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  • C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
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Definitions

• the present techniques generally relate to self-cleaning of a CO 2 reduction system, and more particularly to a self-cleaning system and methods involving application of an unsteady electrochemical forcing.
• CO 2 carbon dioxide
• Gas diffusion electrodes facilitate effective CO 2 mass transport to the cathode catalyst (figure 1), enabling electrolyzers to operate at the current densities required for industrial deployment, e.g., in excess of 100 mA cm -2 .
• Alkali metal cations typically potassium, are implemented broadly in aqueous electrolytes to reduce ohmic losses and improve the CO 2 RR current density and selectivity.
• Performing CO 2 electrolysis at high current densities inevitably produces large quantities of hydroxide ions on the cathode, driving up the local pH and thus encouraging the chemical reaction of dissolved CO 2 with these hydroxide ions to produce bicarbonate ions on route to carbonate ions (figure 2), as mentioned in the studies of Lu X.. et al.
• the present electrochemical techniques address at least some of these challenges to reduce salt formation during conversion of CO 2 into value-added products in comparison to known techniques in the field.
• the present techniques relate to prevention of salt formation by alternating an applied cell voltage between an operational voltage and a lower regeneration voltage.
• the present disclosure relates to a method for reducing CO 2 in an electrolytical system and/or for self-cleaning a gas diffusion electrode in an electrolytical system operating CO 2 reduction, the method comprising: providing an electrolytical system; applying an operational voltage to the electrolytical system to operate CO 2 reduction for a first period of time defining an operation cycle, thereby forming carbonate ions at a cathode side of the electrolytical system and having a local carbonate ion concentration; and subsequently applying a regeneration voltage to the electrolytical system for a second period of time defining a regeneration cycle to force electromigration of the formed carbonate ions to an anode side of the electrolytical system; remarkable in that the regeneration voltage is lower than the operational voltage.
• the present disclosure relates to a method for self-cleaning a gas diffusion electrode in an electrolytical system operating CO 2 reduction, the method comprising: providing an electrolytical system; applying an operational voltage to the electrolytical system to operate CO 2 reduction for a first period of time defining an operation cycle, thereby forming carbonate ions at a cathode side of the electrolytical system and having a local carbonate ion concentration; and subsequently applying a regeneration voltage to the electrolytical system for a second period of time defining a regeneration cycle to force electromigration of the formed carbonate ions to an anode side of the electrolytical system; remarkable in that the regeneration voltage is lower than the operational voltage.
• the regeneration voltage is more negative than the operational voltage.
• the duration of the operation cycle is chosen to maintain the local carbonate ion concentration at the cathode side below a carbonate salt solubility limit.
• the local carbonate ion concentration being determined by solubility calculation, for example via computer simulation (e.g COMSOL).
• the duration of the operation cycle can be chosen to maintain the local carbonate ion concentration at the cathode side below a carbonate salt solubility limit.
• the first period of time is between 1 second and 1200 seconds, preferably between 60 seconds and 300 seconds.
• the duration of the regeneration cycle can be chosen to reduce the local carbonate ion concentration at the cathode side by at least 80 % via electromigration to the anode side.
• the duration of the regeneration cycle can be chosen to reduce the local carbonate ion concentration at the cathode side by at least 90 % via electromigration to the anode side.
• the duration of the regeneration cycle can be chosen to reduce the local carbonate ion concentration at the cathode side by at least 99 % via electromigration to the anode side.
• the second period of time is between 1 second and 60 seconds, preferably between 30 seconds and 60 seconds.
• said method further comprises repeating the operation cycle and the regeneration cycle by alternating a voltage applied to the electrolytic system between the operational voltage and the lower regeneration voltage.
• each operation cycle is performed for the same duration and/or each regeneration cycle is performed for the same duration.
• the duration of each operation cycle varies between 1 second and 1200 seconds, preferably between 60 seconds and 300 seconds.
• the duration of each regeneration cycle varies between 1 second and 60 seconds, preferably between 30 seconds and 60 seconds.
• the regeneration voltage is chosen to obtain a CO 2 reduction rate below 1 mA.cm "
• the operational voltage is between -3.0 and - 4.5 V, preferably between -3.2 and - 4.0 V.
• the operational voltage is -3.6 V.
• the regeneration voltage is between - 2.5 V and -5.0 V, or between -2.5V and - 4.0V, preferably between - 2.1 V and -3.5 V.
• the regeneration voltage is -2.0 V.
• the electrolytical system is a membrane electrode assembly (MEA) comprising a gas diffusion electrode serving as a cathode.
• the electrolytical system is a flow cell system comprising a liquid catholyte and a gas diffusion electrode serving as a cathode.
• the cathode comprises a metal layer deposited on substrate, for example a carbon paper substrate or a PTFE substrate.
• the cathode comprises a silver layer deposited on a carbon paper substrate and/or the cathode comprises a copper layer deposited on a PTFE substrate.
• the electrolytical system comprises an anolyte.
• the anolyte is an aqueous solution of one or more alkaline compounds, said one or more alkaline compounds comprising one alkali metal cations selected from lithium, sodium, potassium, rubidium, cesium and any combination thereof.
• the present disclosure relates to the use of the method according to the first aspect in a an electrolytical system comprising a gas diffusion electrode wherein at an applied cell voltage carbonate ions are formed when the electrolytical system is operating CO 2 reduction; wherein the use comprises self-cleaning the gas diffusion electrode
• the present disclosure relates to a self-cleaning electrolytical system for CO 2 reduction into C2 products, the electrolytical system comprising: a cathode; an anode; an electrolyte; an ion-exchange membrane separating the anode and cathode; an electrical energy source applying a voltage to the electrolytical system; the self-cleaning electrolytic system is remarkable in that it further comprises a controller in operative communication with the electrical energy source to alternate the applied voltage between an operational voltage and a lower regeneration voltage, thereby imposing an operation cycle in alternate with a regeneration cycle.
• the controller is a control amplifier that is programmed or manually actuated.
• the control amplifier and the electrical energy source are combined in a potentiostat.
• a method for reducing CO 2 in an electrolytical system comprises: applying an operational voltage to the electrolytical system to operate CO 2 reduction for a first period of time defining an operation cycle, thereby forming carbonate ions at a cathode side of the electrolytical system and having a local carbonate ion concentration; and subsequently applying a regeneration voltage to the electrolytical system for a second period of time defining a regeneration cycle to force electromigration of the formed carbonate ions to an anode side of the electrolytical system; wherein the regeneration voltage is lower than the operational voltage.
• the duration of the operation cycle can be chosen to maintain the local carbonate ion concentration at the cathode side below a carbonate salt solubility limit.
• the duration of the regeneration cycle can be chosen to reduce the local carbonate ion concentration at the cathode side by at least 80 % via electromigration to the anode side.
• the duration of the regeneration cycle can be chosen to reduce the local carbonate ion concentration at the cathode side by at least 90 % via electromigration to the anode side.
• the duration of the regeneration cycle can be chosen to reduce the local carbonate ion concentration at the cathode side by at least 99 % via electromigration to the anode side.
• the first period of time can be between 60 seconds and 300 seconds.
• the second period of time can be between 30 seconds and 60 seconds.
• the method can further comprise repeating the operation cycle and the regeneration cycle by alternating a voltage applied to the electrolytic system between the operational voltage and the lower regeneration voltage.
• the duration of each regeneration cycle can be chosen to sufficiently reduce the local carbonate ion concentration at the cathode side to remain under the carbonate salt solubility limit during a subsequent operation cycle.
• each operation cycle can be performed for the same duration.
• each regeneration cycle can be performed for the same duration.
• the duration of each operation cycle can vary between 60 seconds and 300 seconds.
• the duration of each regeneration cycle can vary between 30 seconds and 60 seconds.
• the number of operation cycles can be chosen to operate CO 2 reduction during at least 150 hours, while maintaining a CO 2 RR selectivity towards C2 products of at least 80 %.
• a total duration of all operation cycles and regeneration cycles can be 236 hours for an operation duration of 157 hours.
• the regeneration voltage can be chosen to obtain a CO 2 reduction rate below 1 mA.cm -2 .
• the operational voltage can be between -3.0 and - 4.5 V.
• the operational voltage can be -3.6 V.
• the regeneration voltage can be between - 2.5 V and -5.0 V.
• the regeneration voltage can be -2.0 V.
• the electrolytical system can be a membrane electrode assembly (MEA) comprising a gas diffusion electrode serving as a cathode.
• MEA membrane electrode assembly
• the electrolytical system can be a flow cell system comprising a liquid catholyte and a gas diffusion electrode serving as a cathode.
• the cathode can include a metal layer deposited on a substrate, for example a carbon paper substrate or a PTFE substrate.
• the cathode can include a copper layer deposited on a PTFE substrate. In other implementations, the cathode can include a silver layer deposited on a carbon paper substrate.
• the electrolytical system can include an electrolyte liberating alkali metal cations that form alkali metal carbonate salts with the carbonate ions, when above the corresponding carbonate salt solubility limit.
• the alkali metal cations can include lithium, sodium, potassium, rubidium, caesium ions, or any combinations thereof.
• a method for self-cleaning a gas diffusion electrode in an electrolytical cell operating CO 2 reduction at an applied cell voltage and forming carbonate ions including alternating the applied cell voltage between an operational voltage and a lower regeneration voltage.
• alternating the applied cell voltage between the operational voltage and the lower regeneration voltage can include applying the operational voltage for an operation duration maintaining a local carbonate ion concentration at the gas diffusion electrode below a carbonate salt solubility limit.
• the operation duration can be at most 1200 seconds. In another example, the operation duration can be between 60 seconds and 300 seconds.
• alternating the applied cell voltage between the operational voltage and the lower regeneration voltage can include applying the regeneration voltage for a regeneration duration that results in electromigration of at least 80 % of the carbonate ions that are formed at the gas diffusion electrode.
• alternating the applied cell voltage between the operational voltage and the lower regeneration voltage can include applying the regeneration voltage for a regeneration duration that results in electromigration of at least 90 % of the carbonate ions that are formed at the gas diffusion electrode.
• alternating the applied cell voltage between the operational voltage and the lower regeneration voltage can include applying the regeneration voltage for a regeneration duration that results in electromigration of at least 99 % of the carbonate ions that are formed at the gas diffusion electrode.
• alternating the applied cell voltage between the operational voltage and the lower regeneration voltage can include applying the regeneration voltage for a regeneration duration that results in the removal of an amount of carbonate ions allowing remaining under a carbonate salt solubility limit during the subsequent application of the operational voltage.
• the regeneration duration is at most 60 seconds. In another example, the regeneration duration can be between 30 seconds and 60 seconds.
• alternating the applied cell voltage between the operational voltage and the lower regeneration voltage can be performed during 236 hours comprising a total operation duration of 157 hours, while maintaining a CO 2 RR selectivity towards C 2 products of at least 80%.
• the regeneration voltage can be chosen to obtain a CO 2 reduction rate below 1 mA.cm -2 .
• the operational voltage can be between -3.0 and - 4.5 V.
• the operational voltage can be -3.6 V.
• the regeneration voltage can be between - 2.5 V and - 5.0 V.
• the regeneration voltage can be -2.0 V.
• the gas diffusion electrode can serve as a cathode in a membrane electrode assembly (MEA). In other implementations, the gas diffusion electrode can serve as a cathode in a flow cell system.
• MEA membrane electrode assembly
• the gas diffusion electrode can include a silver layer deposited on a carbon paper substrate. In other implementations, the gas diffusion electrode can include a copper layer deposited on a PTFE substrate.
• the electrolytical cell can include an electrolyte liberating alkali metal cations that form alkali metal carbonate salts with the carbonate ions, when above a corresponding carbonate salt solubility limit.
• the alkali metal cations can include lithium, sodium, potassium, rubidium, caesium ions, or any combinations thereof.
• a self-cleaning electrolytical system for CO 2 reduction into C2 products.
• the electrolytical system comprises: a cathode; an anode; an electrolyte; an ion-exchange membrane separating the anode and cathode; an electrical energy source applying a voltage to the electrolytical system; and a controller in operative communication with the electrical energy source to alternate the applied voltage between an operational voltage and a lower regeneration voltage, thereby imposing an operation cycle in alternate with a regeneration cycle.
• the controller can be configured to apply the operational voltage via the electrical energy source for a duration that maintains a local carbonate ion concentration at a cathode side of the system below a carbonate salt solubility limit.
• the controller can be configured to apply the regeneration voltage via the electrical energy source until at least 80 % of carbonate ions that are formed at the cathode cross the ion-exchange membrane.
• the controller can be configured to apply the regeneration voltage via the electrical energy source until at least 80 % of carbonate ions that are formed at the cathode cross the ion-exchange membrane.
• the controller can be configured to apply the regeneration voltage via the electrical energy source until at least 99 % of carbonate ions that are formed at the cathode cross the ion-exchange membrane.
• the controller can be configured to maintain the regeneration voltage during each regeneration cycle to remove an amount of carbonate ions from the cathode side that is sufficient to remain under a carbonate salt solubility limit during the subsequent operation cycle.
• the controller can be configured to maintain each operational cycle for at most 1200 seconds, or between 60 seconds and 1200 seconds.
• the controller can be configured to maintain each regeneration cycle for at most 60 seconds, or between 30 seconds and 60 seconds.
• the controller can be configured to perform each operation cycle for the same duration. In some implementations, the controller can be configured to perform each regeneration cycle for the same duration.
• the controller can be configured to perform a number of operation cycles that allow CO 2 reduction during at least 150 hours, while maintaining a CO 2 RR selectivity towards C 2 products of at least 80 %.
• a total duration of all operation cycles and regeneration cycles can be 236 hours for an operation duration of 157 hours.
• the regeneration voltage can be chosen to obtain a CO 2 reduction rate below 1 mA.cm -2 .
• the operational voltage can be between -3.0 and - 4.5 V.
• the operational voltage can be -3.6 V.
• the regeneration voltage can be between - 2.5 V and - 5.0 V.
• the regeneration voltage can be -2.0 V.
• the electrolytical system can be a membrane electrode assembly (MEA) comprising a gas diffusion electrode serving as the cathode.
• MEA membrane electrode assembly
• the electrolytical system can be a flow cell system comprising a gas diffusion electrode serving as the cathode, wherein the electrolyte is a catholyte and the system further comprises an anolyte in which the anode is immersed.
• the cathode can include a silver layer deposited on a carbon paper substrate. In other implementations, the cathode can include a copper layer deposited on a PTFE substrate.
• the controller can be a control amplifier that is programmed or manually actuated.
• the control amplifier and the electrical energy source can be combined in a potentiostat.
• the electrolyte can comprise alkali metal cations that form alkali metal carbonate salts with the carbonate ions, when above a corresponding carbonate salt solubility limit.
• the alkali metal cations can include lithium, sodium, potassium, rubidium, caesium ions, or any combinations thereof.
• Figure 1 Schematic of the MEA CO 2 electrolyzer.
• Figure 2 CO 2 conversion to bicarbonate and carbonate during regular electrolyzer operation.
• Figure 3 Carbonate migration during cell operation at the regeneration voltage.
• Figure 4 Strategy to mitigate carbonate formation by cycling between operational and regeneration cell voltages.
• Figure 5 COMSOL Multiphysics simulation of pH for different operational times with -3.8 V continuous operation. Salt crystal formation is predicted where salt concentrations in the model exceed the solubility limit (indicated by the dashed line).
• Figure 6 COMSOL Multiphysics simulation of CO 2 concentration for different operational times with -3.8 V continuous operation. Salt crystal formation is predicted where salt concentrations in the model exceed the solubility limit (indicated by the dashed line).
• Figure 7 COMSOL Multiphysics simulation of HCO 3 - concentration for different operational times with -3.8 V continuous operation. Salt crystal formation is predicted where salt concentrations in the model exceed the solubility limit (indicated by the dashed line).
• Figure 8 COMSOL Multiphysics simulation of K + concentration for different operational times with -3.8 V continuous operation. Salt crystal formation is predicted where salt concentrations in the model exceed the solubility limit (indicated by the dashed line).
• Figure 12 COMSOL Multiphysics simulation of pH for different total times when applying the alternating voltage strategy (periodic 60 s of operating and 30 s of regeneration voltage).
• Figure 13 COMSOL Multiphysics simulation of C0 2 concentration for different total times when applying the alternating voltage strategy (periodic 60 s of operating and 30 s of regeneration voltage).
• Figure 14 COMSOL Multiphysics simulation of HCO 3 - concentration for different total times when applying the alternating voltage strategy (periodic 60 s of operating and 30 s of regeneration voltage).
• Figure 15 Carbonate concentrations within the MEA at different operational times for continuous operation at -3.8 V (current density of 172 mA cm -2 ).
• Figure 17 Current density of the different regeneration voltage (cyclic -3.8 V operational voltage for 60 s and regeneration voltage for 30 s).
• Figure 18 The net carbonate ion growth rate averaged during the first 60s of simulated operation at -3.8 V.
• the solid red line is the generation rate;
• the solid blue line is rate of species transport, including convection, diffusion and electromigration;
• the solid black line is the difference between the generation and reduction lines thereby describing the net change of carbonate ion concentration.
• Figure 19 Carbonate concentrations within the MEA and comparison of electromigrative and concentration-driven diffusive effects.
• Figure 20 COMSOL Multiphysics simulation of hydroxide concentration for different total times when applying the alternating voltage strategy (periodic 60 seconds of operating and 10 seconds of regeneration voltage, periodic 60 seconds of operating and 20 seconds of regeneration voltage).
• Figure 21 COMSOL Multiphysics simulation of carbonate concentration for different total times when applying the alternating voltage strategy: periodic 60 seconds of operating and 10 seconds of regeneration voltage.
• Figure 22 COMSOL Multiphysics simulation of carbonate concentration for different total times when applying the alternating voltage strategy: periodic 60 seconds of operating and 20 seconds of regeneration voltage.
• Figure 23 Electrochemical performance of silver catalyst on carbon paper: stability of continuously operated sample at -3.6 V.
• Electrochemical performance of silver catalyst on carbon paper stability of alternating operation sample (60 seconds at operational voltage and 30 seconds at regeneration voltage of - 2.0 V).
• FIG. 25 Electrochemical performance of silver catalyst on carbon paper: selectivity of alternating operation sample at different operational voltages.
• Figure 26 Electrochemical performance of silver catalyst on carbon paper: selectivity of continuous operation sample at different operational voltages.
• Figure 27 Electrochemical performance of silver catalyst on carbon paper: stability of continuously operated sample at -3.4 V.
• FIG. 28 Electrochemical performance of silver catalyst on carbon paper: stability of alternating operation sample (60 seconds at the operational voltage of -3.6 V and 30 seconds at regeneration voltage of -2.0 V) that has the same average current density with -3.4 V continuously operated test.
• Figure 29 Back side of the copper on PTFE electrode after continuous operation at -3.8 V during long-term operation.
• FIG. 30 Electrochemical performance of copper catalyst on PTFE electrode: selectivity of continuously operated sample at -3.8 V during long-term operation.
• Figure 31 Electrochemical performance of copper catalyst on PTFE electrode: current density of continuous operation during long-term operation.
• Figure 32 Raman analysis of the solid phase salt precipitates taken from the continuously operated copper on PTFE electrode.
• Figure 33 Electrochemical performance of silver catalyst on PTFE electrode: selectivity and current density of continuous operation during long-term operation at -3.8 V.
• Figure 34 Electrochemical performance of silver catalyst on PTFE electrode: post-experiment CO 2 gas stream cathode channel.
• FIG. 35 Electrochemical performance of silver catalyst on PTFE electrode: backside of the post- experiment PTFE electrode sample.
• Figure 36 Back side of the copper on PTFE electrode after alternating operation (60 seconds at operational voltage of -3.8 V and 30 seconds at regeneration voltage of -2.0 V).
• FIG. 37 Electrochemical performance of copper catalyst on PTFE electrode: selectivity of alternating operation sample (60 seconds at operational voltage of -3.8 V and 30 seconds at regeneration voltage of -2.0 V) during long-term operation.
• FIG. 38 Electrochemical performance of copper catalyst on PTFE electrode: current density of alternating operation sample during long-term operation.
• FIG 39 Electrochemical performance of copper catalyst on PTFE electrode: magnified early view of current density and late view of current density.
• Figure 40 Ex-situ X-ray photoelectron spectroscopy characterization of a copper on PTFE electrode before electrolysis. Copper (I) oxide, copper (II) oxide, and metallic copper were detected, suggesting that the sputtered copper catalyst was oxidized in ambient air prior to the experiment.
• Figure 41 Ex-situ X-ray photoelectron spectroscopy characterization of a copper on PTFE electrode after electrolysis. The copper catalyst was predominantly in metallic form, suggesting that the copper (I) oxide and copper (II) oxide were reduced to metallic copper at the beginning of operation. The small amount of copper (I) oxide was likely caused by oxidation during reactor disassembly and transport.
• Figure 42 Electrochemical performance of copper catalyst on PTFE electrode: selectivity of alternating operation sample at different operational voltages.
• FIG. 43 Electrochemical performance of copper catalyst on PTFE electrode: selectivity of continuous operation sample at different operational voltages.
• Figure 47 Carbonate concentrations within the MEA: different total times when applying the alternating voltage strategy (periodic 60 seconds of operational voltage and 30 seconds of regeneration voltage). Salt crystal formation is predicted where salt concentrations in the model exceed the solubility limit (indicated by the dashed line).
• the present techniques relate to self-cleaning of a gas diffusion electrode in an electrolytical cell operating CO 2 reduction at an applied cell voltage where carbonate ions are formed.
• the selfcleaning techniques involve alternating the applied cell voltage between an operational voltage and a lower regeneration voltage.
• An operational cycle is defined by application of the operational voltage for an operational duration
• the regeneration cycle is defined by application of the regeneration voltage for a regeneration duration. Duration of each operational cycle and regeneration cycle can be tailored to reduce or avoid carbonate salt precipitation at the gas diffusion electrode side (e.g ., cathode side for CO 2 RR) of the electrolytical cell.
• Carbonate ions that are formed at the cathode side during the operational cycle can be transferred to an anode side of the electrolytical cell via electromigration during the subsequent regeneration cycle.
• the carbonate ions are further changed to CO 2 .
• the techniques proposed herein can be referred to as an alternating voltage approach, an alternating approach, an alternating voltage strategy, an alternating strategy or an unsteady electrochemical forcing strategy.
• the present salt formation prevention strategy includes avoiding reaching the steady state conditions.
• the present techniques include varying the applied cell voltage between two values, and more specifically, applying cyclically an operation voltage for an operation duration, and a regeneration voltage for a regeneration duration.
• the resulting regeneration potential lowers the reaction rate to nearly 0 mA cm -2 , eliminating hydroxide formation, while maintaining a sufficiently negative polarization at the cathode to transport carbonate ions to the anode under electromigration (figure 3).
• the applied cell voltage can be varied in a step-like manner between the operational voltage and the regeneration voltage. In other implementations, the applied cell voltage can be gradually varied to reversibly reach the operational voltage or the regeneration voltage.
• CO 2 electrolysis was performed in a membrane electrode assembly (MEA) electrolyzer, using the present alternating voltage approach. A similar product distribution to that of constant voltage operation was obtained, but demonstrated enhanced stability.
• the copper- PTFE electrodes were able to sustain the product distribution when operated alternatively for 157 hours of operation over 236 hours of total duration, as compared to ⁇ 10 hours of operation when the same copper-PTFE electrodes were operated continuously.
• selection of a duration for each operation cycle and regeneration cycle is based on the variation of a local carbonate ion concentration at the cathode side.
• the local carbonate ion concentration can be maintained below the carbonate salt solubility limit during operation. Additionally, the local carbonate ion concentration can be reduced sufficiently (via electromigration), e.g., by at least 80%, during the regeneration cycle to ensure that the local carbonate ion concentration will not reach the carbonate salt solubility limit during a subsequent operation cycle. For example, selecting the duration for each operation cycle and regeneration cycle can include simulating the local carbonate ion concentration variation history for a specific voltage application scenario.
• a cathode was fabricated by spraying a carbon gas diffusion layer with silver nanoparticles on a substrate and carbon monoxide (CO) was produced from CO 2 in a CO 2 RR MEA electrolyzer including the fabricated cathode.
• the anolyte was 0.1 M potassium bicarbonate and the anode was an iridium-based catalyst that was used to perform oxygen. Referring to figure 23, when performing CO 2 RR at a constant operational voltage of -3.6 V, the CO selectivity dropped from 98% to 76% after just 12 hours of operation.
• the system was cyclically operated with the application of the same operational voltage of -3.6 V for an operation duration of 60 seconds, and further application of a regeneration voltage of -2.0 V for a regeneration duration of 30 seconds (figure 24).
• the alternating system was operated for 12 hours (18 hours total duration including 6 hours regeneration).
• the cyclically operated MEA electrolyzer had no visible salt formation and sustained a high CO selectivity. Comparing operational voltages over short time scales, the alternating sample (figure 25, table 2) exhibited similar selectivities and current densities to that of the sample operated continuously (figure 26).
• Table 2 Product distribution for alternating voltage experiments with silver and copper cathodes.
• the test was stopped after 18 hours (total duration) for direct comparison with the continuously operated system.
• An activation voltage refers herein to the voltage required to reach an onset potential for both cathodic and anodic reactions, thereby generating a current density in accordance with an activation energy of the triggered redox event.
• the regeneration voltage is selected to be below the activation voltage, and thus the regeneration period operates at a negligible current density, which is a much lower current density than during the operational period. Therefore, there is minimal additional energy required to power the regeneration period since the regeneration period can consume less than 1% of the system energy requirements (figure 44).
• the alternating system also reduces the addition of new electrolyte salts, new catalyst materials, and catalyst replacement downtimes, combining for a significant operational advantage in comparison to continuously operated systems.
• the self-cleaning CO 2 reduction method implementations that are proposed herein can circumvent steady state by cycling the applied voltage between an operational voltage and a regeneration voltage.
• the regeneration voltage is applied during the regeneration period in order to maintain an electric field for carbonate ions to migrate to the anode, thereby lowering carbonate ions concentrations at the cathode and avoiding damaging of the cathode via salt formation and plugging.
• the alternating approach was applied to silver and copper catalysts on carbon paper and PTFE based electrodes, respectively.
• the product selectivity resulting from the cyclically operated system was shown to be similar to that of the continuously operated system, with the advantage that alternating operation with regeneration yielded no detectable carbonate formation. More specifically, using the alternating strategy, the copper-PTFE sample in a MEA-based electrolyzer was operated in alternate for 157 hours (236 hours total duration), while maintaining a C 2 product selectivity of 80% and a C 2 partial current density of 138 mA cm -2 with a cost of ⁇ 1% additional system energy input.
• the following part includes information related to the COMSOL Multiphysics simulation results and model mechanism; current density plots of the different regeneration voltages; current density and selectivity plots of continuous operation of silver and copper catalysts; electrochemical performance comparison between continuous and alternating voltage with the same average current density; current density and selectivity of continuous operation of silver catalyst; electrode preparation; operation of the electrochemical MEA cell; and product analysis.
• K sp solubility product constant of potassium carbonate
• the concentrations of the constituent ions can be expressed in E3.
• the basic condition around the cathode pH ⁇ 14
• the concentrations of the [H + ], [HC0 3 ] and [ OH- ] were relatively small and negligible, as compared to [K + ] and [CO 3 2_ ] . Therefore, the concentrations of [K + ] and [CO 3 2- ] maintained the approximate ratio of 2: 1.
• the carbon paper - silver gas diffusion electrode was prepared by airbrushing catalyst inks with a nitrogen carrier gas.
• the catalyst silver ink was prepared with 12 mL ethanol (Greenfield Global Inc., >99.8%), 150 ⁇ L Nafion (Fuel Cell Store D521 Alcohol-based 1100 EW, 5 wt%), and 15 mg silver nanoparticles (Sigma- Aldrich 576832-5G, ⁇ 100 nm particle size).
• the catalyst ink mixtures were sonicated for two hours, and then sprayed on a gas diffusion carbon paper (Fuel Cell Store Sigracet 39 BC, with a microporous layer) with a spray density of 0.15 mL cm -2 .
• the polytetrafluoroethylene (PTFE) based copper electrode used was prepared by plasma sputtering and then airbrushing catalyst inks with a nitrogen carrier gas. Approximately 300 nm of copper catalyst was sputtered onto the PTFE substrate using an AJA International ATC Orion 5 Sputter Deposition System (Toronto Nanofabrication Centre, University of Toronto). An additional copper layer was sprayed on top of the sputtered layer.
• the copper ink was prepared with 12 mL ethanol, 150 ⁇ L Nafion, and 15 mg of copper nanoparticles (Sigma-Aldrich 774081 -5G, 25 nm particle size).
• Catalyst inks were sonicated for two hours and then sprayed on the sputtered PTFE sample with a spray density of 0.15 mL cm -2 . After airbrushing, the GDE was dried for 24 hours at room temperature ( ⁇ 20 °C). A Sustainion anion exchange membrane (Dioxide Materials Sustainion ® 37) was used in the electrolyzer.
• the anode electrode was prepared by spraying iridium chloride (Alfa Aesar, IrC13 xH20 99.8%) on a titanium support (Fuel Cell Store 592795-1, Titanium Felt). The coated electrode was treated by a thermal decomposition method 10 .
• the gas products from C0 2 reduction were analyzed in 1 mL volumes using a gas chromatograph (PerkinElmer Clarus 680), possessing a thermal conductivity detector (TCD) and a flame ionization detector (FID).
• a gas chromatograph PerkinElmer Clarus 680
• TCD thermal conductivity detector
• FID flame ionization detector
• the gas chromatograph was equipped with a Molecular Sieve 5A capillary column and a packed Carboxen- 1000 column. The flow rate of the gas was measured before each 1 mL volume was collected.
• the gas sample was collected by water displacement for one operational and regenerational iteration for alternating voltage tests. Then, we used the integration of total charge passing over the iteration to calculate the gas product Faradaic efficiency.
• the liquid products were quantified using nuclear magnetic resonance spectroscopy (NMR). 'H NMR spectra of freshly acquired samples were collected on an Agilent DD2 500 spectrometer using water suppression mode with dimethyl sulfoxide (DMSO) as an internal standard.
• NMR nuclear magnetic resonance spectroscopy
• the geometry consisted of a gas diffusion electrode (GDE), a cathode catalyst layer (CL), a current collector layer (CCL), an anionic exchange membrane (AEM), an iridium oxide (IrOx) anode catalyst layer and an anolyte layer.
• GDE gas diffusion electrode
• CL cathode catalyst layer
• CCL current collector layer
• AEM anionic exchange membrane
• IrOx iridium oxide
• Anode catalyst layer anode catalyst layer and anolyte layer.
• An electrical potential was applied at the left- hand boundary GDE layer.
• the ground was applied at the anode catalyst/anolyte interface.
• a C0 2 concentration at the GDE/CL interface was specified to be equal to the maximum Solubility in 0.1M KHC 0 3 electrolyte.
• the equilibrium values were specified at the right-hand boundary of the anolyte layer.
• CO 2 Solubility in 0.1M KHCO3 Electrolyte The C0 2 Solub
• the K s represents the Sechenov constant
• C s is the molar concentration of the electrolyte solution.
• the Solubility is determined based on K + , HC0 3 -, C0 3 2- and OH- ions concentration and the specific parameters which are shown in table 4.
• Table 4 Corresponding Sechenov constants in 0.1M KHC0 3 electrolyte - see study of Weisenberger S., et al., entitled “ Estimation of gas solubilities in salt solutions at temperatures from 273 K to 363 K ( AIChEJ 1996, 42 (1), 298-300. Catalyst Electrochemical Reactions:
• Electrochemical reactions were applied within the respective catalyst layers (E9 - El 2): C0 2 reduction to CO , H 2 , C 2 H 4 , C 2 H 5 OH on the cathode and oxygen evolution on the anode catalyst layer (El 3).
• CO 2 RR C0 2 reduction to CO , H 2 , C 2 H 4 , C 2 H 5 OH on the cathode and oxygen evolution on the anode catalyst layer (El 3).
• the electrode and electrolyte potentials were governed by Ohm’s Law (E14).
• the electromigration of the charged species (HC0 3 -, C0 3 2- , H + , OH- and K + ) (El 5) was controlled by the electrolyte potential and the combination of electroneutrality and induced space charge for ion-exchange membrane, which is governed by the Poisson equation (El 6).
• Porous Medium Effective Diffusion All layers except the electrolyte diffusion boundary layer were considered as a porous medium.
• the effective diffusivity was governed by the Bruggeman model.
• the porosity was 0.6 in the Cu cathode catalyst and current collector.
• the porosity was 0.9 in the IrO x Anode catalyst.
• the porosity was 0.1 for the AEM with a 90% reduction in diffusion coefficients for the cations (see studies of Dinh C. T. et al. , entitled “CO 2 Electroreduction to Ethylene via Hydroxide-Mediated Copper Catalysis at an Abrupt Interface ” ⁇ Science, 2018, 360 (6390), 783-787) and of Singh M. R.
• Butler-Volmer Equations The electrode kinetics of CO 2 reduction and water oxidation were modelled by the Butler-Volmer equation (El 7 - E21).
• the species transport equations (E23 - E24) were governed by the Nernst-Planck equations. Diffusion and electromigration terms were considered for the transportation of chemical species.
• Ci, Di and z L represent the species concentration, diffusion coefficient, and charge number, respectively.
• the diffusion coefficient and charge number are listed below in Table 8.
• Table 8 Diffusion coefficients and charge in the MEA system (see Vanysek, P - CRC Handb. Chem. Phys. 1996, 96 (73), 5-98).
• the model predicted a steady-state equilibrium between aqueous C0 2 , HC0 3 -, C0 3 2- , H + ,and OH- by considering several chemical reactions in alkaline conditions (E25 - E28). Water dissociation (E29) was also considered in this system. The reaction rate constants were determined by the temperature and salinity 4 . The corresponding equations are listed below:

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