certain embodiments where the MEA is configured to reduce carbon monoxide to ethylene, the salt concentration, particularly potassium bicarbonate, is about 150 mM to about 250 mM.

[0090] In some embodiments using an MEA containing only an anion-conducting polymer, the salt is provided in aqueous solution at a concentration of about 1 mM to 10 molar. In some embodiments using an MEA containing only anion-conducting polymer, the salt is provided at a concentration of about 100 mM to 5 molar. In certain embodiments using potassium hydroxide as the salt, the salt concentration is about 50-150 mM. In certain embodiments using potassium bicarbonate as the salt, the salt concentration is about 4-10 mM.

[0091] The following concentration ranges are useful for bipolar cells using only anion conducting polymer and anode water containing potassium hydroxide and / or potassium bicarbonate: For certain MEA cells using potassium hydroxide, the salt concentration is about 10 mM to 15 M. For some MEA cells using potassium hydroxide, the salt concentration is about 50-500 mM. For some MEA cells using potassium hydroxide, the salt concentration is about 0.5 M to 15 M. For certain MEA cells using potassium bicarbonate, the salt concentration is about 1 mM to 1 M. For some MEA cells using potassium bicarbonate, the salt concentration is about 1-50 mM. For some MEA cells using potassium bicarbonate, the salt concentration is about 100 mM to 500 mM.

[0092] In certain embodiments using carbon dioxide as a reactant in an MEA cell, the following salt concentration ranges are used: Bipolar membranes (e.g., gold-containing catalysts) for producing carbon monoxide: The salt concentration in the anode water is about 10 μM to 200 mM, or about 100 μM to 20 mM, or about 1 mM to 10 mM, or about 1 mM to 5 mM, or about 2 mM to 5 mM. In certain embodiments, when the salt is sodium bicarbonate, any of these concentration ranges are used. In certain embodiments, any of these concentration ranges are used at about 25 cm 2 is used for MEA cells with a cathode surface area of Bipolar membranes for producing methane (e.g., copper-containing catalysts): The salt concentration in the anode water is about 1 mM to 40 mM, or about 10 mM to 30 mM, or about 3 mM to 20 mM. In certain embodiments, when the salt is sodium bicarbonate, any of these concentration ranges are used. In certain embodiments, any of these concentration ranges are used at about 25 cm 2 is used for MEA cells with a cathode surface area of Bipolar membranes for producing ethylene (e.g., copper-containing catalysts): The salt concentration in the anode water is about 100 μM to 20 mM, or about 1 mM to 10 mM, or about 1 mM to 5 mM, or about 2 mM to 5 mM. In certain embodiments, when the salt is potassium bicarbonate, any of these concentration ranges are used. In certain embodiments, any of these concentration ranges are used at about 25 cm 2 is used for MEA cells with a cathode surface area of Anion-conducting polymer-only MEAs (e.g., copper-containing catalysts) for producing ethylene: The salt concentration in the anode water is about 0.05M to 5M, or about 0.05M to 1M, or about 0.5M to 1M, or about 0.05M to 0.5M. In certain embodiments, when the salt is potassium hydroxide, any of these concentration ranges are used. In certain embodiments, any of these concentration ranges are used at about 25 cm 2 is used for MEA cells with a cathode surface area of

[0093] In certain embodiments using carbon monoxide as a reactant in the MEA cell, the following salt concentration ranges are used: Anion-conducting polymer-only MEA (e.g., copper-containing catalyst) for producing ethylene: The salt concentration in the anode water is about 0.05M to 5M, or about 0.05M to 1M, or about 0.5M to 1M, or about 0.05M to 0.5M, or about 0.5M to 10M. In certain embodiments, when the salt is potassium hydroxide, any of these concentration ranges are used. In certain embodiments, any of these concentration ranges are used at about 25 cm 2 is used for MEA cells with a cathode surface area of Anion-conducting polymer-only MEAs (e.g., copper-containing catalysts) for methane production: The salt concentration in the anode water is about 0.05M to 10M, or about 0.05M to 1M, or about 0.05M to 0.5M, or about 0.5M to 10M, or about 0.5M to 1M. In certain embodiments, when the salt is potassium hydroxide or sodium hydroxide, any of these concentration ranges are used. In certain embodiments, any of these concentration ranges are used at about 25 cm 2 is used for MEA cells with a cathode surface area of Bipolar MEAs for producing ethylene (e.g., copper-containing catalysts): The salt concentration in the anode water is about 20 mM to 2 M, or about 50 mM to 500 mM, or about 50 mM to 250 mM, or about 100 mM to 500 mM. In certain embodiments, when the salt is potassium bicarbonate, any of these concentration ranges are used. In certain embodiments, any of these concentration ranges are used at about 25 cm 2 is used for MEA cells with a cathode surface area of

[0094] The salt concentrations provided herein may be appropriate for any size MEA, but in certain embodiments, they are suitable for an MEA of about 25 cm 2 The ranges listed are appropriate for cells using MEAs with a surface area of ​​1000 m / s, and the ranges listed may be scaled for cells with MEAs with larger surface areas. For example, in some embodiments, the salt concentration increases at a ratio of about 3:4 with increasing MEA area. Thus, for example, a 2 mM salt concentration increases with increasing MEA area by 25 cm. 2 If appropriate for a cell with an MEA area of ​​100 cm 2 The concentration can be increased to 6 mM for a cell with an MEA area of ​​1000 . As used herein, the area of ​​an MEA is the area of ​​the geometric plane of the MEA surface, taking into account pores or other deviations from planarity of the MEA surface.

[0095] In certain embodiments, the concentration of salt in the MEA is between about 1-3 mM / g, in moles of salt per mass of polymer electrolyte, hi certain embodiments, the concentration of salt in the polymer is estimated using conductivity measurements.

[0096] In some implementations, any impurities other than salts introduced into the anode water or cathode water are present at very low concentrations (e.g., on the order of parts per million). This is particularly true for oxidizable anions at the anode and reducible cations at the cathode. In certain embodiments, the water containing one or more introduced salts is substantially free of ions other than those of the salts. For example, the water may contain about 100 ppb or less of any transition metal ions other than any transition metal in the introduced salt. In some cases, the concentration of reducible transition metal ions is 10 ppb or less, 1 ppb or less, or 0.1 ppb or less. In another example, the water contains about 10 ppm or less of any halide ions. In another example, the water contains about 10 ppm or less of any cations other than alkali metal ions and / or alkaline earth metal ions. In another example, the water contains about 10 ppm or less of any cations other than alkali metal ions. In certain embodiments, the salt-containing water contains about 100 ppm or less of unintentionally provided ions. In some cases, the salt-containing water contains about 10 ppm or less of unintentionally provided ions, or about 1 ppm or less of unintentionally provided ions, or about 0.1 ppm or less of unintentionally provided ions.

[0097] In certain embodiments, unwanted ions and / or other impurities are removed from the water before it is delivered to the carbon dioxide reduction cell. This can be accomplished by purifying the water upstream of the anode and / or cathode through which it is delivered. The water can be purified by any of a variety of techniques, such as passing the water through a resin column containing a chelating resin, such as CR11, available from Sigma-Aldrich. Examples of techniques for achieving ultra-high purity water include total filtration of large particles, carbon filtration, water softening, reverse osmosis, exposure to ultraviolet (UV) light for TOC and / or bacterial static control, polishing with ion exchange resins or electrodeionization (EDI), and filtration or ultrafiltration. The specific steps are influenced by the starting water quality. Certain combinations of steps can purify water to resistances greater than about 18 MOhm. In certain embodiments, a resistance of only about 10 MOhm before the intentional addition of salt is sufficient to purify water for CO2 electrolysis.

[0098] The salt concentration values ​​presented herein may define the salt concentration in the aqueous solution supplied to the MEA cell. Such solutions include anode water supplied during cell operation and solutions that immerse or otherwise contact the MEA and inject salt. The salt concentration may differ between the MEA and the solution supplying the MEA. Typically, salt ions permeate the MEA from solution and then migrate through the MEA via one or more transport mechanisms. In one mechanism, salt ions flow into the MEA via the feed solution. This may be when the solution permeates the MEA through pores or other openings in the MEA. Once in the MEA, the solution may move under a pressure gradient. The moving solution carries the salt ions with it. While salt ions are transported in the feed solution, the overall concentration of salt ions in the MEA may decrease because they occupy a larger volume; salt ions occupy the volume of the feed solution in addition to the volume of the MEA polymer.

[0099] Salt ions in solution can move independently of the bulk solution under the influence of a salt concentration gradient (diffusion or osmosis) or an electric field (migration). These transport phenomena can also change the salt concentration within the MEA. Independent of movement within the feed solution, salt ions can move by ionic conduction through the conductive polymer of the MEA. For example, salt cations can move by ionic conduction within the polymer matrix of a cation exchange membrane, such as sulfonated tetrafluoroethylene. Salt anions can move by ionic conduction through the matrix of an anion exchange membrane. Such movement of salt ions within a polymer matrix is ​​sometimes called hopping, where salt ions hop between adjacent charged sites within the polymer matrix. During operation of the MEA cell, salt ions within the polymer matrix have their own concentration, which contributes to the overall salt or salt ion concentration within the MEA.

[0100] Due to the factors mentioned above, and possibly other factors, the salt concentration within the MEA may differ from that within the feed solution. The salt concentration values ​​presented herein typically represent the salt concentration within the feed solution before the feed solution permeates the MEA, but the values ​​may also represent the concentration within the MEA. To the extent that the value represents the concentration within the MEA, it should be considered an average value across the MEA. Note that salt ions may have a different molar concentration than their source salt. For example, a 1 M solution of sodium sulfate, upon complete dissociation, can be considered to provide a 2 M solution of sodium ions.

[0101] [Salt transport through MEA] In certain embodiments, the salt is provided, at least in part, by introduction into one or more components of the MEA prior to operation. For example, the PEM, cathode buffer layer, anode buffer layer, anode catalyst layer, cathode catalyst layer, or any combination thereof, may be preloaded with salt. Preloading can occur before, during, or after assembling individual MEA layers into an MEA stack. In some implementations, preloading is achieved by immersing different layers of the MEA in salt-containing solutions of various preferred concentrations prior to assembly. In some implementations, preloading is achieved by adding small droplets of salt-containing solution onto different layers of the MEA during assembly. In some implementations, preloading is achieved by circulating a salt-containing solution in the anode and / or cathode compartments after assembly.

[0102] In certain embodiments, salt is introduced into the MEA after it has been operating in a carbon dioxide reduction cell for a period of time. In some cases, after a certain amount of use, the MEA is taken out of service and exposed to a composition that introduces salt into the polymer of the MEA. This can be accomplished, for example, by adding salt to the anode water or by introducing salt-containing water through the cathode of the cell.

[0103] [Removing salt from the MEA cell] In certain embodiments, salts may precipitate or otherwise come out of solution and accumulate at specific locations within the cell. For example, salts may deposit in the flow fields and / or MEA layers of the cell, ultimately fouling the cell.

[0104] To address this concern, or for other reasons, the cell can be periodically taken offline and exposed to a flow of water (e.g., deionized water) under hydrodynamic conditions (e.g., flow rate and pressure) that will remove the solid salts from the flow field or other structure they formed in. In some cases, the deionized water is flowed through the cell under thermodynamic conditions that promote the dissolution of the solid salts.

[0105] [Managing salt concentration and water transfer in MEA cells] As previously mentioned, salt can be provided to the MEA from various sources, including the preloaded MEA polymer layer and anode water. The salt supplied to the MEA cell can be depleted during cell operation. This can occur even when salt-containing anode water is recycled to the MEA. Various mechanisms can contribute to this loss. For example, salt from the anode water can be taken up by one or more MEA components, such as the PEM or other cation exchange polymer layer. Furthermore, some salt can migrate from an area of ​​high concentration (anode) to an area of ​​low concentration (cathode) by diffusion, migration, and / or osmosis. Not only the salt content of the anode water, but also the anode water itself, can migrate from the anode to the cathode via osmosis, where it is swept away by a stream of gaseous carbon dioxide.

[0106] Various mechanisms can be used to manage salt concentration during operation of an MEA cell. For example, the anode water may be treated to (a) add salt, (b) remove impurities, and / or (c) add purified water. Such treatment may be achieved by administering concentrated salt solutions and / or purified water to the anode water in the anode water tank. Impurity removal may be achieved by filtration and / or treatment with ion exchange resins.

[0107] Various mechanisms can be used to manage anode water depletion during operation of an MEA cell. One method is to capture water leaving the anode and recirculate it to the anode water inlet. Another method is to recycle water recovered in the cathode product stream. In some implementations, the cathode water contains salts introduced via the anode water.

[0108] 1A shows an example of an electrolytic carbon oxide reduction system that can be used to control the composition and flow of water in an MEA cell. As shown, system 101 includes an MEA cell 103 that includes an anode 105, a cathode 107, and a membrane electrode assembly 109. System 101 also includes an anode water recirculation loop 111 and a gaseous carbon oxide recirculation loop 125.

[0109] In the illustrated embodiment, an anode water recirculation loop 111 delivers water to and removes water from the anode 105. The loop 111 includes an anode water reservoir 113 and water flow paths 115, 117, and 119. The anode water recirculation loop 111 may interface with a water source 121 and / or a concentrated salt solution source 123. These sources may be used to dose purified water and / or concentrated salt solution to the anode water to adjust its composition. In certain embodiments, the water source 121 provides purified water, such as highly purified deionized water, e.g., water having a resistivity of at least about 10 megaohm-cm. In the illustrated embodiment, the concentrated salt solution source 123 is connected directly to the anode water reservoir 113. In some embodiments, the concentrated salt solution source 123 is connected elsewhere on the anode water recirculation loop 111.

[0110] 1A does not show water purification components such as filters, resin columns, or other purifiers configured to remove specific ions, such as iron ions or other transition metal ions, from the anode water. Water purification components may be provided in one of the water flow paths 115, 117, or 119, or may be provided with the water source 121 or between the water source 121 and the anode water recirculation loop 111.

[0111] In certain embodiments, the configuration of the anode water recirculation loop differs from that shown in FIG. 1A. For example, the anode water recirculation loop may not include a separate purified water source and a concentrated salt source. In some embodiments, no purified water source is used. In certain embodiments, both the purified water source and the concentrated salt solution source are connected directly to the anode water tank.

[0112] Regardless of which components are present in the anode water recirculation loop, the loop can be configured to provide or maintain anode water with a salt composition appropriate for operation of the MEA cell, such salt compositions being described elsewhere herein.

[0113] 1A , gaseous carbon oxide recirculation loop 125 provides a gaseous carbon oxide feed stream to cathode 107 and removes a gaseous product stream from cathode 107. In addition to reaction products, the cathode outlet stream may contain a substantial amount of unreacted gaseous carbon oxide. In the illustrated embodiment, recirculation loop 125 includes a water separator component 127 (e.g., a water condenser), a reduction product recovery component 129, a humidifier 131, and flow paths 122, 124, 126, and 128. Fresh carbon oxide reactant gas may be provided from a carbon oxide source 133 that connects to gaseous carbon oxide recirculation loop 125.

[0114] The humidifier 131 can humidify the input stream of carbon oxide gaseous reactant upstream from the cathode 107. The humidifier provides a relatively high water vapor partial pressure of carbon oxide, which can prevent drying out of the cathode 107 or any components of the MEA 109, as described more fully herein. In some embodiments, a humidifier is not present in the system.

[0115] Upon contacting the cathode 107, the gaseous carbon oxide reactant may remove anode water, which traveled from the anode 105 through the MEA 109 to the cathode 107. In the recirculation loop 125, the anode water present in the gaseous carbon oxide stream leaving the cathode 107 is contacted with a water separator component 127, where at least a portion of the water present in the carbon oxide outlet stream is removed. The relatively dry carbon oxide stream leaving the water separator 127 enters a reduction product recovery component 129, which removes one or more reduction products from the carbon oxide outlet stream. Such reduction products may include carbon monoxide, hydrocarbons, and / or other organic compounds.

[0116] A portion of the reduction products produced at the cathode of MEA cell 103 may be dissolved or otherwise contained in the water removed by water separator 127. To address this issue, optionally, the water treated in separator 127 is provided to a reduction product recovery component 135 configured to remove one or more reduction products from the water provided by water separator 127.

[0117] In some implementations, a substantial amount of anode water passes from the anode to the cathode of the MEA 109, potentially resulting in the loss of dissolved salts. Given the high value of salts and other high-purity waters containing dissolved salts, a connection between the two loops that can return water from the gas loop 125 to the water loop 111 can improve the technical and commercial viability of the system. Note that, as discussed, anode water can have very low concentrations (e.g., ppm or ppb levels) of certain inorganic and / or organic materials. For example, the water may have very low concentrations of iron ions and other transition metal ions. Salts may also be intentionally added to the anode water. Any such treated anode water that may be recovered from the cathode side can be returned to the anode.

[0118] In the illustrated embodiment, water removed from gaseous carbon oxide recycle loop 125 is conveyed via line 137 to anode water reservoir 113 where it can re-enter anode water recycle loop 111 .

[0119] In other embodiments, there is no direct connection between the carbon oxide recirculation loop 125 and the anode water recirculation loop 111. Also, note that the reduction product recovery component 135 is optional. In some implementations, the reduction products in the water recirculation loop 125 are not removed but are included in the water provided to the anode water recirculation loop 111. Some or all of the reduction products may be oxidized at the anode 105.

[0120] 1B shows an example of an electrolytic carbon dioxide reduction system that can be used to control the composition and flow of water in an MEA cell. In this diagram, system 151 includes an MEA carbon dioxide reduction cell 153 and two recirculation loops: a salt recirculation loop 155 and a pure water recirculation loop 157. The outputs of the two loops are combined at cell input tank 159, which produces anode water having a salt composition and concentration suitable for use with MEA cell 153, such as those described elsewhere herein. The anode water is supplied from cell input tank 159 to MEA cell 153 via conduit 152.

[0121] The anode water leaving the MEA cell 153 is provided to a salt ion harvester 161 configured to remove some or all of the salts from the anode water. The relatively pure water leaves the salt ion harvester 161 via a conduit 163, which supplies the water to a water purifier component 165, which may remove remaining impurities after the desired salt ions have been harvested. The component 165 may include a small pore filter (e.g., a Milli-DI® filter available from MilliporeSigma, Darmstadt, Germany) and / or an ion exchange resin (e.g., in a bed). The resulting purified water may have very low concentrations (e.g., ppm or ppb levels) of potentially harmful ions, such as transition metal ions and / or halides. The purified water leaving the water purifier 165 is provided to a reservoir 169 via a conduit 167. The purified water in tank 169 is then provided to cell input tank 159 where it is combined with salt or concentrated salt solution, if needed, to prepare anode water for use in MEA cell 153. As shown, the purified water is provided to cell input tank 159 via conduit 171.

[0122] As shown, purified water recirculation loop 157 includes a water purifier 165, a basin 169, a cell input basin 159, and a salt ion harvester 161. In certain embodiments, the purified water loop does not include one or more of these components. For example, some embodiments of the loop do not include a water purifier. Some embodiments of the loop do not include a basin.

[0123] 1B , the salt or concentrated salt solution produced by the salt ion harvester 161 is conveyed via conduit 175 to a salt storage tank 177, which maintains the harvested salt in solid or solution form. Salt is provided from storage tank 177 to cell input tank 159, where it is combined with purified water to prepare anode water for use in MEA cell 153, as needed. The purified water is provided to cell input tank 159 via conduit 179. Salt storage tank 177 can serve as a holding point for pumping desired salt ions into cell input tank 159 as appropriate.

[0124] As shown, the salt recirculation loop 155 includes a storage tank 177 in addition to a cell input tank 159 and a salt ion harvester 161. In certain embodiments, the salt recirculation loop does not include one or more of these components. For example, some embodiments of the loop do not include a tank.

[0125] Examples of salt ion harvesters include devices containing ion-selective membranes and devices composed of salt chelators and releasers. Such devices may select desired salt ions (e.g., potassium ions or sodium ions) in the anode water stream. In certain embodiments, the salt ion harvester produces a solid salt precipitate that is then selectively returned to the salt recirculation loop or other parts of the anode water management system.

[0126] As mentioned above, water may be purified to remove harmful ions by using ion exchange resins. Examples of such resins include (a) Diaion™ CR11, available from Mitsubishi Chemical Corporation of Tokyo, Japan, which captures relatively large multivalent ions (i.e., transition metals) that may deposit on the cathode catalyst and releases sodium ions, and (b) Amberlite™ MB20, available from DuPont de Nemours, Inc. of Wilmington, Germany, which captures all ions (cations and anions) and releases only protons and hydroxides, leaving very pure water. In certain embodiments, the resin is an iminodiacetic acid chelating resin. In certain embodiments, the resin is a mixture of strong acid cation and strong base anion exchange resins.

[0127] 1A and 1B may use a control system including a controller and one or more controllable components such as pumps, sensors, valves, and power supplies. Examples of sensors include pressure sensors, temperature sensors, flow sensors, conductivity sensors, electrolyte composition sensors including electrochemical instruments, chromatography systems, and optical sensors such as absorbance measurement tools. Such sensors may be coupled to the inlets and / or outlets of the MEA cells (e.g., within the flow field), within reservoirs for holding anode water, purified water, salt solutions, etc., and / or other components of the electrolytic carbon reduction system.

[0128] The control system may be configured to supply anode water during operation of the MEA cell. For example, the control system may maintain the salt concentration at a defined level and / or recover and recirculate the anode water. Under the control of the control system, the system may, for example, (a) recirculate the anode water exiting the anode, (b) adjust the composition and / or flow rate of the anode water to the anode, (c) return water from the cathode exit to the anode water, and / or (d) adjust the composition and / or flow rate of water recovered from the cathode stream before returning it to the anode. Note that (d) may take into account carbon oxide reduction products in the water recovered from the cathode. However, in some implementations, this need not be taken into account, as some reduction products may subsequently be oxidized at the anode to harmless products.

[0129] In certain embodiments, the control system is configured to utilize feedback from sensors (e.g., conductivity / ion-specific monitoring) to adjust the mix of pure water and introduced salt ions to ensure bulk conductivity or other anode water parameters are within desired levels. In some embodiments, sensors in the anode water detect salt concentrations, and if the concentration becomes too low, salt from a higher concentration bath may be added. If the salt concentration becomes too high, pure water may be added to dilute the salt and bring it back into the desired concentration range.

[0130] The controller may include any number of processors and / or memory devices. The controller may include control logic, such as software or firmware, and / or may execute instructions provided by another source. The controller may be integrated with electronics to control the operation of the electrolytic cell before, during, and after carbon dioxide reduction. The controller may control various components or subparts of one or more electrolytic carbon dioxide reduction systems. Depending on the process requirements and / or system type, the controller may be programmed to control any of the processes disclosed herein, such as gas delivery, temperature settings (e.g., heating and / or cooling), pressure settings, power settings (e.g., voltage and / or current delivered to electrodes of an MEA cell), liquid flow rate settings, fluid delivery settings, and dosing of purified water and / or salt solution. These controlled processes may be connected to or interact with one or more systems that function in coordination with the electrolytic carbon dioxide reduction system.

[0131] In various embodiments, the controller includes electronics comprising various integrated circuits, logic, memory, and / or software that receive instructions, issue instructions, and control the operations described herein. The integrated circuits may include chips in the form of firmware that store program instructions, digital signal processors (DSPs), chips defined as application-specific integrated circuits (ASICs), and / or one or more microprocessors or microcontrollers that execute the program instructions (e.g., software). The program instructions may be instructions communicated to the controller in the form of various individual settings (or program files) that define operating parameters for carrying out processes in one or more components of the electrolytic oxidative carbon reduction system. In some embodiments, the operating parameters may be part of a recipe defined by a process engineer to accomplish one or more process steps during the production of specific reduction products, such as carbon monoxide, hydrocarbons, and / or other organic compounds.

[0132] In some implementations, the controller may be part of or coupled to a computer that is integrated with, coupled to, or otherwise networked to the system, or a combination thereof. For example, the controller may utilize and / or execute instructions stored remotely (e.g., in the “cloud”). The computer may provide remote access to the system to monitor the current progress of an electrolysis operation, examine the history of past electrolysis operations, and examine trends or performance metrics from multiple electrolysis operations to modify parameters of a current process, configure process steps to follow a current process, or initiate a new process. In some examples, a remote computer (e.g., a server) may provide process recipes to the system over a network, which may include a local network or the Internet. The remote computer may include a user interface that allows entry or programming of parameters and / or settings, which are then communicated from the remote computer to the system. In some examples, the controller receives instructions in the form of data specifying parameters for each of the process steps to be performed during one or more operations.

[0133] The controller may be distributed, such as by including one or more individual controllers networked together and operating toward a common purpose, such as the processes and controls described herein. An example of a distributed controller for such a purpose is a recirculation loop that communicates with one or more integrated circuits on the MEA cell or one or more remotely located integrated circuits (e.g., at the platform level or as part of a remote computer) that couple to control the process in the chamber.

[0134] In certain embodiments, the electrolytic carbon dioxide reduction system is configured and controlled to avoid salt precipitation within the MEA. Precipitated salts can block channels and / or have other effects that degrade the performance of the MEA cell. In some cases, for example, on the cathode side, the cell can become excessively dry because the dry gaseous reactants remove excess water from the MEA, especially on the cathode side. This problem, which can lead to salt precipitation, can be addressed by controlling the partial pressure of water in the gas inlet stream (e.g., by humidifying the gaseous carbon dioxide source gas). In some cases, the salt concentration in the anode water is sufficiently high to promote salt precipitation in the MEA. This problem can be addressed by controlling the salt concentration in the anode water. In some embodiments, the system is taken offline periodically or as needed to address any actual or potential salt accumulation in the MEA cell. While offline, the cathode compartment or other parts of the system can be flushed with water to avoid or remove salt accumulation.

[0135] [EMBODIMENT OF META DESIGN] [MEA Overview] In various embodiments, the MEA contains an anode layer, a cathode layer, an electrolyte, and optionally one or more other layers. The layers may be solid and / or gel. The layers may include a polymer, such as an ion-conducting polymer.

[0136] During use, the cathode of the MEA is heated by CO x , CO x By combining three inputs: ions (e.g., protons) that chemically react with CO, and electrons xThe anode of the MEA promotes the electrochemical reduction of CO, hydrocarbons, and / or organic compounds containing oxygen and hydrogen, such as methanol, ethanol, and acetic acid. In use, the anode of the MEA promotes electrochemical oxidation reactions, such as the electrolysis of water, to produce elemental oxygen and protons. The cathode and anode may each contain a catalyst to promote their respective reactions.

[0137] The composition and arrangement of the layers in the MEA x To this end, the MEA (a) minimizes parasitic reduction reactions (non-CO) at the cathode; x reduction reaction), (b) CO at the anode or elsewhere in the MEA x (c) maintaining the physical integrity of the MEA during the reaction (e.g., preventing delamination of the MEA layers); and (d) reducing CO x The MEA may facilitate any one or more of the following conditions: (a) preventing crossover of reduction products, (b) preventing crossover of oxidation products (e.g., O), (c) maintaining a cathode environment favorable for oxidation, (d) providing a path for desired ions to travel between the cathode and anode while blocking undesired ions, and (e) minimizing voltage losses. As described herein, the presence of salt or salt ions in the MEA can facilitate some of all of these conditions.

[0138] [Considerations for COx reduction] Polymer-based membrane assemblies such as MEAs have been used in a variety of electrolysis systems, such as water electrolyzers, and in a variety of galvanic systems, such as fuel cells. x The reduction of r represents a problem that does not occur, or occurs to a lesser extent, in water electrolyzers and fuel cells.

[0139] For example, in many applications, CO xThe MEA for CO reduction requires a lifespan of about 50,000 hours or more (about 5 years of continuous operation), which is significantly longer than the expected lifespan of a fuel cell for automobile use (for example, about 5,000 hours). x The MEA for CO reduction uses electrodes with a relatively large surface area compared to the MEAs used in fuel cells for automotive applications. x The MEA for reduction should be at least approximately 500 cm 2 Electrodes having a surface area (not considering pores and other non-planar features) of 0.1 mm may be used.

[0140] CO x Reduction reactions may be implemented in an operating environment that facilitates mass transfer of specific reactant and product species and suppresses parasitic reactions. Fuel cell and water electrolyzer MEAs often cannot create such an operating environment. For example, such MEAs may promote undesirable parasitic reactions, such as gaseous hydrogen evolution at the cathode and / or gaseous CO production at the anode.

[0141] In some systems, CO x The rate of reduction is determined by the amount of gaseous CO at the cathode. x Limited by reactant availability. In contrast, the rate of water electrolysis is not significantly limited by reactant availability; liquid water tends to be readily accessible to the cathode and anode, allowing the electrolyzer to operate at close to the highest possible current density.

[0142] [MEA configuration] In certain embodiments, the MEA has a cathode layer, an anode layer, and a polymer electrolyte membrane (PEM) between the anode and cathode layers. The polymer electrolyte membrane provides ionic transfer between the anode and cathode layers while preventing electronic transfer that would cause a short circuit. The cathode layer includes a reduction catalyst and a first ion-conducting polymer. The cathode layer may also include an ionic and / or electronic conductor. The anode layer includes an oxidation catalyst and a second ion-conducting polymer. The anode layer may also include an ionic and / or electronic conductor. The PEM includes a third ion-conducting polymer.

[0143] In certain embodiments, the MEA has a cathode buffer layer between the cathode layer and the polymer electrolyte membrane, the cathode buffer comprising a fourth ion-conducting polymer.

[0144] In certain embodiments, the MEA includes an anode buffer layer between the anode layer and the polymer electrolyte membrane, the anode buffer comprising a fifth ion-conducting polymer.

[0145] With respect to a particular MEA design, there are three available classes of ion-conducting polymers: anion conductors, cation conductors, and mixed cation-anion conductors. In certain embodiments, at least two of the first, second, third, fourth, and fifth ion-conducting polymers are from different classes of ion-conducting polymers.

[0146] [Conductivity and selectivity of ion-conducting polymers for MEA layers] The term "ion-conducting polymer" is used herein to describe a polymer electrolyte having a specific conductivity for anions and / or cations greater than about 1 mS / cm. The term "anion conductor" refers to an ion-conducting polymer that conducts primarily anions (although there may still be some small amount of cation conduction) and has an anion transport number greater than about 0.85 at a thickness of about 100 microns. The terms "cation conductor" and / or "cation-conducting polymer" refer to an ion-conducting polymer that conducts primarily cations (e.g., there may still be an incidental amount of anion conduction) and has a cation transport number greater than about 0.85 at a thickness of about 100 microns. In the case of ion-conducting polymers described as conducting both anions and cations ("cation-anion conductors"), neither the anions nor the cations have a transport number greater than about 0.85 or less than about 0.15 at a thickness of about 100 microns. To say that a material conducts ions (anions and / or cations) is to say that the material is an ion-conducting material or an ionomer. Examples of each class of ion-conducting polymers are given in the table below. [Table 1]

[0147] Some Class A ion-conducting polymers are known by trade names such as 2259-60 (Pall RAI), AHA from Tokuyama Co, fumasep® FAA- (fumatech GmbH), Sustanion®, Morgane ADP from Solvay, or Tosflex® SF-17 from Tosoh anion exchange membrane materials. Additional Class A ion-conducting polymers include HNN5 / HNN8 from Ionomr, FumaSep from Fumatech, TM1 from Orion, and PAP-TP from W7energy. Some Class C ion-conducting polymers are known by trade names such as Nafion® (DuPont™), GORE-SELECT® (Gore), fumapem® (fumatech GmbH), and various formulations of Aquivion® PFSA (Solvay).

[0148] [Bipolar MEA for COx reduction] In certain embodiments, the MEA includes a bipolar interface having an anion-conducting polymer on the cathode side of the MEA and an interacting cation-conducting polymer on the anode side of the MEA. In some implementations, the cathode contains a first catalyst and an anion-conducting polymer. In certain embodiments, the anode contains a second catalyst and a cation-conducting polymer. In some implementations, a cathode buffer layer located between the cathode and the PEM contains an anion-conducting polymer. In some embodiments, an anode buffer layer located between the anode and the PEM contains a cation-conducting polymer.

[0149] In operation, MEAs with bipolar interfaces transport ions through the polymer electrolyte, electrons through the metal and / or carbon in the cathode and anode layers, and liquids and gases through the pores of the layers.

[0150] In embodiments using an anion-conducting polymer in the cathode and / or cathode buffer layer, the MEA can reduce or block undesired reactions that produce undesired products and reduce the overall efficiency of the cell. In embodiments using a cation-conducting polymer in the anode and / or anode buffer layer, the MEA can reduce or block undesired reactions that reduce the production of desired products and reduce the overall efficiency of the cell.

[0151] For example, at the potential levels used for cathodic reduction of CO2, hydrogen ions can be reduced to hydrogen gas. This is a parasitic reaction; the current that could be used to reduce CO2 is instead used to reduce hydrogen ions. Hydrogen ions may be generated by various oxidation reactions performed at the anode of the CO2 reduction reactor, migrate across the MEA, and reach the cathode, where they can be reduced to produce hydrogen gas. The extent to which this parasitic reaction can proceed is a function of the concentration of hydrogen ions present at the cathode. Therefore, the MEA may use anion-conducting materials in the cathode layer and / or cathode buffer layer. The anion-conducting materials at least partially block hydrogen ions from reaching catalytic sites on the cathode. As a result, the parasitic generation of hydrogen gas is reduced, improving the rate of CO2 or other product production and the overall efficiency of the process.

[0152] Another reaction that can be avoided is the reaction of carbonate or bicarbonate ions at the anode to produce CO. Aqueous carbonate or bicarbonate ions can be generated from CO at the cathode. When such ions reach the anode, they can react with hydrogen ions to produce and release gaseous CO. As a result, CO is net transported from the cathode to the anode, where it does not react and is lost along with oxidation products. To prevent carbonate and bicarbonate ions generated at the cathode from reaching the anode, the anode and / or anode buffer layer may include a cation-conducting polymer that at least partially blocks the transport of negative ions, such as bicarbonate ions, to the anode.

[0153] Thus, in some designs, a bipolar membrane structure increases the pH at the cathode to promote CO2 reduction, and a cation-conducting polymer, such as a proton exchange layer, prevents significant amounts of CO2 and CO2 reduction products (e.g., bicarbonate) from passing to the anode side of the cell.

[0154] CO x An example of an MEA 200 for use in reduction is shown in FIG. 2. The MEA 200 has a cathode layer 220 and an anode layer 240 separated by an ion-conducting polymer layer 260, which provides a pathway for ions to travel between the cathode layer 220 and the anode layer 240. In certain embodiments, the cathode layer 220 comprises an anion-conducting polymer and / or the anode layer 240 comprises a cation-conducting polymer. In certain embodiments, the cathode layer and / or the anode layer of the MEA are porous. The pores can facilitate gas and / or fluid transport and can increase the amount of catalyst surface area available for reaction.

[0155] The ion conducting layer 260 may include two or three sublayers: a polymer electrolyte membrane (PEM) 265, an optional cathode buffer layer 225, and / or an optional anode buffer layer 245. One or more layers of the ion conducting layer may be porous. In certain embodiments, at least one layer is non-porous, so that reactants and products of the cathode cannot pass to the anode via gas and / or liquid transport, and vice versa. In certain embodiments, the PEM layer 265 is non-porous. Examples of properties of anode and cathode buffer layers are provided elsewhere herein. In certain embodiments, the ion conducting layer includes only a single layer or two sublayers.

[0156] 3 shows a CO2 electrolyzer 303 configured to receive water and CO2 (e.g., humidified or dried gaseous CO2) as reactants at the cathode 305 and to output CO as a product. The electrolyzer 303 is also configured to receive water as a reactant at the anode 307 and to output gaseous oxygen. The electrolyzer 303 includes a bipolar layer having an anion-conducting polymer 309 adjacent to the cathode 305 and a cation-conducting polymer 311 (shown as a proton exchange membrane) adjacent to the anode 307.

[0157] As shown in the close-up inset of the bipolar interface 313 of the electrolytic cell 303, the cathode 305 includes an anion exchange polymer (in this example, the same anion conducting polymer 309 as in the bipolar layer) that electronically conducts between the carbon support particles 317 and the metal nanoparticles 319 supported on the support particles. CO2 and water are transported through pores, such as pore 321, to reach the metal nanoparticles 319, where they react, in this case, with hydroxide ions to produce bicarbonate ions and reduction reaction products (not shown). CO2 can also reach the metal nanoparticles 319 by transport within the anion exchange polymer 315.

[0158] Hydrogen ions are transported from the anode 307 through the cation-conducting polymer 311 until they reach the bipolar interface 313, where further transport of the hydrogen ions to the cathode is blocked by the anion-exchange polymer 309. At interface 313, the hydrogen ions may react with bicarbonate or carbonate ions to produce carbonic acid (HCO), which may decompose to produce CO and water. As described herein, the resulting CO may be provided in the gas phase and must be routed within the MEA back to the cathode 305 where it can be reduced. The cation-conducting polymer 311 blocks anions, such as bicarbonate ions, from being transported to the anode, where they may react with protons and release CO, which cannot participate in the reduction reaction at the cathode.

[0159] As shown, the cathode buffer layer having an anion-conducting polymer may work in concert with the cathode and its anion-conducting polymer to block the transport of protons to the cathode. An MEA using ion-conducting polymers of appropriate conductivity types in the cathode, anode, cathode buffer layer, and anode buffer layer (if present) may block the transport of cations to the cathode and anions to the anode, although the cations and anions may still come into contact in interior regions of the MEA, e.g., the MEA layers.

[0160] As shown in Figure 3, bicarbonate and / or carbonate ions may combine with hydrogen ions between the cathode and anode layers to form carbonic acid, which may decompose to form gaseous CO. MEAs have sometimes been observed to delaminate, which may be due to the production of this gaseous CO, which has no easy exit route.

[0161] The delamination problem can be addressed by using a cathode buffer layer with an inert filler and associated pores. One possible explanation for its effectiveness is that the pores create a path for gaseous carbon dioxide to return to the cathode where it can be reduced. In some embodiments, the cathode buffer layer is porous, but at least one layer between the cathode and anode layers is non-porous. This can prevent delamination while also preventing the passage of gas and / or bulk liquid between the cathode and anode layers. For example, a non-porous layer can prevent the direct passage of water from the anode to the cathode. The porosity of the various layers in the MEA is further described elsewhere herein.

[0162] [Example of bipolar MEA] As an example, the MEA includes a cathode layer including a reduction catalyst and a first anion-conducting polymer (e.g., Sustainion, FumaSep FAA-3, Tokuyama anion exchange polymer); an anode layer including an oxidation catalyst and a first cation-conducting polymer (e.g., PFSA polymer); a membrane layer including a second cation-conducting polymer and disposed between the cathode and anode layers to conductively connect them; and a cathode buffer layer including a second anion-conducting polymer (e.g., Sustainion, FumaSep FAA-3, Tokuyama anion exchange polymer) and disposed between the cathode and membrane layers to conductively connect them. In this example, the cathode buffer layer can have a porosity of about 1 to 90 volume percent, although it can additionally or alternatively have any suitable porosity (e.g., including no porosity). In other examples, the cathode buffer layer can have any suitable porosity (eg, 0.01 to 95%, 0.1 to 95%, 0.01 to 75%, 1 to 95%, 1 to 90%, etc.).

[0163] Too much porosity can reduce the ionic conductivity of the buffer layer. In some embodiments, the porosity is 20% or less, and in particular embodiments, it is 0.1-20%, 1-10%, or 5-10%. Porosity in these ranges may be sufficient to allow for the movement of water and / or CO2 without losing ionic conductivity. Porosity may be measured as further described below.

[0164] In a related example, the membrane electrode assembly can include an anode buffer layer that includes a third cation-conducting polymer and is disposed between the membrane layer and the anode layer to conductively connect the membrane layer and the anode layer. The anode buffer layer preferably has a porosity of about 1 to 90 volume percent, but can additionally or alternatively have any suitable porosity (e.g., including no porosity). However, in other configurations and examples, the anode buffer layer can have any suitable porosity (e.g., 0.01 to 95%, 0.1 to 95%, 0.01 to 75%, 1 to 95%, 1 to 90%). As with the cathode buffer layer, in some embodiments, the porosity is 20% or less, e.g., 0.1 to 20%, 1 to 10%, or 5 to 10%.

[0165] In one example, an anode buffer layer may be used in an MEA having a cathode catalyst layer with an anion exchange polymer, a cathode buffer layer with an anion exchange polymer, a membrane with a cation exchange polymer, and an anode buffer layer with an anion exchange polymer. In such a structure, the anode buffer layer may be porous to facilitate the transport of water to the membrane / anode buffer layer interface. Water splits at this interface to produce protons, which migrate through the membrane, and hydroxides, which migrate to the anode catalyst layer. One advantage of this structure is that it can be used with a low-cost hydroxide oxidation catalyst (NiFeO) that is stable only in basic conditions. x It is possible to use

[0166] In another specific example, a membrane electrode assembly includes a cathode layer comprising a reduction catalyst and a first anion-conducting polymer (e.g., Sustainion, FumaSep FAA-3, Tokuyama anion exchange polymer), an anode layer comprising an oxidation catalyst and a first cation-conducting polymer, a membrane layer comprising a second anion-conducting polymer (e.g., Sustainion, FumaSep FAA-3, Tokuyama anion exchange polymer) and disposed between the cathode layer and the anode layer to conductively connect the cathode layer and the anode layer, and an anode buffer layer comprising a second cation-conducting polymer and disposed between the anode layer and the membrane layer to conductively connect the anode layer and the membrane layer.

[0167] An MEA containing an anion exchange polymer membrane and an anode buffer layer containing a cation exchange polymer may be used for CO reduction. In this case, water is formed at the membrane / anode buffer layer interface. The pores in the anode buffer layer can facilitate water removal. One advantage of this structure is acid stability (e.g., IrO x ) The use of a hydroxide catalyst.

[0168] In a related example, the membrane electrode assembly can include a cathode buffer layer that includes a third anion-conducting polymer and is disposed between the cathode layer and the membrane layer to conductively connect the cathode layer and the membrane layer. The third anion-conducting polymer can be the same as or different from the first and / or second anion-conducting polymer. The cathode buffer layer preferably has a porosity of about 1 to 90 volume percent, but can additionally or alternatively have any suitable porosity (e.g., including no porosity). However, in other configurations and examples, the cathode buffer layer can have any suitable porosity (e.g., 0.01 to 95%, 0.1 to 95%, 0.01 to 75%, 1 to 95%, 1 to 90%). In some embodiments, the porosity is 20% or less, and in particular embodiments, it is 0.1 to 20%, 1 to 10%, or 5 to 10%.

[0169] In one example, the cathode catalyst layer is composed of 4 nm diameter Au nanoparticles supported on Vulcan XC72R carbon and mixed with TM1 (mTPN-1) anion exchange polymer electrolyte (manufactured by Orion). The layer thickness is approximately 15 μm, with Au / (Au+C) = 20 wt%, a TM1 to catalyst mass ratio of 0.32, a mass loading of 1.4-1.6 mg / cm2 (total Au+C), and an estimated porosity of 0.56. The anion exchange polymer layer is composed of TM1 and PTFE particles. The PTFE diameter is approximately 200 nm. The TM1 molecular weight is 30 kJ / 45 kJ. The layer thickness is approximately 15 μm. The PTFE can result in approximately 8% porosity. The proton exchange membrane layer is composed of a perfluorosulfonic acid polymer (e.g., Nafion 117) and is approximately 125 μm thick. The membrane forms a continuous layer that prevents significant gas (CO2, CO, H2) transport through the layer. The anode catalyst layer consists of 10 μm thick Ir or IrOx nanoparticles (aggregates of 100-200 nm).

[0170] [Anion-exchange membrane-only MEA for COx reduction] In some embodiments, the MEA does not contain a cation-conducting polymer layer. In such embodiments, the electrolyte is not a cation-conducting polymer, and the anode, if it includes an ion-conducting polymer, does not contain a cation-conducting polymer. Examples are provided herein.

[0171] AEM-only MEAs allow anion conduction across the MEA. In embodiments where none of the MEA layers are significantly conductive to cations, hydrogen ions have limited mobility within the MEA. In some implementations, AEM-only membranes can promote CO2 and / or CO2 reduction by providing a high pH environment (e.g., at least about pH 7) and suppressing parasitic hydrogen evolution reactions at the cathode. Like other MEA designs, AEM-only MEAs allow ions, particularly anions such as hydroxide ions, to migrate through the polymer electrolyte. In some embodiments, the pH may be lower; a pH of 4 or higher may be high enough to suppress hydrogen evolution. AEM-only MEAs also allow electrons to migrate to and through the metal and carbon in the catalyst layer. In embodiments, due to the presence of pores in the anode layer, cathode layer, and / or PEM, AEM-only MEAs allow liquid and gas to migrate through the pores.

[0172] In certain embodiments, an AEM-only MEA includes an anion exchange polymer electrolyte membrane with an electrocatalytic layer on either side (cathode and anode). In some embodiments, one or both electrocatalytic layers also contain an anion exchange polymer electrolyte.

[0173] In certain embodiments, an AEM-only MEA is formed by depositing cathode and anode electrocatalyst layers on a porous conductive support, such as a gas diffusion layer, to form gas diffusion electrodes (GDEs), and sandwiching an anion exchange membrane between the gas diffusion electrodes.

[0174] In certain embodiments, AEM-only MEAs are used for CO reduction. The use of an anion-exchange polymer electrolyte avoids the low pH environment that would be unfavorable for CO reduction. Furthermore, when an AEM is used, water is transported away from the cathode catalyst layer, thereby preventing water accumulation (flooding) that could block reactive gas transport at the cell's cathode.

[0175] Water transport in MEAs occurs through various mechanisms, including diffusion and electroosmotic drag. In some embodiments, at the current densities of the CO2 electrolyzers described herein, electroosmotic drag is the dominant mechanism. Water is dragged along with the ions as they move through the polymer electrolyte. For cation-exchange membranes, such as Nafion membranes, the amount of water transport is well characterized and understood to depend on the membrane preconditioning / hydration. Protons move from positive to negative potential (anode to cathode), each carrying two to four water molecules, depending on preconditioning. In anion-exchange polymers, the same type of effect occurs. Hydroxide, bicarbonate, or carbonate ions moving through the polymer electrolyte "dragging" water molecules. In anion-exchange MEAs, ions move from negative to positive potential, i.e., from cathode to anode, carrying water molecules with them and, in the process, moving water from cathode to anode.

[0176] In certain embodiments, AEM-only MEAs are used in CO reduction reactions. Unlike CO reduction reactions, CO reduction does not produce carbonate or bicarbonate anions that can be transported to the anode to release beneficial reactants.

[0177] FIG. 4 shows an example structure of a CO2-reducing MEA 401 having a cathode catalyst layer 403, an anode catalyst layer 405, and an anion-conducting PEM 407. In certain embodiments, the cathode catalyst layer 403 includes metal catalyst particles (e.g., nanoparticles) that are unsupported or supported on a conductive substrate, such as carbon particles. In some implementations, the cathode catalyst layer 403 further includes an anion-conducting polymer. The metal catalyst particles can catalyze CO2 reduction, particularly at a pH greater than 7. In certain embodiments, the anode catalyst layer 405 includes metal oxide catalyst particles (e.g., nanoparticles) that are unsupported or supported on a conductive substrate, such as carbon particles. In some implementations, the anode catalyst layer 403 further includes an anion-conducting polymer. Examples of metal oxide catalyst particles for the anode catalyst layer 405 include iridium oxide, nickel oxide, nickel iron oxide, iridium ruthenium oxide, and platinum oxide. Anion-conducting PEM407 may comprise any of a variety of anion-conducting polymers, such as HNN5 / HNN8 from Ionomr, FumaSep from Fumatech, TM1 from Orion, PAP-TP from W7energy, and Sustainion from Dioxide Materials. These and other anion-conducting polymers may be used, having an ion exchange capacity (IEC) ranging from 1.1 to 2.6, an operating pH range of 0 to 14, tolerable solubility in several organic solvents, adequate thermal and mechanical stability, excellent ionic conductivity / ASR, and acceptable water absorption / swelling ratios. The polymer may be chemically exchanged with a specific anion in place of a halogen anion prior to use.

[0178] As shown in FIG. 4, CO, such as CO gas, may be provided to the cathode catalyst layer 403. In certain embodiments, the CO may be provided via a gas diffusion electrode. In the cathode catalyst layer 403, the CO reacts to form, typically, C x O y H zThe anions produced at the cathode catalyst layer 403 may include hydroxides, carbonates, and / or bicarbonates. These may diffuse, migrate, or otherwise migrate to the anode catalyst layer 405. In the anode catalyst layer 405, oxidation reactions, such as the oxidation of water, may occur to produce diatomic oxygen ions and hydrogen ions. In some applications, the hydrogen ions may react with the hydroxides, carbonates, and / or bicarbonates to produce water, carbonic acid, and / or CO2. The fewer interfaces, the lower the resistance. In some embodiments, a highly basic environment is maintained for C2 and C3 hydrocarbon synthesis.

[0179] 5 shows an example structure of a CO2 reduction MEA 501 having a cathode catalyst layer 503, an anode catalyst layer 505, and an anion-conducting PEM 507. Overall, the structure of MEA 501 may be similar to that of MEA 401 of FIG. 4. However, the cathode catalyst may be selected to promote the CO2 reduction reaction, which means that different reduction catalysts are used in the CO2 and CO2 reduction embodiments.

[0180] In some embodiments, AEM-only MEAs may be advantageous for CO reduction. The water absorption number of the AEM material can be selected to help regulate moisture at the catalyst interface, thereby improving CO availability to the catalyst. Therefore, AEM-only membranes may be advantageous for CO reduction. Bipolar membranes may be advantageous for CO reduction due to their better resistance to CO dissolution and crossover in basic anolyte media.

[0181] In various embodiments, the cathode catalyst layer 503 includes metal catalyst particles (e.g., nanoparticles) that are unsupported or supported on a conductive substrate, such as carbon particles. In some implementations, the cathode catalyst layer 503 further includes an anion-conducting polymer. In particular embodiments, the anode catalyst layer 505 includes metal oxide catalyst particles (e.g., nanoparticles) that are unsupported or supported on a conductive substrate, such as carbon particles. In some implementations, the anode catalyst layer 503 further includes an anion-conducting polymer. Examples of metal oxide catalyst particles for the anode catalyst layer 505 may include those identified for the anode catalyst layer 405 in FIG. 4. The anion-conducting PEM 507 may include any of a variety of anion-conducting polymers, such as those identified for the PEM 407 in FIG. 4.

[0182] As shown in Figure 5, CO gas may be provided to the cathode catalyst layer 503. In certain embodiments, CO may be provided via a gas diffusion electrode. In the cathode catalyst layer 503, CO reacts to form, typically, C x O y H z to produce the reduction product shown as

[0183] The anions produced at the cathode catalyst layer 503 may include hydroxide ions. These may diffuse, migrate, or otherwise migrate to the anode catalyst layer 505. At the anode catalyst layer 505, an oxidation reaction, such as the oxidation of water, may occur to produce diatomic oxygen ions and hydrogen ions. In some applications, the hydrogen ions may react with the hydroxide ions to produce water.

[0184] Although the general configuration of MEA 501 is similar to that of MEA 401, there are certain differences between these MEAs. First, the MEA may be wetter for CO reduction, helping the catalyst surface have more -H. Also, for CO reduction, a significant amount of CO may be dissolved and then transported to the anode of an AEM-only MEA, as shown in Figure 4. For CO reduction, significant CO gas crossover is less likely to occur. In this case, the reaction environment may be very basic. The MEA material, including the catalyst, may be selected to have good stability in a high-pH environment. In some embodiments, a thinner membrane may be used for CO reduction than for CO reduction.

[0185] [Example of an AEM-only MEA] 1.Copper metal (USRN40nm thick Cu, ~0.05mg / cm 2 ) was deposited on a porous carbon sheet (Sigracet39BC gas diffusion layer) by electron beam deposition. Ir metal nanoparticles were deposited at 3 mg / cm via drop casting. 2 An anion exchange membrane (25-50 μm, 80 mS / cm) from Ionomr was deposited on a porous titanium sheet at a loading of 1000 mS / cm. 2 OH- conductivity, 2-3 mS / cm 2 HCO3 - The electrode (high conductivity, 33-37% water absorption) was sandwiched between a porous carbon sheet and a titanium sheet, with the electrode catalyst layer facing the membrane. 2. Sigma Aldrich 80 nm spherical Cu nanoparticles mixed with FAA-3 anion-exchange solid polymer electrolyte from Fumatech, set up as above, mass ratio of FAA-3 to catalyst 0.10.

[0186] Various features and examples of MEAs are described in U.S. Patent Application Publication Nos. US2017 / 0321334, published November 9, 2017, and 20190226103, published July 25, 2019, which are incorporated by reference in their entireties. All publications referenced herein are incorporated by reference in their entirety as if fully set forth herein.

[0187] [Cathode catalyst layer - general structure] As mentioned above, the cathode of the MEA, also known as the cathode layer or cathode catalyst layer, facilitates COx conversion. x It is a porous layer containing a catalyst for the reduction reaction.

[0188] In some embodiments, the cathode catalyst layer contains a blend of reduced catalyst particles, electronically conductive support particles that provide a support for the reduced catalyst particles, and a cathode ion-conducting polymer. In some embodiments, the reduced catalyst particles are blended with the cathode ion-conducting polymer without a support.

[0189] Examples of materials that can be used for the reduction catalyst particles include, but are not limited to, transition metals such as V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Au, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Ir, Pt, and Hg, and combinations thereof, and / or any other suitable materials. Other catalyst materials can include alkali metals, alkaline earth metals, lanthanides, actinides, and post-transition metals such as Sn, Si, Ga, Pb, Al, Tl, Sb, Te, Bi, Sm, Tb, Ce, Nd, and In, or combinations thereof, and / or any other suitable catalyst materials. The choice of catalyst depends on the particular reaction to be performed at the cathode of the CRR.

[0190] The catalyst may be in the form of nanoparticles ranging in size from about 1 to 100 nm, or particles ranging in size from about 0.2 to 10 nm, or particles ranging in size from about 1 to 1000 nm, or any other suitable range. In addition to nanoparticles and larger particles, films and nanostructured surfaces may be used.

[0191] If used, the conductive support particles in the cathode can be carbon particles of various forms. Other possible conductive support particles include boron-doped diamond or fluorine-doped tin oxide. In one configuration, the conductive support particles are vulcanized carbon. The conductive support particles can be nanoparticles. The size range of the conductive support particles is between about 20 nm and 1000 nm, or any other suitable range. This is particularly useful if the conductive support particles are compatible with the chemicals present in the cathode during CRR operation, are reductively stable, and have a high hydrogen generation overpotential so that they do not participate in any electrochemical reactions.

[0192] For composite catalysts such as Au / C, examples of metal nanoparticle sizes may range from about 2 nm to 20 nm, and carbon sizes as support materials may be about 20 to 200 nm. For pure metal catalysts such as Ag or Cu, the crystalline particle size ranges from 2 nm to 500 nm. Agglomerates can be even larger, in the micrometer range.

[0193] Generally, such conductive support particles are larger than the reduced catalyst particles, and each conductive support particle can support many reduced catalyst particles. Figure 6 is a schematic diagram showing possible configurations of two different types of catalyst supported on a catalyst support particle 610, such as a carbon particle. A first type of catalyst particle 630 and a second type of catalyst particle 650 are attached to the catalyst support particle 610. In various configurations, only one type of catalyst particle or more than two types of catalyst particles are attached to the catalyst support particle 610.

[0194] The use of two types of catalysts can be useful in certain embodiments. For example, one catalyst may be suitable for one reaction (e.g., CO → CO) and a second catalyst may be suitable for another reaction (e.g., CO → CH). Together, the catalyst layers perform the conversion of CO to CH, but different steps of the reaction occur over different catalysts.

[0195] Electronically conductive supports may also be in forms other than particles, including tubes (e.g., carbon nanotubes) and sheets (e.g., graphene). High surface area to volume structures are useful for providing sites for the attachment of catalyst particles.

[0196] In addition to the reduction catalyst particles and the electronically conductive support particles, the cathode catalyst layer may include an ionically conductive polymer. There are tradeoffs in selecting the amount of cathode ionically conductive polymer in the cathode. It can be important to include enough cathode ionically conductive polymer to provide sufficient ionic conductivity. However, it is also important that the cathode be porous so that reactants and products can easily move through the cathode and to maximize the amount of catalytic surface area available for reaction. In various configurations, the cathode ionically conductive polymer comprises anywhere in the range of 30-70 wt%, 20-80 wt%, or 10-90 wt%, or any other suitable range, of the material in the cathode layer. The weight percent of the ionically conductive polymer in the cathode can be adjusted to account for CO x The porosity and ionic conductivity of the cathode layer are selected to provide the highest current density for reduction. In some embodiments, the weight percent of the ion-conducting polymer in the cathode can be 20 to 60 weight percent or 20 to 50 weight percent. Example thicknesses of the cathode catalyst layer range from about 80 nm to 300 μm.

[0197] In addition to the reduced catalyst particles, the cathode ion-conducting polymer, and the electron-conducting support (if present), the cathode catalyst layer may contain other additives such as PTFE.

[0198] In addition to the polymer to catalyst mass ratio, the catalyst layer also has a mass loading (mg / cm 2 The thickness of a layer may be characterized by its volumetric density and porosity. Porosity can be determined by a variety of methods. One method is to multiply the loading of each component (e.g., catalyst, support, and polymer) by its respective density. These are summed to determine the thickness of the material that the components occupy. This is then divided by the known total thickness to obtain the fraction of the layer that is filled with material. The resulting fraction is then subtracted from 1 to obtain the fraction of the layer that is assumed to be filled with air, i.e., the porosity. Methods such as mercury porosimetry or image processing of TEM images may also be used.

[0199] Examples of cathode catalyst layers for producing CO, methane, and ethylene / ethanol are shown below. CO generation: 4 nm diameter Au nanoparticles supported on Vulcan XC72R carbon and mixed with TM1 anion-exchange polymer electrolyte from Orion. Layer thickness was approximately 15 μm, Au / (Au+C) = 30%, TM1 to catalyst mass ratio was 0.32, and mass loading was 1.4–1.6 mg / cm. 2 , estimated porosity is 0.47. Methane production: 20-30 nm Cu nanoparticles supported on Vulcan XC72R carbon and mixed with FAA-3 anion-exchange solid polymer electrolyte from Fumatech. The mass ratio of FAA-3 to catalyst was 0.18. 1-100 μg / cm 2 Within the wider range of , the estimated Cu nanoparticle loading is ∼7.1 μg / cm 2 . Ethylene / ethanol production: Cu nanoparticles of 25–80 nm size mixed with FAA-3 anion-exchange solid polymer electrolyte from Fumatech. The mass ratio of FAA-3 to catalyst was 0.10. They were deposited on a Sigracet39BC GDE for pure AEM or on an MEA electrode assembly. The estimated Cu nanoparticle loading was 270 μg / cm. 2 .

[0200] The function, materials, and structure of the components of the cathode catalyst layer are further described below.

[0201] [Water management (cathode catalyst layer)] The cathode catalyst layer can facilitate the movement of water and prevent it from being trapped in the cathode catalyst layer. Trapped water can then be transported to the catalyst. x and / or may impede the migration of reaction products from the cathode catalyst layer.

[0202] Water management challenges are unique to CRRs in many ways. For example, compared to oxygen electrodes in PEM fuel cells, CRRs use much lower gas flow rates. Because gas-phase water removal is determined by the volumetric gas flow rate, much less gas-phase water removal is performed in CRRs. CRRs may operate at higher pressures than fuel cells (e.g., 100 psi to 450 psi). At higher pressures, the same molar flow rate results in lower volumetric flow rates and less gas-phase water removal. As a result, liquid water exists within the MEAs of CRRs to be removed. For some MEAs, the ability to remove gas-phase water is further limited by temperature limitations not present in fuel cells. For example, CO2 reduction to CO2 may be performed at approximately 50°C, and ethylene and methane production may be performed at 20°C to 25°C. This compares to the typical operating temperatures of fuel cells, which are 80°C to 120°C. As a result, more liquid-phase water must be removed.

[0203] Properties that affect the ability of a cathode catalyst layer to remove water include porosity, pore size, pore size distribution, hydrophobicity, the relative amounts of ion-conducting polymer, metal catalyst particles, and electronically conductive support, layer thickness, catalyst distribution throughout the layer, and ion-conducting polymer distribution throughout the layer and around the catalyst.

[0204] The porous layer allows for an egress path for water. In some embodiments, the cathode catalyst layer has a pore size distribution that includes pores between 1 nm and 100 nm in size and pores at least 1 micron in size. This size distribution can aid in water removal. The porous structure can be formed by one or more of the following: pores within the carbon support material, stacking pores between stacked spherical carbon nanoparticles, secondary stacking pores (micrometer-scale) between agglomerated carbon spheres, or an inert filler (e.g., PTFE) introduced at the interface between the carbon and the pores, which also creates irregular pores in the hundreds of nanometers to micrometers range.

[0205] The cathode catalyst layer may have a thickness that contributes to water management. Using a thicker layer allows the catalyst, and therefore the reaction, to be dispersed over a larger volume, which allows for more widespread water distribution and easier management.

[0206] Ion-conducting polymers with non-polar hydrophobic backbones may be used in the cathode catalyst layer. In some embodiments, the cathode catalyst layer may include a hydrophobic polymer, such as PTFE, in addition to the ion-conducting polymer. In some embodiments, the ion-conducting polymer may be a component of a copolymer that also includes a hydrophobic polymer.

[0207] [Gas transport (cathode catalyst layer)] The cathode catalyst layer may be structured for gas transport. x is transported to the catalyst, and gas-phase reaction products (e.g., CO, ethylene, methane, etc.) are transported from the catalyst bed.

[0208] Particular challenges associated with gas transport are inherent in CRR. Gases are transported in and out of the cathode catalyst layer. Namely, CO xis pumped in and products such as CO, ethylene, and methane are pumped out. In PEM fuel cells, gas (O2 or H2) is pumped in and either nothing or product water is pumped out. Also, in PEM water electrolyzers, water is a reactant with the O2 and H2 gas products.

[0209] Operating conditions, including pressure, temperature, and flow rate through the reactor, affect gas transport. Cathode catalyst layer properties that affect gas transport include porosity, pore size and distribution, layer thickness, and ionomer distribution.

[0210] In some embodiments, contact between the ionomer and the catalyst is minimized. For example, in embodiments using a carbon support, the ionomer may form a continuous network along the surface of the carbon while minimizing contact with the catalyst. The ionomer, support, and catalyst may be designed so that the ionomer has a higher affinity for the support surface than for the catalyst surface. This allows the ionomer to conduct ions to and from the catalyst, while facilitating gas transport to and from the catalyst without being blocked by the ionomer.

[0211] [Ionomer (cathode catalyst layer)] The ionomer may have several functions, including holding the particles of the catalyst layer together and allowing the movement of ions through the cathode catalyst layer. In some cases, the interaction of the ionomer with the catalyst surface may result in the formation of CO x It may create an environment favorable for reduction, increasing selectivity to the desired product and / or reducing the voltage required for the reaction. It is important that the ionomer is an ionically conductive polymer that allows the movement of ions through the cathode catalyst layer. For example, hydroxide, bicarbonate, and carbonate ions are converted into CO x It migrates away from the catalyst surface where reduction occurs. In the following description, the ionomer of the cathode catalyst layer may be referred to as the first ion-conducting polymer.

[0212] The first ion-conducting polymer may comprise at least one ion-conducting polymer that is an anion conductor, which may be advantageous as it increases the pH compared to proton conductors.

[0213] In some embodiments, the first ion-conducting polymer can include one or more covalently bonded positively charged functional groups configured to transport mobile negatively charged ions. The first ion-conducting polymer can be selected from the group consisting of aminated tetramethylpolyphenylene, poly(ethylene-co-tetrafluoroethylene)-based quaternary ammonium polymers, quaternized polysulfones, blends thereof, and / or any other suitable ion-conducting polymer. The first ion-conducting polymer can be configured to solubilize bicarbonate or hydroxide salts.

[0214] In some embodiments, the first ion-conducting polymer can include at least one ion-conducting polymer that is a cation-anion conductor. The first ion-conducting polymer can be selected from the group consisting of polyethers capable of transporting cations and anions and polyesters capable of transporting cations and anions. The first ion-conducting polymer can be selected from the group consisting of polyethylene oxide, polyethylene glycol, polyvinylidene fluoride, and polyurethane.

[0215] The cation-anion conductor increases the pH (compared to a pure cation conductor). Furthermore, in some embodiments, it may be advantageous to use a cation-anion conductor to promote acid-base recombination in a larger volume rather than at the 2D interface between the anion- and cation-conducting polymers. This can spread out the water and CO2 production, heat generation, and potentially lower the membrane resistance by reducing the barrier to the acid-base reaction. All of these may be advantageous in helping to avoid product buildup, heat, and lowering resistive losses in the MEA, thereby lowering the cell voltage.

[0216] Typical anion-conducting polymers have a polymer backbone with covalently attached positively charged functional groups attached thereto. In some embodiments, these may include positively charged nitrogen groups. In some embodiments, the polymer backbone is non-polar, as described above. The polymer may be of any suitable molecular weight, for example, from 25,000 g / mol to 150,000 g / mol, although it will be understood that polymers outside this range may be used.

[0217] A particular challenge with ion-conducting polymers in CRRs includes the fact that CO2 can dissolve or solubilize the polymer electrolyte, reducing its mechanical stability and making it more susceptible to swelling, allowing the polymer to move more freely. This reduces the mechanical stability of the entire catalyst layer and the polymer electrolyte membrane. In some embodiments, polymers that are less susceptible to CO2 plasticization are used. Also, unlike in water electrolyzers and fuel cells, carbonate and bicarbonate ion conduction are key parameters for CO2 reduction.

[0218] The introduction of polar functional groups, such as hydroxyl and carboxyl groups, which can form hydrogen bonds, creates a pseudo-crosslinked network. Crosslinkers such as ethylene glycol and aluminum acetylacetonate can be added to strengthen the anion-exchange polymer layer and reduce CO2 plasticization of the polymer. Additives such as polydimethylsiloxane copolymers can also help mitigate CO2 plasticization.

[0219] According to various embodiments, the ion-conducting polymer may have a bicarbonate ion conductivity of at least 12 mS / cm, be chemically and mechanically stable at temperatures up to 80° C., and be soluble in organic solvents used during fabrication, such as methanol, ethanol, and isoproponal. xIt is stable (chemically stable and has stable solubility) in the presence of reduction products. Ion-conducting polymers may be characterized by their ion exchange capacity, the sum of active sites or functional groups that participate in ion exchange, which in some embodiments may range from 2.1 mmol / g to 2.6 mmol / g.

[0220] In the table above, examples of anion-conducting polymers are listed as Class A ion-conducting polymers. A specific example of an anion-conducting polymer is Orion mTPN1, which has a m-triphenylfluoroalkylene backbone and trimethylammonium (TMA+) as the cationic group. The chemical structure is shown below. [ka]

[0221] Further examples include anion exchange membranes manufactured by Fumatech and Ionomr. Fumatech FAA-3 ionomer is provided in the Br-form. Polybenzimidazole-based anion exchange polymers / membranes manufactured by Ionomr are provided in the I-form as AF-1-HNN8-50-X.

[0222] The as-received polymer is reacted with anions (e.g., I - , Br - etc.) with bicarbonate.

[0223] Also, as noted above, in certain embodiments, the ionomer may be a cation-ion conducting polymer, examples of which are shown in the table above as Class B ion conducting polymers.

[0224] [Metal catalyst (cathode catalyst layer)] Metal catalysts catalyze the COx reduction reaction. Metal catalysts are typically nanoparticles, although in some embodiments, larger particles, films, and nanostructured surfaces may be used. The specific morphology of the nanoparticles may expose and stabilize active sites with greater activity.

[0225] Metal catalysts are often composed of pure metals (e.g., Cu, Au, Ag), although certain alloys or other bimetallic systems have high activity and may be used for specific reactions. The choice of catalyst may be guided by the desired reaction. For example, Au may be used for CO production, while Cu may be used for methane and ethylene production. Other metals, including Ag, alloys, and bimetallic systems, may also be used. CO reduction has a high overpotential compared to other well-known electrochemical reactions, such as hydrogen evolution and oxygen evolution, over known catalysts. Small amounts of contaminants can poison catalysts for CO conversion.

[0226] Also, as noted above, metal catalysts such as Cu, Au, and Ag are less developed than catalysts such as platinum used in hydrogen fuel cells.

[0227] Metal catalyst properties that affect the performance of the cathode catalyst layer include size, size distribution, uniformity of coverage on the support particles, shape, loading (characterized as weight of metal / weight of metal + weight of carbon, or mass of particles per geometric area of ​​the catalyst layer), surface area (actual metal catalyst surface area per volume of catalyst layer), purity, and the presence of poisoning surface ligands from the synthesis.

[0228] The nanoparticles may be synthesized by any suitable method, such as those described in, for example, Phan et al., "Role of Capping Agent in Wet Synthesis of Nanoparticles," J.Phys.Chem.A 2018, 121, 17, 3213-3219; Bakshi, "How Surfactants Control Crystal Growth of Nanomaterials," Cryst.Growth Des. 2016, 16, 2, 1104-1133; and Morsy, "Role of Surfactants in Nanotechnology and Their Applications," Int.J.Curr.Microbiol.App.Sci. 2014, 3, 5, 237-260, which are incorporated herein by reference.

[0229] In some embodiments, metal nanoparticles are provided without the presence of poisoning surface ligands. This may be achieved by using an ionomer as a ligand to direct the synthesis of the nanocrystalline catalyst. The surface of the metal nanocatalyst is directly connected to the ion-conducting ionomer. This eliminates the need to treat the catalyst surface to allow the ionomer to contact the metal, improving contact.

[0230] In some embodiments, the metal catalyst may be disposed on a carbon support. For CO production, examples include Premetek 20 wt. % Au supported on Vulcan XC-72R carbon with a 4-6 nm Au particle size and 30% Au / C supported on Vulcan XC-72R with a 5-7 nm Au particle size. For methane, examples include Premetek 20 wt. % Cu supported on Vulcan XC-72R carbon with a 20-30 nm Cu particle size. In some embodiments, the metal catalyst may be unsupported. For ethylene production, examples of unsupported metal catalysts include Sigma-Aldrich unsupported Cu with a 80 nm particle size and thin e-beam or sputter-deposited Cu layers with a 10 nm to 100 nm thickness.

[0231] [Support (cathode catalyst layer)] The support of the cathode catalyst layer can have various functions. The support of the cathode catalyst layer can stabilize the metal nanoparticles to prevent them from agglomerating and distribute catalytic sites throughout the volume of the catalyst layer to spread reactant loss and product formation. The support of the cathode catalyst layer can also electronically form conductive paths to the metal nanoparticles. For example, carbon particles are packed together so that contacting carbon particles provide conductive paths. The void spaces between the particles form a porous network through which gases and liquids can pass.

[0232] In some embodiments, carbon supports developed for fuel cells can be used. Many different types have been developed, typically ranging in size from 50 nm to 500 nm, and can be obtained in a variety of shapes (spheres, nanotubes, sheets (e.g., graphene)), porosity, surface area per volume, electrical conductivity, and functional groups (N-doped, O-doped, etc.).

[0233] The support may be hydrophobic and may have an affinity for the metal nanoparticles.

[0234] Examples of carbon black that can be used are: ·Vulcan XC-72R-density 256mg / cm2, 30~50nm ·Ketjen Black-Hollow structure, density 100~120mg / cm2, 30~50nm Printex carbon, 20-30nm

[0235] [Anode catalyst layer] The anode of the MEA, also called the anode layer or anode catalyst layer, facilitates the oxidation reaction. The anode of the MEA is a porous layer that contains a catalyst for the oxidation reaction. An example reaction is as follows: 2H2O→4H + +4e - +O2 (acidic environment of proton exchange polymer electrolyte - bipolar membrane), or 4OH - →4e - +O2 + 2H2O (basic environment of anion exchange polymer electrolyte)

[0236] The oxidation of other substances may also be carried out, such as hydrocarbons to produce CO2, or chloride ions to produce chlorine gas.

[0237] In some embodiments, referring to FIG. 2, the anode 240 contains a blend of an oxidation catalyst and an anode ion-conducting polymer. Depending on the reactants and anode catalyst supplied to the anode, various oxidation reactions occur at the anode. In one configuration, the oxidation catalyst is selected from the group consisting of Ir, Pt, Ni, Ru, Pd, Au metals and oxides and their alloys, IrRu, PtIr, Ni, NiFe, stainless steel, and combinations thereof. The oxidation catalyst can further contain conductive support particles selected from the group consisting of carbon, boron-doped diamond, and titanium.

[0238] The oxidation catalyst may be in the form of a structured mesh or particles. If the oxidation catalyst is in the form of particles, the particles may be supported by electronically conductive support particles. The conductive support particles may be nanoparticles. This is particularly useful if the conductive support particles are compatible with the chemicals present in the anode 240 during CRR operation and are oxidatively stable so that they do not participate in any electrochemical reactions. This is particularly useful if the conductive support particles are selected with the voltage and reactants at the anode in mind. In some arrangements, the conductive support particles are titanium, which is well suited for high voltages. In other configurations, the conductive support particles are carbon, which may be most useful at low voltages. Generally, such conductive support particles are larger than the oxidation catalyst particles, and each conductive support particle can support many oxidation catalyst particles. An example of such an arrangement is shown in FIG. 3 and described above in connection with the cathode catalyst layer.

[0239] In one configuration, the oxidation catalyst is iridium ruthenium oxide. Examples of other materials that can be used for the oxidation catalyst include, but are not limited to, those listed above. It should be understood that many of these metal catalysts may be in the form of an oxide, particularly under reaction conditions.

[0240] In some embodiments, the MEA has an anode layer including an oxidation catalyst and a second ion-conducting polymer. The second ion-conducting polymer can include one or more polymers containing covalently bound negatively charged functional groups configured to transport mobile positively charged ions. The second ion-conducting polymer can be selected from the group consisting of ethanesulfonyl fluoride, 2-[1-[difluoro-[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2,-tetrafluoro-, tetrafluoroethylene, tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octene sulfonic acid copolymer, other perfluorosulfonic acid polymers, and blends thereof. Examples of cation-conducting polymers include, for example, Nafion 115, Nafion 117, and / or Nafion 211.

[0241] There are trade-offs in selecting the amount of ion-conducting polymer in the anode. It is important to include enough anode ion-conducting polymer to provide sufficient ionic conductivity. However, it is also important that the anode be porous so that reactants and products can easily transport through the anode and maximize the amount of catalytic surface area available for reaction. In various configurations, the ion-conducting polymer in the anode constitutes about 50% by weight of the layer, or about 5-20%, 10-90%, 20-80%, 25-70%, or any suitable range. This is particularly useful when the anode 240 can withstand high voltages, such as voltages greater than about 1.2 V versus the reversible hydrogen electrode. This is particularly useful when the anode 240 is porous to maximize the amount of catalytic surface area available for reaction and facilitate gas and liquid transport.

[0242] In one example of a metal catalyst, Ir or IrOx particles (100-200 nm) and Nafion ionomer form a porous layer approximately 10 μm thick. The loading of the metal catalyst is approximately 0.5-3 g / cm. 2 is.

[0243] In some embodiments, NiFeOx is used for basic reactions.

[0244] [PEM] The MEA includes a polymer electrolyte membrane (PEM) disposed between and conductively coupled to the anode and cathode catalyst layers. Referring to FIG. 2, the polymer electrolyte membrane 265 has high ionic conductivity (greater than about 1 mS / cm) and is mechanically stable. Mechanical stability can be demonstrated in various ways, such as high tensile strength, modulus, elongation at break, and tear resistance. Many commercially available membranes can be used for the polymer electrolyte membrane 265. Examples include, but are not limited to, various Nafion® formulations, GORE-SELECT, FumaPEM® (PFSA) (FuMA-Tech GmbH), and Aquivion® (PFSA) (Solvay).

[0245] In one configuration, the PEM includes at least one ion-conducting polymer that is a cation conductor. The third ion-conducting polymer can include one or more covalently bound negatively charged functional groups configured to transport mobile positively charged ions. The third ion-conducting polymer can be selected from the group consisting of ethanesulfonyl fluoride, 2-[1-[difluoro-[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2,-tetrafluoro-, tetrafluoroethylene, tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octene sulfonic acid copolymer, other perfluorosulfonic acid polymers, and blends thereof.

[0246] [Cathode buffer layer] Referring to FIG. 2 , note that the polymer electrolyte membrane 265, when it is a cation conductor and conducting protons, contains a high concentration of protons during CRR operation, while the cathode 220 operates best when a low concentration of protons is present. It can be useful to include a cathode buffer layer 225 between the polymer electrolyte membrane 265 and the cathode 220 to provide a transition region from a high concentration of protons to a low concentration of protons. In one configuration, the cathode buffer layer 225 is an ion-conducting polymer that has many of the same properties as the ion-conducting polymer in the cathode 220. The cathode buffer layer 225 provides a region for the proton concentration to transition from the polymer electrolyte membrane 265, which has a high proton concentration, to the cathode 220, which has a low proton concentration. Within the cathode buffer layer 225, protons from the polymer electrolyte membrane 265 meet anions from the cathode 220, and they neutralize each other. The cathode buffer layer 225 helps ensure that a detrimental number of protons from the polymer electrolyte membrane 265 do not reach the cathode 220 and increase the proton concentration. If the proton concentration at the cathode 220 is too high, COx reduction will not occur. A high proton concentration is considered to be in the range of about 10 to 0.1 molar, and a low concentration is considered to be less than about 0.01 molar.

[0247] The cathode buffer layer 225 can include a single polymer or multiple polymers. When the cathode buffer layer 225 includes multiple polymers, the multiple polymers can be mixed together or arranged in separate adjacent layers. Examples of materials that can be used for the cathode buffer layer 225 include, but are not limited to, FumaSep FAA-3, Tokuyama anion exchange membrane materials, and polyether-based polymers such as polyethylene oxide (PEO), as well as blends thereof. Further examples are provided above in the description of the cathode catalyst layer.

[0248] The thickness of the cathode buffer layer is selected to be sufficient to provide a low proton concentration and therefore high COx reduction activity. This sufficiency may vary depending on the material of the cathode buffer layer. Typically, the thickness of the cathode buffer layer is between about 200 nm and 100 μm, between 300 nm and 75 μm, between 500 nm and 50 μm, or any suitable range.

[0249] In some embodiments, the cathode buffer layer is less than 50 μm, e.g., between 1 and 25 μm, such as between 1 and 5 μm, 5 and 15 μm, or 10 and 25 μm. Using a cathode buffer layer in this range of thickness can reduce the proton concentration in the cathode while maintaining the overall conductivity of the cell. In some embodiments, ultrathin layers (100 nm to 1 μm, and in some embodiments, submicron) may be used. Also, as noted above, in some embodiments, the MEA does not have a cathode buffer layer. In some such embodiments, the anion-conducting polymer in the cathode catalyst layer is sufficient. The thickness of the cathode buffer layer may be characterized relative to the thickness of the PEM.

[0250] Water and CO2 formed at the interface between the cathode buffer layer and the PEM can delaminate the MEA to which the polymer layer connects. The delamination problem can be addressed by using a cathode buffer layer with inert filler particles and associated pores. One possible explanation for its effectiveness is that the pores create a pathway for gaseous carbon dioxide to return to the cathode where it can be reduced.

[0251] Suitable materials for the inert filler particles include, but are not limited to, TiO2, silica, PTFE, zirconia, and alumina. In various configurations, the size of the inert filler particles is between 5 nm and 500 μm, between 10 nm and 100 μm, or any suitable size range. The particles may be generally spherical.

[0252] Too much PTFE (or other filler) dilutes the polymer electrolyte to the point where ionic conductivity is low. Too much polymer electrolyte dilutes the PTFE to the point where it is not useful for porosity. In many embodiments, the polymer electrolyte / PTFE mass ratio is 0.25 to 2, more specifically, 0.5 to 1. The polymer electrolyte / PTFE volume ratio (or, more generally, polymer electrolyte / inert filler) can be 0.25 to 3, 0.5 to 2, 0.75 to 1.5, or 1.0 to 1.5.

[0253] In other configurations, porosity is achieved by using specific processing methods when the layer is formed. One example of such a processing method is laser ablation, where nano- to micro-sized channels are formed in the layer. Another example is mechanically drilling holes in the layer to form channels through the layer.

[0254] In one configuration, the cathode buffer layer has a porosity of 0.01% to 95% (e.g., by weight, volume, mass, etc., and anywhere therebetween). However, in other configurations, the cathode buffer layer can have any suitable porosity (e.g., 0.01 to 95%, 0.1 to 95%, 0.01 to 75%, 1 to 95%, 1 to 90%). In some embodiments, the porosity is 50% or less, such as 0.1 to 50%, 5 to 50%, 20 to 50%, 5 to 40%, 10 to 40%, 20 to 40%, or 25% to 40%. In some embodiments, the porosity is 20% or less, such as 0.1 to 20%, 1 to 10%, or 5 to 10%.

[0255] Porosity may be measured as described above in connection with the catalyst layer, including using component mass loading and thickness with methods such as mercury porosimetry, X-ray diffraction (SAXS or WAXS), and image processing of TEM images to calculate filled versus empty space. Porosity is measured when the MEA is completely dry, because materials swell to varying degrees when exposed to water during operation.

[0256] The porosity of the layers of the MEA, including the cathode buffer layer, is discussed further below.

[0257] [Anode buffer layer] In some CRR reactions, bicarbonate is produced at the cathode 220. Preventing bicarbonate from migrating away from the cathode can be useful if there is a polymer somewhere between the cathode 220 and the anode 240 that blocks bicarbonate transport. As the bicarbonate migrates, it can pick up some CO, potentially reducing the amount of CO available for reaction at the cathode. In one configuration, the polymer electrolyte membrane 265 includes a polymer that blocks bicarbonate transport. Examples of such polymers include, but are not limited to, Nafion® formulations, GORE-SELECT, FumaPEM® (PFSA) (FuMA-Tech GmbH), and Aquivion® (PFSA) (Solvay). In another configuration, there is an anode buffer layer 245 between the polymer electrolyte membrane 265 and the anode 240, which blocks bicarbonate transport. If the polymer electrolyte membrane is an anion conductor or does not block bicarbonate transport, an additional anode buffer layer to prevent bicarbonate transport may be useful. Materials that can be used to block bicarbonate transport include, but are not limited to, Nafion® formulations, GORE-SELECT, FumaPEM® (PFSA) (FuMA-Tech GmbH), and Aquivion® (PFSA) (Solvay). Of course, if there is no bicarbonate in the CRR, it is not particularly desirable to include a bicarbonate blocking function in the ion exchange layer 260.

[0258] In another embodiment of the present invention, the anode buffer layer 245 provides a region for the proton concentration to transition between the polymer electrolyte membrane 265 and the anode 240. The proton concentration in the polymer electrolyte membrane 265 depends on both its composition and the ions it is conducting. For example, a Nafion polymer electrolyte membrane 265, which conducts protons, has a high proton concentration. A FumaSep FAA-3 polymer electrolyte membrane 265, which conducts hydroxides, has a low proton concentration. For example, if the desired proton concentration in the anode 240 differs from that of the polymer electrolyte membrane 265 by more than three orders of magnitude, the anode buffer layer 245 can be useful for creating a transition from the proton concentration in the polymer electrolyte membrane 265 to the desired proton concentration in the anode. The anode buffer layer 245 can include a single polymer or multiple polymers. If the anode buffer layer 245 includes multiple polymers, the multiple polymers can be mixed together or arranged in separate, adjacent layers. Materials that may be useful for providing a region for pH transition include, but are not limited to, Nafion, FumaSep FAA-3, Sustainion®, Tokuyama anion exchange polymers, and polyether-based polymers such as polyethylene oxide (PEO), blends thereof, and / or any other suitable material.

[0259] A high proton concentration is considered to be in the range of about 10 to 0.1 molar, and a low concentration is considered to be less than about 0.01 molar. Ion-conducting polymers can be divided into various classes based on the type of ions they conduct, as discussed in detail above. There are three classes of ion-conducting polymers listed in Table 4 above. In one embodiment of the present invention, at least one of the ion-conducting polymers in the cathode 220, anode 240, polymer electrolyte membrane 265, cathode buffer layer 225, and anode buffer layer 245 is of a different class than at least one of the others.

[0260] [Layer Porosity] It can be useful if some or all of the layers, such as the cathode 220, cathode buffer layer 225, anode 240, and anode buffer layer 245, are porous. In some configurations, porosity is achieved by combining inert filler particles with the polymers in these layers. Suitable materials for the inert filler particles include, but are not limited to, TiO2, silica, PTFE, zirconia, and alumina. In various configurations, the size of the inert filler particles ranges from 5 nm to 500 μm, 10 nm to 100 μm, or any suitable size range. In other configurations, porosity is achieved by using specific processing methods when the layers are formed. One example of such a processing method is laser ablation, in which nano- to micro-sized channels are formed in the layer. Laser ablation can additionally or alternatively achieve porosity in the layer through subsurface ablation. Subsurface ablation focuses a beam at a point within the layer, thereby vaporizing the layer material near that point, creating voids within the layer. This process can be repeated to form voids throughout the layer, thereby achieving porosity in the layer. The volume of the voids is preferably determined by the laser power (e.g., higher laser power corresponds to a larger void volume), but may additionally or alternatively be determined by the beam focal size or any other suitable laser parameter. Another example is mechanically drilling holes in the layer to form channels through the layer. The porosity can have any suitable distribution within the layer (e.g., uniform, an increasing porosity gradient through the layer, a random porosity gradient, a decreasing porosity gradient through the layer, periodic porosity, etc.).

[0261] The porosity of the above and other examples and variations (e.g., the porosity of the cathode buffer layer, anode buffer layer, membrane layer, cathode layer, anode layer, other suitable layer, etc.) preferably has a uniform distribution, but can additionally or alternatively have any suitable distribution (e.g., a random distribution, a gradient of increasing pore size through or across the layer, a gradient of decreasing pore size through or across the layer, etc.). The porosity can be formed by any suitable mechanism, such as inert filler particles (e.g., diamond particles, boron-doped diamond particles, polyvinylidene fluoride / PVDF particles, polytetrafluoroethylene / PTFE particles, etc.), and any other suitable mechanism for forming substantially non-reactive regions within the polymer layer. The inert filler particles can have any suitable size, such as a minimum of about 10 nanometers and a maximum of about 200 nanometers, and / or any other suitable dimension or dimension distribution.

[0262] As described above, the cathode buffer layer preferably has a porosity of about 1 to 90 volume percent, but can additionally or alternatively have any suitable porosity (e.g., including no porosity). However, in other configurations and examples, the cathode buffer layer can have any suitable porosity (e.g., 0.01 to 95%, 0.1 to 95%, 0.01 to 75%, 1 to 95%, 1 to 90%, etc.). In some embodiments, the porosity is 20% or less, e.g., 0.1 to 20%, 1 to 10%, or 5 to 10%.

[0263] In some embodiments, the cathode buffer layer is porous, but at least one layer between the cathode and anode layers is non-porous. This can prevent delamination while also preventing the passage of gases and / or bulk liquids between the cathode and anode layers. For example, the non-porous layer can prevent the direct passage of water from the anode to the cathode.

[0264] [MEA manufacturing] CO xMEAs for reduction can be manufactured using a variety of techniques. In various embodiments, MEA manufacturing uses multiple stages. Small variations in the manufacturing process parameters can result in large differences in performance.

[0265] In certain embodiments, MEA fabrication uses a polymer electrolyte membrane (e.g., Nafion PEM) layer, with an anion exchange polymer electrolyte layer and cathode catalyst layer deposited or otherwise formed on the cathode, and an anode catalyst layer deposited or otherwise formed on the anode. Another route is to fabricate the catalyst layer on a porous gas diffusion layer (e.g., carbon for the cathode or titanium for the anode) and sandwich the membrane (which may include an anion exchange layer) between the catalyst-containing porous layers. In certain embodiments, the catalyst layer is fabricated by dispersing an ink of solid catalyst and support particles and polymer electrolyte in a solvent. The ink can be applied to the polymer electrolyte membrane or GDL by various methods. The solvent is then evaporated, leaving a porous, solid catalyst layer.

[0266] Imaging methods may be used to characterize thickness and uniformity: thickness should be consistent and controllable, and uniformity should be smooth and as defect-free as possible.

[0267] Various techniques may be used to form the individual layers of the MEA. Generally, these techniques form a layer, such as a PEM layer or a GDL, as referred to herein, on a substrate. Examples of such techniques include ultrasonic spray deposition, doctor blade coating, gravure, screen printing, and decal transfer.

[0268] Catalyst inks using anion exchange polymers have not been well studied (especially for certain polymers) and do not have the same solution structure as typical Nafion-based inks used in fuel cells and electrolyzers. The formulations and steps required to form well-dispersed, stable catalyst inks were unknown. Nafion is believed to form micelle-like structures that allow for relatively easy suspension in aqueous media. Other ion-conducting polymers, particularly some anion-conducting polymers, do not form such structures and are therefore more difficult to provide in suspension.

[0269] In certain embodiments, the catalyst layer ink is prepared by mixing a metal or a metal supported on a carbon catalyst with an ion-conducting polymer (e.g., an anion-conducting polymer) and dispersing it in a solvent (such as an alcohol) by ultrasonication.

[0270] As indicated, specific manufacturing techniques use doctor blade application, screen printing, decal transfer, electrospinning, etc. Roll-to-roll techniques such as gravure or microgravure may be used for high throughput processing.

[0271] [MEA post-processing] After an MEA is fabricated, further processing can be used to improve performance. Examples of types of performance improvements include lifetime and voltage. In some embodiments, the post-processing introduces salt or specific salt ions into the MEA. In some embodiments, the post-processing produces an MEA with structural modifications resulting from the processing, including better adhesion between layers.

[0272] Hot pressing: The MEA is heated under pressure to bond the layers. Hot pressing helps to "melt" the layers together and prevent delamination. Time: Approximately 2 to 10 minutes (MEA only), 1.5 to 2 minutes (MEA + gas distribution layer (GDL)). Press "MEA + GDL" at least twice to allow a stable assembly to form. Temperature: Approximately 100°C to 150°C. Pressure: Approximately 300 psi to 600 psi (for a 3 x 3 inch (7.62 x 7.62 cm) 1 / 2 MEA), but the MEA can withstand approximately 2500 psi without a GDL. Hydration: Prior to cell assembly, the MEA is immersed in water or an aqueous solution to wet the polymer electrolyte. In some embodiments, the aqueous solution is a salt solution as described herein.

[0273] Boiling a Nafion or other polymer electrolyte MEA permanently changes the polymer electrolyte macrostructure and increases the amount of water within the polymer matrix, which increases ionic conductivity but also increases the water transport coefficient.

[0274] Heat drying, which reduces the moisture content and reduces the amount of water transported through the polymer electrolyte during operation.

[0275] [Stabilized interface between MEA layers] Water and CO2 formed at the interface between an anion-conducting layer (e.g., cathode buffer layer) and a cation-conducting membrane (e.g., PEM) can separate or delaminate the two layers at the location where the polymer layers connect. The reactions at the bipolar interface are shown in Figures 3 and 7.

[0276] Additionally, because it is desirable for the CO2 to return to the cathode of the cell where it can be reduced to the anode instead of being lost, the pathways (e.g., pores) in the anion exchange layer (e.g., the cathode buffer layer and / or cathode layer) provide both a way to remove water and CO2 from the interface to prevent delamination and a way to return the CO2 to the cathode where it can react.

[0277] The structure shown in Figure 7 is similar to that shown in Figure 3, but Figure 7 includes additional information related to mass transfer and the production of CO2 and water at the bipolar interface. For example, hydroxide and CO2 react on the cathode side to produce bicarbonate ions, which are shown migrating toward the bipolar interface 713. On the anode side, hydrogen ions produced by the oxidation of water migrate toward the bipolar interface 713, where they react with the bicarbonate ions to produce water and CO2. Both water and CO2 must be able to escape without damaging the bipolar layers.

[0278] 7 also shows water transport pathways including (a) electroosmotic drag due to anions from the cathode to the interface 713, (b) electroosmotic drag due to cations from the anode to the interface 713, and (c) diffusion. Water evaporates at the anode and cathode.

[0279] Various MEA designs include features that resist delamination and optionally provide a path for reaction products to leave the interface region. In some embodiments, the bipolar interface is flat. However, in some designs, the interface comprises a compositional gradient and / or an interlocking structure. These are further described below with reference to Figures 8A, 8B, 8C, and 8D, which show bipolar interfaces of MEA designs configured to resist delamination.

[0280] In some embodiments, the interface comprises a gradient. The gradient may be formed, for example, by using two nozzles during spray deposition and adding anion exchange polymer with varying relative amounts of polymer during deposition of the cation exchange layer. Similarly, cation exchange polymer may be added during deposition of the anion exchange layer. For example, with reference to FIG. 7 , the gradient may extend through substantially all or a portion of the anion exchange region and cation exchange region, such that the anion exchange region has primarily anion exchange polymer adjacent to the cathode, and the relative amount of cation exchange polymer increases from the cathode toward the interface 713. Similarly, the cathode exchange region has primarily cation exchange polymer adjacent to the anode, and the relative amount of anion exchange polymer increases from the anode toward the interface 713. In some embodiments, there are pure anion exchange regions and pure cation exchange regions, with a gradient between the two.

[0281] In some embodiments, the layers of the bipolar membrane are fused together. This may be achieved by choosing an appropriate solvent. For example, Nafion is at least slightly soluble in a water / ethanol mixture. By using that mixture (or another solvent in which the cation-conducting polymer is soluble) as the solvent for the anion-conducting polymer, Nafion or other cation-conducting polymer can be at least slightly dissolved and melted into the interface. In some embodiments, this results in a thin gradient, e.g., a gradient extending 0.5-10% into the thickness of the anion-conducting polymer layer.

[0282] In some embodiments, the interface comprises a mixture of polymers. Figure 8A shows a bipolar interface 813 where a cation-conducting polymer 821 and an anion-conducting polymer 819 are mixed. In the example of Figure 8A, a portion of an anion-conducting polymer layer 809 and a portion of a cation-conducting polymer layer 811 are shown. Anion-conducting polymer layer 809 may be a pure anion-conducting polymer, and cation-conducting polymer layer 811 may be a pure cation-exchange polymer. Cation-conducting polymer 821 may be the same or a different cation-conducting polymer as cation-conducting polymer layer 811. Anion-conducting polymer 819 may be the same or a different anion-conducting polymer as anion-conducting polymer layer 809.

[0283] In some embodiments, the interface includes a third material that physically reinforces the interface. For example, FIG. 8B shows an example of material 830 spanning interface 813. That is, material 830 is partially present in anion-conducting polymer layer 809 and cation-conducting polymer layer 811. Material 830 may thus bond these two layers in a manner that resists delamination. In one example, material 830 is a porous, inert material such as porous PTFE. Such an interface may be fabricated, for example, by casting or otherwise applying a cation-conducting polymer and an anion-conducting polymer to opposite sides of a PTFE or similar porous film, followed by hot pressing.

[0284] FIG. 8C shows a bipolar interface 813 with cation-conducting polymer protrusions 840 extending from the cation-conducting polymer layer 811 into the anion-conducting polymer layer 809. These protrusions can mechanically reinforce the interface 813 to prevent delamination when CO2 and water are produced at the interface. In some embodiments, the protrusions extend from the anion-conducting polymer layer 809 into the cation-conducting polymer layer 811. In particular embodiments, the protrusions extend in both directions. Example dimensions include in-plane dimensions of 10 μm to 1 mm, although smaller dimensions (e.g., 500 nm to 1 μm) are also contemplated. The out-of-plane dimensions can be, for example, 10 to 75% or 10 to 50% of the total thickness of the polymer layer through which they extend. The protrusions may be fabricated by any suitable technique, such as, for example, lithography techniques, or by spraying the polymer into a patterned mesh that is subsequently removed. Surface roughening techniques may also be used to form the protrusions. In some embodiments, the protrusions may be formed from a different material, for example, a metal, to help interlock the polymer layers and mechanically strengthen the interface.

[0285] 8D shows a bipolar interface 813 having a third material 850 disposed between the cation-conducting polymer layer 811 and the anion-conducting polymer layer 809 or mixed with one or more of the cation-conducting polymer layer 811 and the anion-conducting polymer layer 809. In some embodiments, for example, the third material 850 can be an additive, as described further below. In some embodiments, the third material 850 can be a blend of an anion-conducting ionomer and a cation-conducting ionomer at the interface. For example, it can be a mixture of 5 wt% Nafion ionomer and 2 wt% Orion mTPN1. In some embodiments, the third material can include an ion acceptor and donor, mixed together or provided as separate layers.

[0286] In some embodiments, the interface contains an additive to promote acid-base reactions and prevent delamination. In some embodiments, the additive can facilitate acid-base recombination over a larger volume than just the 2D interface between the anion-conducting polymer and the cation-conducting polymer. This spreads the generation of water and CO2 and heat, and can lower the membrane resistance by lowering the barrier to acid-base reactions. These effects can be advantageous in helping to avoid product accumulation and heat, and in reducing resistive losses in the MEA and lowering cell voltage. Additionally, they help to avoid material degradation at the interface due to heat and gas generation.

[0287] Examples of additives that promote acid-base reactions include molecules that are both proton and anion acceptors, such as hydroxide-containing ionic liquids with 1-butyl-3-methylimidazolium hydroxide. Other ionic liquids may also be used. In some embodiments, an ionomer different from the ionomers of the anion-conducting polymer layer and the cation-conducting polymer layer may be used. For example, a relatively high conductivity anion exchange material, such as Sustainion, may be used. While such anion exchange materials may not be selective enough for use as a cathode buffer layer, they may be used at the interface.

[0288] Further examples of materials that may be present at the interface include block copolymers with differently charged groups (e.g., both cationic and anionic fixed charged groups), cation-anion conducting polymers, resin materials, ion donors such as oxides including graphene oxide, catalysts for acid / base recombination, catalysts for reacting H2 diffusing from the anode and cathode with O2, water splitting catalysts, CO2 absorption materials, and H2 absorption materials.

[0289] In some embodiments, a crosslinking agent may be added to covalently crosslink the two polymers of the bipolar membrane. An example of a crosslinking group includes xylene, which may be provided on the ionomer. Other crosslinking groups may also be used. For example, the crosslinking agent may be provided on the cation-conducting polymer, and the anion-conducting polymer may be spray-deposited thereon, followed by heating to induce the crosslinking reaction and introduce crosslinks across the interface.

[0290] In some embodiments, the anion-conducting polymer and the cation-conducting polymer of the bipolar membrane have the same backbone with different fixed charged groups. As an example, Orion ionomers may be used with different fixed charged groups. Ionomers are more compatible and less prone to peeling.

[0291] In the above example, interface 813 may be a three-dimensional volume having a thickness of 1% to 90% of the total thickness of the bipolar membrane, or a thickness of 5% to 90%, or 10% to 80%, or 20% to 70%, or 30% to 60% of the total thickness of the bipolar membrane. In some embodiments, interface 813 is less than half of the total thickness, including 1% to 45%, 5% to 45%, 5% to 40%, or 5% to 30%.

[0292] Hot pressing may be used in manufacturing any of the above bipolar interface designs.

[0293] [Relative sizes of MEA layers] In certain embodiments, the polymer electrolyte membrane and the adjacent cathode buffer layer or other anion-conducting polymer layer may have relative thicknesses that facilitate the manufacture and / or operational performance of the MEA.

[0294] FIG. 9 shows an example of a partial MEA including an anion-conducting polymer layer (AEM) 903, which may be a cathode buffer layer, and a polymer electrolyte membrane (PEM) 905, which may be a cation-conducting polymer layer (e.g., a proton exchange polymer layer) or an anion-conducting polymer layer. In this example, the PEM 905 is relatively thicker than the anion-conducting polymer layer 903, which may be a cathode buffer layer, and the polymer electrolyte membrane (PEM) 905, which may be a cation-conducting polymer layer (e.g., a proton exchange polymer layer) or an anion-conducting polymer layer. In this example, the PEM 905 is relatively thicker than the anion-conducting polymer layer 903. For example, the PEM 905 may be 120 micrometers thick, compared to the approximately 10-20 micrometer thickness of the AEM 903.

[0295] In some cases, anion-conducting polymers, such as those used in anion-conducting polymer layer 903, are substantially less conductive than cation-conducting polymers, such as those used in PEM 905. Therefore, a relatively thin cathode buffer is used to provide the benefits of a cathode buffer layer (e.g., anion-conducting polymer layer 903) without substantially increasing the overall resistance of the MEA. However, an excessively thin cathode buffer layer can be difficult to handle during MEA fabrication and other situations. Therefore, in certain embodiments, a thin cathode buffer layer is fabricated on a relatively thicker PEM layer, such as a cation-conducting polymer layer. The anion-conducting polymer layer may be fabricated on the PEM layer, for example, using any of the fabrication techniques described elsewhere herein.

[0296] In various embodiments, the polymer electrolyte membrane layer is about 20-200 micrometers thick. In some embodiments, the polymer electrolyte membrane layer is about 60-120 micrometers thick. In some embodiments, a thin polymer electrolyte membrane layer is used, about 20-60 micrometers thick. In some embodiments, a relatively thick polymer electrolyte layer is used, about 120-200 micrometers thick.

[0297] In some embodiments, a thinner cathode buffer layer is used with a thinner polymer electrolyte membrane, which may facilitate migration of CO formed at the interface back to the cathode instead of the anode. In some embodiments, a thicker cathode buffer layer is used with a thicker polymer electrolyte membrane, which may result in a reduced cell voltage in some embodiments.

[0298] Factors that can affect the thickness of the cathode buffer layer include the ion selectivity of the anion-conducting polymer, the porosity of the anion-conducting polymer, and the suitability of the anion-conducting polymer to coat the polymer electrolyte membrane.

[0299] Many anion-conducting polymers are in the 95% selectivity range for anions, with about 5% of the current being cations. Higher selectivity anion-conducting polymers, with selectivity greater than 99% for anions, may allow for a significant reduction in thickness while still providing sufficient buffering.

[0300] The mechanical strength of the anion-conducting layer can also affect its thickness, with stronger layers allowing for thinner layers. Reducing the porosity of the anion-conducting polymer can reduce the thickness of the anion-conducting layer.

[0301] In some implementations, the cathode buffer layer or other anion-conducting polymer layer adjacent to the polymer electrolyte membrane is about 10-20 micrometers thick. In some embodiments, using a polymer with a selectivity greater than 99% may allow the cathode buffer layer to be reduced to 2-10 microns.

[0302] In some cases, the thickness ratio of the polymer electrolyte membrane to the adjacent anion-conducting polymer layer is about 3:1 to 90:1, with the upper ratios being used with highly selective anion-conducting polymer layers. In some embodiments, the ratio is about 2:1 to 13:1, about 3:1 to 13.1, or about 7:1 to 13.1.

[0303] In certain embodiments, a relatively thin PEM improves some aspects of MEA performance. Referring to FIG. 9 , for example, the polymer electrolyte membrane 905 may have a thickness of approximately 50 micrometers, while the anion-conducting layer may have a thickness of approximately 10-20 micrometers. A thin PEM facilitates the migration of water generated at the AEM / PEM interface toward the anode. The gas pressure on the cathode side of the cell may be approximately 80-450 psi, which causes the interfacial water to migrate to the anode. However, in some cases, a thick PEM can cause a large portion of the water to migrate through the AEM to the cathode, resulting in flooding. Using a thin PEM can avoid flooding.

[0304] [CO x Reduction Reactor (CRR)] FIG. 10 illustrates a CO x FIG. 1 is a schematic diagram showing the major components of a catalytic reduction reactor (CRR) 1005. The CRR 1005 has a membrane electrode assembly 1000, such as any of those described elsewhere herein. The membrane electrode assembly 1000 has a cathode 1020 and an anode 1040 separated by an ion exchange layer 1060. The ion exchange layer 1060 may include sublayers. The illustrated embodiment has three sublayers: a cathode buffer layer 1025, a polymer electrolyte membrane 1065, and an optional anode buffer layer 1045. Additionally, the CRR 1005 has a cathode support structure 1022 adjacent to the cathode 1020 and an anode support structure 1042 adjacent to the anode 1040.

[0305] The cathode support structure 1022 has a cathode polarity plate 1024, made of, for example, graphite, to which a voltage can be applied. There may be flow field channels, such as serpentine channels, cut into the inner surface of the cathode polarity plate 1024. There is also a cathode gas diffusion layer 1026 adjacent to the inner surface of the cathode polarity plate 1024. In some configurations, there are multiple cathode gas diffusion layers (not shown). The cathode gas diffusion layer 1026 facilitates the flow of gas into and out of the membrane electrode assembly 1000. An example of a cathode gas diffusion layer 1026 is carbon paper with a carbon microporous layer.

[0306] The anode support structure 1042 has an anode polarity plate 1044, typically fabricated from metal, to which a voltage can be applied. There may be flow field channels, such as serpentine channels, cut into the inner surface of the anode polarity plate 1044. There is also an anode gas diffusion layer 1046 adjacent to the inner surface of the anode polarity plate 1044. In some configurations, there are multiple anode gas diffusion layers (not shown). The anode gas diffusion layer 1046 facilitates the flow of gas into and out of the membrane electrode assembly 1000. Examples of the anode gas diffusion layer 1046 include titanium mesh or titanium felt. In some configurations, the gas diffusion layers 1026, 1046 are microporous.

[0307] There are also inlets and outlets (not shown) associated with the support structures 1022, 1042 that allow the flow of reactants and products, respectively, into the membrane electrode assembly 1000. There are also various gaskets (not shown) that prevent leakage of reactants and products from the cell.

[0308] In one embodiment, a direct current (DC) voltage is applied to the membrane electrode assembly 1000 via the cathode polarity plate 1024 and the anode polarity plate 1042. Water is supplied to the anode 1040 and is oxidized over an oxidation catalyst to form molecular oxygen (O) and release protons (H) and electrons (e). The protons migrate through the ion exchange layer 1060 toward the cathode 1020. The electrons flow through an external circuit (not shown). In one embodiment, the reaction is described as follows: 2H2O→4H + +4e - +O2

[0309] In other embodiments, other reactants can be supplied to the anode 1040 and other reactions can occur.

[0310] Although the illustrated embodiment shows an ion exchange layer having three sublayers, certain embodiments use an ion exchange layer having only a single layer (e.g., a cation-conducting polymer layer or an anion-conducting polymer layer), while other embodiments have only two sublayers.

[0311] FIG. 11 illustrates the flow of reactants, products, ions, and electrons through a CRR 1105 reactor, according to one embodiment. The CRR 1105 includes a membrane electrode assembly 1100, such as any of the MEAs described elsewhere herein. The membrane electrode assembly 1100 includes a cathode 1120 and an anode 1140 separated by an ion exchange layer 1160. In certain embodiments, the ion exchange layer 1160 includes three sublayers: a cathode buffer layer 1125, a polymer electrolyte membrane 1165, and an optional anode buffer layer 1145. Additionally, the CRR 1105 includes a cathode support structure 1122 adjacent to the cathode 1120 and an anode support structure 1142 adjacent to the anode 1140.

[0312] The cathode support structure 1122 has a cathode polarity plate 1124, which may be made of graphite, to which a voltage can be applied. There may be flow field channels, such as serpentine channels, cut into the inner surface of the cathode polarity plate 1124. There is also a cathode gas diffusion layer 1126 adjacent to the inner surface of the cathode polarity plate 1124. In some configurations, there are multiple cathode gas diffusion layers (not shown). The cathode gas diffusion layer 1126 facilitates the flow of gas into and out of the membrane electrode assembly 1100. An example of a cathode gas diffusion layer 1126 is carbon paper with a carbon microporous layer.

[0313] The anode support structure 1142 has an anode polarity plate 1144, which may be made of metal, to which a voltage can be applied. There may be flow field channels, such as serpentine channels, cut into the inner surface of the anode polarity plate 1144. There is also an anode gas diffusion layer 1146 adjacent to the inner surface of the anode polarity plate 1144. In some configurations, there are multiple anode gas diffusion layers (not shown). The anode gas diffusion layer 1146 facilitates gas flow into and out of the membrane electrode assembly 1100. Examples of the anode gas diffusion layer 1146 include titanium mesh or titanium felt. In some configurations, the gas diffusion layers 1126, 1146 are microporous.

[0314] There may also be inlets and outlets associated with the support structures 1122, 1142 that allow the flow of reactants and products, respectively, into the membrane electrode assembly 1100. Various gaskets may also be present to prevent leakage of reactants and products from the cell.

[0315] CO x is supplied to the cathode 1120 and reacts with CO in the presence of protons and electrons. x CO can be reduced over a reduction catalyst. x CO may be supplied to the cathode 1120 at a pressure between 0 psig and 1000 psig, or any other suitable range. x CO may be supplied to the cathode 1120 at a concentration of less than 100% or any other suitable percentage, along with a mixture of other gases.x The concentration can be reduced to about 0.5%, 5%, or 20% or any other suitable percentage.

[0316] In one embodiment, unreacted CO x Approximately 10% to 100% of the oxidation product at the anode 1140 is collected at an outlet adjacent the cathode 1120, separated from the reduction reaction products, and then recycled back to the inlet adjacent the cathode 1120. In one embodiment, the oxidation product at the anode 1140 is compressed to a pressure of 0 psig to 1500 psig.

[0317] In one embodiment, multiple CRRs (such as those shown in FIG. 10) are arranged in an electrochemical stack and operate together. The CRRs that make up the individual electrochemical cells of the stack can be electrically connected in series or parallel. Reactants are supplied to the individual CRRs, and then the reaction products are collected.

[0318] According to some embodiments, the inputs and outputs to the reactor are shown in Figure 12. The reactor contains CO x Anode feed material and electricity are supplied. x reduction products and any unreacted CO x leaves the reactor. Unreacted CO x is separated from the reduction product and can be recycled back to the input side of the reactor. The anodic oxidation product and any unreacted anode feed material leave the reactor in a separate stream. The unreacted anode feed material can be recycled back to the input side of the reactor.

[0319] Various catalysts at the cathode of the CRR x The reduction reaction can produce a variety of products or mixtures of products. Examples of CO reduction reactions that can occur at the cathode are described below. CO2+2H + +2e - →CO+H2O 2CO2+12H + +12e - →CH2CH2+4H2O 2CO2+12H ++12e - →CH3CH2OH+3H2O CO2+8H + +8e - →CH4+2H2O 2CO+8H + +8e - →CH2CH2+2H2O 2CO+8H + +8e - →CH3CH2OH+H2O CO+6H + +8e - →CH4+H2O

[0320] In some embodiments, CO x A method of operating the reduction reactor includes applying a DC voltage to the cathode and anode polarity plates, supplying an oxidation reactant to the anode to cause an oxidation reaction, supplying a reduction reactant to the cathode to cause a reduction reaction, collecting the oxidation reaction product from the anode, and collecting the reduction reaction product from the cathode.

[0321] In one configuration, the DC voltage is greater than about −1.2 V. In various arrangements, the oxidation reactant can be hydrogen, methane, ammonia, water, or a combination thereof, and / or any other suitable oxidation reactant. In one configuration, the oxidation reactant is water. In various arrangements, the reduction reactant can be carbon dioxide, carbon monoxide, or a combination thereof, and / or any other suitable reduction reactant. In one configuration, the reduction reactant is carbon dioxide.

[0322] [Example: aqueous salts in an operating MEA cell] [Improvement of lifespan and Faraday yield] Adding salt to the anode water can improve the faradaic yield, lower the cell voltage, and reduce the rate of performance decay. Figures 13A and 13B show performance plots of two carbon dioxide electrolyzers, one with no salt in the anode water and one with 2 mM NaHCO3 in the anode water. Both electrolyzers used bipolar MEAs and gold catalysts (cathode). In this example, adding NaHCO3 salt improves cell performance. The cell with no salt in the anode water achieved a current of 0.5 A / cm. 2 The average voltage for the first hour was 3.86 V, the average CO faradaic yield was 0.53, and the current was 500 mA / cm. 2 The decay rate from 2 to 5 hours is 144 mV / hr, and the CO faradaic yield / hr is 0.018. In comparison, the cell with 2 mM NaHCO3 has a decay rate of 0.5 A / cm 2 The average voltage for the first hour was 3.52 V, the average CO faradaic yield was 0.936, and the average current was 500 mA / cm. 2 The decay rate from 2 to 5 hours is 15.5 mV / hour, and the CO faradaic yield / hour is 0.001.

[0323] The presence of salt has also been shown to have a performance-enhancing effect on the faradaic yield and voltage efficiency of a CO electrolyzer system producing methane and ethylene. In this example, the presence of NaHCO3 increased the voltage from 5.19 V to 3.86 V at 0.2 A / cm2, and improved the total detectable CO2 faradaic yield (including CO, CH4, and C2H4) from 1% to 38%. The cell used a bipolar MEA and copper catalyst (cathode). See Figure 14.

[0324] Small variations in salt concentration can have a large effect. In the plot below, various concentrations of NaHCO3 were added to the anode water of a 100 cm2 CO2 electrolysis cell. In this example, 6 mM NaHCO3 showed the greatest performance improvement, resulting in a higher faradaic yield than lower (2 mM) or higher (8 and 10 mM) concentrations. The optimal salt concentration also depends on the size of the electrolysis cell. In this example, 2 mM NaHCO3 was added to the anode water of a 25 cm2 CO2 electrolysis cell. 2The best performance for the electrolyzer was achieved. All MEA cells used bipolar MEAs and gold catalysts (cathode). See Figure 15.

[0325] Similarly, the effect of salt concentration was observed in copper-based catalyst systems where CO electrolyzers convert CO to methane, ethylene, ethanol, and other small-chain hydrocarbons and derivative organic compounds. All MEA cells used bipolar MEAs and copper catalysts (cathode). The effect of KHCO salt concentration was screened to optimize C2 hydrocarbon production at levels from 1.5 mM to 30 mM. A total hydrocarbon yield of 70% was observed when the concentration was set at 3 mM KHCO. This setting was found to improve ethanol yield (40%) and ethylene yield (24%) compared to lower and higher concentrations. See Figure 16.

[0326] [Product selectivity changes depending on the identity of the salt] Varying the identity of the salt can alter product selectivity. For example, comparing product selectivity for CO electrolysis using a copper catalyst at the cathode and 3 mM KHCO or 3 mM NaHCO in the anode water reveals that the presence of KHCO slightly improves methane selectivity and increases selectivity to ethanol and ethylene. Figure 17 shows that when the anode water was changed from NaHCO to KHCO during the reaction, methane selectivity decreased from 40% to 11%, while ethylene selectivity increased from 20% to 35%. Switching the anion from bicarbonate to sulfate showed less sensitivity to product selectivity. All cells in this example used a bipolar MEA and a copper catalyst (cathode).

[0327] In Figure 18, improved selectivity towards ethanol was also observed when using the larger cation salt KHCO3 compared to the smaller cation NaHCO3.

[0328] In some implementations, ion concentrations decrease over long periods of operation. Adding extra salt or replacing the electrolyte reservoir with a fresh salt solution helps restore selectivity and voltage. Figures 19A and 19B (table) show the improvement in selectivity and voltage after adding new salt solution or replacing the old solution in the anolyte reservoir. Selectivity improves in the range of 0.3–2%, while voltage decreases in the range of 10–100 mV. In this example, all cells used bipolar MEAs and gold catalysts (cathode). The salt composition was 2 mM NaHCO3. In one test, the salt solution was replenished by directly exchanging the solution.

[0329] A salt concentration scan of the selectivity of the copper catalyst for methane was performed in the range of 1 mM to 30 mM NaHCO. The selectivity for methane was 100 mA / cm. 2 The effect was greatest at low current densities of 250 mA / cm, showing an increase from 55% to 73% methane production at the expense of hydrogen production. At salt concentrations above 20 mM, the effect of increasing the amount of salt did not appear to have a positive impact on performance. 2 In the case of HCl, a similar yield improvement was observed from approximately 52% to 62% methane, with the voltage improving from 4.25 to 4.00 V and the salt concentration increasing to 20 mA / cm. 2 See Figure 20, which shows that increasing the salt concentration up to 20 mM NaHCO3 in the bipolar MEA cell improves methane selectivity.

[0330] Various salts were tested for their effect on ethylene selectivity at various concentrations. An anion-conducting polymer-only MEA was used with a copper catalyst (cathode). A concentration dependence of potassium bicarbonate salt was observed at 3 mM and 6 mM levels, with the yield of ethylene from carbon monoxide being 33% at 6 mM. See Figure 21A. This figure shows the effect of potassium bicarbonate salt in the anolyte on the CORR ethylene yield in an anion-conducting polymer-only MEA. In contrast, potassium hydroxide salt was shown to improve the voltage of the same reaction, with the voltage decreasing as higher concentrations of KOH were used. Good ethylene yield performance was observed at lower KOH concentrations of approximately 0.1 and 0.01 M. See Figure 21B. This figure shows the effect of potassium hydroxide salt concentration in the anolyte on the CORR ethylene yield in an anion-conducting polymer-only setup.

[0331] [Other embodiments] Although omitted for brevity, system and / or method embodiments may include any combination and permutation of the various system components and various method processes, and one or more instances of the methods and / or processes described herein may be performed asynchronously (e.g., sequentially), simultaneously (e.g., in parallel), or in any other suitable order by and / or using one or more instances of the systems, elements, and / or entities described herein.

[0332] As those skilled in the art will recognize from the foregoing detailed description, drawings, and claims, modifications and variations to the preferred embodiments of the invention can be made without departing from the scope of the invention, which is defined in the following claims. [Item 1] A membrane electrode assembly (MEA), a cathode layer including a carbon oxide reduction catalyst that promotes the reduction of carbon oxide; an anode layer including a catalyst that promotes water oxidation; a polymer electrolyte membrane layer (PEM layer) disposed between and in contact with the cathode layer and the anode layer; salt ions from a salt solution in contact with the MEA; Equipped with The salt in the salt solution has a concentration of at least about 10 μM. MEA. [Item 2] 2. The MEA of claim 1, wherein the carbon oxide is carbon dioxide and the carbon oxide reduction catalyst comprises gold, silver, copper, or a combination thereof. [Item 3] 2. The MEA of claim 1, wherein the carbon oxide is carbon monoxide and the carbon oxide reduction catalyst comprises gold, silver, copper, or a combination thereof. [Item 4] 4. The MEA of any one of items 1 to 3, wherein the cathode layer comprises an anion-conducting polymer. [Item 5] 5. The MEA of any one of items 1 to 4, wherein the anode layer comprises a cation-conducting polymer. [Item 6] 6. The MEA according to any one of items 1 to 5, wherein the MEA is bipolar and comprises at least one layer of a cation-conducting polymer and at least one layer of an anion-conducting polymer. [Item 7] 7. The MEA of any one of items 1 to 6, wherein the PEM layer has a polymer electrolyte layer and a cathode buffer layer. [Item 8] 8. The MEA of item 7, wherein the PEM layer comprises a cation-conducting polymer and the cathode buffer layer comprises an anion-conducting polymer. [Item 9] 2. The MEA of item 1, wherein the PEM layer comprises an anion-conducting polymer. [Item 10] 10. The MEA of any one of items 1 to 9, wherein the salt ions comprise alkali metal ions. [Item 11] 11. The MEA of any one of items 1 to 10, wherein the salt ions comprise anions selected from the group consisting of phosphate, sulfate, carbonate, bicarbonate, and hydroxide. [Item 12] 12. The MEA of any one of items 1 to 11, wherein the MEA is a bipolar MEA, the carbon oxide reduction catalyst comprises copper, and the salt comprises (i) an alkali metal cation, and (ii) a bicarbonate, sulfate, or hydroxide anion. [Item 13] 13. The MEA of claim 12, wherein the salt is present in the salt solution at a concentration of about 1 mM to about 1 M. [Item 14] 13. The MEA of claim 12, wherein the salt is present in the salt solution at a concentration of about 1 mM to about 50 mM. [Item 15] Item 13. The MEA according to item 12, wherein the MEA is configured to produce methane by reducing carbon dioxide and / or carbon monoxide in the cathode layer, and the salt ions are sodium ions. [Item 16] Item 13. The MEA of item 12, wherein the MEA is configured to produce one or more organic compounds having two or more carbon atoms by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt ions include ions of potassium, cesium, rubidium, or any combination thereof. [Item 17] 2. The MEA of claim 1, wherein the MEA is a bipolar MEA, the carbon oxide reduction catalyst comprises gold, and the salt comprises (i) an alkali metal cation and (ii) a bicarbonate, hydroxide, or sulfate anion. [Item 18] 18. The MEA of item 17, wherein the salt is present in the salt solution at a concentration of about 10 μM to about 200 mM. [Item 19] 18. The MEA of item 17, wherein the salt is present in the salt solution at a concentration of about 100 μM to about 20 mM. [Item 20] Item 18. The MEA of item 17, wherein the MEA is configured to produce carbon monoxide by reducing carbon dioxide in the cathode layer, and the salt ions include alkali metal ions. [Item 21] 18. The MEA of item 17, which is substantially free of transition metal ions. [Item 22] 2. The MEA of claim 1, wherein all polymers in the MEA are anion conducting polymers, the carbon oxide reduction catalyst comprises copper, and the salt comprises (i) an alkali metal cation, and (ii) a bicarbonate or hydroxide anion. [Item 23] 23. The MEA of claim 22, wherein the salt is present in the salt solution at a concentration of about 10 mM to about 15 M. [Item 24] 23. The MEA of claim 22, wherein the salt is present in the salt solution at a concentration of about 50 mM to about 1 M. [Item 25] 23. The MEA of claim 22, wherein the MEA is configured to produce methane by reducing carbon dioxide and / or carbon monoxide in the cathode layer, and the salt ions include sodium ions. [Item 26] 23. The MEA of claim 22, wherein the MEA is configured to produce one or more organic compounds having two or more carbon atoms by reducing carbon dioxide and / or carbon monoxide in the cathode layer, and the salt ions include ions of potassium, cesium, rubidium, or any combination thereof. [Item 27] 1. An electrochemical system configured for the electrolytic reduction of carbon oxides, comprising: (a) a membrane electrode assembly (MEA) including: (i) a cathode layer including a carbon oxide reduction catalyst that promotes the reduction of carbon oxides; (ii) an anode layer including a catalyst that promotes the oxidation of water; and (iii) a polymer electrolyte membrane layer (PEM layer) disposed between the cathode layer and the anode layer and in contact with the cathode layer and the anode layer; (b) an anode water source containing a salt having a concentration of at least about 10 μM in the anode water, the anode water source being connected to the MEA in a manner that allows the anode water to contact the anode layer and provide the salt to the MEA; An electrochemical system comprising: [Item 28] 28. The electrochemical system of claim 27, wherein the carbon oxide is carbon dioxide and the carbon oxide reduction catalyst comprises gold, silver, copper, or a combination thereof. [Item 29] 28. The electrochemical system of claim 27, wherein the carbon oxide is carbon monoxide and the carbon oxide reduction catalyst comprises gold, silver, copper, or a combination thereof. [Item 30] 30. The electrochemical system of any one of items 27 to 29, wherein the cathode layer comprises an anion-conducting polymer. [Item 31] 31. The electrochemical system of any one of items 27 to 30, wherein the anode layer comprises a cation-conducting polymer. [Item 32] 32. The electrochemical system of any one of items 27 to 31, wherein the MEA is bipolar and has at least one layer of a cation-conducting polymer and at least one layer of an anion-conducting polymer. [Item 33] 33. The electrochemical system of any one of items 27 to 32, wherein the PEM layer comprises a polymer electrolyte layer and a cathode buffer layer. [Item 34] 34. The electrochemical system of claim 33, wherein the PEM layer comprises a cation-conducting polymer and the cathode buffer layer comprises an anion-conducting polymer. [Item 35] 28. The electrochemical system of claim 27, wherein the PEM layer comprises an anion conducting polymer. [Item 36] 36. The electrochemical system of any one of items 27 to 35, wherein the salt comprises an alkali metal ion. [Item 37] 37. The electrochemical system of any one of items 27 to 36, wherein the salt comprises an anion selected from the group consisting of phosphate, sulfate, carbonate, bicarbonate, and hydroxide. [Item 38] 37. The electrochemical system of any one of items 27 to 36, wherein the MEA is a bipolar MEA, the carbon oxide reduction catalyst comprises copper, and the salt comprises (i) an alkali metal cation and (ii) a bicarbonate, sulfate, or hydroxide anion. [Item 39] 39. The electrochemical system of claim 38, wherein the salt is present in the anode water at a concentration of about 1 mM to about 1 M. [Item 40] 39. The electrochemical system of claim 38, wherein the salt is present in the anode water at a concentration of about 1 mM to about 50 mM. [Item 41] Item 39. The electrochemical system of item 38, wherein the MEA is configured to produce methane by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt comprises sodium ions. [Item 42] Item 39. The electrochemical system of item 38, wherein the MEA is configured to produce one or more organic compounds having two or more carbon atoms by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt comprises ions of potassium, cesium, rubidium, or any combination thereof. [Item 43] 37. The electrochemical system of any one of items 27 to 36, wherein the MEA is a bipolar MEA, the carbon oxide reduction catalyst comprises gold, and the salt comprises (i) an alkali metal cation and (ii) a bicarbonate, hydroxide, or sulfate anion. [Item 44] 44. The electrochemical system of claim 43, wherein the salt is present in the anode water at a concentration of about 10 μM to about 200 mM. [Item 45] 44. The electrochemical system of claim 43, wherein the salt is present in the anode water at a concentration of about 100 μM to about 20 mM. [Item 46] Item 44. The electrochemical system of item 43, wherein the MEA is configured to produce carbon monoxide by reducing carbon dioxide at the cathode layer, and the salt comprises an alkali metal ion. [Item 47] 44. The electrochemical system of claim 43, wherein the MEA is substantially free of transition metal ions. [Item 48] 37. The electrochemical system of any one of items 27 to 36, wherein all polymers in the MEA are anion conducting polymers, the carbon oxide reduction catalyst comprises copper, and the salt comprises (i) an alkali metal cation, and (ii) a bicarbonate or hydroxide anion. [Item 49] 49. The electrochemical system of claim 48, wherein the salt is present in the anode water at a concentration of about 10 mM to about 15 M. [Item 50] 49. The electrochemical system of claim 48, wherein the salt is present in the anode water at a concentration of about 50 mM to about 1 M. [Item 51] Item 49. The electrochemical system of item 48, wherein the MEA is configured to produce methane by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt comprises sodium ions. [Item 52] Item 49. The electrochemical system of item 48, wherein the MEA is configured to produce one or more organic compounds having two or more carbon atoms by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt comprises ions of potassium, cesium, rubidium, or any combination thereof. [Item 53] 53. The electrochemical system of any one of items 27 to 52, further comprising a recirculation loop connected to the MEA configured to recover anode water from the MEA, store and / or treat the recovered anode water, and supply the stored or treated anode water to the MEA. [Item 54] Item 54. The electrochemical system of item 53, wherein the recirculation loop includes a reservoir for storing the anode water. [Item 55] Item 54. The electrochemical system of item 53, wherein the recirculation loop includes an anode water purification element configured to remove impurities from the anode water. [Item 56] Item 54. The electrochemical system of item 53, wherein the recirculation loop has an inlet for receiving purified water. [Item 57] Item 54. The electrochemical system of item 53, wherein the recirculation loop is connected to the anode water source. [Item 58] Item 54. The electrochemical system of item 53, further comprising a cathode water conduit connected to the recirculation loop and configured to provide to the recirculation loop water recovered from the carbon oxide stream after the carbon oxide stream contacts the cathode layer of the MEA. [Item 59] Item 59. The electrochemical system of item 58, further comprising a water separator coupled to the cathode water conduit and configured to separate cathode water from the carbon oxide stream. [Item 60] 1. A method for electrolytically reducing carbon oxides, comprising: providing a salt solution to a membrane electrode assembly (MEA) comprising: (a) a cathode layer comprising a carbon oxide reduction catalyst that promotes the reduction of carbon oxides; (b) an anode layer comprising a catalyst that promotes the oxidation of water; and (c) a polymer electrolyte membrane layer (PEM layer) disposed between the cathode layer and the anode layer and in contact with the cathode layer and the anode layer, wherein the salt solution comprises at least about 10 μM salt; electrolytically reducing carbon oxides at the cathode layer of the MEA while the MEA is in contact with the salt solution; A method for providing the above. [Item 61] Item 61. The method of item 60, wherein the carbon oxide is carbon dioxide and the carbon oxide reduction catalyst comprises gold, silver, copper, or a combination thereof. [Item 62] Item 61. The method of item 60, wherein the carbon oxide is carbon monoxide and the carbon oxide reduction catalyst comprises gold, silver, copper, or a combination thereof. [Item 63] 63. The method of any one of items 60 to 62, wherein the cathode layer comprises an anion-conducting polymer. [Item 64] 64. The method of any one of items 60 to 63, wherein the anode layer comprises a cation-conducting polymer. [Item 65] 65. The method of any one of items 60 to 64, wherein the MEA is bipolar and has at least one layer of a cation-conducting polymer and at least one layer of an anion-conducting polymer. [Item 66] 66. The method of any one of items 60 to 65, wherein the PEM layer comprises a polymer electrolyte layer and a cathode buffer layer. [Item 67] Item 67. The method of item 66, wherein the PEM layer comprises a cation-conducting polymer and the cathode buffer layer comprises an anion-conducting polymer. [Item 68] Item 61. The method of item 60, wherein the PEM layer comprises an anion-conducting polymer. [Item 69] 69. The method of any one of items 60 to 68, wherein the salt comprises an alkali metal ion. [Item 70] 70. The method of any one of items 60 to 69, wherein the salt comprises an anion selected from the group consisting of phosphate, sulfate, carbonate, bicarbonate, and hydroxide. [Item 71] 71. The method of any one of items 60 to 70, wherein the MEA is a bipolar MEA, the carbon oxide reduction catalyst comprises copper, and the salt comprises (i) an alkali metal cation and (ii) a bicarbonate, sulfate, or hydroxide anion. [Item 72] 72. The method of claim 71, wherein the salt is present in the salt solution at a concentration of about 1 mM to about 1 M. [Item 73] 72. The method of claim 71, wherein the salt is present in the salt solution at a concentration of about 1 mM to about 50 mM. [Item 74] Item 72. The method of claim 71, wherein the MEA is configured to produce methane by reducing carbon dioxide and / or carbon monoxide in the cathode layer, and the salt comprises sodium ions. [Item 75] Item 72. The method of item 71, wherein the MEA is configured to produce one or more organic compounds having two or more carbon atoms by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt comprises ions of potassium, cesium, rubidium, or any combination thereof. [Item 76] 71. The method of any one of items 60 to 70, wherein the MEA is a bipolar MEA, the carbon oxide reduction catalyst comprises gold, and the salt comprises (i) an alkali metal cation and (ii) a bicarbonate, hydroxide, or sulfate anion. [Item 77] 77. The method of claim 76, wherein the salt is present in the salt solution at a concentration of about 10 μM to about 200 mM. [Item 78] 77. The method of claim 76, wherein the salt is present in the salt solution at a concentration of about 100 μM to about 20 mM. [Item 79] Item 77. The method of item 76, wherein the MEA is configured to produce carbon monoxide by reducing carbon dioxide at the cathode layer, and the salt comprises an alkali metal ion. [Item 80] Item 77. The method of item 76, wherein the MEA is substantially free of transition metal ions. [Item 81] 71. The method of any one of items 60 to 70, wherein all polymers in the MEA are anion conducting polymers, the carbon oxide reduction catalyst comprises copper, and the salt comprises (i) an alkali metal cation, and (ii) a bicarbonate or hydroxide anion. [Item 82] 82. The method of claim 81, wherein the salt is present in the salt solution at a concentration of about 10 mM to about 15 M. [Item 83] 82. The method of claim 81, wherein the salt is present in the salt solution at a concentration of about 50 mM to about 1 M. [Item 84] Item 82. The method of item 81, wherein the MEA is configured to produce methane by reducing carbon dioxide and / or carbon monoxide in the cathode layer, and the salt comprises sodium ions. [Item 85] Item 82. The method of item 81, wherein the MEA is configured to produce one or more organic compounds having two or more carbon atoms by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt comprises ions of potassium, cesium, rubidium, or any combination thereof. [Item 86] 86. The method of any one of items 60 to 85, wherein providing the MEA with a salt solution comprises supplying anode water to the anode layer of the MEA. [Item 87] Item 87. The method according to item 86, further comprising: (i) recovering the anode water supplied to the MEA; and (ii) recycling the recovered anode water to the anode layer of the MEA. [Item 88] Item 88. The method of item 87, further comprising storing and / or treating the recovered anode water before recycling the recovered anode water to the anode layer of the MEA. [Item 89] Item 88. The method of item 87, further comprising purifying the anode water and / or the recovered anode water to remove impurities from the anode water. [Item 90] 89. The method of claim 68, further comprising: (i) recovering water from the carbon oxide stream after the carbon oxide stream contacts the cathode layer of the MEA; and (ii) providing the water recovered from the carbon oxide stream to the anode layer of the MEA.

Claims

1. 1. An electrochemical system configured for the electrolytic reduction of carbon oxides, comprising: (a) a membrane electrode assembly (MEA) including: (i) a cathode layer including a carbon oxide reduction catalyst that promotes the reduction of carbon oxides; (ii) an anode layer including a catalyst that promotes the oxidation of water; and (iii) a bipolar polymer electrolyte membrane (PEM) layer disposed between the cathode layer and the anode layer and in contact with the cathode layer and the anode layer; (b) a source of anode water connected to the MEA in a manner that allows the anode water to contact the anode layer and provide salt to the MEA; (c) a recirculation loop connected to the MEA, the recirculation loop configured to recover anode water from the MEA, store and / or treat the recovered anode water, and supply the stored or treated anode water to the MEA; Equipped with the bipolar PEM layer comprises at least one anion-conducting polymer layer, at least one cation-conducting polymer layer, and a bipolar interface between the anion-conducting polymer layer and the cation-conducting polymer layer; the cation-conducting polymer layer is disposed between the anode layer and the anion-conducting polymer layer; The concentration of sodium ions in the anode water is 20 mM or less; The concentration of potassium ions in the anode water is 6 mM or less. Electrochemical systems.

2. The electrochemical system of claim 1 , wherein the cathode layer comprises an anion-conducting polymer.

3. The electrochemical system of claim 1 , wherein the anode layer comprises a cationically conductive polymer.

4. The electrochemical system of claim 1 , wherein the salt comprises an alkali metal ion.

5. 10. The electrochemical system of claim 1, wherein the salt comprises an anion selected from the group consisting of phosphate, sulfate, carbonate, bicarbonate, and hydroxide.

6. 10. The electrochemical system of claim 1, wherein the carbon oxide reduction catalyst comprises copper and the salt comprises (i) an alkali metal cation and (ii) a bicarbonate, sulfate, or hydroxide anion.

7. 7. The electrochemical system of claim 6, wherein the salt is present in the anode water at a concentration of 10 μM to 1 M.

8. 7. The electrochemical system of claim 6, wherein the salt is present in the anode water at a concentration of 1 mM to 50 mM.

9. 10. The electrochemical system of claim 1, wherein the carbon oxide reduction catalyst comprises gold and the salt comprises (i) an alkali metal cation and (ii) a bicarbonate, hydroxide, or sulfate anion.

10. 10. The electrochemical system of claim 9, wherein the salt is present in the anode water at a concentration of 10 μM to 200 mM.

11. 10. The electrochemical system of claim 9, wherein the salt is present in the anode water at a concentration of 100 μM to 20 mM.

12. 10. The electrochemical system of claim 9, wherein the MEA is substantially free of transition metal ions.

13. 10. The electrochemical system of claim 1, wherein the carbon oxide reduction catalyst comprises copper and the salt comprises (i) an alkali metal cation and (ii) a bicarbonate or hydroxide anion.

14. 14. The electrochemical system of claim 13, wherein the salt is present in the anode water at a concentration of 10 mM to 15M.

15. 14. The electrochemical system of claim 13, wherein the salt is present in the anode water at a concentration of 50 mM to 1 M.

16. The electrochemical system of claim 1 , wherein the recirculation loop includes a reservoir for storing the anode water.

17. The electrochemical system of claim 1 , wherein the recirculation loop comprises an anode water purification element configured to remove impurities from the anode water.

18. 10. The electrochemical system of claim 1, further comprising a cathode water conduit connected to the recirculation loop and configured to provide to the recirculation loop water recovered from the carbon oxide stream after the carbon oxide stream contacts the cathode layer of the MEA.

19. 20. The electrochemical system of claim 18, further comprising a water separator coupled to the cathode water conduit and configured to separate cathode water from the carbon oxide stream.

20. 10. The electrochemical system of claim 1, wherein the salt comprises (i) an alkali metal cation and (ii) a bicarbonate, sulfate, or hydroxide anion.

21. 10. The electrochemical system of claim 1, wherein the carbon oxide reduction catalyst comprises gold and the salt comprises sodium bicarbonate or potassium bicarbonate.

22. 10. The electrochemical system of claim 1, wherein the salt is present in the salt solution at a concentration of 100 μM to 20 mM.

23. 10. The electrochemical system of claim 1, wherein the salt is present in the salt solution at a concentration of 500 μM to 10 mM.

24. 10. The electrochemical system of claim 1, wherein the salt is present in the salt solution at a concentration of 1 mM to 5 mM.

25. 1. A method for electrolytically reducing carbon oxides, comprising: providing a salt solution to a membrane electrode assembly (MEA) comprising: (a) a cathode layer including a carbon oxide reduction catalyst that promotes the reduction of carbon oxides; (b) an anode layer including a catalyst that promotes the oxidation of water; and (c) a bipolar polymer electrolyte membrane (PEM) layer disposed between the cathode layer and the anode layer and in contact with the cathode layer and the anode layer; electrolytically reducing carbon oxides at the cathode layer of the MEA while the MEA is in contact with the salt solution; recovering the anode water supplied to the MEA; recycling recovered anode water to the anode layer of the MEA; Equipped with The concentration of sodium ions in the salt solution is 20 mM or less; The concentration of potassium ions in the salt solution is 6 mM or less. method.

26. the bipolar PEM layer comprises at least one anion-conducting polymer layer, at least one cation-conducting polymer layer, and a bipolar interface between the anion-conducting polymer layer and the cation-conducting polymer layer; 26. The method of claim 25, wherein the cation-conducting polymer layer is disposed between the anode layer and the anion-conducting polymer layer.

27. 27. The method of claim 25 or 26, wherein the carbon oxide reduction catalyst comprises gold, silver, copper, or a combination thereof.

28. 28. The method of any one of claims 25 to 27, wherein the anode layer comprises a cationically conductive polymer.

29. 29. The method of any one of claims 25 to 28, wherein the bipolar PEM layer comprises a polymer electrolyte layer and a cathode buffer layer.

30. 30. The method of claim 29, wherein the bipolar PEM layer comprises a cation-conducting polymer and the cathode buffer layer comprises an anion-conducting polymer.

31. 31. The method of any one of claims 25 to 30, wherein the salt solution comprises alkali metal ions.

32. 32. The method of any one of claims 25 to 31, wherein the salt solution comprises anions selected from the group consisting of phosphate, sulfate, carbonate, bicarbonate, and hydroxide.

33. 33. The method of any one of claims 25 to 32, wherein the carbon oxide reduction catalyst comprises copper and the salt solution comprises (i) an alkali metal cation, and (ii) a bicarbonate, sulfate, or hydroxide anion.

34. 34. The method of claim 33, wherein the MEA is configured to produce methane by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt solution comprises sodium ions.

35. 34. The method of claim 33, wherein the MEA is configured to produce one or more organic compounds having two or more carbon atoms by reducing carbon dioxide and / or carbon monoxide at the cathode layer, and the salt solution comprises ions of potassium, cesium, rubidium, or any combination thereof.

36. 36. The method of any one of claims 25 to 35, wherein the carbon oxide reduction catalyst comprises gold and the salt solution comprises (i) an alkali metal cation, and (ii) a bicarbonate, hydroxide, or sulfate anion.

37. 37. The method of claim 36, wherein the MEA is configured to produce carbon monoxide by reducing carbon dioxide at the cathode layer.

38. 37. The method of claim 36, wherein the MEA is substantially free of transition metal ions.

39. 39. The method of any one of claims 25 to 38, wherein providing the MEA with a salt solution comprises supplying anode water to the anode layer of the MEA.

40. 26. The method of claim 25, further comprising storing and / or treating the recovered anode water prior to recycling the recovered anode water to the anode layer of the MEA.

41. 26. The method of claim 25, further comprising purifying the anode water and / or the recovered anode water to remove impurities from the anode water.

42. 42. The method of any one of claims 25 to 41, further comprising: (i) recovering water from the carbon oxide stream after the carbon oxide stream contacts the cathode layer of the MEA; and (ii) providing water recovered from the carbon oxide stream to the anode layer of the MEA.

43. 43. The method of any one of claims 25 to 42, wherein salt is present in the salt solution at a concentration of 10 μM to 30 mM.

44. 44. The method of claim 43, wherein the salt is present in the salt solution at a concentration of 100 μM to 20 mM.

Images