Carbon oxide electrolyzer recovery procedure

Abstract

A method and / or system for operating a carbon oxide reduction electrolyzer may include (a) performing normal operation with the electrolyzer, (b) performing a recovery or protection process including (i) applying a modified current and / or voltage to the electrolyzer, and (ii) flowing a recovery gas to the cathode while applying a reverse current to the electrolyzer, and (c) resuming normal operation with the electrolyzer. Applying the modified current and / or voltage may include applying a short circuit to the electrolyzer, holding electrodes of the electrolyzer at an open circuit voltage, and / or applying a reverse current to the electrolyzer.

Description

[Technical field]

[0001] [Statement of Government Support] This invention was made with Government support under Award No. DE-FE0031712 awarded by the National Energy Technology Laboratory. The Government has certain rights in this invention. [INCORPORATION BY REFERENCE]

[0002] The PCT Request Form is being filed contemporaneously herewith as part of this application. Each application to which this application claims benefit or priority identified in the contemporaneously filed PCT Request Form is hereby incorporated by reference in its entirety for all purposes. [Background technology]

[0003] An electrolytic carbon dioxide reactor requires balancing of various operating conditions, such as 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 effect on the operating voltage of the electrolytic reactor, the faradaic yield, and the mix of products produced at the cathode, including carbon monoxide (CO) and / or other carbon-containing products (CCPs) and hydrogen.

[0004] The background and contextual discussion contained herein is provided solely for the purpose of generally setting forth 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 is presented as context elsewhere herein does not mean that such work is admitted as prior art. Summary of the Invention

[0005] Some aspects of the present disclosure relate to methods of operating an electrolyzer for carbon oxide reduction, which may be characterized by the following operations: (a) performing normal operation with the electrolyzer; (b) performing a recovery or protection process including creating a short circuit between the cathode and anode of the electrolyzer and (ii) flowing a recovery gas to the cathode while the cathode and anode are shorted; and (c) resuming normal operation with the electrolyzer.

[0006] In certain embodiments, normal operation includes injecting a reactant gas comprising carbon oxides into a cathode of the electrolytic cell and applying an electric current to the electrolytic cell at a first current density to reduce the carbon oxides to produce a carbon-containing reduction product, In some embodiments, resuming normal operation includes stopping or modifying the flow of recovery gas to the cathode and removing the short circuit.

[0007] In certain embodiments, normal operation includes flowing reactant gas to the cathode at a first flow rate and at a first pressure. In certain embodiments, normal operation includes periodically pausing and / or pulsing electrical current to the electrolytic cell. In certain embodiments, performing normal operation on the electrolytic cell includes performing normal operation for a period of at least about 100 hours.

[0008] In certain embodiments, the method additionally includes, after the step of resuming normal operation with the electrolytic cell, continuing to perform normal operation with the electrolytic cell for at least about 100 hours before again performing a recovery or protection process or terminating operation of the electrolytic cell.

[0009] In certain embodiments, the carbon oxide is CO2 and / or CO, and the carbon-containing reduction products include CO, hydrocarbons, or organic oxygen-containing compounds.

[0010] In certain embodiments, the recovery gas has a different composition than the reactive gas. In certain embodiments, the recovery or protection process is carried out for a period of about 5 to 300 minutes.

[0011] In some embodiments, the method additionally includes determining that an event that may harm the performance of the electrolyzer is occurring or is likely to occur and implementing a protective process.

[0012] Some aspects of the present disclosure relate to methods of operating an electrolyzer for carbon oxide reduction, such methods may be characterized by the following operations: (a) performing normal operation with the electrolyzer; (b) performing a recovery or protection process including (i) transitioning the electrolyzer to a state in which an open circuit voltage exists between the cathode and anode of the electrolyzer, and (ii) flowing recovery gas to the cathode while the electrolyzer maintains the open circuit voltage; and (c) resuming normal operation with the electrolyzer.

[0013] In some embodiments, normal operation includes injecting a reactant gas comprising carbon oxides into a cathode of the electrolytic cell and applying an electric current to the electrolytic cell at a first current density to generate an operating potential between the cathode and anode of the electrolytic cell to reduce the carbon oxides and generate carbon-containing reduction products. In some embodiments, resuming normal operation includes stopping or modifying the flow of recovery gas to the cathode and returning to the operating potential.

[0014] In certain embodiments, normal operation includes flowing reactant gas to the cathode at a first flow rate and at a first pressure. In certain embodiments, normal operation includes periodically pausing and / or pulsing electrical current to the electrolytic cell. In certain embodiments, performing normal operation on the electrolytic cell includes performing normal operation for a period of at least about 100 hours.

[0015] In certain embodiments, the method additionally includes, after the step of resuming normal operation with the electrolytic cell, continuing to perform normal operation with the electrolytic cell for at least about 100 hours before again performing a recovery or protection process or terminating operation of the electrolytic cell.

[0016] In certain embodiments, the carbon oxide is CO2 and / or CO, and the carbon-containing reduction products include CO, hydrocarbons, or organic oxygen-containing compounds.

[0017] In certain embodiments, the recovery gas has a different composition than the reactive gas. In certain embodiments, the recovery or protection process is carried out for a period of about 5 to 300 minutes.

[0018] Some aspects of the present disclosure relate to methods of operating an electrolyzer for carbon oxide reduction, which may be characterized by the following acts: (a) performing normal operation with the electrolyzer; (b) performing a recovery or protection process including (i) applying a reverse current to the electrolyzer, and (ii) flowing a recovery gas to the cathode while the reverse current is being applied to the electrolyzer; and (c) resuming normal operation with the electrolyzer.

[0019] Normal operation may include injecting a reactant gas comprising carbon oxides into a cathode of the electrolytic cell and applying a current to the electrolytic cell at a first current density to generate an operating potential between the cathode and an anode of the electrolytic cell to reduce the carbon oxides and generate carbon-containing reduction products. Resuming normal operation may include stopping or modifying the flow of recovery gas to the cathode and ceasing to apply a reverse current to the electrolytic cell.

[0020] In certain embodiments, applying a reverse current reduces the anodic current to a maximum of about -50 mA / cm 2 (the flat surface area of ​​the cathode).

[0021] In certain embodiments, normal operation includes flowing reactant gas to the cathode at a first flow rate and at a first pressure. In certain embodiments, normal operation includes periodically pausing and / or pulsing electrical current to the electrolytic cell. In certain embodiments, performing normal operation on the electrolytic cell includes performing normal operation for a period of at least about 100 hours.

[0022] In certain embodiments, the method additionally includes, after the step of resuming normal operation with the electrolytic cell, continuing to perform normal operation with the electrolytic cell for at least about 100 hours before again performing a recovery or protection process or terminating operation of the electrolytic cell.

[0023] In certain embodiments, the carbon oxide is CO2 and / or CO, and the carbon-containing reduction products include CO, hydrocarbons, or organic oxygen-containing compounds.

[0024] In certain embodiments, the recovery gas has a different composition than the reactive gas. In certain embodiments, the recovery or protection process is carried out for a period of about 5 to 300 minutes.

[0025] In some embodiments, the method additionally includes determining that an event that may harm the performance of the electrolyzer is occurring or is likely to occur and implementing a protective process.

[0026] Certain aspects of the disclosure relate to a carbon oxide reduction electrolyzer that may be characterized by the following features: (a) at least one membrane electrode assembly (MEA) including: (i) a cathode including a carbon oxide reduction catalyst that promotes the reduction of carbon oxides, (ii) an anode including a catalyst that promotes the oxidation, and (iii) a polymer electrolyte membrane (PEM) layer disposed between the cathode and the anode; (b) a power source configured to control a current applied to the carbon oxide reduction electrolyzer; and (c) one or more controllers, the one or more controllers being configured to control the electrolyzer by: (1) controlling a current that is normally applied to the MEA; (1) performing a recovery or protection process, where normal operation includes injecting a reactant gas including carbon oxide into the cathode of the MEA and applying an electric current to the MEA at a first current density; (2) performing a recovery or protection process including (i) creating a short circuit between the cathode and anode of the electrolyzer and (ii) flowing recovery gas to the cathode while the cathode and anode are shorted; and (3) resuming normal operation with the MEA by stopping or modifying the flow of recovery gas to the cathode and removing the short circuit.

[0027] In certain embodiments, the one or more controllers are configured to cause the electrolyzer to flow reactant gases to the cathode at a first flow rate and at a first pressure during normal operation. In certain embodiments, the one or more controllers are configured to cause the electrolyzer to periodically pause and / or pulse current to the MEA during normal operation. In certain embodiments, the one or more controllers are configured to cause the electrolyzer to perform normal operation for a period of at least about 100 hours.

[0028] In certain embodiments, the one or more controllers are further configured to cause the electrolyzer to continue performing normal operations with the MEA for at least about 100 hours after resuming normal operations with the MEA and before again performing a recovery or protection process or terminating operation of the MEA.

[0029] In certain embodiments, the recovery gas has a different composition than the reactive gas.

[0030] In certain embodiments, the one or more controllers are configured to cause the electrolytic cell to perform a recovery or protection process for a period of about 5 to 300 minutes.

[0031] In certain embodiments, the one or more controllers are configured to determine that an event that may be harmful to the performance of the electrolyzer is occurring or is likely to occur, and to implement a protective process.

[0032] Certain aspects of the disclosure relate to a carbon oxide reduction electrolyzer, which may be characterized by the following features: (a) at least one membrane electrode assembly (MEA) including: (i) a cathode including a carbon oxide reduction catalyst that promotes the reduction of carbon oxides, (ii) an anode including a catalyst that promotes the oxidation, and (iii) a polymer electrolyte membrane (PEM) layer disposed between the cathode and the anode; (b) a power source configured to control a current applied to the carbon oxide reduction electrolyzer; and (c) one or more controllers, the one or more controllers configured to control the electrolyzer to: (1) perform normal operations at the MEA; (2) performing a recovery or protection process including (i) transitioning the electrolyzer to a state where an open circuit voltage exists between the cathode and anode of the electrolyzer, and (ii) flowing recovery gas to the cathode while the electrolyzer maintains the open circuit voltage, and (3) resuming normal operation with the MEA by stopping or modifying the flow of recovery gas to the cathode and returning it to an operating potential.

[0033] In certain embodiments, the one or more controllers are configured to cause the electrolyzer to flow reactant gases to the cathode at a first flow rate and at a first pressure during normal operation.

[0034] In certain embodiments, the one or more controllers are further configured to cause the electrolyzer to continue performing normal operations with the MEA for at least about 100 hours after resuming normal operations with the MEA and before again performing a recovery or protection process or terminating operation of the MEA.

[0035] In certain embodiments, the recovery gas has a different composition than the reactive gas.

[0036] Certain aspects of the disclosure relate to carbon oxide reduction electrolyzers that may be characterized by the following features: (a) at least one membrane electrode assembly (MEA) including: (i) a cathode including a carbon oxide reduction catalyst that promotes the reduction of carbon oxides, (ii) an anode including a catalyst that promotes the oxidation, and (iii) a polymer electrolyte membrane (PEM) layer disposed between the cathode and the anode; (b) a power source configured to control a current applied to the carbon oxide reduction electrolyzer; and (c) one or more controllers, the one or more controllers controlling the electrolyzer: (1) a current source connected to the MEA; (1) performing normal operation at a first current density, where normal operation includes injecting a reactant gas including carbon oxide into the cathode of the MEA and applying a current to the MEA at a first current density; (2) performing a recovery or protection process including (i) applying a reverse current to the electrolyzer, and (ii) flowing a recovery gas to the cathode while the reverse current is being applied to the electrolyzer; and (3) resuming normal operation with the MEA by stopping or modifying the flow of recovery gas to the cathode and by discontinuing the application of the reverse current to the electrolyzer.

[0037] In certain embodiments, the one or more controllers are configured to cause the electrolyzer to flow reactant gases to the cathode at a first flow rate and at a first pressure during normal operation. In certain embodiments, the one or more controllers are configured to cause the electrolyzer to periodically pause and / or pulse current to the MEA during normal operation. In certain embodiments, the one or more controllers are configured to cause the electrolyzer to perform normal operation for a period of at least about 100 hours.

[0038] In certain embodiments, the one or more controllers are further configured to cause the electrolyzer to continue performing normal operations with the MEA for at least about 100 hours after resuming normal operations with the MEA and before again performing a recovery or protection process or terminating operation of the MEA.

[0039] In certain embodiments, the recovery gas has a different composition than the reactive gas.

[0040] In certain embodiments, the one or more controllers are configured to cause the electrolytic cell to perform a recovery or protection process for a period of about 5 to 300 minutes.

[0041] In certain embodiments, the one or more controllers are configured to determine that an event that may be harmful to the performance of the electrolyzer is occurring or is likely to occur, and to implement a protective process.

[0042] Yet another aspect of the present disclosure relates to a method of operating a carbon oxide reduction electrolytic cell, the method may be characterized by the following actions: (a) performing normal operations with the electrolytic cell, where normal operations include injecting a reactant gas containing carbon oxide into a cathode of the electrolytic cell and applying an electric current to the electrolytic cell at a first current density to reduce the carbon oxide and produce a carbon-containing reduction product; (b) performing a recovery process including (i) ceasing or significantly interrupting the electric current applied to the electrolytic cell, and (ii) contacting the cathode with a liquid while ceasing or significantly reducing the electric current applied to the electrolytic cell; and (c) resuming normal operations with the electrolytic cell. In some embodiments, resuming normal operations includes ceasing contacting the cathode with the liquid and reapplying the electric current to the electrolytic cell.

[0043] In some implementations, contacting the cathode with a liquid comprises flowing the liquid to the cathode. In some embodiments, the liquid comprises water. In some embodiments, the water comprises dissolved salts. In some embodiments, the dissolved salts comprise bicarbonate at a concentration of about 20 mM or less.

[0044] In some embodiments, the rejuvenation process additionally includes flowing a dry gas to the cathode after contacting the cathode with the liquid and before at least partially resuming normal operation, In such embodiments, the dry gas may include carbon oxides, an inert gas, air, or any combination thereof.

[0045] In some embodiments, the recovery process further includes flowing a recovery gas to the cathode prior to contacting the cathode with the liquid. In such embodiments, the recovery gas may have a different composition than the reactant gas, may flow to the cathode at a different flow rate than the reactant gas during normal operation, may contact the cathode at a different pressure than the reactant gas during normal operation, or may include any combination thereof.

[0046] For any of the multiple methods of performing recovery or protection operations, normal operation may be performed as described above. Further, in some embodiments, during normal operation, no liquid contacts the cathode. Additionally, for any of the multiple embodiments of performing recovery or protection operations, carbon oxides and carbon-containing reaction products may be characterized as described above.

[0047] In some embodiments, the recovery process is carried out for a period of about 5 to 300 minutes.

[0048] In some embodiments, the method additionally includes, after the step of resuming normal operation with the electrolytic cell, continuing to perform normal operation with the electrolytic cell for at least about 100 hours before again performing the recovery process or terminating operation of the electrolytic cell.

[0049] In some embodiments, significantly reducing the current applied to the electrolytic cell comprises reducing the current to the cathode by a maximum of about 100 mA / cm 2 (flat surface area of ​​the cathode). In some embodiments, significantly reducing the current applied to the electrolytic cell includes applying the current to the cathode in an anodic direction. In some examples, applying the current in an anodic direction reduces the anodic current to a maximum of about 1 mA / cm. 2 (flat surface area of ​​the cathode).

[0050] In some embodiments, while stopping or significantly reducing the current applied to the electrolytic cell, the method additionally includes a voltage or current scan across the electrolytic cell. In some such embodiments, the voltage or current scan is performed cyclically.

[0051] Certain other aspects of the present disclosure relate to a carbon oxide reduction electrolyzer that may be characterized by the following features: (a) at least one membrane electrode assembly (MEA) including (i) a cathode including a carbon oxide reduction catalyst that promotes reduction of carbon oxides, (ii) an anode including a catalyst that promotes oxidation, and (iii) a polymer electrolyte membrane (PEM) layer disposed between the cathode and the anode; (b) a power source configured to control a current or voltage applied to the carbon oxide reduction electrolyzer; and (c) one or more controllers configured to cause the electrolyzer to: (1) perform normal operation with the MEA; (2) perform a recovery process that includes (i) ceasing or significantly reducing the current applied to the MEA, and (ii) contacting the cathode with a liquid while ceasing or significantly reducing the current applied to the MEA; and (3) resuming normal operation with the MEA, where resuming normal operation includes ceasing contact of the cathode with the liquid and reapplying current to the MEA. Normal operation may include injecting a reactant gas including carbon oxide into the cathode of the MEA and applying a current to the MEA at a first current density, in certain embodiments, the one or more controllers are configured to cause the electrolyzer to flow liquid to the cathode.

[0052] For any system that performs recovery or protective actions, normal operation may be controlled by the system as described above. Additionally, for any of the multiple manners in which recovery or protective actions may be performed, carbon oxides and carbon-containing reaction products may be characterized as described above.

[0053] In certain embodiments, the one or more controllers are further configured to cause the electrolyzer to continue performing normal operations with the MEA for at least about 100 hours after resuming normal operations with the MEA and before again performing the recovery process or terminating operation of the MEA, hi certain embodiments, the one or more controllers are configured to cause the electrolyzer to perform the recovery process for a period of about 5 to 300 minutes.

[0054] In certain embodiments, the one or more controllers are further configured to cause the electrolyzer to flow a recovery gas to the cathode prior to contacting the cathode with the liquid. In such embodiments, the recovery gas may have a different composition than the reactant gas, may flow to the cathode at a different flow rate than the reactant gas during normal operation, may contact the cathode at a different pressure than the reactant gas during normal operation, or any combination thereof.

[0055] In certain embodiments, the one or more controllers are further configured to cause the electrolyzer to flow a dry gas to the cathode after contacting the cathode with the liquid and before at least partially resuming normal operation.

[0056] In some implementations, contacting the cathode with a liquid comprises flowing the liquid to the cathode. In some embodiments, the liquid comprises water. In some embodiments, the water comprises dissolved salts. In some embodiments, the dissolved salts comprise bicarbonate at a concentration of about 20 mM or less.

[0057] In certain embodiments, the one or more controllers may be configured to cause the electrolytic cell to provide a current to the cathode of up to about 100 mA / cm 2 (the flat surface area of ​​the cathode) to significantly reduce the current applied to the MEA.

[0058] In certain embodiments, the one or more controllers are configured to cause the electrolyzer to apply a current in the anodic direction to the cathode, thereby significantly reducing the current applied to the MEA. In some such embodiments, the current in the anodic direction is at most about 1 mA / cm. 2 (flat surface area of ​​the cathode).

[0059] In certain embodiments, the one or more controllers are configured to cause the electrolyzer to perform voltage or current scans on the MEA while stopping or significantly reducing current applied to the MEA, hi some cases, the one or more controllers are configured to cause the electrolyzer to perform voltage or current scans cyclically.

[0060] Some further aspects of the present disclosure relate to methods of operating an electrolytic cell for carbon oxide reduction, the method may be characterized by the following actions: (a) performing normal operation with the electrolytic cell; (b) performing a recovery process including (i) stopping or significantly reducing the current applied to the electrolytic cell, and (ii) flowing recovery gas to the cathode while the current applied to the electrolytic cell is stopped or significantly reduced; and (c) resuming normal operation with the electrolytic cell, where resuming normal operation includes stopping or modifying the flow of recovery gas to the cathode and reapplying current to the electrolytic cell.

[0061] Normal operation may include injecting a reactant gas containing carbon oxides into a cathode of the electrolytic cell and applying a current to the electrolytic cell at a first current density to reduce the carbon oxides and produce carbon-containing reduction products. For any of the multiple methods of performing recovery or protection operations, normal operation may be performed as described above. Further, in some embodiments, during normal operation, no liquid contacts the cathode. Additionally, for any of the multiple embodiments of performing recovery or protection operations, the carbon oxides and carbon-containing reaction products may be characterized as described above.

[0062] In certain embodiments, the recovery gas has a different composition than the reactive gas. In certain embodiments, the recovery process is carried out for a period of about 5 to 300 minutes.

[0063] In some embodiments, the method additionally includes, after the step of resuming normal operation with the electrolytic cell, continuing to perform normal operation with the electrolytic cell for at least about 100 hours before again performing the recovery process or terminating operation of the electrolytic cell.

[0064] In certain embodiments, significantly reducing the current applied to the electrolytic cell includes reducing the current to the cathode to a maximum of about 100 mA / cm 2 (flat surface area of ​​the cathode). In certain embodiments, significantly reducing the current applied to the electrolytic cell includes applying the current to the cathode in an anodic direction. As an example, applying the current in an anodic direction may reduce the anodic current to a maximum of about 1 mA / cm 2 (flat surface area of ​​the cathode).

[0065] In certain embodiments, the method performs a voltage or current scan on the electrolytic cell while stopping or significantly reducing the current applied to the electrolytic cell. By way of example, the voltage or current scan is performed cyclically.

[0066] Yet another aspect of the present disclosure relates to a system, the system characterized by the following elements: (a) at least one membrane electrode assembly (MEA) including: (i) a cathode including a carbon oxide reduction catalyst that promotes reduction of carbon oxides, (ii) an anode including a catalyst that promotes oxidation, and (iii) a polymer electrolyte membrane (PEM) layer disposed between the cathode and the anode; (b) a power source configured to control a current applied to the carbon oxide reduction electrolyzer; and (c) one or more controllers, the one or more controllers controlling the electrolyzer to: a. perform normal operations at the MEA; b. (i) stopping or significantly reducing the current applied to the MEA, and (ii) performing a recovery process that includes flowing a recovery gas to the cathode while stopping or significantly reducing the current applied to the MEA; c. Configured to cause the MEA to resume normal operation.

[0067] Normal operation may include injecting a reactant gas including carbon oxide into a cathode of the MEA and applying an electric current to the MEA at a first current density, and resuming normal operation may include removing the cathode from contact with the liquid and reapplying the electric current to the MEA.

[0068] For any of the systems that perform recovery or protective actions, normal operation may be controlled by the system as described above. Additionally, for any of the systems that may perform recovery or protective actions, carbon oxides and carbon-containing reaction products may be characterized as described above.

[0069] In certain embodiments, the one or more controllers are configured to cause the electrolyzer to perform the recovery process for a period of about 5-300 minutes. In certain embodiments, the one or more controllers are further configured to cause the electrolyzer to continue performing normal operations with the MEA for at least about 100 hours after resumption of normal operations with the MEA and before again performing the recovery process or terminating operation of the MEA. In some embodiments, the recovery gas has a different composition than the reactant gas.

[0070] In certain embodiments, the one or more controllers may be configured to direct a current to the electrolytic cell at a maximum of about 100 mA / cm 2 (the flat surface area of ​​the cathode) is configured to significantly reduce the current applied to the MEA.

[0071] In certain embodiments, the one or more controllers are configured to cause the electrolyzer to significantly reduce the current applied to the MEA by applying a current in the anodic direction to the cathode. As an example, the current in the anodic direction may be at most about 1 mA / cm. 2 (flat surface area of ​​the cathode).

[0072] In certain embodiments, the one or more controllers are configured to cause the electrolyzer to perform voltage or current scans on the MEA while stopping or significantly reducing current applied to the MEA. As an example, the one or more controllers are configured to cause the electrolyzer to perform voltage or current scans cyclically.

[0073] These and other features of the present disclosure will now be presented in further detail with reference to the associated drawings. [Brief description of the drawings]

[0074] [Figure 1A] FIG. 1 illustrates an example of a current pause schedule or profile that may be implemented during operation of a carbon oxide reduction electrolyzer according to various embodiments of the present disclosure.

[0075] [Figure 1B] 4A-4C show schematic examples of current profiles as the current decreases from the operating current density to the pause current density at the beginning of a current pause period, in accordance with various embodiments of the present disclosure.

[0076] [Figure 1C] 4A-4C show schematic examples of current profiles returning to the operating current density at the end of a current pause period, according to various embodiments of the present disclosure.

[0077] [Diagram 2] 1 illustrates an exemplary feature of a cyclic voltage scan used during recovery or protection mode.

[0078] [Diagram 3] Experimental results are presented that show some of the effects of the recovery sequence. This example used a carbon dioxide electrolyser with a 25 cm2 cathode using Au / C catalyst particles.

[0079] [Figure 4]1 illustrates a system for controlling the operation of a carbon oxide reduction reactor that may include a cell containing an MEA.

[0080] [Diagram 5] 1 shows an example MEA for use in COx reduction. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0081] [Introduction and context] Carbon oxide electrolysers containing polymer-based membrane electrode assemblies (MEAs) are designed to produce oxygen from water at the anode and one or more carbon-based compounds through the electrochemical reduction of carbon dioxide or other carbon oxides at the cathode. As used herein, carbon oxides include carbon dioxide (CO2), carbon monoxide (CO), carbonate ions (CO3 2- ), hydrogen carbonate ion (HCO3 - ), and any combination thereof. Various examples of MEAs and MEA-based carbon oxide electrolyzers are described in the following references: PCT Application Publication No. 2017 / 192788, published on November 9, 2017, and entitled "Reactor With Advanced Architecture for Electrochemical Reaction of CO2, CO and Other Compounds," PCT Application Publication No. 2019 / 144135, published on July 25, 2019, and entitled "System and Method for Carbon Dioxide Reactor Control," and U.S. Provisional Patent Application No. 62 / 939,960, filed on November 25, 2019, and entitled "CO x No. 6,393,962, and U.S. Pat. No. 6,393,962, each of which is incorporated herein by reference in its entirety. In some cases, the MEA has a bipolar interface, i.e., an interface between a layer of a first ion exchange polymer that is substantially more conductive for anions than for cations, and a layer of a second ion exchange polymer that is substantially more conductive for cations than for anions. In some cases, the MEA includes only an anion exchange polymer or multiple anion exchange polymers (optionally provided as multiple layers). [Operation parameter type]

[0082] Various operating conditions for MEAs and MEA-based carbon dioxide electrolysers are disclosed herein. Among the types of operating parameters are the following: Carbon dioxide gas flow parameters, e.g., CO2 flow rate (molar and volumetric), pressure and composition; Anode water flow parameters, e.g., water flow rate, pressure, temperature and composition; Electrical parameters, e.g. current density and voltage; MEA and cell temperatures; Electrolyzer start-up conditions; and Transient variations in operating conditions (e.g., pulsed currents and / or gas flows). [Gas management] [Introduction]

[0083] In a carbon oxide reduction cell, carbon oxide is supplied to the cathode. The carbon oxide serves any one or more of several possible purposes. For example, the carbon oxide functions as a reactant. The carbon oxide may also function as a purge gas to remove water and / or to remove reduction products from the cathode.

[0084] Parameters characterizing the gas flow to the cathode include the gas composition at the inlet to the cell, the gas composition at the outlet from the cell, the volumetric flow rate of the gas stream to the cathode, the velocity of the gas stream to the cathode, the molar flow rate of the reactant gas to the cathode, the pressure of the gas at the inlet to the cell, the gas distribution pattern across the cathode, the total cross-sectional area of ​​the flow channel and the pressure drop of the gas as it flows through the cell. The term "input gas stream" refers to the gas at the inlet to the electrolytic carbon dioxide reduction cell. In an MEA electrolyzer with a flow field, the inlet gas stream may be the gas entering the cell upstream of the MEA and the flow field. An example of a cell stack including an MEA, gas diffusion layer, and flow field is described in PCT Patent Application Publication No. 2019144135, published on July 25, 2019, which is incorporated herein by reference in its entirety. [Role of gas flowing to the cathode]

[0085] At least a portion of the gas flowing to the cathode is consumed by the reduction reaction. The inlet gas may be characterized by the molar flow rate of the reactant carbon oxide entering the cell. Typically, the molar flow rate is large enough to support a specified reaction rate. The reaction rate may be determined, at least in part, by the current density at the cathode and the efficiency of the reduction reaction at the cathode. A non-exhaustive list of cathode reduction reactions is provided here:

[0086] Electrolysis reaction of CO and CO2 where water is the product. CO2+2H + +2e - →CO+H2O(2 electrons) 2CO2+12H + +12e - →CH2CH2+4H2O(12 electrons) 2CO2+12H + +12e - →CH3CH2OH+3H2O(12 electrons) CO2+8H + +8e - →CH4+2H2O(8 electrons) 2CO+8H + +8e - →CH2CH2+2H2O(8 electrons) 2CO+8H + +8e - →CH3CH2OH+H2O (8 electrons) CO+6H + +6e - →CH4+H2O(6 electrons)

[0087] Electrolysis of CO and CO2 when water is the proton source. CO2+H2O+2e - →CO+2OH - (2 electrons) 2CO2+8H2O+12e - →CH2CH2+12OH - (12 electrons) 2CO2+9H2O+12e - →CH3CH2OH+12OH - (12 electrons) CO2+6H2O+8e - →CH4+8OH - (8 electrons) 2CO+6H2O+8e - →CH2CH2+8OH - (8 electrons) 2CO+7H2O+8e - →CH3CH2OH+8OH - (8 electrons) CO+5H2O+6e - →CH4+6OH - (6 electrons)

[0088] Another role of the inlet gas stream to the cathode may be to flush away reaction products produced at the cathode, which may be liquids (e.g., ethanol, formic acid, acetic acid, 1-propanol) or gases (e.g., CO, methane, ethylene and / or hydrogen).

[0089] Another role of the inlet gas stream to the cathode may be to force water out of the cathode, preventing it from being flooded with water, which may interfere with the reaction at the catalytic sites. The water may be a gas or a liquid (e.g., water droplets). Some or all of the water may originate from the anode side of the MEA. The water may be generated from some carbon oxide reduction reaction. The inlet gas may carry the water to the cathode outlet. The carried water may be in gas and / or liquid phase (e.g., mist or droplets). The inlet gas stream may also exert pressure on the cathode side of the MEA, forcing water to permeate into the MEA toward the anode and / or resisting water transport from the anode to the cathode across the MEA.

[0090] In certain embodiments, the inlet gas stream may carry moisture (through the humidified inlet gas) that delivers water to at least a portion of the cathode of the MEA. Preferentially, the moisture in the inlet gas may humidify portions of the MEA that are susceptible to drying, such as portions of the MEA closest to the gas inlet. The moisture may also act as a proton source for certain carbon oxide reduction reactions. [Molar Flow Rate]

[0091] The molar flow rate of the reactant gas together with the current may set the carbon oxide reduction reaction rate. In some embodiments, the molar flow rate of the inlet carbon oxide may be set by the current at the cathode and / or the efficiency of the carbon oxide reduction reaction. Note that carbon oxide reduction cells often operate at electrochemical conversion efficiencies less than 100%. Thus, in some embodiments, the molar flow rate of carbon oxide to the cathode is greater than that required for theoretical complete conversion of the input carbon oxide to the desired reduction products. Furthermore, even if all of the incoming carbon oxide is electrochemically reducible, excess gas may be required to serve one or more other purposes, such as flushing water from the cathode or flushing reaction products. In other words, the incoming carbon oxide stream may require excess carbon oxide beyond that required for complete stoichiometric conversion. Even if the carbon oxide reduction reaction is 100% efficient, the system cannot have negligible amounts of input gas reactants. This is because some gas is needed to push the water and / or certain reaction products away from the cathode.

[0092] Because the molar flow rate is so high, the cathode output stream may have a relatively low concentration of reduction products (e.g., a relatively low concentration of carbon monoxide in carbon dioxide, e.g., about 30% molar or less), even though most of the inlet carbon oxide is unreacted. In some contexts, this may require extensive and / or expensive purification of the reduction products.

[0093] One way to relate the molar flow rate to the current is by a parameter referred to herein as the "stoichiometric value" with the corresponding flow rate. The molar flow rate of carbon oxide in the input stream may be defined in terms of the flow rate per unit of reaction expected for a particular current. The term "stoichiometric" value herein refers to the fraction or multiple of the flow rate of the reactant carbon oxide required to fully utilize all the current at the cathode, assuming that for a particular reaction, the reduction reaction of carbon oxide at the cathode is 100% efficient. A carbon oxide flow rate having a stoichiometric value of "1" is the flow rate required to consume all the electrons provided at the cathode, or less, for a particular reduction reaction at the cathode. The stoichiometric value represents the amount of excess (or deficit) reactant present over (or below) that which could theoretically react if the current efficiency for a particular reaction were 100%. It is a dimensionless number or fraction.

[0094] Carbon dioxide reduction reaction (CO2 + 2H + +2e - For CO2+H2O, a carbon dioxide flow rate with a stoichiometry of 1 provides 1 mole of carbon dioxide for every 2 moles of electrons provided by the cell. Stated another way, a cell with a current providing 2 moles of electrons per second and a carbon dioxide flow rate providing 1 mole of carbon dioxide molecules per second has a stoichiometry of 1. At the same current and a flow rate of 0.5 moles of carbon dioxide per second, the cell has a stoichiometry of 0.5. Stated again, at the same current and a flow rate of 1.5 moles of carbon dioxide per second, the cell has a stoichiometry of 1.5. The molar or volumetric flow rate required to achieve a stoichiometry of 1 may be calculated as follows:

[0095] Stoichiometric value 1-flow rate (sccm) = [60 (s / min)] * molar gas volume at STP (mL / mol) / [Faraday constant (C / mol e-) * #e-'s / mol CO 2]*The current supplied to the electrolyser in amperes. If the electrolyser system comprises a stack of two or more cells in series, the flow rate is multiplied by the number of cells in the stack.

[0096] In one example, 500mA / cm 2 100 cm to perform electrochemical reduction of CO2 to CO at a current density of 2 If an electrolytic cell has a total current of 50 A and the reaction requires that 2 moles e- / mole CO be produced, then the stoichiometric flow rate, which has a stoichiometric value of 1, is [60*22,413] / [9,6485*2]*50=348.4sccm

[0097] In this example, the flow rate that results in a stoichiometry of 0.5 is 0.5*348.4=174.2 sccm.

[0098] The flow rate that results in stoichiometry 2 is 2*348.4=696.8 sccm.

[0099] In another example of a cell that produces ethylene from carbon dioxide, 12 moles of electrons are required to reduce 2 moles of carbon dioxide to 1 mole of ethylene. 2 For an electrolytic cell, 300mA / cm 2 The stoichiometric flow rate that results in a stoichiometric value of 1 at a current density of [60*22,413] / [96,485*6]*1350=3,136sccm.

[0100] In certain embodiments, the stoichiometric value is at least about 1. In certain embodiments, the stoichiometric value is between about 1 and 400. In certain embodiments, the stoichiometric value is between about 1 and 20. Particular ranges of stoichiometric values ​​for particular operating regimes are disclosed elsewhere herein (e.g., high ratios of CO:CO2 in the output stream).

[0101] As an example, for a reaction to produce CO from CO2, the stoichiometric value may be about 1 to 30. As another example, for a reaction to produce ethylene or other C2 products in a bipolar MEA configuration, the stoichiometric value may be about 1 to 180. As another example, for a reaction to produce ethylene or other C2 products in an AEM-only MEA configuration, the stoichiometric value may be about 1 to 90. As another example, for a reaction to produce methane, the stoichiometric value may be about 1 to 230. [Volumetric flow rate and velocity]

[0102] The volumetric flow rate and corresponding flow velocity of the input gas are related to the molar flow rate of the reactant gas, but they may be set independently. Furthermore, different criteria may be applied to set the volumetric flow rate and the corresponding velocity. Considerations for determining the volumetric flow rate and flow velocity may include the gas pressure and composition of the input gas stream as well as the molar flow rate of the reactant gas. The volume occupied by a particular mass of input gas decreases with increasing pressure. Thus, for a fixed molar flow rate, the volumetric flow rate, which is proportional to the velocity of a particular cross-sectional area, decreases with increasing pressure. Furthermore, if the reactant gas is diluted with a non-reactant gas, the volumetric flow rate is determined not only by the molar flow rate of the reactant gas and the pressure of the input gas stream, but also by the flow rate of the non-reactant gas in the inlet stream.

[0103] The volumetric flow rate and flow velocity of the input gas stream can affect the rate of removal of water and / or reaction products from the cathode. Higher velocities remove more water and / or reaction products from the cathode. These materials may be in gas or liquid form. The input gas stream picks up and carries these materials in either phase from the cathode and pushes them, for example, through a flow field to the exhaust. For example, the gas stream may push liquid water (droplets) through a flow channel. Note that the water at the cathode may arrive from the anode via the anode water passage.

[0104] Water moving from the anode to the cathode in an MEA-based carbon oxide reduction electrolyzer may contain valuable components, such as salts, introduced into the cell. Thus, in certain embodiments, the electrolyzer system is configured to recycle water from the gas leaving the cathode and returning to the anode. In certain embodiments, the water removed from the cathode by the gas stream has one or more liquid-phase reduction reaction products (e.g., formate or ethanol). In certain embodiments, the water recovered from the cathode gas stream is treated to remove the reduction reaction products. Such treatment may be performed prior to reintroduction to the anode. In certain embodiments, the water removed from the cathode by the gas stream has salt ions. In some cases, the concentration of salt ions removed by the gas stream at the cathode differs from the concentration of salt ions in the anode water. In certain embodiments, the water recovered from the cathode gas stream is treated to adjust its salt concentration prior to reintroduction to the anode.

[0105] In certain embodiments, the volumetric flow rate of carbon dioxide is about 1.4E-6 to 1.66E-4 liters / (cm of active surface area of ​​the MEA cathode). 2 In some cases, the volumetric flow rate of carbon dioxide is about 1.4E-6 to 5.53E-5 liters / cm of active surface area of ​​the MEA cathode. 2 In certain embodiments, the velocity of carbon oxide over the active surface area of ​​the cathode is about 0.2 to 4 m / s.

[0106] In certain embodiments using AEM-only MEAs used to produce hydrocarbons such as CH4, C2H4, ethanol, etc., there is minimal water recovered at the cathode since in AEM-only MEAs water migrates from the cathode to the anode side, therefore, in such embodiments, lower volumetric input gas flow rates and corresponding gas velocities may be used. [Gas pressure at the inlet to the cathode cell]

[0107] The pressure of the inlet gas stream may be set or adjusted based on various considerations. Some considerations suggest a relatively high pressure. For example, a relatively high pressure inlet gas stream may result in a relatively high molar flow rate at the cathode and allow for a relatively high reaction rate. In other words, a relatively high pressure inlet gas may increase the performance of the electrolyzer by providing a relatively high feed rate of carbon oxide reactant to the catalyst. As indicated, practical considerations may require excess carbon oxide beyond that required for full stoichiometric conversion, even for a 100% efficient reaction.

[0108] In some implementations, a relatively high pressure gas stream may increase the ability to remove water from the cathode: pressurized carbon dioxide on the cathode may push water through permeation toward the anode of the MEA, especially where the water pressure at the anode is less than the pressure of the gas stream at the cathode.

[0109] However, applying pressure to the inlet carbon dioxide gas stream increases the gas density, thereby reducing the volumetric flow rate and velocity for a particular molar flow rate. The water removed depends on the flow rate. Increasing the pressure reduces the amount of volumetric flow and therefore the amount of water that can be removed. By compressing the inlet gas stream, the system can remove less water from the cathode.

[0110] Additionally, the gas stream washes products away from the cathode. A reduced volumetric gas flow rate may wash away less product, which may shift the reaction equilibrium towards the reactants.

[0111] Furthermore, if the pressure of the gas stream is too high, the gas pressure differential between the inlet and outlet of the cathode of the cell may be insufficient to push water droplets out of the flow field of the cell.

[0112] In certain embodiments, the feed gas may be diluted to increase or maintain a relatively high volumetric flow at high inlet gas pressures. For example, an inlet gas may be provided having a molar flow rate of carbon oxide sufficient to support a desired reaction rate, and the inlet gas may have a diluent that maintains an overall relatively high volumetric flow rate and corresponding flow velocity.

[0113] In some embodiments, the relatively high gas outlet gas pressure (defined by the inlet gas pressure and the pressure drop through the electrolyzer) provides the gas electrolyzer product at a pressure suitable for downstream processing (e.g., high pressure gas reactant to a Fischer-Tropsch reactor).

[0114] The gas pressure may be limited by structural or mechanical constraints imposed by the electrolyzer cell. For example, in certain embodiments using bipolar MEAs (e.g., with a Nafion® layer), the gas pressure is limited to about 100 psig or less. In certain embodiments using AEM-only MEAs, the gas pressure is limited to about 20 psig or less due to the limited mechanical stability of the anion exchange membrane.

[0115] The gas pressure may be limited by the physical properties of one or more components of the inlet gas stream. For example, carbon dioxide liquefies at room temperature at .about.800 psi. In certain embodiments, the pressure of the gas stream is maintained below the point at which carbon oxides and / or any other components condense or deposit as solids.

[0116] In some implementations, the gas pressure on the cathode side of the cell is balanced by the water pressure on the anode side or on the cell. Typically, as the pressure of the cathode gas stream increases, more of the cathode side gas (e.g., CO2, CO and / or H2) may pass through to the anode side of the cell. This can result in dangerous process conditions. For example, H2 mixed with CO and / or O2 can create a flammable mixture. It was noted that any polymer, such as the polymer electrolyte membrane in the MEA, may have some permeability to gases such as CO2, CO and H2.

[0117] In certain embodiments, the pressure of the gas stream at the inlet to the cathode side of the cell is about 10-400 psig. In certain embodiments, the pressure of the gas stream at the inlet to the cathode side of the cell is about 25-400 psig. As an example, the pressure of the gas stream at the inlet to the cathode is about 100 psig. As another example, the pressure of the gas stream at the inlet to the cathode is about 10-20 psig. Note that the inlet gas pressure is measured at the inlet to the cell stack, i.e., upstream from the gas diffusion layers and flow field. Note that in some implementations, there may be a significant pressure drop from the inlet of the cell stack to the MEA (e.g., about 0.5-15 psi in a serpentine pattern flow field). Also note that the listed pressures may be particularly appropriate for bipolar MEAs.

[0118] In certain embodiments, the pressure of the gas stream at the inlet to the cathode side of an AEM-only cell is about 0-100 psig. In certain embodiments, the pressure of the gas stream at the inlet to the cathode side of an AEM-only cell is about 0-20 psig due to the poor mechanical stability of the AEM. [Pressure drop across the cathode side of the cell (inlet P to outlet P)]

[0119] Gaseous reaction products emerge from the cell at a pressure different from the inlet pressure of the gas stream. The outlet pressure is given by the inlet pressure minus the pressure drop caused by the gas flowing through the cathode side of the cell. The pressure drop across the cell depends on various parameters, including, for example, the flow field configuration, the gas flow rate, the reduction products produced, the presence of water or precipitate in the flow field, the GDL type, the flow transition elements (e.g., manifolds) into the flow field, and the gas tubing diameter. In various embodiments, the flow rate is first set and the desired pressure drop (for the set flow rate) is tuned (using the flow field design).

[0120] In certain embodiments, the pressure drop of the gas stream flowing through the electrolyzer cathode is about 0.5-20 psi. In certain embodiments, the pressure drop of the gas stream flowing through the electrolyzer cell is about 2-7 psi. The pressure drop is measured between the cathode inlet tube and the cathode outlet tube of the electrolyzer. These pressure drop values ​​may be appropriate for a variety of reduction reactions, including those that produce carbon monoxide, methane, and / or ethylene. In cases where low flow rates are required, a higher pressure drop may be desired. [Inlet composition - reactant (carbon oxide), water (optional), inert gas (optional)]

[0121] As explained, the molar flow rate of the carbon oxide reactant may be determined, at least in part, by the current supplied to the cell, and the flow may be characterized by a parameter called the "stoichiometric" flow rate.

[0122] The process may use large amounts of excess reactants, however at some point the unreacted reactants in the product stream become a processing burden. For example, large amounts of unreacted carbon dioxide in a carbon monoxide product stream can result in large separation costs. For many applications, the carbon monoxide needs to be purified. For gas streams with carbon monoxide concentrations less than about 30% molar, the separation burden can rise significantly.

[0123] The degree of humidification of the inlet gas stream may be adjusted depending on the requirements of the electrolyzer. In general, a humidified carbon oxide inlet gas stream will remove less water from the cathode than a dry inlet gas stream, and removal of a significant amount of water via the cathode gas stream is often desired. However, in some electrolyzers, a humidified input gas stream results in relatively uniform hydration of the MEA across the electrode area. In certain embodiments, electrolyzers using AEM-only MEAs employ a humidified inlet gas stream, since the cathodes in such MEAs tend to be relatively dry. It has also been observed that humidification can affect the selectivity of the reduction reaction.

[0124] In various MEA implementations, in bipolar membrane systems, water is transferred from the anode to the cathode. As an example, about 2-5E-8 moles of water are transferred per mA / cm2 per second. In some implementations, for each proton transferred from the anode to the cathode, 2-4 water molecules are transferred toward the cathode. In some implementations, at least a portion of this water is separated from the exit gas stream and recycled to the anode.

[0125] In certain embodiments, the carbon dioxide gas stream contains one or more additives that have been intentionally or unintentionally added. By way of example, the carbon dioxide feed gas may be mixed with carbon monoxide or an inert gas (e.g., nitrogen) or impurities. In some combustion processes, the waste carbon dioxide contains nitrogen, oxygen, carbon monoxide, nitrogen oxides, sulfur oxides, etc.

[0126] In certain embodiments, the inlet carbon dioxide concentration is at least about 20 mole percent, or at least about 40 mole percent, or at least about 75 mole percent, or at least about 90 mole percent. In certain embodiments, the carbon dioxide provided to the carbon dioxide reduction reactor has a concentration of about 40-60 mole percent. [Water management] [Introduction]

[0127] In various embodiments, water is supplied to the anode of the electrolytic carbon dioxide reduction cell. In some implementations, water constantly flows past the anode during operation of the cell. In some cases, some water is removed from the cathode. The term "anode water" refers to the water at the inlet to the anode in an electrolytic carbon dioxide reduction cell.

[0128] Among the parameters that may characterize the water in an electrolytic carbon reduction cell are the composition of the water supplied to the anode, the composition of the water present at (or recovered from) the cathode, the mass flow rate of water to the anode, the mass flow rate of water recovered from the cathode, the pressure of the water at the inlet to the anode, the respective temperatures of the water at the inlet and outlet of the anode, and the pressure drop of the water as it flows through the cell. [Role of water flowing to or through the electrolysis cell]

[0129] Water in the cell may serve any one or more of a variety of purposes. For example, water may function as a reactant that is oxidized at the anode. In some cases, water functions as a transport medium for insoluble or soluble reaction products, such as hydrogen ions, hydroxide ions, and / or bicarbonate ions. In some cases, water functions as a flushing agent to remove anode reaction products. In some embodiments, water controls the temperature of one or more components in the electrolytic cell. In certain embodiments, water functions as a conductivity enhancer or activity enhancer for MEA components, such as one or more of the individual ion-conducting polymer layers of the MEA.

[0130] As a reactant at the anode, water is oxidized and provides electrons to the anode. In some embodiments, the anode half reaction is 2H2O→4H + +O2+4e - is given by:

[0131] The water flowing past the anode may provide a medium for purging the anode oxidation products, such as oxygen. As an example, the flowing water may remove oxygen bubbles from the anode side of an operating electrolyzer.

[0132] Anode water may be provided under conditions that supply water across the MEA to the cathode, for example by diffusion, electroosmosis, and / or osmosis. Water directed to the cathode may hydrate the CO2 feedstock and / or the cathode. A hydrated cathode may have increased activity and / or conductivity compared to a dry cathode.

[0133] The water may heat or cool the electrolysis cell. In certain embodiments, the system including the electrolyzer includes a heater and / or cooler configured to heat or cool the water upstream from the anode.

[0134] The pressure of the water at the anode may be balanced with the inlet gas pressure applied at the cathode side of the MEA. This pressure applied by the anode water may provide protection against damage that may occur to the MEA or other cell components as a result of uncompensated gas pressure on the cathode side of the cell. The anode water pressure may also provide a driving force for species transport across the MEA from the anode and / or balance the driving force for species transport from the cathode to the anode.

[0135] The anode water may serve as a source of other species required by the electrolytic cell, including salts and other additives used by the MEA to facilitate electrolysis. [Water flow rate]

[0136] In certain embodiments, the flow rate of water to the anode is determined at least in part by the rate of reaction at the anode, which in turn is determined at least in part by the current at the anode. The molar flow rate of water determines how much reactant is provided to the anode, and perhaps more importantly, how much reduction product can be produced at the cathode. In certain embodiments, the molar flow rate is at least as high as the theoretical amount of water (dictated by the current at the cathode) required to support the rate of reaction (oxidation of water with oxygen and hydrogen ions) dictated by the current at the anode.

[0137] In many embodiments, the reaction rate of the water at the anode is not a major consideration in setting the water flow rate to the anode. Other factors that may affect the water flow rate to the anode include removal of oxygen (e.g., gaseous oxygen and / or dissolved oxygen) from the anode, control of the temperature of the MEA and / or cell stack as a whole, and / or the supply of salts or other additives to the anode water.

[0138] In certain embodiments, the volumetric flow rate (and associated velocity) is at least high enough to ensure that oxygen gas and / or other products produced at the anode are swept away. Generally, under normal operating conditions, the molar flow rate is proportional to the volumetric flow rate, since water is essentially incompressible.

[0139] In certain embodiments, the volumetric water flow rate of the anode is about 0.2 to 60 milliliters / cm of active surface area of ​​the anode of the MEA. 2 In some cases, the anode water volumetric flow rate is about 1.2 to 4.8 liters / cm of anode active surface area of ​​the MEA. 2 (seconds per second). [Water pressure](inlet)

[0140] In certain embodiments, pressure is set at the inlet and / or outlet on the flow field. The pressure at the inlet to the anode flow field may be set based on the desired water flow rate and / or the desired pressure at the anode. In various embodiments, the flow rate is first set and the desired pressure drop (for a set flow rate) is tuned (using the flow field design). For a particular flow rate, the pressure drop is set by the flow field configuration. In some cases, the water inlet pressure is balanced with high pressure inlet gas on the cathode side. This may protect the MEA from damage and / or affect the permeation rate of one or more species across the MEA.

[0141] In certain embodiments, the water pressure at the inlet to the anode side of the MEA cell is about 0.5-20 psig. In some embodiments, the water pressure at the anode inlet is about 10 psig. In some embodiments using an AEM-only MEA, the water pressure at the anode inlet is about 0-5 psig. [Water pressure drop across the MEA]

[0142] In some embodiments, the pressure difference between the anode and cathode sides of the MEA is a controlled parameter. In some implementations, the cell may operate to balance the water pressure at the anode against the gas pressure at the cathode. In some cases, a pressure gradient is maintained that results in a higher pressure at the cathode than the anode. However, if the pressure gradient is reversed (anode higher), the pressure difference across the MEA may drive some permeation through the membrane. In certain embodiments, the pressure gradient is intentionally kept higher on the cathode side (pushing water back to the anode side) to reduce flooding at the cathode. This may be effective in managing water on the cathode side, for example, driving water away from the cathode so that catalytic sites remain available for reaction of the carbon oxide reactants. However, if the carbon dioxide feedstock is too dry, the pressure gradient may be driven in the opposite direction (from anode to cathode).

[0143] In certain embodiments, the pressure differential across the MEA (cathode pressure minus anode pressure) is about 5-400 psig. In certain embodiments, the pressure differential across the MEA (cathode pressure minus anode pressure) is about 70-400 psig. [Anode water composition]

[0144] Any of a variety of factors may be related to the composition of the anode water. In some embodiments, the anode water is maintained at a specified purity, particularly with respect to certain ions. In some embodiments, the anode water has very low concentrations of impurities such as iron and possibly other ions that may poison the carbon oxide reduction catalyst at the cathode.

[0145] In certain embodiments, the anodic water includes one or more salts or ions, and the composition of the anodic water may be controlled to maintain such salts or ions within particular concentration ranges.

[0146] In certain embodiments, the anodic water contains a salt or salt ion having a concentration of at least about 10 μM. In some implementations, the salt ions include alkali metal ions, hi some implementations, the salt ions include phosphate ions, sulfate ions, carbonate ions, bicarbonate ions, hydroxide ions, or any combination thereof.

[0147] In one example, the anode water is used with a bipolar MEA having a reduction catalyst comprising copper, and the salt comprises (i) an alkali metal cation, and (ii) a bicarbonate, sulfate, or hydroxide anion. In this example, the salt may be present at a concentration of about 1 mM to about 1 M, or about 1 mM to about 50 mM. In some cases, the MEA is configured to produce methane by reducing carbon dioxide and / or carbon monoxide at the cathode, and the salt ions comprise sodium ions. In some cases, the MEA is configured to produce organic compounds having two or more carbon atoms at the cathode, and the salt ions comprise ions of potassium, ions of cesium, ions of rubidium, or any combination thereof.

[0148] In one example, the anode water is used with a bipolar MEA having a gold-containing reduction catalyst, and the salt includes (i) an alkali metal cation and (ii) a bicarbonate, hydroxide, or sulfate anion. In this example, the salt may be present at a concentration of about 10 uM to about 200 mM, or about 100 uM to about 20 mM.

[0149] In one example, the anode water is used with a bipolar MEA where all polymers in the MEA are anion conducting polymers, and the carbon oxide reduction catalyst includes copper. In this example, the salt may include (i) an alkali metal cation and (ii) a bicarbonate or hydroxide anion. The salt may be present at a concentration of about 10 mM to about 15 M, or about 50 mM to about 1 M. In some cases, the MEA is configured to produce methane by reducing carbon dioxide and / or carbon monoxide at the cathode, and the salt ions include sodium ions. In some cases, the MEA is configured to produce organic compounds having two or more carbon atoms at the cathode by reducing carbon dioxide and / or carbon monoxide, and the salt ions include ions of potassium, ions of cesium, ions of rubidium, or any combination thereof.

[0150] Several examples of salts and methods and apparatus for controlling salt supply are described in PCT Patent Application Publication No. 2020 / 112919, published June 4, 2020, entitled "Electrolytic Cells and Methods of Use," which is incorporated herein by reference in its entirety. [temperature]

[0151] In some embodiments, the carbon oxide electrolyzer is operated at a specified temperature range, which may facilitate certain operations. In some embodiments, the carbon oxide electrolyzer generates heat during operation. This may be due to a relatively high overvoltage. In some embodiments, the carbon oxide electrolyzer is heated by flowing heated anode water to the anode. In some embodiments, the carbon oxide electrolyzer is heated by heat from an end plate on the cell stack. In some embodiments, the carbon oxide electrolyzer is cooled during operation. In some implementations, the cells use air cooling to maintain the temperature within the specified range. Some air-cooled electrolyzers have about 50 or fewer carbon oxide reduction cells in the stack. In some implementations, the cells use water cooling to maintain the temperature within the specified range. Some water-cooled electrolyzers have about 50 or more than 50 carbon oxide reduction cells in the stack.

[0152] One or more of the various physical effects in the carbon oxide electrolyzer may change with increasing temperature. Increasing the temperature may result in a decrease in the solubility of gases, which may reduce the rate or fraction of incoming carbon oxide reaching the catalyst. Increasing the temperature in the electrolyzer may increase the conductivity of the polymer electrolyte. Increasing the temperature in the electrolyzer may increase mass transport, such as by increasing the diffusion of species in the water and / or in the ion-conducting polymer of the MEA and / or by increasing the flow of water or other gas species by permeation into the MEA. Increasing the temperature in the electrolyzer may increase the reaction rates at the cathode. These may include one or both of the carbon oxide reduction reaction and the hydrogen gas evolution reaction. Increasing the temperature in the electrolyzer may increase the decomposition rate of the MEA material, including, for example, the catalyst and / or the polymer electrolyte. Increasing the temperature in the electrolyzer may increase the gas phase concentration of water in the gas stream leaving the cathode. Any one or more of these temperature effects may be enhanced or controlled by maintaining the cell operating temperature above or below a threshold value, or within a particular range.

[0153] By comparison, in carbon oxide reduction systems, the desired temperature of the reaction is often relatively low, e.g., below 20° C. In polymer-based MEA carbon oxide reduction systems, the desired temperature may be relatively high, e.g., above about 20° C.

[0154] In certain embodiments, the temperature of the bipolar MEA in the operating cell is about 20-90° C. In certain embodiments, the temperature of the bipolar MEA in the operating cell is about 30-80° C.

[0155] In some embodiments, the temperature of the bipolar MEA in the operating cell is about 45° C. or higher. In some embodiments, the temperature of the bipolar MEA in the operating cell is about 70° C. or higher and the cell uses a relatively modest cooling system, such as, for example, one that uses an air-cooled system.

[0156] In certain embodiments, the temperature of the AEM-only MEA in the operating cell is between about 20-45° C. In certain embodiments, the temperature of the AEM-only MEA in the operating cell is between about 25-35° C.

[0157] The optimum operating temperature may be limited by the thermal stability of the cell's components. [Electrical conditions]

[0158] In some implementations, the carbon oxide electrolysis system uses a power supply configured to provide a constant current and / or a constant voltage to the carbon oxide reduction cell. Constant current operation may provide a generally constant rate of product generation at the cathode and anode. Under some operating conditions, constant voltage operation may result in variable amounts of product since the current density may be varied while maintaining a constant voltage. In some implementations, the selectivity of the reduction products at the cathode may be tuned by varying the cell voltage.

[0159] In certain embodiments, the constant current or nominal current density at the cathode of a single electrolytic cell is about 10-2000 mA / cm 2 In a particular embodiment, the density of the constant current at the cathode of a single electrolytic cell is about 20-600 mA / cm 2 In these ranges, the current density is defined for a geometrically smooth cathode active surface without considering pores or other surface texture.

[0160] In some cases, the current density may affect the selectivity of the products produced. Some products may not be produced at low current densities and low cell voltages, so a higher or lower current density may be selected to favor or de-favor certain products. For example, at about 200 mA / cm 2 may promote the production of methane and / or ethylene (versus carbon monoxide) in a bipolar MEA configuration, or ethylene in an AEM-only configuration. In some implementations, current densities of about 270-330 mA / cm 2 or about 300mA / cm 2 At about 200 mA / cm, selectivity for methane and / or ethylene (e.g., major products) is promoted. 2 Below this, CO and H2 may be the main products.

[0161] In some implementations, the carbon oxide electrolyzer power supply is configured to regulate the current by stepping up and / or stepping down the current of the cell, by ramping up and / or ramping down the current to the cell, and / or by pulsing the current to the cell. In some implementations, the carbon oxide electrolyzer power supply is configured to regulate the voltage by stepping up and / or stepping down the voltage of the cell, by ramping up and / or ramping down the voltage of the cell, and / or by pulsing the voltage of the cell.

[0162] In certain embodiments, the electrolyzer controller is configured to temporarily apply a positive current (i.e., temporarily run the cathode as the anode and vice versa). This may deplete (or oxidize away) impurities such as transition metals that may become coated with metal on the cathode during operation. As an example, a source of such impurities may be the water at the anode. Reversing the current may remove intermediate carbon oxide reduction products that may poison the cathode catalyst.

[0163] PCT patent application PCT / US2019 / 067169, filed December 18, 2019, entitled "Electrolyzer and Method of Use," describes embodiments related to controlling electrical conditions in a carbon oxide electrolysis cell, and is incorporated herein by reference in its entirety. [Start-up conditions and interruptions during procedures]

[0164] The carbon oxide reduction electrolyzer may be subjected to one or more start-up conditions that differ from normal operating conditions. For example, the electrolyzer may be heated during start-up. As another example, the electrolyzer may be subjected to an adverse pressure gradient across the MEA (from anode to cathode) compared to normal operating conditions.

[0165] In certain embodiments, the start-up process of the electrolyzer includes first heating the electrolyzer to an operating temperature of the electrolyzer (e.g., see temperature ranges above), etc., without application of electrical energy to drive the reduction reaction, followed by application of operating electrical energy.

[0166] In some embodiments, the carbon electrolysis cell is subjected to an interruption in the procedure prior to full operation. Such an interruption in the procedure may involve controlling the electrical energy to the cell, controlling the supply of oxidized carbon feedstock to the cell, and / or controlling other operating parameters of the cell. [Electric pulse] [Operational context and stage]

[0167] In some embodiments, the current applied to the MEA has a non-constant profile. The current profile may vary according to the operating mode, as described further below. The operating modes may include hydration (pre-break-in), break-in, normal operation, planned shutdown, extended shutdown or storage, or any combination thereof. Other cell operating parameters (often related to adjusting the current) that may be adjusted during these operating modes include: (a) cathode gas composition, flow rate and pressure, (b) anode water composition and flow rate, (c) temperature, or (d) any combination thereof. In some embodiments, the voltage is controlled.

[0168] The applied current may be paused or pulsed during operation of the cell. A current pause may be referred to as an off / on cycle, where the current is switched off and then on one or more times. In some embodiments, the applied current is reduced to zero (i.e., turned off) during the current pause. In some embodiments, the current pause reduces the current to a non-zero level.

[0169] In some embodiments, before applying current to the cell, the MEA is put through a hydration step. This may include starting the reactant flows and optionally heating the cell (or stack) so that a steady state can be reached before applying current. In some implementations, before assembling the stack or cell, the MEA is submerged in water to start hydration of the MEA. After assembly, the anode water and cathode CO2 flows and pressures are set. Dry or humidified CO2 flows can be beneficial in this step, even if dry CO2 is used as the input during relatively long-term operation. The anode outlet may be observed to ensure that there are no air bubbles exiting this outlet. If air bubbles are present, this indicates significant CO2 crossover (from pinholes in the membrane) or leakage in the hardware. If the desired operating temperature is higher than ambient, the cell may be heated to the desired temperature after starting the anode water flow. During this step, the MEA remains hydrated at the desired temperature.

[0170] Break-in period refers to a procedure applied to an MEA or stack for the first time until the operating conditions and performance meet the desired long-term settings. In some embodiments, when an MEA is used for the first time, a procedure different from normal operation may be used. An MEA that has not been operated before may not be fully hydrated or may have changes in structure due to temperature increase during operation. In some embodiments, the current ramps up from a lower value to a higher value in a series of steps rather than jumping in a straight line to the desired operating value. A gradual linear ramp-up may be used. An example of a current profile is shown in FIG. 1A. The number of intermediate steps in a multi-step ramp-up may be, for example, 1, 2, 3, 4, 5, or 6. The duration of each step may be the same or different.

[0171] In embodiments where the operating temperature is reached prior to pre-break-in (e.g., during the hydration period), the temperature may be maintained constant at this temperature, while in other embodiments, the temperature may be ramped up during the break-in procedure.

[0172] Cycling the stack off and on during normal operation may be useful to maintain performance over time. Examples of performance improvements include increasing the current efficiency of the electrolyzer, increasing the voltage efficiency of the electrolyzer, providing single pass conversion (less frequent pulsing increases the overall conversion / utilization of the electrolyzer), increasing the life of the electrolyzer MEA, increasing the life of other cell components (such as gas diffusion layers GDLs), and increasing selectivity for certain reactions.

[0173] In some embodiments, the current profile or current pause schedule is such that the current on period is significantly longer than the pause period. FIG. 1A shows a schematic example of a current pause schedule, which may also be referred to as a current profile. Current density is shown on the y-axis and time on the x-axis. In some embodiments, for high throughput, the duration of the current pause is much shorter than the current on period. For example, the current on period may be at least 3 times, 5 times, 10 times, 20 times, 50 times, 100 times, or 500 times longer than the current pause period.

[0174] 1A, the current pause schedule is constant for the duration of normal operation. In other embodiments, the intervals and / or pause durations may vary throughout the course of operation. For example, current pauses may be programmed to be more frequent during advanced operating steps.

[0175] Also, in the example of FIG. 1A, a single step is used to decrease the current density at the beginning of the pause period and to ramp back to the operating density at the end of the pause period. Similar to increasing or decreasing the current in other operating modes described herein, the current may be ramped in multiple steps or continuously at the beginning and / or end of the current pause period. FIG. 1B shows a schematic example of decreasing the current from the operating current density to the pause current density at the beginning of the current pause period. Similarly, FIG. 1C shows a schematic example of returning to the operating current density at the end of the current pause period. The current profile at the beginning may be selected independently of that at the end of the pause period. For example, the current may be decreased in a single step and increased in multiple steps.

[0176] During the current pause, the cell voltage may be held at any of a variety of values. In some cases, during the current pause, the anode and cathode are shorted (e.g., through a power source or by connecting the electrodes to a metal or other conductor). In this case, the cell voltage is 0 or near 0 volts. In some cases, during the current pause, the anode and cathode are allowed to float and the cell's voltage is its open circuit voltage under prevailing conditions, e.g., 0.8V to 1.4V, 0.8V to 1.2V, or 0.9V to 1.1V. According to various embodiments, the cathode and / or anode flow may be stopped or allowed to continue during the current pause.

[0177] Sometimes, CO x Depending on the use of the electrolysis system, planned outages may be performed. In a planned outage, the system is shut down for a short period of time and then turned back on. Examples of reasons for planned outages include maintenance of parts of the system (e.g., changing a filter on the anode water recycle loop, replacing a flow controller, or testing a temperature sensor), planned power outages, and CO x This includes a temporary stop in a downstream process that uses the products of the reduction. A planned outage may have a relatively short downtime, lasting, for example, from a few minutes to a few days.

[0178] At times, it may be desirable to shut down a system or stack for an extended period of time. For example, a facility holiday shutdown, moving a system to a new facility, or CO x Supply interruption. During this time, it is expected that the system can be completely disconnected from external inputs. In this case, a different gas or aqueous solution than that used during normal operation can be sealed in the anode or cathode. The start-up procedure after an extended shutdown or storage period can be the same as the break-in procedure described above. [Pulse-triggered mechanism]

[0179] The electrical pulse may affect the carbon oxide reduction electrolyzer through one or more mechanisms. Without wishing to be bound by theory, one such mechanism may include relieving the internal stress of the electrodes caused by the strong polarity. This may be accomplished by relieving the overpotential of charging the carbon material in the cathode caused by the strong polarity. The porous carbon cathode support material may create a high capacity electrochemical double layer at the carbon electrolyzer interface (effectively forming a supercapacitor). Over time, constant charging may continually increase the overpotential to charge our electrode. The resulting electric field may affect the restructuring of the surface and may affect the charging of the carbon (or other) material on the electrode. The pulse or pause may cause some of the charge on the electrode interface to bleed off.

[0180] The electrical pulse may also modify the surface environment of the electrode by suspending mass transport, altering adsorption / desorption at different voltages, and / or depleting the electrode of impurities that may otherwise adsorb onto the electrode and degrade performance. Examples of such impurities include main or side reaction intermediates and impurities from the cell hardware or reactants (i.e., water, CO2). The electrical pulse may modify the cathode by restructuring the surface morphology of the catalyst and / or by repositioning the catalytic sites in a more favorable orientation for carbon oxide reduction.

[0181] Additionally, the electrical pulse may affect the quality of water at the cathode: the electrical pulse may reduce the amount of water transported from the anode (e.g., at lower current densities), while the water removal rate at the cathode may remain unchanged due to the same gas flow rate, thereby improving the mass transport of carbon oxides. [Parameter Value]

[0182] In the context of some embodiments, and in some embodiments, normal operation of the carbon dioxide reduction cell is at a voltage of about 0-10 V (electrolysis) and / or at a current of about 0-2000 mA / cm 2 The electrolysis may be carried out at a cathodic current density of about 0 V. The cell may have a normal open circuit voltage (rest voltage) in the range of about 0 to 2.5 V. Note that unless otherwise stated herein, all currents and voltages provided to the electrolysis cell have positive values ​​(i.e., cathodic current flows at the positive electrode and carbon oxides are reduced at the positive electrode).

[0183] The following parameters may characterize the electrical pulse. Unless otherwise stated, the parameters may be implemented by controlling the current and / or the voltage. It should be noted that if the electrolyzer operates under current control, the applied current pulse will have a corresponding voltage pulse, which may have a different profile than the current pulse. Similarly, if the electrolyzer operates under voltage control, the applied voltage pulse may have a corresponding but different current pulse.

[0184] [magnitude and duration of the pulse or pause] The current pulse is about 100 to 2000mA / cm 2 or about 200~600mA / cm 2The current density cycles may be performed using current density cycles with a high current density of about 1000 mA / cm. The high current density states may be held for about 30 minutes to 1000 minutes, with such states being separated by reductions or pauses in the current, respectively. According to various embodiments, the current is paused at relatively frequent intervals (e.g., less than about 10 hours, or less than about 2 hours) or at relatively infrequent intervals (e.g., more than about 10 hours). The reduced current between pulses may be about 1-100 mA / cm. 2 and may be held for a period of about 0.5 seconds to 60 minutes. This cycle may be repeated for the duration of normal operation. Note that the low current density pause may have a reversed current flow, e.g., positive (oxidizing) at the cathode.

[0185] In some embodiments, for high throughput, the duration of the current pause is much shorter than the current-on period. For example, the current-on period may be at least 2 times, at least 3 times, at least 5 times, at least 10 times, at least 20 times, at least 50 times, at least 100 times, or at least 500 times longer than the current pause period. In certain embodiments, the periodic pulse / pulses have a duty cycle of about 0.2 to 1.

[0186] As mentioned, the cell voltage may be controlled to affect the pulses or pauses. As an example, the voltage pulses are implemented using cycles in which the high voltage state ranges from about 2.7 to 3.9V. In these or other examples, the low voltage state ranges from about 1.5 to 2.7V. In some examples, the high voltage is held for about 30 minutes to 1000 minutes, and / or the low voltage is held for about 5 minutes to 100 minutes. Such cycles may be repeated for the duration of normal operation. In certain embodiments, the periodic pulse / pulses have a duty cycle of about 0.2 to 1.

[0187] In certain embodiments, the current pulses aid in the removal of liquid water from the cathode. A reduced current density may reduce the water transported to the cathode. The operating current density is about 200-600 mA / cm for the majority of the operating time. 2 The majority of the operating time may be, for example, in the range of about 65% to 95% of the total time. The suspended current density may be, for example, about 1 to 100 mA / cm 2 and accordingly, for a small portion of the total time, for example, about 5% to 35%.

[0188] [Gradual and Ramp Changes] Step changes or ramps (up and down) may be used during an initial break-in protocol, or during a transition protocol between different power densities during a pulse, or before or after a planned outage. Step changes may include 2-10 steps (e.g., about 2-5 steps). In some embodiments, the step size is about 50-300 mA / cm. 2 In some embodiments, the duration of a step is about 1 minute to 300 minutes (e.g., about 30 to 150 minutes or about 60 to 120 minutes). The ramp protocol may include ramping to or from a target current in between about 1 second and 200 minutes. In some implementations, the ramp is linear.

[0189] In some embodiments, periods during which electrical pulses or pauses occur are separated by periods during which no pulses or pauses occur. Such alternating periods of pulses / pauses and no pulses / pauses may occur during normal operation, break-in, planned outages, etc. The no pulse periods may be used as a second stage break-in protocol prior to normal operation. By way of example, a current of about 200-400 mA / cm2 may be used. 2 A constant medium current density over a range of 10-20 Hz may be applied for about 50-100 hours before the pulse protocol begins.

[0190] The pulse may have a reverse cell current (or polarity) where the cathode is temporarily operated at an oxidizing current and voltage. The reverse potential pulse may range from about 0 to 3.5 V with a corresponding current density of -10 to 0 mA / cm. 2 The reverse pulse may range from about 0 to 60 minutes in duration. The reverse pulse may be implemented with the same period and / or other parameters as described herein for the forward electrical pulse. In some embodiments, the reverse electrical pulse is interleaved with the forward electrical pulse.

[0191] Some relevant values ​​of the pause or pulse parameters are set forth in U.S. Patent Application Publication No. 2020 / 022185, filed December 18, 2019, which is incorporated by reference in its entirety herein.

[0192] The following parameters may characterize the planned shutdown cycles: The shutdown cycles may be placed every 100-10,000 operating hours, and the "off" current state may be absolute zero current (OCV mode) or minimum current state (short mode). [Non-electrical parameter pulse] [context]

[0193] Current is not the only reactor condition that may be pulsed or paused. Examples of other reactor conditions that may be pulsed or paused include gas flow rate to the cathode, gas pressure to the cathode, cell temperature, and water flow to the anode. Pulsing of non-electrical parameters may be performed synchronously with the electrical pulses, or may be performed independently of the electrical pulses, if electrical pulses are used. In some implementations, CO x Flow rate, electrical parameters, cell temperature, and CO x The pressures may be pulsed independently, all together, or in different combinations.

[0194] The mechanisms and effects of pausing or pulsing non-electrical parameters may overlap with the mechanisms and effects of pausing or pulsing electrical parameters. In certain embodiments, the mechanisms involve "water management," where water management is CO x It may improve mass transfer. Water management may include removing water from the flow field, gas diffusion layers, catalyst layers (pores as above) and / or the MEA. In certain embodiments, water management removes undesirable intermediates in liquid form. In certain embodiments, water management removes potential salt plugging when reducing gas flow. [Non-electrical parameter pulse range]

[0195] The following are non-limiting examples of non-electrical parameter values ​​that can be used in pulse or pause embodiments: [Pulse pressure magnitude]

[0196] The reactor gas pressure may have a normal operating set point ranging from about 90 to 150 psi that is maintained for operating periods ranging from about several minutes to several hundred hours.

[0197] During the pulse or pause, the reactor gas pressure may have a lower gas pressure ranging from about 0 to 70 psi, which is maintained for a period ranging from about several minutes to an hour, with or without the application of electrical current.

[0198] Such a cycle may be repeated multiple times during normal operation, for example, at least about five times, or at least about ten times. [Pulse duration]

[0199] The electrolysis reactor may be operated at normal (high) cathode gas pressure for operation periods ranging from about 30 minutes to 1000 hours.

[0200] The reactor may be operated at a lower cathode gas pressure, with or without the application of electrical current, for a period ranging, for example, from about 5 minutes to 60 minutes.

[0201] As an example, a CO2 reduction cell may be operated at about 90 psi for about 45 minutes, followed by about 0 psi (gauge) for about 5 minutes. Pulsing from 0 to 70 psi, typically in operation, has been found to be useful for water management. [Variation of volumetric flow rate during the pulse]

[0202] The gas flow rate to the cathode of the electrolysis reactor is, for example, about 2 to 80 sccm (1 cm of the cathode) for a duration of about 30 minutes to 1000 hours. 2 In some embodiments, the reactor gas flow rate may be between about 12 and 120 sccm (1 cm of cathode), for example. 2 In some embodiments, the reactor gas flow rate is increased to a higher flow rate over a planar surface area of ​​about 0.4-4 sccm (1 cm of cathode). 2 The reactor is then reduced to a lower flow rate, such as a lower flow rate (scalable for a flat surface area of ​​1000 sq. m). The duration of the gas flow rate deviation (higher or lower than the normal operating set point) may be shorter than the duration of the normal gas flow rate. For example, the deviating gas flow rate may be for a period of about 0.1 seconds to 12 hours, with or without electrical current applied. As with other parameter variations, the reactor gas flow rate cycle may be repeated multiple times.

[0203] In one example, the gas flow rate cycle includes a carbon oxide flow rate set point of about 1000 sccm, which is maintained for about 45 minutes. In the example, the carbon oxide flow rate is then increased to about 2000 sccm for about 5 minutes. This cycle is repeated during normal operation. [Temperature Pulse]

[0204] In certain embodiments, the carbon oxide reduction electrolysis cell has varying temperatures during normal operation. In some cases, the normal operating temperature is about 30-70° C. and the lower pause or pulse temperature is about 20-40° C. In some cases, the normal operating temperature is maintained for about 1-100 days and the lower temperature is maintained for about 1 hour to 1 day.

[0205] As an example, a carbon oxide reduction electrolyzer may use the following temperature variations: the electrolyzer is operated at about 50° C. for about 10 days, followed by about 30° C. for about 1 day. This cycle may be repeated multiple times during normal operation of the electrolyzer. Adjusting the cell temperature may improve catalyst selectivity and modify properties of the polymer electrolyte, such as water uptake and chemical transport rates, thereby facilitating efficient water management. [Pulse ramp rate (rise and fall; linear and / or step)]

[0206] The gas pressure pulse may be achieved by a step change or a ramp.

[0207] The gas flow rate pulse may be accomplished by a step change or a ramp.

[0208] The temperature pulse may be achieved by a step change or a ramp. [Recovery and Process] [Introduction]

[0209] In certain embodiments, the operating sequence involves temporary deviations from normal operating conditions to flow water or other liquid to the cathode and / or to flow gas to the cathode under non-standard conditions. It has been found that flowing water to the cathode and / or flowing gas (e.g., gas other than the normal carbon oxide reactant) to the cathode can facilitate recovery of carbon oxide electrolyzer performance. This alternating operating sequence is sometimes referred to as a "recovery process" or "recovery sequence."

[0210] The recovery process may be performed after the carbon oxide electrolyser has been placed into service and operated for a period of time, such as thousands of hours, under normal conditions. After the recovery process is completed, the electrolyser may be transitioned back to normal operation. The recovery process may be performed repeatedly over the useful life of the electrolyser or over the life of one or more of the electrolyser's components, such as the electrolyser's associated MEAs, gas diffusion layers (GDLs), and flow fields. For example, the recovery process may be performed every 1000-10,000 hours of the useful life.

[0211] While many embodiments disclosed herein are presented as procedures for restoring lost performance of a carbon oxide electrolyzer, some embodiments relate to protecting the carbon oxide electrolyzer from the adverse effects of some unexpected event, such as loss of power to the electrolyzer. If it is determined that an unexpected event is occurring or may soon occur, the carbon oxide electrolyzer may be placed in a protection mode. If such an unexpected event is not mitigated, it may cause damage to the electrolyzer or the infrastructure supporting the electrolyzer.

[0212] In some implementations, any of the actions described herein for performing recovery, or any combination of such actions, may be used for protection of the carbon oxide electrolyzer.

[0213] In some embodiments, the electrolyser and / or associated control system implements a protective mode by (a) determining that an unexpected and potentially harmful event is occurring or may occur in the future and that if such unexpected event is not mitigated, the unexpected event may damage or degrade the carbon oxide electrolyser, and (b) taking one or more protective actions on the carbon oxide electrolyser that reduce the likelihood that the electrolyser will be damaged or degraded if the unexpected event continues to occur or actually occurs in the future.

[0214] Examples of unexpected events that may trigger protective action include a sudden reduction or loss of input material such as anolyte or carbon oxide (e.g., CO2) gas, a reduction or loss of heating or cooling, and a loss of power to the electrolyzer. A significant reduction or loss of input material may require the power to the electrolyzer to be adjusted to result in an open circuit voltage or no current. Loss of power to the electrolyzer may cause the electrolyzer to discharge, either rapidly or gradually, from the operating voltage to an uncontrolled voltage such as the open circuit voltage or zero voltage.

[0215] Examples of protective actions to mitigate the effects of an unexpected event include applying a relatively low current density to the electrolyzer, transitioning the electrolyzer voltage to an open circuit voltage, and reducing or ramping down the current applied to the electrolyzer. Any of these protective actions may be applied for a limited time, such as only while the unexpected event continues to occur or until the likelihood of such an event occurring has been significantly reduced.

[0216] In some embodiments, the protective action is to reduce the current density of the electrolytic cell to about 1 to 50 mA / cm 2 or about 5 to 25 mA / cm 2 (For example, about 10mA / cm 2 ) to a relatively small (compared to normal operation) forward current density, or to approximately 0.3% to 20% of the current density under normal operating conditions.

[0217] In some embodiments, the protective action is to ramp down the current to the electrolyzer. The ramp may have any shape or slope. In some cases, the average ramp rate from full current (normal operation) to final current is about 0.1-1 mA / cm. 2 / min, or approximately 1 to 10mA / cm 2 / min. In some cases, the ramp is stepped. The number of steps, the duration of the steps, and the magnitude of the change in current density of the steps can be variable. By way of example, the ramp may have about 2-50 steps, or about 5-30 steps. By way of further example, the duration of a step may be about 1-100 seconds, or about 5-50 seconds. By way of further example, the magnitude of the current of the steps may be about 0.1-10 mA / cm. 2 or about 0.5 to 5 mA / cm 2 It may be.

[0218] In one example, the step profile reduces the current density to the electrolytic cell to a value below that of normal operation (e.g., about 300 mA / cm 2 to 2 A / cm) through a sequence of steps, each of which reduces to a much smaller value (e.g., about 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 mA / cm 2 ), each having a defined duration (e.g., about 30 seconds each), after which the final current output is set to maintain the electrolytic cell at open circuit for about 5-10 minutes.

[0219] In some embodiments, the electrolyzer returns from a recovery or protection mode to normal operating conditions via a current ramp. Such a return ramp may have any of the features just identified for ramping down the current in the opposite direction, i.e., from a low current density to a higher current density. [Exemplary Recovery Process]

[0220] In one example, the recovery process involves the following sequence: pausing the current to the electrolyzer, then flowing water to the cathode, then resuming current flow to the electrolyzer. In another example, the recovery process involves the following sequence: pausing the current to the electrolyzer, then flowing gas to the cathode, then flowing water to the cathode, then re-flowing gas to the cathode, and finally resuming normal operation by flowing current through the cell. Some further examples of recovery sequences are included below.

[0221] In some examples, the recovery operation involves contacting the cathode with water while no current is flowing to the cathode. In some implementations, a relatively small amount of current flows while water is present at the cathode. In some cases, this current flows in the reverse direction (anode in a carbon oxide reduction cathode). As an example, only about 1 mA / cm 2 flows while water is present. 2 A current of 1000 volts flows in the reverse direction to the cathode. In some examples, instead of statically contacting the cathode during a portion of the recovery process, water flows to the cathode. Examples of recovery operations that include water flow or water contact include Examples 1, 2, 3, and 4.

[0222] [Example Recovery Sequence 1] Normal operation, optionally including pulsing current or voltage; Turn off or significantly reduce the current; introducing water to the cathode by flowing it or by contacting it with the cathode; The current is turned on or increased to normal operating levels to re-establish normal operation (eg, flow reactant gases at normal operating pressures and flow rates).

[0223] In some embodiments, the recovery operation includes stopping the current, flowing water through the cathode, and then drying the cathode. The drying operation is optionally performed using a different gas, such as a reactive gas, a modified reactive gas, or an inert gas.

[0224] [Example Recovery Sequence 2] Normal operation, optionally including pulsing current or voltage; Turn off or significantly reduce the current; introducing water to the cathode by flowing it or by contacting it with the cathode; Drying the cathode (e.g., by flowing gas through the cathode); The current is turned on or increased to normal operating levels to re-establish normal operation (eg, flow reactant gases at normal operating pressures and flow rates).

[0225] In some embodiments, the recovery operation involves flowing water to the cathode while no gas (reactant or other gas) is flowing to the cathode.

[0226] [Illustrative Recovery Sequence 3] Normal operation, optionally including pulsing current or voltage; Turn off or significantly reduce the current; Releasing the reactant gas backpressure to the cathode and stopping the flow of the reactant gas; Water is introduced to the cathode; Drying the cathode (e.g., by flowing gas through the cathode); The current is turned on or increased to normal operating levels to re-establish normal operation (eg, flowing reactant gases at normal operating pressures and flow rates).

[0227] Note that stopping the gas flow and releasing or reducing the back pressure can occur in either order: if the process uses gases different from the reactants, it may be necessary to release the gas pressure first and then stop the flow.

[0228] [Illustrative Recovery Sequence 4] Normal operation, optionally including pulsing current or voltage; Turning off or significantly reducing the cell current; Releasing the reactant gas backpressure to the cathode and stopping the flow of the reactant gas; flowing gas to the cathode under alternative conditions; Stop the gas flow; introducing water to the cathode by flowing it or by contacting it with the cathode; Remove contact of the cathode with water; Flow dry gas to the cathode; Flow reactant gases to the cathode; The current is turned on or increased to normal operating levels to re-establish normal operation.

[0229] In some embodiments, the recovery process includes stopping the flow of current followed by flowing a gas (which may be a reactive gas or a non-reactive gas). If the gas is not a reactive gas, after the gas flows for a period of time, the flow of the reactive gas is resumed and the current is turned back on. In some embodiments, the non-reactive gas includes air, an oxidizing gas, an inert gas, combinations thereof, or modified compositions of reactive gases. In some such embodiments, water is not flowed to the cathode for at least a portion of the time that the gas flows. Examples of recovery operations involving gas flows that do not contact water for at least a portion of the time that the gas flows include Examples 5, 6, and 7.

[0230] [Illustrative Recovery Sequence 5] Normal operation, optionally including pulsing current or voltage; Turn off or significantly reduce the current; Release reactant gas backpressure and turn off reactant gas to the cathode; flowing a gas other than the reactant gas through the cathode for a period of time; reintroducing reactant gases at normal operating pressures and flow rates; The current is turned on or increased to re-establish normal operation.

[0231] In some embodiments, the recovery process involves stopping the flow of electrical current followed by flowing the reactant gases at reduced pressures and / or reduced flow rates for a period of time, followed by increasing the pressure and / or flow rate of the reactant gases to normal operating pressures, and finally returning the electrical current to normal levels. In some such embodiments, water is not flowed to the cathode.

[0232] [Illustrative Recovery Sequence 6] Normal operation, optionally including pulsing current or voltage; Turn off or significantly reduce the current; flowing gas (optionally a reactant gas) to the cathode under alternative conditions, such as at a reduced flow rate; The current is turned on or increased to re-establish normal operation.

[0233] [Illustrative Recovery Sequence 7] Normal operation, optionally including pulsing current or voltage; Turn off or significantly reduce the current; Reducing the pressure of the reactant gas to the cathode for a period of time; increasing the pressure of the reactant gas to normal operating pressure; The current is turned on or increased to re-establish normal operation.

[0234] [Illustrative Recovery Sequence 8] Normal operation, optionally including pulsing current or voltage; Ramp down the current to the electrolyser and optionally apply a low reverse current; flowing gas (optionally reactant gas) to the cathode under alternative conditions, such as reduced flow rate and / or pressure (while applying a low current, which may be a low reverse current); Ramp up positive current; Full normal operating conditions are re-established, including full positive current and full flow of reactant gas.

[0235] [Illustrative Recovery Sequence 9] Normal operation, optionally including pulsing current or voltage; Applying low reverse currents (ramp or direct transition); flowing gas (optionally reactant gas) to the cathode under alternative conditions, such as reduced flow rate and / or pressure (while applying a low current, which may be a low reverse current); Ramp up positive current; Full normal operating conditions are re-established, including full positive current and full flow of reactant gas.

[0236] [Example Recovery Sequence 10] Normal operation, optionally including pulsing current or voltage; The power supply or associated circuitry is adjusted to short circuit the electrolytic cell or to reach a potential below the open circuit voltage. flowing gas (optionally reactant gas) to the cathode under alternative conditions such as reduced flow rate and / or pressure (while holding the electrolyser at open circuit voltage or in a short circuit condition); Full normal operating conditions are re-established, including normal operating electrolyzer potentials and full flow of reactant gases.

[0237] [Example protection sequence 11] Normal operation, optionally including pulsing current or voltage; Determine the likelihood of unexpected, adverse events occurring; transitioning the electrolyzer into a protection mode (examples of protection mode operating conditions include applying a relatively low current density to the electrolyzer, transitioning the electrolyzer voltage to the open circuit voltage, reducing or ramping down the current applied to the electrolyzer); Determine that the adverse event is no longer a threat; Re-establish full normal operating conditions.

[0238] [Illustrative Recovery Sequence 12] Normal operation, optionally including pulsing current or voltage; The power supply or associated circuitry is adjusted to reach and maintain the open circuit voltage. flowing gas (optionally reactant gas) to the cathode under alternative conditions such as reduced flow rate and / or pressure (while holding the electrolyser at open circuit voltage or in a short circuit condition); Full normal operating conditions are re-established, including normal operating electrolyzer potentials and full flow of reactant gases. [Process parameters associated with the recovery process]

[0239] The various operations associated with the recovery process are described in the following sequence. [Normal operation]

[0240] As mentioned, before a recovery sequence is expected, the carbon oxide electrolyzer may be operated normally for a period of time. Normal operation may include a set of normal operating conditions as described elsewhere herein. Such conditions may include (a) normal reactant gas flow, which may be characterized by normal levels of reactant gas pressure and flow rate or velocity at the cathode, (b) reactant gas composition, (c) a set temperature or temperature profile, (d) a current or voltage magnitude (optionally with a non-constant waveform), or (e) any combination thereof. In some embodiments, during normal operation, the current or voltage has a pulsed or paused profile in which the magnitude of the current in the electrolyzer is periodically temporarily reduced or increased.

[0241] Normal operation may include converting carbon oxides in the reactant gas to carbon-containing products. In some embodiments, the carbon oxides are CO2 and / or CO, and the carbon-containing reduction products include CO, hydrocarbons, and / or organic oxygen-containing compounds. Typically, during normal operation, no liquid (e.g., water) is introduced to the cathode via the carbon oxide inlet or other source external to the MEA. However, during normal operation, liquid in the form of a mist or droplets may contact the cathode along with the inlet gas.

[0242] In various embodiments, prior to the recovery sequence, the electrolytic cell operates normally for a period of time. For example, prior to performing the recovery sequence, the electrolytic cell may operate under normal conditions for a period of at least about 100 hours. In some cases, the period of normal operation prior to performing the recovery sequence lasts for at least about 1000 hours, or at least about 2000 hours, or at least about 5000 hours, or at least about 10,000 hours. After the recovery sequence, the electrolytic cell may return to normal operation for an extended period of time, such as at least about 100 hours. The period of normal operation may not be continuously interrupted by another recovery process or by terminating the operation of the electrolytic cell. In some implementations, the recovery process is performed periodically during the life of the electrolytic cell, but multiple instances of the recovery process are separated by a minimum period of normal operation, such as a period of at least about 100 hours, or at least about 500 hours, or at least about 1000 hours. [Reduce or stop current]

[0243] In certain implementations, the recovery sequence stops the flow of current to the electrolyzer, reduces the magnitude of the current density, or reverses the direction of the current at the cathode. The reduction in current can be large, i.e., the reduction in current does not adversely affect the electrolyzer or any of its components, such as the cathode catalyst layer. For example, the current should not corrode or degrade catalytic components, such as metals, carbon support materials, or polymers. In one example, the current is reduced by at least about 50%. In some examples, the reduced current density at the cathode is at most about 100 mA / cm. 2 (flat cathode surface area). This current density may be applied when gas and / or water is supplied to the cathode from outside the MEA.

[0244] In some cases, the recovery sequence involves at least temporarily applying a current in the reverse direction (i.e., an anodic current flows at the cathode side of the cell). A small amount of anodic current (at the cathode side) may contribute to the recovery of performance at the cathode of the electrolyzer during subsequent normal operation. The small amount of anodic current may refresh the surface of the catalyst.

[0245] In some embodiments, a current or voltage ramp is applied to the carbon oxide electrolyzer for recovery or protection. As discussed above in connection with the description of the protection mode, the ramp may have any shape or slope. In some cases, the average ramp rate from full current (normal operation) to final current is about 20 mA / cm per minute. 2 or less, or about 1-10 mA / cm per minute 2 , or about 0.5 to 1 mA / cm per minute 2 In some cases, the ramp is stepped. The number of steps, the duration of the steps, and the magnitude of the change in current density of the steps can be variable. By way of example, the ramp may have about 2-50 steps, or about 5-30 steps. By way of further example, the duration of a step may be about 1-100 seconds, or about 5-50 seconds. By way of further example, the magnitude of the current of the steps may be about 0.1-10 mA / cm. 2 or about 0.5 to 5 mA / cm 2 It may be. [Current stop duration]

[0246] In certain embodiments, the duration of the current cessation in the recovery sequence is about 5 minutes to several days (e.g., 10 days). In certain embodiments, the duration of the current cessation is about 10 minutes to about 300 minutes, or about 15 minutes to 60 minutes.

[0247] When the current is changed from normal operation upon initiation of the recovery process, the current may be reduced by an abrupt shutdown (single step), a ramp down, and / or multiple steps.

[0248] In certain embodiments in which a reverse current is applied, the duration for which the reverse current is applied to the cathode is from about 5 seconds to 60 minutes, or from about 5 minutes to 60 minutes. [Short circuit]

[0249] In some embodiments, during recovery or protection mode, the power source for powering the carbon oxide electrolyzer is short-circuited. A short circuit can occur when the electrical resistance in the circuit is not large enough to prevent current flow between the anode and the cathode. In such a case, the potentials of the anode and cathode are equal. In other words, the cell voltage is 0 volts. When short-circuited, the electrolyzer discharges from normal operating conditions or from the open circuit voltage. During a short circuit of the electrolyzer, the cell voltage transitions to a level below the open circuit voltage.

[0250] During the recovery or protection mode, the short circuit condition may be maintained for a predetermined period of time. In some embodiments, the short circuit mode is maintained for about 30 minutes or less, or about 10 minutes or less, or about 5 minutes or less, or about 1-5 minutes. After exiting the short circuit condition, the electrolyzer may return to normal operating voltage, and optionally return to full normal operation.

[0251] A short circuit, where current flows in the reverse direction, and the associated release process, may create oxidizing conditions on the catalyst (e.g., a gold catalyst) that increase catalytic effectiveness, for example, by creating extra active surface area and / or by removing potential impurities or intermediates. A short circuit may also reduce the K + , H + , COOH - , HCO3 - , CO3( 2- ), OH -, or any combination of positive and / or negative ions. The short circuit may also result in water electrolysis at the MEA interface (e.g., between the PEM and AEM in a bipolar MEA) and / or at the cathode, resulting in dry conditions for better mass transfer of carbon oxides (e.g., CO2). The short circuit may also improve the selectivity of CO2 reduction by temporarily changing the local environment of the cathode, such as its pH or ion concentration. Changing from a lower to a higher pH, or from a lower to a higher concentration of potassium, may improve the selectivity for a particular reaction, such as the CO production reaction.

[0252] In certain embodiments, the system applies a short circuit when the selectivity of the CO2 to CO reduction is below a certain target value and / or when it is desirable to extend the life of the electrolyzer, for example by several hundred hours. In certain embodiments, for periodic (e.g., cyclical) recovery of reaction selectivity, the system applies a short circuit until a certain decay rate target is reached within a certain time range. For example, the system may perform a recovery operation whenever the selectivity decays to a certain value, for example, below about 90%. Another approach performs a recovery operation whenever the electrolyzer operates normally for a defined period of time (e.g., about 200-500 hours). Another approach performs a recovery operation whenever the electrolyzer exhibits a threshold drop in selectivity (e.g., below about 10%). [Open circuit voltage]

[0253] In some embodiments, the carbon oxide electrolyzer is allowed to reach or maintain an open circuit voltage during recovery or protection. Open circuit voltage refers to the voltage difference between the anode and cathode when no net electrochemical reaction is occurring at the anode or cathode. The voltage difference may result when no current flows between the anode and cathode. The open circuit potential may be achieved when a switch in a circuit including the electrolyzer and power source opens by breaking the conductive path between the anode and / or cathode. The open circuit potential may also be achieved by using a very high impedance element in a circuit including the power source and the electrolyzer. Such a resistor has the effect of completely shutting off or nearly shutting off all current between the anode and cathode of the electrolyzer.

[0254] In some cases, setting or allowing the electrolyzer to reach the OCV while maintaining a similar CO2 gas flow rate at the cathode may provide relatively dry conditions, thereby allowing for improved mass transport of CO2. Setting or allowing the electrolyzer to reach the OCV when no electrochemical reaction is occurring may flush out potential intermediates or impurities on the surface of the catalyst. In some cases, setting or allowing the electrolyzer to reach the OCV temporarily alters the local environment of the cathode, such as pH or ion concentration, thereby improving CO2 reduction selectivity, for example, for CO production.

[0255] Setting the electrolyzer to an OCV or allowing it to reach the OCV may be advantageous in various scenarios and applications. In some cases, an OCV is used when a modest recovery of selectivity is desired and / or the electrolyzer voltage needs to be maintained at a relatively high level compared to short circuit conditions. [Reverse current]

[0256] As already indicated, in some embodiments, during recovery or protection, a reverse current is applied to the carbon oxide electrolyser. A forward current is a current applied to the carbon oxide electrolyser during normal operation. Electrons are supplied from the power source to the cathode, allowing reduction to occur, and electrons are extracted from the anode, allowing oxidation to occur. During reverse current, the flow of electrons is reversed, so that an electrode that functions as a cathode during normal operation functions as an anode during application of the reverse current, and an electrode that functions as an anode during normal operation functions as a cathode during application of the reverse current.

[0257] Typically, the reverse current is maintained below a level that would oxidize or corrode the carbon and / or other materials in the cathode catalyst. In some embodiments, the reverse current is about -100 mA / cm 2 Less than or equal to -5 to -100mA / cm 2 In some embodiments, the reverse current is applied for a duration of about 100 minutes or less, or about 50 minutes or less, about 30 seconds to 20 minutes. By way of further example, the reverse current may be about 1 mA / cm 2 (surface area of ​​the cathode) or about 0.5 mA / cm 2 (surface area of ​​the cathode). In some cases, the reverse current flow is maintained at or below a level where the cell voltage does not exceed about 1.25 V (for reactive gases) and 2.5 V (for oxidizing gases such as air), or does not exceed about 0.5 V (for reactive gases) and 2 V (for oxidizing gases such as air). In certain embodiments using reverse current, such current is no more than about 0.6 Coulombs / cm 2 (surface area of ​​the cathode) is the limit.

[0258] After application of the reverse current has ceased, the electrolyzer may return to normal operating current, and may optionally return to full normal operation. In certain embodiments, the final value of the reverse current is achieved by ramping to that final value. The ramp rates and / or stepped ramp procedures described herein for the protection mode or for achieving the reduced current value may be used.

[0259] Potential advantages of exposing a carbon oxide electrolyser to a reverse current include the advantages of short circuit applications described herein, as well as the advantages of exposing a carbon oxide electrolyser to a reverse current that may find similar applications as short circuit applications. [Type of gas flowing to cathode (pre-contact with water)]

[0260] In some implementations of the recovery process, a gas flows to the cathode for a certain period of time after the current is stopped or reduced. This gas may be referred to herein as the "recovery gas." In some cases, the recovery gas has the same composition as the carbon oxide reactant that flows during normal operation (optionally at a pressure and / or flow rate different from that used in normal operation). For example, the gas that flows during normal operation and during the recovery process includes a defined concentration of carbon dioxide or carbon monoxide. In some cases, the recovery gas that flows to the cathode during the recovery process has a different composition than the composition of the reactant gas. In some cases, compared to the reactant gas, the recovery process gas has a lower concentration of the carbon oxide reactant. In some cases, the recovery process gas includes an inert gas that is not present (or is present at a different concentration) in the normal process gas. Examples of inert gases include noble gases (e.g., Ar, He, or Kr) or nitrogen. In some cases, the recovery process gas is or includes air. In some cases, the recovery gas includes an oxidizing gas, such as oxygen. In some cases, the oxidizing gas is simply air, which may contain about 21% oxygen. In other cases, the oxidizing gas is oxygen or another oxidizing substance provided separately from air. For example, oxygen produced at the anode of the electrolyzer during normal operation may be used as the oxidizing recovery gas. In some implementations, the recovery gas is humidified. In some embodiments, the component gas includes carbon dioxide, air, water, an inert gas, or any combination thereof.

[0261] In some examples, the recovery gas is 100% or pure reactive gas. In some examples, the recovery gas is 100% or pure inert gas. In some examples, the recovery gas includes reactive gas and inert gas in any ratio. In some examples, the recovery gas includes oxidizing gas and inert gas in any ratio. In some examples, the recovery gas is a humidified gas having water vapor present at a concentration of about 0-2% by volume. In some cases, the humidified gas includes reactive gas, inert gas, oxidizing gas, or any combination thereof. [Gas pressure at cathode (pre-contact with water)]

[0262] In some embodiments, after the current is stopped or reduced, the pressure of the recovery gas flowing to the cathode may be at a level up to the normal operating pressure of the electrolyzer cell. In some embodiments, after the current is stopped or reduced, the cathode gas backpressure is reduced, for example to 0 psig. The cathode gas backpressure may be controlled by a pressure regulator located downstream from the cathode in the gas flow path. After the cathode gas backpressure is reduced, recovery gas may be present, optionally flowing under a pressure of about 0-600 psig, or about 0-400 psig, or about 0-50 psig. [Gas flow rate through the cathode (pre-contact with water)]

[0263] In certain embodiments where recovery gas flows after the current is reduced or stopped, the recovery gas flows at a rate of about 0-50 sccm / cm 2 (surface area of ​​flat cathode) or about 10-30sccm / cm 2 (flat cathode surface area). Note that the flow rate values ​​given herein are specified in units per cathode surface area (e.g., cm of flat cathode surface area). 2 As a single example, a cathode with a surface area of ​​25 cm 2For an electrolytic cell having a cathode surface area, the gas flow rate may be about 500 sccm. The gas flow rate may scale linearly or non-linearly with the cathode surface area. The flow rate values ​​given herein may be instantaneous flow rates or average flow rates. [Duration of gas flow at cathode (pre-contact with water)]

[0264] In embodiments of the recovery sequence in which recovery gas flows to the cathode, the gas may flow for a period of time after the current is stopped or reduced. In certain embodiments, the duration of gas flowing or present at the cathode is about 30 minutes to 10 days, or about 1 hour to 2 days. The duration of gas flow in the recovery sequence may depend, at least in part, on the flow rate of the gas (e.g., the average flow rate in the case of variable flow rates). As an example, low or zero flow for several days and high flow for several minutes or hours may have a similar effect. As a further example, the gas flow and / or exposure time range may range from 30 minutes to even several days. However, a 50 sccm / cm 2 At a flow rate of 1000 s, the maximum contact / flow time may be about 5 hours.

[0265] In some embodiments, the recovery gas flows to the cathode and then contacts the cathode with water or other liquid, and the recovery gas flow may be stopped prior to contact with the liquid. [Composition of water in contact with the cathode]

[0266] As shown, various rejuvenation processes involve contacting the cathode with a liquid, such as water. When referring to water herein, it is understood that the rejuvenation process may use water over a wide range of purity. In some embodiments, the water is deionized water, such as deionized water having a resistivity of at least about 1 megaohm, or at least about 10 megaohms, or at least about 18 megaohms. In some embodiments, the water includes one or more dissolved solutes or suspended components. Examples of dissolved solutes include bicarbonate, carbonate, sulfate, hydrogen, sulfate, formate, acetate, and halides. By way of example, the solute may be a metal (e.g., sodium, potassium, or cesium) or an ammonium salt of these anions). In certain embodiments, the rejuvenation process water includes bicarbonate at a concentration of up to about 10 mM bicarbonate ion, or up to about 5 mM bicarbonate ion, or up to about 2 mM bicarbonate ion. In some embodiments, the rejuvenation process water has a composition that matches or is similar to the composition of the anode water used during normal operation. Examples of anode water compositions are provided in PCT patent application PCT / US2019 / 063471, filed November 26, 2019, which is incorporated by reference in its entirety. [Water flow rate at cathode]

[0267] Optionally, water flows through the cathode. In some embodiments, the flow rate of water to the cathode is up to cm of flat cathode surface area. 2 As an example, the water flow rate is about 20 ml / min per cm of surface area of ​​a flat cathode. 2 The flow rate is about 2-10 ml / min per MEA. In some embodiments, the flow rate is limited based on the pump and associated hardware. Note that the water supplied to the cathode as part of the recovery process is supplied from the anode through the MEA, whereas it is typically supplied from outside the MEA, for example, via a carbon dioxide inlet to the cathode. [Duration of water flow through the cathode]

[0268] In certain embodiments, the duration of the water flow through the cathode is about 1 to 100 minutes. In certain embodiments, the duration of the water flow through the cathode is about 2 to 50 minutes, or about 5 to 15 minutes. [Drying stage after water flows (gas flows)]

[0269] A drying operation may be performed when water is not in contact with the cathode, which may be performed after water has contacted the cathode and before resumption of normal operation, or possibly during an initial period of resumption of normal operation.

[0270] Drying may be performed using a gas of any composition that removes water from the cathode. Such gases may be referred to herein as "drying gases." Examples of gases that may be present in the drying gas include air, reactive gases, and inert gases. Examples of inert gases include noble gases (e.g., He, Ar, Kr) and nitrogen. When a reactive gas is used, it is optionally used at a concentration different from that used during normal operation. For example, carbon dioxide may be present at 50% (molar) in the reactants and about 20% (molar) in the drying gas.

[0271] The physical mechanism by which the dry gas removes water from the cathode may include pushing or otherwise forcing the water from the cathode by contacting the cathode with a gas at a pressure and / or velocity sufficient to remove liquid water.

[0272] Water may also, or alternatively, be removed by evaporation into a dry gas. To this end, the relative humidity of the dry gas entering the cathode may be controlled to promote evaporation. In some embodiments, the dry gas may have a very low relative humidity (e.g., about 0-100%), while in other embodiments the dry gas may have a higher value, which may be desirable to ensure that the MEA does not lose moisture to such an extent that it dries out and its performance deteriorates. [Drying duration]

[0273] In certain embodiments, the duration of drying the cathode is from about 0 to 500 minutes, or from about 2 to 100 minutes, or from about 5 to 30 minutes.

[0274] In some embodiments, the drying operation continues until there is no additional liquid water (e.g., water droplets or mist) downstream from the cathode, hi some embodiments, the drying operation continues until the humidity of the drying gas entering the cathode is approximately the same as the humidity of the drying gas exiting the cathode. [Resume flow or reactant gas]

[0275] If drying with a reactive gas is not performed, the recovery process transitions from flowing a drying gas to flowing a reactive gas. This may include re-establishing the normal operating gas pressure at the cathode, for example, by adjusting the setting on a pressure regulator downstream of the cathode. If the drying gas is the reactive gas, the transition from flowing a drying gas to flowing a reactive gas need not occur, or optionally does occur, but represents only a change in gas flow rate, pressure, composition, or a combination thereof. [Current ramp rate after recovery]

[0276] In some embodiments, the recovery action results in the current being resumed directly back to its original value, or with some ramp or step increase. [Optional voltage scan]

[0277] In some embodiments, the recovery process is performed in conjunction with a voltage or current scan. The voltage or current scan may be performed repeatedly in alternating directions between two endpoints. The voltage scan may be performed in a cyclic voltammetric manner. In some cases, the voltage or current scan is performed in the recovery process while the current is otherwise stopped. In some cases, the voltage or current scan is performed while gas is flowing to the cathode and water is not contacting the cathode. For example, the voltage or current scan may be performed during a gas drying operation in any of the recovery sequence examples 2-4 described above. In some examples, the voltage or current scan is performed during a gas contacting operation that occurs before contact with water. See recovery sequence examples 3 and 4 described above. In some examples, the voltage or current scan is performed during a gas contacting operation that is not associated with a water contacting operation. See recovery sequence examples 5-7 described above.

[0278] The voltage or current scan may have various effects on the electrolyzer cell or its cathode. Examples include: - The overpotential of the charging of the carbon material in the electrodes caused by strong polarization is mitigated through current interruption for different lengths of time. -Cleaning of the electrode surface (removal of impurities / undesirable intermediates) by changing the adsorption / desorption environment under zero or slight anodic voltage or exposure to air. - Cathode catalytic sites CO X Realign to a more favorable orientation for reduction. - Keep the MEA hydrated.

[0279] The voltage scan may be characterized by an initial cathode voltage E0 (V), an upper cathode voltage limit V1 (V), a lower cathode voltage limit V2 (V), and a scan rate S (mV / s). In some embodiments, E0 has a range of about -1V to 1.2V vs. RHE. In some embodiments, E1 has a range of about -1V to 1.2V. In some embodiments, E2 has a range of about -1V to 1.2V. In some embodiments, S has a range of about -10000 to -0.1mV / s, or about 0.1 to 10000mV / s. The scan direction may be positive or negative, with positive meaning that the first sweep is toward the positive direction, while negative means that the first sweep is toward the positive direction. The scan rates in the positive and negative directions may be the same or different. The number of scans (n) may range from about 1 to 1000.

[0280] FIG. 2 illustrates an exemplary feature of a cyclic voltage scan. [Temperature change during recovery operation]

[0281] In certain embodiments, the temperature of the electrolyzer cell does not change during the recovery run. In certain embodiments, the temperature of the electrolyzer cell changes during the recovery run. The temperature change may be driven by a temperature controller, water flowing to the cathode, gas flowing to the cathode, or any combination thereof. In some embodiments, the temperature of the electrolyzer cell increases during the recovery run. In some embodiments, the temperature of the electrolyzer cell decreases during the recovery run, for example, by about 5-25°C. [Effects of recovery and protection actions]

[0282] In certain embodiments, when a recovery process is used, the current efficiency of the electrolytic cell may increase immediately after the recovery process, and often for an extended period of time thereafter. In some cases, the current efficiency increases by at least about 20%, or by at least about 35%, or by at least about 50% after the recovery process.

[0283] In certain embodiments, when the recovery process is used, the voltage efficiency of the electrolytic cell is not reduced. For example, the voltage efficiency of the electrolytic cell may increase by at least about 1% or at least about 3% after the recovery process. In certain embodiments, when the recovery process is used, the cell voltage of the electrolytic cell is not increased. For example, the cell voltage of the electrolytic cell may decrease by at least about 50 mV or at least about 100 mV after the recovery process.

[0284] In certain embodiments, when a recovery process is used, the operational life of the electrolytic cell may be increased by at least about 100 hours, or by at least about 1000 hours, or by at least about 5000 hours, or by at least about 10,000 hours, as compared to an electrolytic cell that does not undergo a recovery process. In certain embodiments, when a recovery process is used, the operational life of the electrolytic cell may be increased by at least about 50% as compared to an electrolytic cell that does not undergo a recovery process.

[0285] In certain embodiments, when using the recovery process, the single pass conversion of the electrolyzer is increased. The single pass conversion may be the mole fraction of reactant gases that convert to the intended product. In certain embodiments, the single pass conversion of the carbon dioxide electrolyzer is increased by at least 3%, or by at least about 5%, or by at least about 10% after the recovery process. [example]

[0286] Figure 3 shows a preliminary experiment that shows some of the influence of the recovery sequence. In this example, a 25 cm 2 A carbon dioxide electrolysis cell having a cathode of 1000 .mu.m was used.

[0287] The graph in FIG. 3 shows the applied current density (lower plot), the experimentally observed faradaic yield for converting carbon dioxide to carbon monoxide (middle plot) and the experimentally observed voltage of the electrolyzer cell.

[0288] The experiment used a start-up sequence (not to be confused with a recovery sequence) that was driven at a continuous current density of 300 mA / cm 2 and a first stage with a cell temperature of 50°C and 10 mA / cm at 40°C for 5 minutes every 45 minutes. 2 Low current of 300mA / cm, interrupted by pause 2 Note that the first stage had a second stage with a higher current density of 10 mA / cm at 40 °C for 5 min. The first stage ran for 72 h and the second phase ran for 163 min. When the second stage was completed, the experiment was repeated every 45 min with a current density of 10 mA / cm at 40 °C for 5 min. 2 Interrupted by low current pause of 200mA / cm 2 The device was then switched to normal operation using a normal current density of 1000 mA.

[0289] As shown, after approximately 3000 hours of experimentation, normal operation was temporarily terminated and a recovery sequence was applied, which included turning off the cell current, releasing the backpressure and stopping the flow of reactant gases to the cathode, exposing the cathode to air at atmospheric pressure, flowing deionized water to the cathode, removing contact of the cathode with the water, flowing air to the cathode, flowing reactant gases to the cathode, and reversing.

[0290] As can be seen, the electrolyzer cell voltage gradually increased over the period of normal operation while the electrolyzer's faradaic yield of carbon monoxide production decreased. After performing the recovery sequence, the cell voltage decreased by about 0.04 volts and the faradaic yield of carbon monoxide production increased by about 20%.

[0291] Later in the experiment, at about 3600 hours, a second recovery sequence was performed that included turning off the cell current, releasing the back pressure and stopping the reactant gas flow to the cathode, exposing the cathode to air at atmospheric pressure, flowing deionized water to the cathode, removing contact of the cathode with water, flowing air to the cathode, flowing reactant gas to the cathode, turning on the current, or increasing the current to normal operating levels. After performing the second recovery sequence, the cell voltage decreased by about 0.05 volts and the faradaic yield of carbon monoxide production increased by about 23%. [Process Window]

[0292] The electrolyzer design and operating conditions can be tuned for a particular application and to produce a cathode output having a specified composition. In some implementations, one or more general principles may be applied to operate to produce a desired output stream composition.

[0293] 1. Limiting the availability of carbon dioxide reactant at the cathode active sites and / or increasing the current density at the cathode. These ranges of operating conditions tend to result in the following: (a) initially, as the availability of carbon dioxide reactant is reduced and / or the current density is increased, the ratio of CO2 converted to CO increases (i.e., CO:CO2 in the output stream increases); (b) at some point, as the availability of carbon dioxide reactant is further reduced and / or the current density is increased, the hydrogen ion reduction reaction becomes more prominent (i.e., H2:CO increases). Electrolyzers configured to operate with relatively low carbon dioxide input / availability may be simply designed to control the flow rate of carbon dioxide upstream of the electrolyzer. In some cases, the electrolyzer is configured with flow fields or gas diffusion components that limit carbon dioxide from reaching the active sites on the cathode of the electrolyzer. In certain embodiments, flow field designs that are not interdigital and that have long paths, such as serpentine paths, between the CO2 source and the cathode result in higher ratios of CO:H2. The interdigital flow field forces the input gas (carbon oxide) to flow through the gas diffusion layer and then exit at a different location in the flow field. Non-interdigital designs have long continuous paths for the carbon oxide feed gas to flow into and out of the cathode. The channels on the inlet side are spaced apart from the channels on the outlet side. A relatively thick gas diffusion electrode can restrict CO2 mass transport to the cathode active sites, thus tending to increase the ratio of CO:CO2 and / or H2:CO.

[0294] 2. Making hydrogen ions relatively more available at the cathode. Making hydrogen ions relatively more available at the cathode may result in a cathode product stream with a relatively high H2:CO ratio. Electrolyzers configured to produce a relatively hydrogen-rich product may use designs that (a) starve the cathode of carbon dioxide reactant (as described in 1), (b) allow a relatively high flow of hydrogen ions to be transported from the anode (where they are produced) to the cathode, and / or (c) operate at a relatively high cell temperature. Electrolyzers that can operate with a relatively high flow of hydrogen ions to the cathode may have an MEA with a cation conducting polymer and / or a mixed ion conducting polymer at the cathode. Alternatively or additionally, in an MEA with a cathode buffer layer, the cathode buffer layer is designed to be relatively thin and / or to have a relatively high hydrogen ion transport number.

[0295] 3. Making hydrogen ions less available at the cathode. Making hydrogen ions less available at the cathode may result in a cathode product stream with a relatively high CO:H2 ratio. Electrolyzers configured to produce a relatively low concentration of hydrogen in the product may (a) provide excess carbon dioxide reactant at the cathode for a particular current density, (b) use an MEA design that prevents hydrogen ions from reaching the cathode, and / or (c) use a design that operates at a relatively low cell temperature. [Operating parameter regime with high ratio of CO2 reduction products to CO2]

[0296] In certain embodiments, the electrolyzer is configured to, and during operation, produces an output stream having a CO:CO molar ratio of at least about 1:1, or at least about 1:2, or at least about 1:3. Alternatively, the CO-rich output stream may be characterized as having a CO concentration of at least about 25 mol%, or at least about 33 mol%, or at least about 50 mol%.

[0297] In certain embodiments, a high carbon monoxide output concentration (ranges in any of the above examples) is obtained by operating the carbon dioxide electrolyser to provide any one of the following operating conditions or any combination of these operating conditions: At least about 100 mA / cm at the cathode 2 current density, a CO stoichiometric flow rate (as described elsewhere herein) of at most about 4, or at most about 2.5, or at most about 1.5; Temperatures up to about 80°C or up to about 65°C, Pressure range of approximately 25 to 400 psig, anode water composition containing about 0.1-50 mM salt, such as formate and / or bicarbonate; and Anode water pH of at least about 1.

[0298] In certain embodiments, the electrolyser may be constructed to favor high molar ratios or concentrations of CO:CO2 by using a carbon dioxide electrolyser having any one of the following characteristics, or any combination thereof, as exemplified herein: a relatively small nanoparticle cathode catalyst (e.g., having a maximum dimension of about 0.1-15 nm on average); Gold as a cathode catalyst material; The thickness of the cathode catalyst layer is about 5 to 20 um. a cathode gas diffusion layer (GDL) with a microporous layer (MPL); a cathode GDL having PTFE present at about 1-20 wt%, or about 1-10 wt%, or about 1-5 wt%; A GDL having a thickness of at least about 200 um; a bipolar MEA having an anion exchange cathode buffer layer having a thickness of at least about 5 um; and Cathode flow field with parallel and / or serpentine flow paths. [Operating parameter regime with high ratio of reduction products (H2+CO) to CO2]

[0299] In certain embodiments, the electrolyser is configured to, and does during operation, produce an output stream having a molar ratio of (H2+CO):CO2 of at least about 2:1, or at least about 1:2, or at least about 1:3.

[0300] In certain embodiments, relatively high reduction product output concentrations (using any of the (H+CO):CO ratios described above) are obtained by operating the carbon dioxide electrolyzer to provide any one or any combination of the following operating conditions: At least about 300mA / cm 2 current density, a CO2 stoichiometric flow rate of at most about 4, or at most about 2.5, or at most about 1.5; Maximum temperature is about 125℃. Up to about 800 psi pressure, anode water composition of 0 to about 500 mM bicarbonate, and Anode water pH of approximately 0 to 15

[0301] In certain embodiments, the electrolyser may be constructed to favor high molar ratios or concentrations of (CO+H2):CO2 by using a carbon dioxide electrolyser having any one of the following characteristics, or any combination thereof, as defined herein: a nanoparticle cathode catalyst (e.g., having a maximum dimension of about 0.1 to 1000 nm on average); Transition metals as cathode catalyst materials, A thickness of the cathode catalyst layer of about 0.1 to 100 um; a cathode gas diffusion layer with or without a microporous layer (MPL); GDL with approximately 0-70 wt% PTFE, a GDL having a thickness of about 10 to 1000 um; and A bipolar MEA with an anion exchange cathode buffer layer that is approximately 0-100um thick. [Operating parameter regime for hydrogen-rich product stream]

[0302] In certain embodiments, the carbon dioxide electrolyser is configured to produce, and does during operation, produce an output stream having a molar ratio of H2:CO of at least about 1:1.

[0303] In certain embodiments, such hydrogen-rich output concentrations are obtained by operating the carbon dioxide electrolyser to provide any one of the following operating conditions, or any combination thereof: At least about 300mA / cm 2 current density, CO2 mass transfer to the cathode at a maximum stoichiometric flow rate of about 2; a temperature of at least about 65° C. or at least about 80° C.; Pressure range of approximately 75 to 500 psig, an anode water composition of pure water or at least about 50 mM bicarbonate; and Anode water pH of approximately 1 maximum.

[0304] In certain embodiments, the electrolyser may be constructed to favor a hydrogen rich molar ratio or concentration by using a carbon dioxide electrolyser having any one of the following characteristics, or any combination thereof, as defined herein: a relatively large nanoparticle cathode catalyst (e.g., having an average maximum dimension of at least about 80 nm); Silver, palladium or zinc as the cathode catalyst material; a cathode catalyst layer having a thickness of at most about 5 um, or at least about 25 um; A cathode gas diffusion layer that does not have a microporous layer (MPL); a cathode GDL having no PTFE or at least about 20 wt% PTFE; a cathode GDL having a thickness of at most about 200 um or at least about 500 um; and A bipolar MEA with an anion exchange cathode buffer layer with a thickness of approximately 0-5 um. [Stream operating parameter regimes for highly reduced products versus hydrogen products]

[0305] In certain embodiments, the carbon dioxide electrolyser is configured to produce, and during operation does produce, an output stream having a molar ratio of CO:H2 of at least about 2:1.

[0306] In certain embodiments, such product rich output concentrations are obtained by operating the carbon dioxide electrolyser to provide any one of the following operating conditions, or any combination thereof: At least about 300 mA / cm at the cathode 2 current density, a CO mass transport stoichiometric flow rate to the cathode of at least about 1.5, or at least about 2.5, or at least about 4; Maximum temperature is about 80℃, Pressures in the range of approximately 75 to 400 psig, an anode water composition of about 0.1 mM to 50 mM bicarbonate, and The pH of the anode water is greater than approximately 1.

[0307] In certain embodiments, the electrolyser may be constructed to favor product rich molar ratios or concentrations by using a carbon dioxide electrolyser having any one of the following characteristics, or any combination thereof, as defined herein: Relatively small nanoparticle catalysts (e.g., having a maximum dimension on average of about 0.1-15 nm); Gold as a cathode catalyst material; The thickness of the cathode catalyst layer is about 5 to 20 um. a cathode gas diffusion layer with a microporous layer (MPL); a cathode GDL having PTFE present at about 1-20 wt %, or about 1-10 wt %, or about 1-5 wt %; a cathode GDL having a thickness of at least about 200 um; and A bipolar MEA having an anion exchange layer with a thickness of at least about 5 um. [Reduced need for recovery actions]

[0308] The present disclosure relates to methods of performing recovery and / or protection on a carbon oxide electrolyzer, as well as methods of reducing the likelihood or frequency of performing recovery actions. Such methods may include operating the electrolyzer in a manner that is unlikely to result in a problem requiring recovery.

[0309] One way to reduce the likelihood that recovery operations will be required or the frequency with which recovery operations are performed is to use anode water that has limited tendency to precipitate solids during normal operation. The anode water may use salts with concentrations well below their solubility limits. Examples include sodium salts, such as baking soda, at a concentration of about 10 mM or less during normal operation. In some embodiments, the anode water uses potassium salts, such as potassium bicarbonate, at a concentration of about 15 mM or less. In some cases, the anode water uses primarily or exclusively salts with relatively high solubility compared to other potential salts. For example, the anode water may use primarily or exclusively potassium salts and relatively little or no sodium salts. Additional examples of anode water compositions are provided in PCT Publication No. 2020112919, published June 4, 2020, which is incorporated herein by reference in its entirety. [System embodiment]

[0310] 4 illustrates a system 401 for controlling the operation of a carbon oxide reduction reactor 403, which may include cells including MEAs such as any one or more of those described herein. The reactor may include multiple cells or MEAs arranged in a stack. The system 401 includes an anode subsystem that interfaces with the anode of the reduction reactor 403 and a cathode subsystem that interfaces with the cathode of the reduction reactor 403. The system 401 is an example of a system that may be used with or implement any of the methods or operating conditions described above.

[0311] As shown, the cathode subsystem includes a carbon oxide source 409 configured to provide a carbon oxide feed stream to the cathode of the reduction reactor 403, which during operation may produce an output stream that includes products of the reduction reaction at the cathode. The product stream may include unreacted carbon oxides and / or hydrogen, see 408.

[0312] The carbon oxide source 409 is coupled to a carbon oxide flow controller 413 configured to control a volumetric or mass flow rate of carbon oxide to the reduction reactor 403. One or more other components may be disposed on the flow path from the flow carbon oxide source 409 to the cathode of the reduction reactor 403. For example, an optional humidifier 404 may be provided on the path and configured to humidify the carbon oxide feed stream. The humidified carbon oxide may wet one or more polymer layers of the MEA, thereby preventing drying of such layers. Another component that may be disposed on the flow path is a purge gas inlet coupled to a purge gas source 417. In certain embodiments, the purge gas source 417 is configured to provide purge gas during periods when the current to the cells of the reduction reactor 403 is suspended. In some implementations, flowing purge gas over the MEA cathode promotes recovery of catalytic activity and / or selectivity. This may be due, at least in part, to flushing certain reaction intermediates from the catalytically active sites and / or removing water from the cathode. Examples of purge gases include carbon dioxide, carbon monoxide, hydrogen, nitrogen, argon, helium, oxygen, and mixtures of any two or more of these.

[0313] During operation, the output stream from the cathode flows through a conduit 407 that connects to a backpressure controller 415 that is configured to maintain the pressure at the cathode side of the cell within a defined range (e.g., about 50-800 psig, depending on system configuration). The output stream may provide reaction products 408 to one or more components (not shown) for separation and / or concentration.

[0314] In certain embodiments, the cathode subsystem is configured to controllably recycle unreacted carbon oxides from the outlet stream back to the cathode of the reduction reactor 403. In some implementations, the output stream is treated to remove reduction products and / or hydrogen before recycling the carbon oxides. Depending on the configuration and operating parameters of the MEA, the reduction products may be carbon monoxide, hydrogen, hydrocarbons such as methane and / or ethylene, oxygen-containing organic compounds such as formic acid, acetic acid, and any combination thereof. In certain embodiments, one or more components (not shown) for removing water from the product stream are disposed downstream form the cathode outlet. Examples of such components include a phase separator configured to remove liquid water from the product gas stream and / or a condenser configured to cool the product stream gas and thereby provide a dry gas, for example, to a downstream process if required. In some implementations, the recycled carbon oxides may be mixed with fresh carbon oxides from a source 409 upstream of the cathode.

[0315] As shown in FIG. 4, the anode subsystem is configured to provide an anode feed stream to the anode side of the carbon oxide reduction reactor 403. In certain embodiments, the anode subsystem includes an anode water source (not shown) configured to provide fresh anode water to a recirculation loop that includes an anode water reservoir 419 and an anode water flow controller 411. The anode water flow controller 411 is configured to control the flow rate of the anode water to or from the anode of the reduction reactor 403. In the illustrated embodiment, the anode water recirculation loop is coupled to components for adjusting the composition of the anode water. These may include a water reservoir 421 and / or an anode water additive source 423. The water reservoir 421 is configured to provide (and circulate in the anode water recirculation loop) water having a different composition than that in the anode water reservoir 419. In one example, the water in the water reservoir 421 is pure water that may dilute solutes or other components in the circulating anode water. The pure water may be conventional deionized water or even ultrapure water, for example having a resistivity of at least about 15 MOhm-cm or greater than 18.0 MOhm-cm. The anode water additive source 423 is configured to supply solutes, such as salts and / or other components, to the circulating anode water.

[0316] During operation, the anode subsystem may provide water or other reactants to the anode of the reactor 403 where the water or other reactants at least partially react to produce oxidation products, such as oxygen. The products, along with any unreacted anode feed material, are provided in the reduction reactor outlet stream. Although not shown in FIG. 4, an optional separation component may be provided in the path of the anode outlet stream and configured to condense or separate the oxidation products from the anode product stream.

[0317] Other control functions may be included in the system 401. For example, a temperature controller may be configured to heat and / or cool the carbon oxide reduction reactor 403 at appropriate times during its operation. In the illustrated embodiment, the temperature controller 405 is configured to heat and / or cool the anode water provided to the anode water recirculation loop. For example, the temperature controller 405 may include or be coupled to heaters and / or coolers that may heat or cool the water in the anode water reservoir 419 and / or the water in reservoir 421. In some embodiments, the system 401 includes a temperature controller configured to directly heat and / or cool components other than the anode water component. Examples of such other components in the cell or stack and carbon oxide flow to the cathode.

[0318] Depending on the phase of electrochemical operation, including whether or not current to the carbon oxide reduction reactor 403 is suspended, certain components of the system 401 may operate to control non-electrical operation. For example, the system 401 may be configured to adjust the flow rate of carbon oxide to the cathode and / or the flow rate of anode feed to the anode of the reactor 403. Components that may be controlled for this purpose may include a carbon oxide flow controller 413 and an anode water controller 411.

[0319] Also, depending on the phase of the electrochemical operation, including whether or not the current is paused, certain components of the system 401 may operate to control the composition of the carbon oxide feed stream and / or the anode feed stream. For example, the water reservoir 421 and / or the anode water additive source 423 may be controlled to adjust the composition of the anode feed stream. In some cases, the additive source 423 may be configured to adjust the concentration of one or more solutes, such as one or more salts, in the aqueous anode feed stream.

[0320] In some cases, a temperature controller, such as controller 405, is configured to adjust the temperature of one or more components of system 401 based on the phase of operation. For example, the temperature of cells 403 may be increased or decreased during break-in, current suspension during normal operation, and / or storage.

[0321] In some embodiments, the carbon oxide electrolytic reduction system is configured to facilitate removal of the reduction cell from other system components. This may be useful when the cell needs to be removed for storage, maintenance, refurbishment, etc. In the illustrated embodiment, isolation valves 425a and 425b are configured to block fluid communication of cell 403 to the carbon oxide source and to the cathode and backpressure controller 415, respectively. Additionally, isolation valves 425c and 425d are configured to block fluid communication of cell 403 to the anode water inlet and outlet, respectively.

[0322] The carbon oxide reduction reactor 403 may also operate under the control of one or more power supplies and associated controllers. See block 433. The power supplies and controllers 433 may be programmed or otherwise configured to control the current provided to and / or the voltage applied to the electrodes in the reduction reactor 403. The current and / or voltage may be controlled to implement a current schedule and / or current profile described elsewhere herein. For example, the power supplies and controllers 433 may be configured to periodically pause the current applied to the anode and / or cathode of the reduction reactor 403. Any of the current profiles described herein may be programmed into the power supplies and controllers 433.

[0323] In certain embodiments, the power supply and controller 433 performs some, but not all, of the operations necessary to implement a desired current schedule and / or profile in the carbon oxide reduction reactor 403. A system operator or other responsible party may act in conjunction with the power supply and controller 433 to completely define the schedule and / or profile of current applied to the reduction reactor 403. For example, the operator may institute one or more current pauses in addition to the set of current pauses programmed into the power supply and controller 433.

[0324] In certain embodiments, the power supply and controller operates in coordination with one or more other controllers or control mechanisms associated with other components of the system 401. For example, the power supply and controller 433 may operate in coordination with controllers for controlling the supply of carbon oxide to the cathode, the supply of anode water to the anode, the addition of pure water or additives to the anode water, and any combination of these functions. In some implementations, the one or more controllers are configured to control or operate in a coordinated manner to control any combination of the following functions: application of current and / or voltage to the reduction cell 403, control of backpressure (e.g., via backpressure controller 415), supply of purge gas (e.g., using purge gas component 417), supply of carbon oxide (e.g., via carbon oxide flow controller 413), humidification of carbon oxide in the cathode feed stream (e.g., via humidifier 404), flow of anode water to and / or from the anode (e.g., via anode water flow controller 411), and anode water composition (e.g., via anode water source 405, pure water reservoir 421, and / or anode water additive component 423).

[0325] In the illustrated embodiment, the voltage monitoring system 434 is used to determine the voltage across the anode and cathode of the MEA cell or across any two electrodes of the cell stack, e.g., across all cells in a multi-cell stack. The voltages thus determined can be used to control the cell voltage during a current pause, inform the duration of the pause, etc. In certain embodiments, the voltage monitoring system 434 is configured to work in coordination with the power supply 433 to keep the reduction cell 403 within a specified voltage range. For example, the power supply 433 can be configured to apply a current and / or a voltage to the electrodes of the reduction cell 403 during the current pause to maintain the cell voltage within the specified range. For example, during the current pause, if the open circuit voltage of the cell deviates from a specified range (as determined by the voltage monitoring system 434), the power supply can be configured to apply a current or a voltage to the electrodes to maintain the cell voltage within the specified range.

[0326] A condition that may trigger a protection mode is loss of power to the electrolyzer. Under such conditions, it may be desirable to apply a small amount of current to the electrolyzer while current would otherwise be unavailable. To achieve this, some electrolyzer systems include an uninterruptible power supply (UPS), which may include a power source such as a battery or battery pack that has sufficient capacity to provide at least a limited amount of current to the electrolyzer. As shown, providing such current may mitigate problems or potential problems caused by unexpected interruptions such as power outages.

[0327] In some embodiments, the UPS is directly integrated with the carbon oxide electrolyzer or battery of electrolyzers. Some industrial-scale carbon oxide electrolyzer systems may use a dedicated UPS. Examples of industrial-scale electrolyzers include those configured to consume at least about 100 kg of carbon dioxide per day. In some cases, such industrial-scale carbon oxide electrolyzer systems may operate without the need for power of about 100 kW or more.

[0328] Electrolytic carbon reduction systems such as that shown in FIG. 4 may employ a control system including one or more controllers and one or more controllable components such as pumps, sensors, dispensers, valves, and power supplies. Examples of sensors include pressure sensors, temperature sensors, flow sensors, conductivity sensors, voltmeters, ammeters, electrolyte composition sensors including electrochemical instruments, optical sensors such as chromatography systems, absorbance measurement tools, and the like. Such sensors may be coupled to the inlets and / or outlets of the MEA cells (e.g., in the flow field), in reservoirs for holding anode water, pure water, salt solutions, and the like, and / or other components of the electrolytic carbon reduction system.

[0329] Various functions that may be controlled by one or more controllers include applying current and / or voltage to the carbon oxide reduction cells, controlling back pressure on the cathode outlet on such cells, supplying purge gas to the cathode inlet, supplying carbon oxide to the cathode inlet, humidifying carbon oxide in the cathode feed stream, flowing anode water to and / or from the anode, and controlling the anode feed composition. Any one or more of these functions may have a dedicated controller to control that function alone. Any two or more of these functions may share a controller. In some embodiments, a hierarchy of controllers is used, with at least one master controller providing instructions to two or more component controllers. For example, the system may include a master controller configured to provide high-level control instructions to (i) the power supplies to the carbon oxide reduction cells, (ii) the cathode feed stream flow controller, and (iii) the anode feed stream flow controller. For example, a programmable logic controller (PLC) may be used to control the individual components of the system.

[0330] In certain embodiments, the control system is configured to apply a current to a carbon oxide reduction cell comprising an MEA according to a current schedule, which may have any of the features described herein. For example, the current schedule may provide periodic pauses in the applied current. In some cases, the control system provides current pauses having a defined profile, such as a ramp and / or step change as described herein.

[0331] In certain embodiments, the control system is configured to control the flow rate of one or more feedstreams (e.g., a cathode feedstream and an anode feedstream, such as a carbon oxide flow) in coordination with the current schedule. For example, the flow of carbon oxide or a purge gas may be switched on, off, or otherwise adjusted when the current applied to the MEA cell is suspended.

[0332] In certain embodiments, a control system may be configured to implement the recovery sequences described herein. Such a control system may be configured to suspend or reduce current, flow recovery gas, flow water or other liquid, dry the cathode, resume normal operation, or any combination thereof. The controller may be configured to control the initialization of the recovery sequence, control the duration of any operation in the recovery sequence, etc.

[0333] 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 execute instructions provided from another source. The controller may be integrated with electronics to control the operation of the electrolytic cell before, during, and after carbon oxide reduction. The controller may control various components or subparts of one or more electrolytic carbon oxide reduction systems. The controller may be programmed to control any of the processes disclosed herein, such as gas supply, temperature settings (e.g., heating and / or cooling), pressure settings, power settings (e.g., voltage and / or current supplied to the electrodes of the MEA cell), liquid flow rate settings, fluid supply settings, and dosing of purified water and / or salt solutions, depending on the process requirements and / or type of system. These controlled processes may be connected or interfaced with one or more systems that function in concert with the electrolytic carbon oxide reduction system.

[0334] In various embodiments, the controller comprises electronics having 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 the operating parameters for carrying out a process on one or more components of the electrolytic oxidation 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 a particular reduction product, such as carbon monoxide, hydrocarbons, and / or other organic compounds.

[0335] 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 enable remote access to the system to monitor the current progress of the electrolysis operation, examine the history of past electrolysis operations, examine trends or performance metrics from multiple electrolysis operations, and modify parameters of the current process, set process steps to follow the current process, or start a new process. In some examples, a remote computer (e.g., a server) may provide a process recipe 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 for 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 that specify parameters for each of the process steps to be executed during one or more operations.

[0336] The controller may be distributed, such as by including one or more separate controllers networked together and functioning toward a common purpose, such as application of current to the MEA cells and other process control as described herein. One example of a distributed control system for such purposes includes one or more processors on the system for electrolytic reduction of carbon oxides and one or more remotely located processors (such as at the platform level or as part of a remote computer) that are combined to control the process.

[0337] Controllers and various associated computational elements, including processors, memories, instructions, routines, models, or other components, are often described or claimed as being "configured to" perform a task. In this context, the phrase "configured to" is used to imply structure by indicating that the components include structure (e.g., stored instructions, circuitry, etc.) that perform a task(s) during operation. Thus, a controller and / or associated components may be said to be configured to perform a task even if a particular component is not necessarily currently operating (e.g., not on).

[0338] Controllers and other components "configured to" perform an operation may be implemented as hardware, including, for example, circuits, memories storing executable program instructions to implement the operation, etc. Additionally, controllers and other components "configured to" perform an operation may be implemented as hardware that is operated by software and / or firmware (e.g., an FPGA or a general-purpose processor running software) to operate in a manner that enables it to perform the described tasks. Additionally, "configured to" may refer to one or more memories or memory elements that store computer-executable instructions to perform the described tasks. Such memory elements may include memory on a computer chip with processing logic. MEA Design Examples [Outline of MEA]

[0339] In various embodiments, the MEA includes an anode layer, a cathode layer, an electrolyte, and optionally one or more other layers. The layers may be solid and / or gel. These layers may include polymers, such as ion-conducting polymers.

[0340] In use, the cathode of the MEA is x , CO xBy combining three inputs: CO, ions (e.g., protons), which react chemically with CO, and electrons. x The anode of the MEA promotes the electrochemical reduction of CO, hydrocarbons, and / or organic compounds such as methanol, ethanol, and acetic acid to oxygen and hydrogen. In use, the anode of the MEA promotes an electrochemical oxidation reaction, such as the electrolysis of water, to produce elemental oxygen and protons. The cathode and anode may each include a catalyst to promote the respective reaction.

[0341] The composition and organization of the layers within the MEA is x To this end, the MEA can promote high yields of reduction products under the following conditions: (a) no parasitic reduction reaction (non-CO) at the cathode; x The MEA may facilitate any one or more of: (a) minimizing COx reduction reactions, (b) minimizing loss of COx reactant at the anode or elsewhere in the MEA, (c) maintaining the physical integrity of the MEA during the reaction (e.g., preventing delamination of the MEA layers), (d) preventing crossover of COx reduction products, (e) preventing crossover of oxidation products (e.g., O2), (f) maintaining a suitable environment at the cathode for oxidation, (g) providing a path for desired ions to travel between the cathode and anode while blocking undesired ions, and (h) minimizing voltage losses. As described herein, the presence of salt or salt ions in the MEA can facilitate some of all of these conditions. [COx reduction considerations]

[0342] 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 β-aminoethyl ketone presents a problem that does not occur, or occurs to a lesser extent, in water electrolyzers and fuel cells.

[0343] For example, in many applications, CO xThe MEA for reduction requires a life of about 50,000 hours or more (about 5 years of continuous operation), which is significantly longer than the expected life of a fuel cell for automobile use (for example, about 5,000 hours). x The MEA for reduction uses electrodes with a relatively large surface area compared to the MEAs used in fuel cells for automotive applications. For example, CO x The MEA for reduction should be at least about 500 cm 2 An electrode having a surface area (not considering voids and other non-planar features) of 0.1 mm may be used.

[0344] CO x The reduction reaction may be implemented in an operating environment that facilitates mass transfer of certain 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 CO2 production at the anode.

[0345] In some systems, CO x The rate of reduction is determined by the amount of gaseous CO at the cathode. x Limited by the availability of reactants. In contrast, the rate of water electrolysis is not significantly limited by the availability of reactants. Liquid water tends to be readily accessible to the cathode and anode, allowing the electrolyzer to operate near the highest possible current density. [MEA configuration]

[0346] In certain embodiments, the MEA has a cathode layer, an anode layer, and a polymer electrolyte membrane (PEM) between the anode layer and the cathode layer. The polymer electrolyte membrane provides ionic transfer between the anode layer and the cathode layer 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.

[0347] In certain embodiments, the MEA has a cathode buffer layer between the cathode layer and the polymer electrolyte membrane. The cathode buffer comprises a fourth ion-conducting polymer.

[0348] In certain embodiments, the MEA has an anode buffer layer between the anode layer and the polymer electrolyte membrane, the anode buffer comprising a fifth ion-conducting polymer.

[0349] 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. [Ion-conducting polymers for MEA layers]

[0350] The term "ion-conducting polymer" is used herein to describe a polymer electrolyte having a specific conductivity greater than about 1 mS / cm for anions and / or cations. 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 an ion-conducting polymer described as conducting both anions and cations ("cation-anion conductor"), neither the anions nor the cations have transport numbers 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 ionically conductive material or an ionomer. Examples of each class of ionically conductive polymers are provided in Table 1 below. [Table 1] [Polymer structure]

[0351] Examples of polymer structures that may include ionizable or ionic moieties and can be used as ion-conducting polymers in the MEAs described herein are given below. The ion-conducting polymers may be used as appropriate in any of the multiple MEA layers that include ion-conducting polymers. Charge conduction through the material may be controlled by the type and amount of charge provided on the ionizable / ionic moieties (e.g., anionic and / or cationic charges on the polymer structure). The composition may also include polymers, homopolymers, copolymers, block copolymers, polymeric blends, other polymer-based forms, or other useful combinations of monomeric repeat units. As described below, according to various embodiments, the ion-conducting polymer layer may include one or more of a bridge, a linking moiety, and an arylene group. In some embodiments, two or more ion-conducting polymers (e.g., in two or more ion-conducting polymer layers of an MEA) may be crosslinked.

[0352] Non-limiting monomeric units can include one or more of the following: [ka] where Ar is an optionally substituted arylene or aromatic; Ak is an optionally substituted alkylene, haloalkylene, aliphatic, heteroalkylene, or heteroaliphatic; and L is a linking moiety (e.g., any of those described herein) or -C(R 7 )(R 8 )-. Still other non-limiting monomeric units can include optionally substituted arylene, aryleneoxy, alkylene, or combinations thereof, such as an optionally substituted (aryl)(alkyl)ene (e.g., -Ak-Ar- or -Ak-Ar-Ak- or -Ar-Ak-, where Ar is an optionally substituted arylene and Ak is an optionally substituted alkylene). One or more of the monomeric units can be optionally substituted with one or more ionizable or ionic moieties (e.g., as described herein).

[0353] One or more monomer units may be combined to form a polymer unit. Non-limiting polymer units include any of the following: [ka] where Ar, Ak, L, n, and m can be any of those described herein. In some embodiments, each m is independently an integer of 0 or greater than 1. In other embodiments, Ar can include two or more arylene or aromatic groups.

[0354] Other alternative configurations are also encompassed by the compositions herein, such as branched configurations, diblock copolymers, triblock copolymers, random or statistical copolymers, steroblock copolymers, gradient copolymers, graft copolymers, and combinations of any of the blocks or regions described herein.

[0355] Exemplary polymeric structures include those according to any one of formulas (I)-(V) and (X)-(XXXIV) or salts thereof. In some embodiments, the polymeric structure is a copolymer and includes a first polymeric structure selected from any one of formulas (I)-(V) or salts thereof; a second polymeric structure includes an optionally substituted aromatic, an optionally substituted arylene, a structure selected from any one of formulas (I)-(V) and (X)-(XXXIV) or salts thereof.

[0356] In one embodiment, the MW of the ion-conducting polymer is a weight average molecular weight (Mw) of at least 10,000 g / mol or about 5,000 to 2,500,000 g / mol, in another embodiment, the Mw is a number average molecular weight of at least 20,000 g / mol or about 2,000 to 2,500,000 g / mol.

[0357] In any embodiment herein, n, n2, n3, n4, m, m1, m2, or m3 are each independently 1 or more, 20 or more, 50 or more, 100 or more, and 1 to 1,000,000, for example, 10 to 1,000,000, 100 to 1,000,000, 200 to 1,000,000, 500 to 1,000,000, or 1,000 to 1,000,000.

[0358] Non-limiting polymer structures include: [ka] or a salt thereof. R 7 , R 8 , R 9 , R 10 each is independently an electron-withdrawing moiety, H, an optionally substituted aliphatic, alkyl, heteroaliphatic, heteroalkylene, aromatic, aryl, or arylalkylene, where R 7 or R 8 At least one of R is an electron-withdrawing moiety, or 7 and R 8 or R 9 and R 10 , which form an optionally permuted cyclic group; Ar includes or is an optionally substituted aromatic or arylene (e.g., any of those described herein); Each n is independently an integer greater than or equal to 1; Each of rings a-c is optionally substituted; and Ring a~c, R 7 , R 8 , R 9 and R 10 optionally includes an ionizable or ionic moiety.

[0359] Further non-limiting polymer structures include one or more of the following: [ka] or a salt thereof. R 7 may be any of those described herein (e.g., formulas (I)-(V)). n is 1 or greater; Each L 8A , L B' and L B'' is independently a linking moiety; and each X 8A , X 8A' , X 8A' ' , X B' and X B'' is independently an ionizable or ionic moiety.

[0360] Further alternative polymer structures include [ka] JPEG2024507368000008.jpg251123 or a salt thereof. R 1 , R 2 , R 3 , R 7 , R 8 , R 9 and R 10 each is independently an electron-withdrawing moiety, H, an optionally substituted aliphatic, alkyl, heteroaliphatic, heteroalkylene, aromatic, aryl, or arylalkylene, where R 7 or R 8 may contain an electron-withdrawing moiety, or 7 and R 8 or R 9 and R 10 may be combined to form optionally permuted cyclic groups; Each Ak is or includes an optionally substituted aliphatic, alkylene, haloalkylene, heteroaliphatic, or heteroalkylene; each Ar is or includes an optionally substituted arylene or aromatic; L 1 , L 2 , L 3and L 4 each of which is independently a linking moiety; Each of n, n1, n2, n3 and n4, m, m1, m2 and m3 is independently an integer of 1 or more; q is 0, 1, 2 or more; Each ring a-i is optionally substituted; and Rings a~i, R 7 , R 8 , R 9 and R 10 may optionally include an ionizable or ionic moiety.

[0361] In certain embodiments (e.g., with respect to formula (XIV) or (XV)), each of the nitrogen atoms on ring a and / or ring b is substituted with an optionally substituted aliphatic, alkyl, aromatic, aryl, ionizable or ionic moiety. In some embodiments, one or more hydrogen or fluorine atoms (e.g., in formula (XIX) or (XX)) may be substituted to include an ionizable or ionic moiety (e.g., any of those described herein). In other embodiments, an oxygen atom present in the polymer structure (e.g., in formula (XXVIII)) is substituted with an alkali dopant (e.g., K + ) may be associated with

[0362] In certain examples, Ar, rings a-i (e.g., rings a, b, f, g, h, or i), L, L 1 , L 2 , L 3 , L 4 , Ak, R 7 , R 8 , R 9 and / or R 10 One or more of Ar, a ring (e.g., rings a-i), L, Ak, R may be optionally substituted with one or more ionizable or ionic moieties and / or one or more electron-withdrawing groups. 7 , R 8 , R 9 and R 10Further other non-limiting substituents for include one or more of those described herein, e.g., cyano, hydroxy, nitro, and halo, and optionally substituted aliphatic, alkyl, alkoxy, alkoxyalkyl, amino, aminoalkyl, aryl, arylalkylene, aryloyl, aryloxy, arylalkoxy, hydroxyalkyl, and haloalkyl.

[0363] In some embodiments, R 1 , R 2 and R 3 Each of R is independently H, an optionally substituted aromatic, aryl, aryloxy, or arylalkylene. In other embodiments (e.g., in Formulas (I)-(V) or (XII)), R 7 In yet another embodiment, R 8、 R 9 and / or R 10 comprises an ionizable or ionic moiety.

[0364] In one example, a polymer subunit may not have an ionic moiety. Alternatively, a polymer subunit may include an ionic moiety that may be integrated into the Ar group, the L group, both the Ar and L groups, or as part of the L group. Non-limiting examples of ionizable or ionic moieties include the cationic groups, anionic groups, and multi-ionic groups described herein.

[0365] In any embodiment herein, the electron withdrawing moiety may be an optionally substituted haloalkyl, cyano (CN), phosphate (e.g., -O(P=O)(OR P1 )(OR P2 ) or -O-[P(=O)(OR P1 )-O] P3 -R P2 ), sulfates (e.g., -OS(=O)2(OR S1), sulfonate (-SO3H), sulfonyl (e.g., -SO2-CF3), difluoroboranyl (-BF2), borono (B(OH)2), thiocyanato (-SCN), or piperidinium. Further other non-limiting phosphate groups may include derivatives of phosphoric acid, such as orthophosphate, pyrophosphate, tripolyphosphate, tetrapolyphosphate, trimetaphosphate, and / or phosphoric anhydride, or combinations thereof.

[0366] Further other polymer units include poly(benzimidazole) (PBI), polyphenylene (PP), polyimide (PI), poly(ethyleneimine) (PEI), sulfonated polyimide (SPI), polysulfone (PSF), sulfonated polysulfone (SPSF), poly(ether ether ketone) (PEEK), PEEK with cardo groups (PEEK-WC), polyethersulfone (PES), sulfonated polyethersulfone (SPES), sulfonated poly(ether ether ketone) (SPEEK), SPEEK with cardo groups (SPEEK-WC), poly(p-phenylene oxide) (PPO), sulfonated polyphenylene oxide (SPPO), ethylene tetrafluoroethylene (ETFE), poly poly(vinyl alcohol) (PVA), poly(phosphazene), poly(aryloxyphosphazene), polyetherimide, and combinations thereof. [Bipolar MEA for COx reduction]

[0367] In certain embodiments, the MEA comprises a bipolar interface having an anion conducting polymer on the cathode side of the MEA and an interfacial 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 is disposed between the anode and the PEM and includes a cation conducting polymer.

[0368] During operation, MEAs with bipolar interfaces transfer 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.

[0369] 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, and 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.

[0370] For example, at the level of potential used for cathodic reduction of CO2, hydrogen ions may be reduced to hydrogen gas. This is a parasitic reaction, and 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 carried out at the anode of the CO2 reduction reactor, and may travel 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. Thus, 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 the catalytic sites on the cathode. As a result, the parasitic generation of hydrogen gas evolution is reduced, increasing the rate of production of CO or other products and the overall efficiency of the process.

[0371] Another reaction that can be avoided is the reaction of carbonate or bicarbonate ions at the anode to produce CO2. Aqueous carbonate or bicarbonate ions can be generated from CO2 at the cathode. When such ions reach the anode, they can react with hydrogen ions to produce and release gaseous CO2. As a result, CO2 moves net from the cathode to the anode, where it does not react and is lost together with the oxidation products. To prevent carbonate and bicarbonate ions generated at the cathode from reaching the anode, the anode and / or anode buffer layer can include a cation-conducting polymer, which at least partially blocks the movement of negative ions, such as bicarbonate ions, to the anode.

[0372] Thus, in some designs, a bipolar membrane structure raises 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.

[0373] CO xAn example of an MEA 500 for use in reduction is shown in FIG. The MEA 500 has a cathode layer 520 and an anode layer 540 separated by an ion-conducting polymer layer 560 that provides a path for ions to move between the cathode layer 520 and the anode layer 540. In certain embodiments, the cathode layer 520 comprises an anion-conducting polymer and / or the anode layer 540 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 movement and can increase the amount of catalytic surface area available for reaction.

[0374] The ion conducting layer 560 may include two or three sublayers, namely, a polymer electrolyte membrane (PEM) 565, an optional cathode buffer layer 525, and / or an optional anode buffer layer 545. 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 by gas and / or liquid transport, and vice versa. In certain embodiments, the PEM layer 565 is non-porous. Examples of properties of the anode and cathode buffer layers are provided elsewhere herein. In certain embodiments, the ion conducting layer is exclusively a single layer or includes two sublayers.

[0375] In some embodiments, the anode of the carbon oxide electrolysis cell contains a blend of an oxidation catalyst and an anode ion-conducting polymer. Depending on the reactants and anode catalyst supplied to the anode, there are various oxidation reactions that can 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.

[0376] The oxidation catalyst may be in the form of a structured mesh or in the form of particles. If the oxidation catalyst is in the form of particles, the particles may be supported by electronically conductive support 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. The conductive support particles may be compatible with the chemicals present at the electrolytic cell anode during operation of the CRR and are oxidatively stable, so that the conductive support particles do not participate in the electrochemical reaction. This is particularly useful when 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. 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 mentioned above. It should be understood that many of these metal catalysts may be in the form of an oxide, especially under the reaction conditions.

[0377] In some embodiments, the MEA has an anode layer that includes an oxidation catalyst and a second ion-conducting polymer. The second ion-conducting polymer can include one or more polymers that contain 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 ethanesulfonic acid fluoride, 2-[1-[difluoro-[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2,-tetrafluoro- with tetrafluoroethylene, tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octanesulfonic acid copolymers, other perfluorosulfonic acid polymers, and blends thereof. Examples of cation-conducting polymers include, for example, Nafion 115, Nafion 117, and / or Nafion 211.

[0378] There may be a trade-off in choosing the amount of ion-conducting polymer in the anode. It is important to include enough anode ion-conducting polymer to provide sufficient ion conductivity. However, it is also important that the anode is porous so that reactants and products can easily move through the anode and to 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. It is particularly useful if the anode 240 can withstand high voltages, such as voltages greater than about 1.2 V versus the reversible hydrogen electrode. It is particularly useful if the anode 240 is porous to maximize the amount of available catalytic surface area and facilitate gas and liquid movement.

[0379] In one example of a metal catalyst, Ir or IrOx particles (100-200 nm) and Nafion ionomer form a porous layer about 10 μm thick. The metal catalyst loading is approximately 0.5-3 g / cm. 2 It is.

[0380] In some embodiments, NiFeOx is used for the basic reaction. [PEM]

[0381] The MEA may include a polymer electrolyte membrane (PEM) disposed between and conductively coupled to the anode and cathode catalyst layers. In certain embodiments, the polymer electrolyte membrane has high ionic conductivity (e.g., greater than about 1 mS / cm) and is mechanically stable. Mechanical stability may be demonstrated in a variety of ways, such as high tensile strength, modulus, elongation at break, and tear resistance. Many commercially available membranes may be used for the polymer electrolyte membrane. Examples include, but are not limited to, various Nafion® formulations, GORE-SELECT, FumaPEM® (PFSA) (FuMA-Tech GmbH), and Aquivion® (PFSA) (Solvay).

[0382] 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. [Cathode buffer layer]

[0383] If the polymer electrolyte membrane is a cation conductor (e.g., it conducts protons), it may contain a high concentration of protons during operation of the CRR, while the cathode may operate better when a low concentration of protons is present. A cathode buffer layer may be provided between the polymer electrolyte membrane and the cathode to provide a transition region from a high concentration of protons to a low concentration of protons. In one configuration, the cathode buffer layer is an ion-conducting polymer that has many of the same properties as the ion-conducting polymer in the cathode. The cathode buffer layer may provide a region for the proton concentration to transition from the polymer electrolyte membrane, which has a high proton concentration, to the cathode, which has a low proton concentration. Within the cathode buffer layer, protons from the polymer electrolyte membrane may encounter anions from the cathode, which may neutralize each other. The cathode buffer layer may help ensure that a detrimental number of protons from the polymer electrolyte membrane do not reach the cathode and increase the proton concentration. If the proton concentration in the cathode is too high, CO x No reduction occurs. A high proton concentration may be a concentration in the range of about 10 to 0.1 molar, and a low proton concentration may be a concentration less than about 0.01 molar.

[0384] The cathode buffer layer can include a single polymer or multiple polymers. When the cathode buffer layer includes multiple polymers, the multiple polymers can be mixed together or can be arranged in separate adjacent layers. Examples of materials that can be used for the cathode buffer layer include, but are not limited to, FumaSep FAA-3, Tokuyama anion exchange membrane material, and polyether-based polymers such as polyethylene oxide (PEO), and blends thereof. Further examples are described above in the description of the cathode catalyst layer.

[0385] The thickness of the cathode buffer layer is determined by the low proton concentration and the CO x The thickness is selected to be sufficient to provide high reduction activity. This sufficiency may vary depending on the material of the cathode buffer layer. In general, 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.

[0386] In some embodiments, the cathode buffer layer is less than 50 μm, for example, between 1 and 25 μm, such as between 1 and 5 μm, 5 and 15 μm, or 10 and 25 μm. By using a cathode buffer layer of this range of thickness, the proton concentration in the cathode can be reduced while maintaining the overall conductivity of the cell. In some embodiments, ultra-thin 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 can be characterized relative to that of the PEM.

[0387] Water and CO2 formed at the interface of the cathode buffer layer and the PEM can delaminate the MEA to which the polymer layer connects. The delamination issue 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 path for gaseous carbon dioxide to escape back to the cathode where it can be reduced.

[0388] 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.

[0389] 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 does not contribute to porosity. In many embodiments, the polymer electrolyte / PTFE mass ratio is 0.25-2, more specifically 0.5-1. The polymer electrolyte / PTFE volume ratio (or, more generally, polymer electrolyte / inert filler) can be 0.25-3, 0.5-2, 0.75-1.5, or 1.0-1.5.

[0390] In other configurations, porosity is achieved by using certain processing methods when the layers are formed. One example of such a processing method is laser ablation, where nano- to micro-sized channels are formed in the layers. Another example is mechanically drilling holes in the layers to form channels through them.

[0391] In one configuration, the cathode buffer layer has a porosity between 0.01% and 95% (e.g., by weight, volume, mass, etc., about therebetween). However, in other configurations, the cathode buffer layer can have any suitable porosity (e.g., 0.01-95%, 0.1-95%, 0.01-75%, 1-95%, 1-90%). In some embodiments, the porosity is 50% or less, e.g., 0.1-50%, 5-50%, 20-50%, 5-40%, 10-40%, 20-40%, or 25%-40%. In some embodiments, the porosity is 20% or less, e.g., 0.1-20%, 1-10%, or 5-10%.

[0392] Porosity may be measured as described above for the catalyst layer, including using the mass loading and thickness of the components 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, since materials swell to different degrees when exposed to water during operation.

[0393] The porosity of the layers of the MEA, including the cathode buffer layer, is discussed further below. [Anode buffer layer]

[0394] In some CRR reactions, bicarbonate is produced at the cathode. It may be useful to have a polymer that blocks bicarbonate transport somewhere between the cathode and anode, preventing bicarbonate migration from the cathode. Bicarbonate may pick up some CO2 as it migrates, reducing the amount of CO2 available for reaction at the cathode. In some MEA configurations, the polymer electrolyte membrane contains 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 some MEA configurations, there is an anode buffer layer between the polymer electrolyte membrane and the anode, 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 no bicarbonate is present in the CRR, it is not particularly desirable to include a bicarbonate blocking function in the ion exchange layer.

[0395] In certain embodiments, the anode buffer layer provides a region for the transition of proton concentration from the polymer electrolyte membrane to the anode. The concentration of protons in the polymer electrolyte membrane depends on both its composition and the ions it conducts. For example, a Nafion polymer electrolyte membrane that conducts protons has a high proton concentration. A FumaSep FAA-3 polymer electrolyte membrane that conducts hydroxide has a low proton concentration. For example, if the desired proton concentration at the anode differs from the polymer electrolyte membrane by more than three orders of magnitude, the anode buffer layer can be useful to affect the transition from the proton concentration of the polymer electrolyte membrane to the desired proton concentration of the anode. The anode buffer layer can include a single polymer or multiple polymers. When the anode buffer layer includes multiple polymers, the multiple polymers can be mixed together or can be arranged in separate adjacent layers. Materials that may be useful for providing regions 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 materials. 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 may be classified into different classes based on the type of ion they conduct. This was discussed in detail above. There are three classes of ion-conducting polymers listed in Table 1 above. In one embodiment of the invention, at least one of the ion-conducting polymers in the cathode, anode, polymer electrolyte membrane, cathode buffer layer, and anode buffer layer is of a different class than at least one of the others. [Layer porosity]

[0396] It may be useful if some or all of the following layers are porous: cathode, cathode buffer layer, anode, and anode buffer layer. In some configurations, porosity is achieved by combining inert filler particles with the polymers of these layers. Suitable materials for the inert filler particles include, but are not limited to, TiO2, silica, PTFE, zirconia, and alumina. In various arrangements, 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. In other configurations, porosity is achieved by using a particular processing method 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. Laser ablation can additionally or alternatively achieve porosity in the layer by sub-surface ablation. Sub-surface ablation can form voids in the layer when a beam is focused at a point in the layer, thereby vaporizing the layer material near that point. 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 focal size of the beam, 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 in 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.).

[0397] The porosity of the above and other examples and variations (e.g., the porosity of the cathode buffer layer, the anode buffer layer, the membrane layer, the cathode layer, the anode layer, other suitable layers, etc.) preferably has a uniform distribution, but can additionally or alternatively have any suitable distribution (e.g., a randomized 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, a maximum of about 200 nanometers, and / or any other suitable size or size distribution.

[0398] As noted above, the cathode buffer layer preferably has a porosity of about 1-90 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., 0.01-95%, 0.1-95%, 0.01-75%, 1-95%, 1-90%, etc.). In some embodiments, the porosity is 20% or less, e.g., 0.1-20%, 1-10%, or 5-10%.

[0399] In some embodiments, the cathode buffer layer is porous, but at least one layer between the cathode layer and the anode layer is non-porous. This can prevent the passage of gas and / or bulk liquid between the cathode layer and the anode layer while still preventing delamination. For example, the non-porous layer can prevent the direct passage of water from the anode to the cathode. [Other embodiments]

[0400] Although omitted for simplicity, system and / or method embodiments may include any combination and permutation of the various system components and various method processes, and one or more examples 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 with one or more example systems, elements, and / or entities described herein.

[0401] 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.

Claims

1. 1. A method of operating an electrolytic cell for carbon oxide reduction, the method comprising: performing normal operations with the electrolytic cell, the normal operations including injecting a reactant gas comprising carbon oxides into a cathode of the electrolytic cell and applying an electric current to the electrolytic cell at a first current density to generate an operating potential between the cathode and an anode of the electrolytic cell to reduce the carbon oxides and generate a carbon-containing reduction product; (i) transitioning the electrolytic cell to a state in which an open circuit voltage exists between the cathode and the anode of the electrolytic cell, and (ii) performing a recovery or protection process comprising flowing a recovery gas to the cathode while the electrolytic cell maintains the open circuit voltage; and resuming normal operation on the electrolytic cell, where resuming normal operation includes stopping or modifying the flow of recovery gas to the cathode and returning to the operating potential.

2. The method of claim 1 , wherein the normal operation includes flowing the reactant gas to the cathode at a first flow rate and at a first pressure.

3. The method of claim 1 , wherein the normal operation comprises performing the normal operation for a period of at least about 100 hours.

4. 4. The method of any one of claims 1 to 3, further comprising, after the step of resuming normal operation with the electrolytic cell, continuing to perform the normal operation with the electrolytic cell for at least about 100 hours before again performing the recovery or protection process or terminating operation of the electrolytic cell.

5. 5. The method of claim 1, wherein the recovery gas has a different composition than the reaction gas.

6. A carbon oxide reduction electrolytic cell comprising: (a) at least one membrane electrode assembly (MEA) including: (i) a cathode including a carbon oxide reduction catalyst that promotes the reduction of carbon oxides; (ii) an anode including a catalyst that promotes the oxidation; and (iii) a polymer electrolyte membrane (PEM) layer disposed between the cathode and the anode; (b) a power source configured to control a current applied to the carbon oxide reduction electrolyzer; and (c) one or more controllers, the one or more controllers controlling the carbon oxide reduction electrolyzer to: performing normal operation with the MEA, where the normal operation includes injecting a reactant gas comprising carbon oxide into the cathode of the MEA and applying an electric current to the MEA at a first current density; (i) transitioning the carbon oxide reduction electrolyzer to a state in which an open circuit voltage exists between the cathode and the anode of the carbon oxide reduction electrolyzer, and (ii) performing a recovery or protection process comprising flowing a recovery gas to the cathode while the carbon oxide reduction electrolyzer maintains the open circuit voltage; and and configured to cause said normal operation to be resumed with said MEA by stopping or modifying the flow of said recovery gas to said cathode and by returning to an operating potential.

7. 7. The carbon oxide reduction electrolyzer of claim 6, wherein the one or more controllers are configured to cause the carbon oxide reduction electrolyzer to flow the reactant gas to the cathode at a first flow rate and at a first pressure during the normal operation.

8. 8. The carbon oxide reduction electrolyzer of claim 6 or 7, wherein the one or more controllers are further configured to cause the carbon oxide reduction electrolyzer to continue performing the normal operation with the MEA for at least about 100 hours after resuming the normal operation with the MEA and before again performing the recovery or protection process or terminating operation of the MEA.

9. 9. A carbon oxide reduction electrolyser according to any one of claims 6 to 8, wherein the recovery gas has a different composition to the reaction gas.

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