Electrochemically converting aqueous carbonate and bicarbonate

Abstract

An anode is on a first side of a selectively permeable membrane. A cathode is on a second side of the selective permeable membrane. An anode housing defines an anode fluid chamber configured to be filled with a carbonate or bicarbonate solution. A cathode housing defines a cathode fluid chamber configured to be filled with the same carbonate or bicarbonate solution as the anode chamber.

Description

ELECTROCHEMICALLY CONVERTING AQUEOUS CARBONATE AND BICARBONATECROSS-CITATION TO RELATED APPLICATION

[0001] This application claims priority to USPTO provisional application 63 / 611,935 under 35 U.S.C. 119(e). The contents of which are hereby incorporated by reference in their entirety.TECHNICAL FIELD

[0002] The subject matter described with in this disclosure relates to electrochemically converting carbonates and bicarbonates into useful products.BACKGROUND

[0003] Captured CO2 can either be stored or converted into a carbon-containing product. Both processes require energy to liberate CO2 from the sorbent. One way to bypass the energy-intensive regeneration of CO2 is to integrate CO2 capture and conversion. For example, OH reacts spontaneously with waste CO2 to form (bi)carbonate-rich liquids, which can then be electrochemically upgraded into carbon-containing products (e.g., CO, CH4) while regenerating OH for further CO2 capture. The challenge is that electrolyzers designed to mediate this process are characterized by high voltages (e.g., >4 V at 100 mA cm2) and, thus, low energy efficiencies. In this study, we report a bicarbonate electrolyzer that operates at 2.9 V at 100 mA cm2at room temperature and 2.7 V at 100 mA cm2at 50°C. This lower voltage was made possible by designing a bicarbonate electrolyzer with a cationic exchange membrane that uses a neutral anolyte (3 M KHCO3(aq)) to minimize the thermodynamic potential for the oxygen evolution reaction. We also show that pressure-drop-induced desorption of CO2 from the electrolyte benefits electrolysis. This bicarbonate electrolyzer does not require the same purification and pressurization steps of other processes that convert CO2 into syngas, andtherefore enables a carbon capture and utilization energy efficiency for an electrochemical process that is comparable to thermochemical CO2 conversion for syngas production.SUMMARY

[0004] An example implementation of the subject matter described in this disclosure includes an electrolyzer cell with the following features. An anode is on a first side of a selectively permeable membrane. A cathode is on a second side of the selective permeable membrane. An anode housing defines an anode fluid chamber configured to be filled with a carbonate or bicarbonate solution. A cathode housing defines a cathode fluid chamber configured to be filled with the carbonate or bicarbonate solution.

[0005] Aspects of the example electrolyzer cell, which can be combined with the example electrolyzer alone or in combination with other aspects, include the following. The anode abuts the selectively permeable membrane.

[0006] Aspects of the example electrolyzer cell, which can be combined with the example electrolyzer alone or in combination with other aspects, include the following. The cathode abuts the selectively permeable membrane.

[0007] Aspects of the example electrolyzer cell, which can be combined with the example electrolyzer alone or in combination with other aspects, include the following. The selectively permeable membrane is a cation exchange membrane.

[0008] Aspects of the example electrolyzer cell, which can be combined with the example electrolyzer alone or in combination with other aspects, include the following. The anode housing further defines an anode inlet and an anode outlet. The anode inlet is configured to receive the carbonate or bicarbonate solution. The anode housing is configured to direct a flow of the carbonate or bicarbonate solution across the anode. The anode outlet is configured to emit the carbonate or bicarbonate solution from the anode housing.

[0009] Aspects of the example electrolyzer cell, which can be combined with the example electrolyzer alone or in combination with other aspects, include the following. The cathode housing further defines a cathode inlet and a cathode outlet. The cathode inlet is configured to receive the carbonate or bicarbonate solution. The cathode housing is configuredto direct a flow of the carbonate or bicarbonate solution across the cathode. The cathode outlet is configured to emit the carbonate or bicarbonate solution from the cathode housing.

[0010] Aspects of the example electrolyzer cell, which can be combined with the example electrolyzer alone or in combination with other aspects, include the following. The cathode includes Ag or C.

[0011] Aspects of the example electrolyzer cell, which can be combined with the example electrolyzer alone or in combination with other aspects, include the following. The anode includes Ir, O, or C. In some implementations, the anode is made of a material comprising IrCh, RUO2, mixed oxides of Ir-Ru, MnOx, CO3O4, NiFe, and / or BiVC .

[0012] An example implementation of the subject matter described within this disclosure is a system with the following features. An electrolyzer cell is configured to receive a carbonate or bicarbonate solution by an anode section and a cathode section. The electrolyzer is configured to produce one or more products, by a current exchanged between the anode section and the cathode section, using the received carbonate or bicarbonate solution. A source is configured to provide the carbonate or bicarbonate solution. A pump is arranged to circulate the carbonate or bicarbonate solution between the electrolyzer cell and the source. The carbonate or bicarbonate solution is circulated to both the anode section and the cathode section.

[0013] Aspects of the example system, which can be combined with the example system alone or in combination with other aspects, include the following. A carbon capture tower is fluidically coupled to the reservoir. A second pump circulates the carbonate or bicarbonate solution between the carbon capture tower and the reservoir.

[0014] Aspects of the example system, which can be combined with the example system alone or in combination with other aspects, include the following. A direct current power supply is coupled to the anode section and the cathode section. The direct current power supply is configured to provide sufficient current and voltage to produce products from the carbonate or bicarbonate solution.

[0015] Aspects of the example system, which can be combined with the example system alone or in combination with other aspects, include the following. The power supply provides three volts.

[0016] Aspects of the example system, which can be combined with the example system alone or in combination with other aspects, include the following. A separator is downstream of the cathode section. The separator configured to separate gas from liquid.

[0017] An example implementation of the subject matter described within this disclosure is a method with the following features. A carbonate or bicarbonate solution is received by an anode section of an electrolyzer cell. The carbonate or bicarbonate solution is received by a cathode section of the electrolyzer cell. The anode section is separated from the cathode section by a selectively permeable membrane. One or more products are formed by a current exchanged between the anode section and the cathode section.

[0018] Aspects of the example method, which can be combined with the example method alone or in combination with other aspects, include the following. The carbonate or bicarbonate solution is recirculated from an outlet of the anode section and an outlet of the cathode section, through a common reservoir, and back to the anode section and the cathode section.

[0019] Aspects of the example method, which can be combined with the example method alone or in combination with other aspects, include the following. The selectively permeable membrane is a cation exchange membrane.

[0020] Aspects of the example method, which can be combined with the example method alone or in combination with other aspects, include the following. The one or more products include O2 formed by the anode section.

[0021] Aspects of the example method, which can be combined with the example method alone or in combination with other aspects, include the following. The one or more products include CO, H2, and CO2 formed by the cathode section.

[0022] Aspects of the example method, which can be combined with the example method alone or in combination with other aspects, include the following. The one or more products include liquid products.

[0023] Aspects of the example method, which can be combined with the example method alone or in combination with other aspects, include the following. The one or more products include gas products.BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 A are flow diagrams of four different carbon capture and utilization (CCU) pathways. The differentiating feature of each pathway is the utilization step (marked with cross hatch lines) where CO2 is converted into syngas with a 2: 1 H2:CO molar ratio thermochemically or electrochemically.

[0025] FIG. IB is a chart illustrating CCU energy efficiencies and total energy consumption of four different pathways. The total energy consumption is divided into five different steps: CO2 capture and solvent regeneration; CO2 pressurization; CO2 conversion; product separation; and water electrolysis for H2 production. CCU energy efficiency is defined as the ratio of the minimum theoretical energy required to convert CO2 into 1 mol of syngas, to the total energy actually required to capture and convert CO2 into 1 mol of syngas.

[0026] FIG. 2A illustrates an example two-compartment H-cell setup to visualize CO2 bubbles formed at the membrane|catholyte interface. The Ni foam anode and the cation exchange membrane (green; Nafion™ Nl 17) were compressed against each other to simulate the zero-gap environment in a bicarbonate electrolyzer. A carbon paper (marked with cross hatch lines; Freudenberg H23) was used as the cathode.

[0027] FIG. 2B illustrates CO2 bubbles formed at a membrane|catholyte interface.

[0028] FIG. 2C illustrates a peak signal from the flame ionization detector (FID) of the gas chromatography (GC) confirming that the bubbles formed at the membrane|catholyte interface shown in FIG. 2B included CO2 gas.

[0029] FIG. 3 A is an exploded view of an example bicarbonate electrolyzer. The anolyte and catholyte are mixed after each circulation to reset ion concentrations.

[0030] FIG. 3B is a graph illustrating Full cell voltages (Eceii) of bicarbonate electrolyzers that use a CEM or BPM at various current densities.

[0031] FIG. 3C is a graph illustrating FEco of a bicarbonate electrolyzer with a CEM at various current densities. Bicarbonate electrolysis was conducted at two different temperatures: room temperature and 50°C for the CEM-based electrolyzer.

[0032] FIG. 4A illustrates two scenarios with gaseous N2 being passed along the headspace of a round-bottom flask containing 3 M KHCO3(aq) solution. A first scenario only includes N2 being passed along the headspace of a round-bottom flask containing 3 M KHCO3(aq) solution. The second scenario is similar, but the 3 M KHCC>3(aq) solution is circulated through the porous cathode of the electrolyzer, without an applied current. The [ / -CO2] is measured to be higher in the case where the electrolyte was circulated through the electrolyzer.

[0033] FIG. 4B is a graph illustrating the concentration of Z-CO2 that is generated in a bicarbonate electrolyzer over time (hr).

[0034] FIG. 5 is a block diagram of a system that uses the subject matter described throughout this disclosure.DETAILED DESCRIPTION

[0035] Carbon capture and utilization (CCU) is typically a sequential process that captures and upgrades CO2 from a point source or air into value-added fuels and chemicals. Examples of such processes are illustrated in FIG. 1 A. One method of upgrading the captured CO2 is to electrochemically reduce it at a cathode in the presence of a proton source. The challenge is that the captured CO2 needs to be released from the CO2 capture solution, then purified and pressurized prior to delivery into an electrolyzer (see “CO2 Electrolyzer Pathway” 102). These steps are both capital- and energy-intensive.

[0036] This situation motivated us to develop a bicarbonate electrolyzer to couple the CO2 capture and upgrading processes (“Bicarbonate Electrolyzer Pathway” 104). By using alkaline solutions (e.g., KOH or K2CO3) in the carbon capture unit to react with CO2 to form “reactive carbon solutions” (e.g, (bi)carbonates; Eq. 1), we recognized that an electrolyzer that leverages both pH and electrochemistry could convert these reactive carbon solutions into useful products (e.g., CO) while regenerating the carbon capture solution. A key feature of the bicarbonate electrolyzer is that it delivers protons (H+) to react with (bi)carbonate ions to form high concentrations of CO2 in situ (Eq. 2). This in situ generated CO2 (“Z-CO2”) is then electrochemically reduced into carbon-containing products such as CO, just as what would occur in an electrolyzer fed with gaseous CO2. Importantly, bicarbonate electrolyzers convert more of the CO2 feedstock into CO than gas-fed CO2 electrolyzers. Bicarbonate electrolyzers are also less sensitive to impurities and oxygen.CO2(g) + OH’(aq)HCO3-(aq) (Eq. 1)HCO3 (aq) + H+(aq) i-CO2(g) + H2O(1) (Eq. 2)

[0037] To deliver acid into the cathode compartment, we previously built bicarbonate electrolyzers containing bipolar membranes (BPMs) to separate the cathode and anode. A BPM includes a cation exchange membrane (CEM) pressed against an anion exchange membrane (AEM). Water is dissociated into OH and H+at a catalyst layer at the interface of these two membrane layers, and OH and H+are delivered into the anode and cathode compartments, respectively. The challenge is that these BPMs are characterized by large voltages at meaningful current densities (>4 V at 100 mA cm2).

[0038] To put this high voltage penalty into broader perspective, we calculated the energy required to capture and convert CO2into syngas when using a bicarbonate electrolyzer (“Bicarbonate Electrolyzer Pathway” 104). We targeted syngas with a H2:CO molar ratio of 2: 1, a feedstock for the Fischer-Tropsch process that produces hydrocarbons and oxygenates. We then benchmarked this CCU energy efficiency against three other CCU pathways widely considered to be commercially viable: the reverse water gas shift (“RWGS Pathway” 106); high-temperature CO2electrolysis with a solid oxide electrolysis cell (“SOEC Pathway” 108); and low-temperature CO2electrolysis (“CO2Electrolyzer Pathway” 102). All of these three pathways require purified gaseous CO2feed. We define CCU energy efficiency as the ratio of the minimum theoretical energy required to convert CO2into 1 mol of syngas to the total energy actually required to capture and convert CO2into 1 mol of syngas. FIG. IB illustrates a chart 150 of efficiencies for each of these pathways. The CCU energy efficiencies of each of these pathways are primarily differentiated by the utilization step (the conversion of CO2into CO); the CO2capture (using amines), solvent regeneration, and pressurization steps were held at parity for the analysis. All four pathways necessitate the use of a water electrolyzer to generate H2(g) to maintain the required 2: 1 H2:CO molar ratio, as these specific utilization steps produce CO with high selectivity. The Bicarbonate Electrolyzer Pathway assumes CO2capture by alkaline solutions that are fed directly into the bicarbonate electrolyzer. Note that the selectivity for CO formation along each pathway governs how much CO2must be captured to produce 1 mol of CO, thus the energy required for the same step may differ for each pathway.We assumed unreacted CO2 leaving each CO2 conversion step would be recycled back as the inlet feedstock. Input parameters for the analysis are summarized in Table 1 and Table 2.Table 1. A summary of heat duty values for carbon capture solvent regeneration.Solvent Heat duty (kJ / mol) NoteMEA 150MEA 167 Lab-scale dataMEA 238MDEA 123MEA 176154 Survey from experts264KS-1 119MEA 136 ModelKS-1 110KS-1 112 Pilot dataMEA 187 ModelDEA 136DGA 123MEA 396 ModelDEA 163DGA 185DEA / AMP blend 133 ModelMEA 198 Pilot dataAMP / PZ blend 216DETA 97 Lab-scale dataMEA 166 ModelPZ 88 ModelPZ 105MEA 136MEA 170 Pilot dataMEA 136 Optimized modelAMP / PZ / MEA blend 170 ModelMEA 120 Optimized modelMEA 199 Base case modelMEA 244 Pilot dataMEA 227 Model validationMEA 123 ModelTable 2: Input and output parameters for four different carbon capture and utilization pathways.Miscellaneous

[0039] This analysis shows CCU energy efficiencies of 51%, 47%, and 40% for the SOEC, RWGS, and CO2 Electrolyzer Pathways (cell voltage, Eceii = 3 V; Faradaic efficiency of CO formation, FEco = 90%; carbon utilization = 20%), respectively. Note that even in a scenario where the energy cost for the capture solvent regeneration is decreased, the Bicarbonate Electrolyzer Pathway largely yields the highest CCU energy efficiency. The CCU energy efficiency for the pathway with a bicarbonate electrolyzer that uses a BPM (ceii = 4 V; FEco = 90%; carbon utilization = 50%) would be 45% (not shown in Fig. IB). This analysis indicated that bicarbonate electrolyzers need to operate Eceii <3.1 V (in cases where FEco = 90%) to achieve comparable energy efficiencies to the RWGS or SOEC Pathways. The voltage of bicarbonate electrolyzers needs to be lowered substantially from the current benchmark of 4 V.

[0038] We therefore set out to build a bicarbonate electrolyzer with a CEM instead of a BPM to increase the energy efficiency of the CO2 utilization step. CEMs mediatethe transport of cations (e.g., H+) to the cathode for the requisite reaction with bicarbonate to form Z-CO2 (Eq. 2). We previously showed that a Nafion™ CEM does indeed lower the bicarbonate electrolyzer voltage, but we needed to feed h(g) into the anode compartment. This configuration resulted in a strikingly low full cell voltage of 1.7 V at 100 mA cm2. However, the use of a gaseous H2 feedstock requires additional energy and capital costs.

[0040] In this disclosure, we show that a bicarbonate electrolyzer with a CEM separating a IrCE nanoparticle anode spray-coated on a carbon paper substrate (IrCE / C) and a silver nanoparticle cathode spray-coated on a carbon paper substrate (Ag / C) can operate at 2.7 V at 100 mA cm2. In some implementations, the anode is made of a material comprising EO2, RUO2, mixed oxides of Ir-Ru, MnOx, CO3O4, NiFe, and / or BiVCU. The higher energy efficiency of the bicarbonate electrolyzer is achieved with: (i) the use of a neutral anolyte (3 M KHCO3) that decreases the thermodynamic potential for the oxygen evolution reaction (OER) relative to that in strongly acidic media; and (ii) the use of a CEM to avoid the high voltage penalty associated with BPMs for dissociating water. Importantly, the Bicarbonate Electrolyzer Pathway with this new bicarbonate electrolyzer design exhibits a high CCU energy efficiency of about 51% (Fig. IB). This value is comparable to the SOEC pathway and represents a 1.25-fold improvement over the CO2 Electrolyzer Pathway, where the maximum CCU energy efficiency is modeled to be about 40%. In some implementations, the anolyte comprises 0.5-3 M bicarbonate (KHCO3). In some implementations, the anolyte comprises 0.25-3 M carbonate (K2CO3). The anolyte may comprise other monovalent cations such as Na+, K+, H+, NH / . In some implementations, the anolyte additionally comprises a CO2 capture promoter (such as an amine- based carbonic anhydrase).

[0041] An acidic anolyte is typically used with cation exchange membranes to create the acidic environment required for Z-CO2 generation in the cathode compartment. However, in this configuration, the anolyte becomes more acidic over time because more water molecules are transported per 1 mol of proton migrated from the anode to the cathode compartment (i.e., electro-osmotic drag). This increase in acidity leads to an increase in cell voltage over time. This situation prompted us to test neutral anolytes with a CEM. The relatively lower pH of the neutral medium decreases the thermodynamic potential for OER.

[0042] We first performed two-electrode galvanostatic electrolysis in a two- compartment H-cell 200 using 3 M KHCO3 for both the anolyte and catholyte solutions. The IrCh / C anode 202 was compressed against a Nafion™ N117 CEM 204 using a hydraulic press prior to H-cell 200 assembly to simulate the zero-gap environment in a bicarbonate electrolyzer as shown in FIG. 2A. Porous carbon paper was used as the cathode 206. A stream of N2 gas was passed through the headspace of the cathode compartment at 80 seem to transport the CO2 product from the cathode chamber to the gas chromatograph (GC). We initially applied no current to measure the amount of CO2 degassed from the catholyte by purging the stream of N2 gas into the cathode compartment. We observed that 1.5 seem of CO2 was released from the catholyte by purging the N2 gas to the cathode compartment. Next, we applied a current of 100 mA to investigate whether CO2 could be generated electrochemically. Under this current, we observed the formation of CO2 bubbles 226 at the surface of the CEM facing the cathode compartment (FIG. 2B). A GC analysis confirmed an increase in CO2 flow rate as shown with chart 250 of FIG. 2C. Further increasing the current to 200 mA resulted in an additional increase in CO2 flow rate. These results indicate that the CO2 was generated electrochemically in the H- cell.

[0043] We then performed a control experiment where the IrCE / C anode was separated from the CEM by placing the IrCE / C anode in the anode compartment. All other components and experimental conditions were kept the same as the previous H-cell experiment. CO2 bubbles did not form at the surface of the CEM when the anode and the membrane were separated from each other. This experiment signaled to us that it is only when the anode is in physical contact with the CEM that the protons are transferred from the anode to the cathode compartment.

[0044] With confirmation that CO2 could be sourced from an anode immersed in a neutral anolyte, we designed and built a zero-gap flow reactor 300, shown in FIG. 3A, for electrolyzing 3 M KHCO3 solutions. This solution was supplied to both the anolyte and catholyte from the same reservoir 302. This configuration enables the anolyte and catholyte solutions to be mixed after being passed through the flow cell, thereby rebalancing the ionic concentrations of each electrolyte prior to entering the electrolyzer again. The electrolyzer 300 included of grade 3 titanium anode and cathode flow plates with serpentine patterns sandwichingthe membrane electrode assembly (MEA). The use of titanium helps reduce the risk of corrosion by the 3 M KHCO3 solution. The MEA included iridium oxide nanoparticles spray-coated on a carbon paper with a microporous layer (anode) and silver nanoparticles spray-coated on a carbon paper (cathode) sandwiching a Nafion™ NR211 CEM. Nafion™ NR211 was selected because it is relatively thin (25.4 pm), which helps minimize the membrane Ohmic resistance and reduce the cell voltage. The geometric surface area of the anode and cathode were both 4 cm2. A 3 M KHCO3 solution was delivered to both cathode and anode flow plates at 100 mL min-1. The reservoir was sparged with 175 seem of N2 gas to purge gaseous products from the electrolyzer into a GC. The GC andJH NMR were each used to determine the Faradaic efficiencies of gaseous and liquid products, respectively, at various current densities. The gaseous product stream was analyzed once after 5 minutes of electrolysis. Liquid products were collected after 20 minutes of electrolysis forJH NMR analysis. All experiments were performed in triplicate, unless otherwise noted. We conducted experiments using an electrolyzer with a BPM to benchmark the performance of an electrolyzer with a CEM. For electrolysis experiments with a BPM, a 1 M KOH solution was fed to the anode compartment and a 3 M KHCO3 solution was delivered to the cathode compartment. Other conditions were kept the same as the electrolysis experiments with a CEM. CO and H2 were the only gaseous reduction products detected by the GC analysis. No significant quantities of liquid products were detected by 'H NMR analysis.

[0045] As shown in Chart 350 of FIG. 3C, the bicarbonate electrolyzer with the CEM yielded an Eceii of 2.9 ± 0.1 V at an applied current density of 100 mA cm2. This value is much lower than the 4.0 ± 0.3 V measured for the same electrolyzer containing the BPM. Both electrolyzers yielded similar FEco values at 100 mA cm2: 65% for the electrolyzer with the BPM, and 61% for the electrolyzer with the CEM. The carbon efficiencies of both electrolyzers were also similar at 100 mA cm2: 49% ± 5% for the CEM; and 47% ± 1% for the BPM.

[0046] Our energy consumption analysis assumed a 90% FEco in the bicarbonate electrolyzer. On an industrial scale, when electrolyzers with large active areas operate at high current densities, significant heat is generated due to resistance (Joule heating). We explored the possibility of utilizing this waste heat to increase the FEco of the bicarbonate electrolyzer reported in this work. We also performed electrolysis experiments at various current densities with the electrolyte temperature set at 50 °C, to better understand how temperatures of anelectrolyzer operating at commercial conditions impacts voltage as shown in chart 350. With an inlet electrolyte temperature of 50 °C, we measured an Eceii of 2.7 V ± 0.1 V at 100 mA cm2. This value is 0.2 V lower than the Eceii measured at room temperature. The FEco value increased from 61% at room temperature to 84% at 50 °C.

[0047] We then evaluated the durability of the bicarbonate electrolyzer containing the CEM over 16 hours at room temperature. These experiments used an electrolyte sparged with 100 seem of CO2 to simulate the CO2 capture process. Over the course of the 16-hr experiment, both the anolyte and catholyte were fed to the electrolyzer from the same reservoir to facilitate the mixing of the anolyte and catholyte solutions. All other experimental conditions were otherwise kept the same as the 5-min electrolysis experiments. The Eceii were measured to be 3.0 ± 0.1 V over the course of the 16-hr experiment. Moreover, the bulk pH of the electrolyte solution and the rate of Z-CO2 formation remained constant at 8 and 5.3 seem, respectively, for the duration of the experiment. The only parameter that changed substantially was FEco and CO2 utilization, which began to decline after 30 min of electrolysis, the same behavior that we observed with nickel as the anode.

[0048] We then set out to find an alternative mechanism for Z-CO2 generation. We hypothesized that CO2 could be drawn from the bicarbonate solution through the physical desorption of gaseous CO2 from the liquid electrolyte (Eq. 3). To test this hypothesis, we performed a series of experiments 400, graphically represented in FIG. 4A, without any applied electrical bias. We first flowed N2 at a rate of 175 seem through the headspace of a vessel containing 3 M KHCO3 catholyte and used GC to measure a relative [Z-CO2] of 7900 ppm exiting the vessel. We then performed a similar experiment, but instead circulated the 3 M KHCO3 electrolyte through the cathode compartment of a bicarbonate electrolyzer without any applied electrical bias. The [Z-CO2] was measured to be 13000 ppm in this case. This increase from 7900 ppm to 13000 ppm is consistent with the MEA increasing the generation of Z-CO2. When an external bias was applied to the electrolyzer, the amount of [i-CCE] exiting the cathode compartment increased substantially. These findings are illustrated in chart 450 of FIG. 4B.HCO3(aq) CO2(g) + OH (aq) (Eq. 3)

[0049] To produce syngas, bicarbonate electrolyzers need to operate at cell voltages (Eceii) less than 3.1 V (FEco = 90%, carbon utilization = 50%) to achieve comparableenergy efficiencies to the RWGS Pathway. We show here how to meet these performance targets by designing a bicarbonate electrolyzer that utilizes a nearly pH neutral anolyte (i.e., 3 M KHCO3) to reduce the thermodynamic potential for the OER. The anodic OER proceeds through the coupling of (i) hydroxide ions at high pH (“alkaline OER”; Eq. 4) or (ii) water at low pH (“acidic OER”; Eq. 5). At near neutral value, the pH at an anode surface can be as much as 5 units lower than the bulk solution because of rapid OH depletion. Thus, the OER is better expressed by the consumption of H2O (Eq. 5) rather than OH (Eq. 4). Consequently, the chemical products that form at the anode at high current densities should be O2(g) and H+. We, therefore, initially hypothesized that these locally generated H+at the acidic anode surface were migrating to the cathode and forming CO2 upon reacting with HCO3 . However, when we performed sustained electrolysis with separate anolyte and catholyte chambers, we observed that salt crystals began to precipitate in the catholyte compartment just as we observed with the KOH anolyte. This observation indicates that even when a neutral anolyte is employed, K+migration from the anode dominates over H+migration.40H (aq) — 02(g) + 2H2O(i) + 4e (OER under alkaline conditions) E° = 0.40 V vs. SHE (Eq. 4) 2H2O(i) — 02(g) + 4H+(aq) + 4e (OER under acidic conditions) E° = 1.23 V vs. SHE (Eq. 5)

[0050] Moreover, if the CO2 at the cathode was formed exclusively from the reaction of HCO3 with H+from the CEM, we would expect comparable Z-CO2 production between the electrolyzer with the CEM and the one with the BPM during sustained electrolysis. However, during sustained electrolysis runs without the additional supply of CO2, we observed a more pronounced decline in Z-CO2 generation over time when using the CEM compared to the BPM. We also ruled out that this difference in Z-CO2 generation was due to different rates of hydrogen evolution reaction (HER; eqs. 6 & 7) by performing galvanostatic electrolysis experiments at 100 mA cm2using catholyte containing dodecyltrimethylammonium bromide (DTAB), a cationic surfactant known to suppress HER. These experiments with varying concentrations of DTAB did not yield meaningful differences in [Z-CO2] when using the CEM or BPM. The similar rates of HER using both membranes are likely because the local pH at the cathode of a bicarbonate electrolyzer is alkaline. Intermediate steps in the HER therefore do not involve H+generation or consumption (Eq. 7) and consequently would not impact Z-CO2generation. These observations collectively show that the Z-CO2 generation in the bicarbonate electrolyzer with the CEM is not due to the bicarbonate equilibrium (Eq. 2).2H+(aq) + 2e- — Eh(g) (HER under acidic conditions) (Eq. 6)2H2O(i) + 2e- — H2(g) + 2OH (aq) (HER under alkaline conditions) (Eq. 7)

[0051] After ruling out the other factors, we converged on the so-called “saltingout effect” as a key factor in generating Z-CO2. This effect describes a scenario where a high local concentration of salts in aqueous media reduces the solubility of gasses in water. There are two key experimental observations that point to the salting-out effect impacting Z-CO2 generation in the electrolyzer containing the CEM: (i) the rate of Z-CO2 generation increased with increasing current density; and (ii) the formation of salt crystals were observed within the catholyte reservoir when electrolysis was performed without mixing the anolyte and catholyte. Moreover, a [K+] concentration as high as 6 M at 200 mA cm2is expected. These collective observations suggest that the salting-out effect governs Z-CO2 formation. We therefore conclude that CC g) desorbed from the bicarbonate solution at the membrane|cathode interface increases with increasing concentrations of K+.

[0052] While the salting-out effect is often unfavorable in electrochemical processes, it is actually advantageous to our system because it make more Z-CO2 available for conversion into product.

[0053] The bicarbonate electrolyzer containing a CEM, and the mixing of the anolyte and catholyte for each cycle, was capable of sustained electrolysis over a 16-hour period with a consistent Eceii of 3.0 V at 100 mA cm2. This experiment teaches that the new bicarbonate electrolyzer configuration addresses K+depletion in the anode chamber. The Eceii value was maintained throughout the experiment, which supports the K+ions being replenished after each circulation.

[0054] Another factor that could increase Z-CO2 generation is a pressure-drop- induced desorption of CO2 from the electrolyte. This phenomenon involves a pressure drop within the porous MEA that increases the production of Z-CO2 at the cathode surface and, in turn, the production of CO (FIG. 4 A). An independent set of experiments where 175 seem of N2 waspassed through the headspace of a vessel (not connected to an electrolyzer) that contained the 3 M KHCO3 catholyte showed an Z-CO2 concentration of 7900 ppm (FIGS. 4A-4B). This Z-CO2 concentration increased to 13000 ppm when the catholyte was circulated through the porous cathode of the electrolyzer (without an applied potential) and back to the reservoir (FIG. 4A). We interpret this result to track with the physical desorption of CO2 from HCO3 increasing when 3 M KHCO3 electrolyte is transported through the porous electrode, consistent with a pressure drop as the electrolyte is transported through the pores. Put differently, this pressure drop causes CO2 generation by shifting the bicarbonate equilibrium towards CO2 (Eq. 2). The [z- CO2] increased to 35000 ppm when 100 mA cm2of current was applied to the electrolyzer (FIG. 4B).

[0055] The positive electrolysis results withstanding, we observed a marked decrease in FEco after 30 min of electrolysis, despite sustained Z-CO2 formation rates. We attribute the decrease in FEco to the instability of the anode material or oxidation of the anode flow plate, which would diminish CO2 utilization efficiency. To indirectly test this hypothesis, we performed X-ray fluorescence (XRF) analysis on both the pre- and post-electrolysis Ag cathode, which had been spray-coated onto a carbon paper substrate. The XRF analysis indicates that in both cases, where either Ni or IrOxwas used as the anode, the anode material and Ti from the flow plate were oxidized and electrodeposited onto the cathode. To further support the hypothesis, we conducted a 2-hr bicarbonate electrolysis experiment in a zero-gap electrolyzer with a CEM at 100 mA cm2using a CCE-sparged cathode feed, but without mixing the 3 M KHCO3 anolyte and 3 M KHCO3 catholyte, and we observed FEco values that remained steady for the duration of the experiment.

[0056] On a final note, we employed a fully saturated bicarbonate solution (e.g., 3 M KHCO3) as the electrolyte to simulate the outlet conditions of an alkaline solvent-based carbon capture process. However, it is important to recognize that under realistic settings, the process outlet is expected to comprise a mixture of bicarbonate and carbonate in varying ratios influenced by the pH. We have previously demonstrated that at near-neutral pH, where dissolved inorganic carbon is predominantly bicarbonate, there is a preference for CO formation. Nevertheless, it is crucial to note that alternative studies have indicated that carbonates can also be efficiently converted into CO through the use of specially designed cathode materials.

[0057] FIG. 5 illustrates a block diagram of an industrial system 500 that can use the electrolyzer arrangements described throughout this disclosure. The system 500 includes a carbon capture tower 502 that acts as a source of enriched carbonate or bicarbonate solution 504. The tower 502 enriches the carbonate or bicarbonate solution by passing air, which includes CO2, across un-enriched carbonate or bicarbonate solution 506. In some implementations, such a tower receives the un-enriched carbonate of bicarbonate solution 506 at or near an upper end of the tower 502, and allows the un-enriched carbonate of bicarbonate solution 506 to flow across contactors, such as fins, packing, weirs, or louvers to produce enriched carbonate of bicarbonate solution to be used by the electrolyzer 508. In some implementations, a circulation pump 510 is used to direct the enriched carbonate or bicarbonate solution 504 and / or un-enriched carbonate or bicarbonate solution 506 through the system 500. While illustrated as a single pump between an outlet of the tower 502 and inlets of the electrolyzer 508, multiple pumps 510 can be used without departing from this disclosure. Similarly, one or more pumps 510 can be located in various parts of the system 500 without departing from this disclosure.

[0058] The enriched carbonate or bicarbonate solution 504 is then directed into both an anode section 512 of the electrolyzer 508 and the cathode section 514 of the electrolyzer 508. Note, the electrolyzer 508 is substantially similar to the other electrolyzers described throughout this disclosure, including using a CEM 516 between the anode section 512 and the cathode section 514. In some implementations, the CEM 516 is the only selectively permeable membrane separating the anode section 512 and the cathode section 514. Within the electrolyzer 508, the enriched carbonate or bicarbonate solution 504 is turned into products 518.

[0059] The outlet of the anode section 512 and the cathode section 514 each direct their respective products 518 into an anode separator 520 and a cathode separator 522. Both the anode separator 520 and the cathode separator 522 are sized to provide sufficient retention times, temperatures, and other parameters that allow for the products to be separated into gas and liquid products. In some implementations, gas products from the anode separator 520 include O2, which can be vented to the atmosphere or captured to be compressed and used. In some implementations, gas products from the cathode separator 522 include CO, H2, and / or CO2. These products, in some implementations, are then captured and used as syngas in a Fischer-Tropsch process.

[0060] The liquid products produced by both the anode separator 520 and the anode separator 522 include un-enriched carbonate or bicarbonate solutions 506 that are recirculated back to the tower to be enriched again.

[0062] While this disclosure contains many specific implementation details, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features specific to particular implementations of particular inventions. Certain features that are described in this disclosure in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

[0063] Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described components and systems can generally be integrated together in a single product or packaged into multiple products.

[0064] Thus, particular implementations of the subject matter have been described. Other implementations are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. I n addition, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results.

[0065] Other implementations can be within the scope of the following claims.

Claims

What is claimed:

1. An electrolyzer cell comprising: a selectively permeable membrane; an anode on a first side of the selectively permeable membrane; a cathode on a second side of the selective permeable membrane; an anode housing defining an anode fluid chamber configured to be filled with a carbonate or bicarbonate solution; and a cathode housing defining a cathode fluid chamber configured to be filled with the carbonate or bicarbonate solution.

2. The electrolyzer cell of claim 1, wherein the anode abuts the selectively permeable membrane.

3. The electrolyzer cell of claim 1 or 2, wherein the cathode abuts the selectively permeable membrane.

4. The electrolyzer cell of any one of claims 1 to 3, wherein the selectively permeable membrane is a cation exchange membrane.

5. The electrolyzer cell of any one of claims 1 to 4, wherein the anode housing further defines an anode inlet and an anode outlet, the anode inlet configured to receive the carbonate or bicarbonate solution, wherein the anode housing is configured to direct a flow of the carbonate or bicarbonate solution across the anode, wherein the anode outlet is configured to emit the carbonate or bicarbonate solution from the anode housing.

6. The electrolyzer cell of any one of claims 1 to 5, wherein the cathode housing further defines a cathode inlet and a cathode outlet, the cathode inlet configured to receive the carbonate or bicarbonate solution, wherein the cathode housing is configured to direct a flow of the carbonate or bicarbonate solution across the cathode, wherein the cathodeoutlet is configured to emit the carbonate or bicarbonate solution from the cathode housing.

7. The electrolyzer cell of any one of claims 1 to 6, wherein the cathode comprises Ag or C.

8. The electrolyzer cell of any one of claims 1 to 7, wherein the anode comprises Ir, O, or C.

9. A system comprising: an electrolyzer cell configured to receive a carbonate or bicarbonate solution, by an anode section and a cathode section, the electrolyzer configured to produce one or more products, by a current exchanged between the anode section and the cathode section, using the received carbonate or bicarbonate solution; a source from which the carbonate or bicarbonate solution originates; and a pump arranged to circulate the carbonate or bicarbonate solution between the electrolyzer cell and the source, the carbonate or bicarbonate solution being circulated to both the anode section and the cathode section.

10. The system of claim 9, wherein the source comprises: a carbon capture tower fluidically coupled to the reservoir; and a second pump circulating the carbonate or bicarbonate solution between the carbon capture tower and the reservoir.

11. The system of claim 9 or 10, further comprising a direct current power supply coupled to the anode section and the cathode section, the direct current power supply configured to provide sufficient current and voltage to produce products from the carbonate or bicarbonate solution.

12. The system of claim 11, wherein the power supply provides three volts.

13. The system of any one of claims 9 to 12, further comprising a separator downstream of the cathode section, the separator configured to separate gas from liquid.

14. A method comprising: receiving a carbonate or bicarbonate solution by an anode section of an electrolyzer cell; receiving the carbonate or bicarbonate solution by a cathode section of the electrolyzer cell, the anode section being separated from the cathode section by a selectively permeable membrane; and forming, by a current exchanged between the anode section and the cathode section, one or more products.

15. The method of claim 14, further comprising recirculating the carbonate or bicarbonate solution from an outlet of the anode section and an outlet of the cathode section, through a common reservoir, and back to the anode section and the cathode section.

16. The method of claim 14 or 15, wherein the selectively permeable membrane is a cation exchange membrane.

17. The method of any one of claims 14 to 16, wherein the one or more products comprise O2 formed by the anode section.

18. The method of any one of claims 14 to 17, wherein the one or more products comprise CO, H2, and CO2 formed by the cathode section.

19. The method of any one of claims 14 to 18, wherein the one or more products comprise liquid products.

20. The method of any one of claims 14 to 19, wherein the one or more products comprise gas products. 1

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