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  • C25B11/03—Electrodes; Manufacture thereof not otherwise provided for characterised by shape or form perforated or foraminous
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  • C25B15/00—Operating or servicing cells
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  • C25B15/023—Measuring, analysing or testing during electrolytic production
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  • C25B15/00—Operating or servicing cells
  • C25B15/02—Process control or regulation
  • C25B15/023—Measuring, analysing or testing during electrolytic production
  • C25B15/025—Measuring, analysing or testing during electrolytic production of electrolyte parameters
  • C25B15/029—Concentration
  • C25B15/031—Concentration pH
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  • C25B15/08—Supplying or removing reactants or electrolytes; Regeneration of electrolytes
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  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B3/00—Electrolytic production of organic compounds
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  • C25B3/00—Electrolytic production of organic compounds
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  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
  • C25B9/19—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms
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  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
  • C25B9/19—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms
  • C25B9/21—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms two or more diaphragms

Definitions

• the technical field generally relates to electrocatalytic conversion in electrolyzers, and more particularly to electroreduction process and system combining a bipolar membrane and a stationary catholyte layer facilitating efficient conversion of CO2 and/or CO into multicarbon (C 2+ ) products.
• BACKGROUND Gigatons of CO 2 need to be avoided or removed from the atmosphere each year (see reference 1).
• CO2RR electrochemical reduction of CO2
• C 2+ multi-carbon
• CO 2 RR in electrolyzers operating both with alkaline and neutral electrolytes incur significant CO2 loss to carbonate formation and crossover, leading to low CO 2 utilization.
• the industrial implementation of the CO 2 RR for C 2+ production requires the simultaneous achievement of high production rates, high energy efficiencies, and high carbon efficiencies (see references 3 and 4).
• Known CO2RR electrolyzers based on alkaline bulk electrolytes e.g.
• Certain flow cells and MEAs have been designed to use neutral electrolytes (e.g., KHCO 3 ) rather than strong alkaline ones in order to reduce CO 2 absorption.
• Neutral media flow cells and MEAs have lower CO 2 absorption than do alkaline cells, and yet, since the reaction drives up the local pH and creates locally alkaline conditions, carbonate and bicarbonate formation remain a problem (see reference 11).
• Carbonate/bicarbonate ions migrate to the anode via the AEM, to combine with protons provided from the anode oxygen evolution reaction, thereby releasing CO 2 into the anode gas stream (SI1).
• SI1 anode gas stream
• a single pass CO2 utilization (SPU, the fraction of the CO2 feed been transformed to products) of C 2 + producing electrolyzers has remained in the range 3% to 30% (Table S1) (see references 6, 11 , 13 to 16).
• the present techniques relate to carbon oxides-to-C 2 + electrochemical reduction strategies that overcome previously-observed limits of carbon efficiency by designing an electroreduction system that inhibits carbon oxides crossover from cathode to anode and reverts formed carbonate/bicarbonate ions to carbon oxides via acidification of the catholyte.
• Carbon oxides as encompassed herein are selected from selected from CO, CO 2 or any mixture thereof.
• an electroreduction system for converting carbon oxides into multicarbon (C2+) products comprising:
• a cathodic compartment having a reactant inlet for receiving CO, CO2 or any mixture thereof, and comprising a cathode, the cathode comprising a catalyst layer that is in contact with a catholyte solution: an anodic compartment, the anodic compartment comprising an anode and accommodating a flowing anolyte solution; a bipolar membrane being positioned between the cathodic compartment and the anodic compartment, the bipolar membrane comprising: a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution; an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions at a surface of the anode; and an interfacial layer defined between the cation-exchange layer and the anion-exchange layer for splitting water into the protons and the hydroxide ions; wherein the cathodic compartment and/or the anodic compartment and
• the thickness of the stationary catholyte layer can be between 20 ⁇ m and 250 ⁇ m as measured by a spiral micrometer; preferably between 40 ⁇ m and 200 ⁇ m; and more preferably between 50 ⁇ m and 150 ⁇ m; and even more preferably between 65 and 125 ⁇ m.
• the solid porous support can be sandwiched between the catalyst layer of the cathode and the CEL for direct contact therewith.
• the cathodic compartment further comprises a solid porous support in between the CEL and the catalyst layer, the solid porous support being configured to be saturated with the catholyte solution to form the stationary catholyte layer.
• the solid porous support can include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polycarbonate, nylon, cellulose acetate, cellulose nitrate, polypropylene, alumina, or any combinations thereof.
• the solid porous support can have a mean pore diameter (i.e., a mean pore size) between 0.05 and 50 ⁇ m as determined by scanning electron microscopy (SEM), optionally between 0.2 and 1 ⁇ m, and further optionally of 0.45 ⁇ m.
• the stationary catholyte layer can have a liquid content between 5 and 50 ⁇ L.cm -2 preferably between 6 and 40 ⁇ L.cm -2 ; more preferably between 8 and 30 ⁇ L.cm -2 ; even more preferably between 8 and 30 ⁇ L.cm -2 ; most preferably between 10 and 25 ⁇ L.cm -2 ; even most preferably between 10 and 20 ⁇ L.cm -2 or between 12 and 18 ⁇ L.cm-2; when the solid porous support is saturated with the catholyte solution; the liquid content being determined by weighting.
• the stationary catholyte layer can have a liquid content about 10 ⁇ L.cm-2.
• the stationary catholyte layer has a thickness that is selected to maximize a mass transport of regenerated CO 2 or CO to the catalyst layer of the cathode while maintaining a resistance to compression of the solid porous support.
• the catholyte solution has a concentration of cations between 0.25 M and 3 M; preferably between 0.4 M and 4 M; and more preferably between 0.5 M and 2 M, and for example about 1M.
• the catholyte solution can be a solution of K 2 SO 4 with a K+ concentration equal to or greater than 0.5M.
• the cations in the catholyte solution can be one or more selected from K + , Na+, Cs+, Rb+, NH4+, Mg2+, Ca2+, Al3+.
• the catholyte solution is a solution with a cations concentration equal to or greater than 0.5M.
• the catholyte solution is a buffered solution.
• the buffered solution can be a solution comprising one or more selected from KHCO3, K3PO4, K2HPO4, KH2PO4, the buffered solution is a mixture of glycine and sodium hydroxide, or a mixture of H3BO3 and sodium hydroxide.
• the catholyte solution is a non-buffered solution.
• the non-buffered solution can be or comprise K2SO4, KCl or any other combinations of the Cl- anions or SO 4 2- anions with Na + , Cs + , Rb + , NH 4+ , Mg 2+ , Ca 2+ , or Al 3+ cations.
• the anolyte solution can have an anolyte concentration between 2.0 M and 0.01M, optionally about 0.1 M.
• the anolyte solution is a neutral solution.
• the anolyte solution can have a pH between 7 and 10.
• the anolyte solution is a neutral solution hen having a pH between 7 and 10.
• the anolyte (neutral) solution can be a KHCO 3, K 2 SO 4 , or K 2 HPO 4 solution.
• the anolyte solution is an acidic solution.
• the anolyte solution has a pH between 1 and 4.
• the anolyte solution has a bulk pH between 1 and 4.
• the acidic solution can be a H 3 PO 4 solution, H 2 SO 4 solution or a combination thereof.
• the catalyst layer of the cathode comprises copper (Cu), silver (Ag), platinum (Pt), carbon (C), or any combination thereof.
• the cathode further comprises a gas diffusion layer for contacting the stream of CO, CO2 or any mixture thereof, and the catalyst layer is deposited onto the gas diffusion layer.
• the gas diffusion layer can be a hydrophobic carbon paper, or a copper sputtered hydrophobic PTFE layer.
• hydrophobic means a water contact angle following ISO 19403-6:2017 of at least 30°.
• the anode comprises an anodic catalyst layer and an anodic current collector layer.
• the anodic catalyst layer can include one or more selected from IrO2, Pt, Pd, Ni, NiOx, CoOx.
• the anodic current collector layer can include Ti felt, hydrophilic carbon paper, or Ni foam.
• hydrophilic means a water contact angle following ISO 19403-6:2017 below 30°.
• the interfacial layer of the bipolar membrane comprises a water dissociation catalyst.
• the water dissociation catalyst can be present as nanoparticles.
• the water dissociation catalyst can comprise one or more selected from TiO 2 , IrO 2 , NiO, SnO 2 , graphene oxide, CoOx, ZrO2, Al2O3, Fe(OH)3, MnO2, Ru, Rh, RuPt alloy, PtIr alloy, Ir, Pt.
• the AEL is a membrane comprising poly(aryl piperidinium), polystyrene methyl methylimidazolium, or polystyrene tetramethyl methylimidazolium.
• the CEL can comprise or consist of a sulfonated tetrafluoroethylene based fluoropolymer- copolymer.
• the system can include a temperature controller configured to maintain an operating temperature between 20°C and 50°C, optionally about 35°C.
• a single-pass utilization of the stream of CO, CO 2 or any mixture thereof can be of at least 50% for an inlet flowrate between 1 sccm and 15 sccm.
• the single-pass utilization of the stream of CO, CO 2 or any mixture thereof can be of at least 60% for an inlet flowrate between 1 sccm and 8 sccm.
• the system can have a Faradeic Efficiency (FE) for conversion into the C2+ products of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 mA.cm -2 .
• the FE for conversion into the C2+ products can be of at least 25% during at 30 hours of operation and the applied current density of 350 mA.cm-2.
• a carbon oxides electroreduction process for converting CO, CO 2 or any mixture thereof into C 2+ products.
• the process includes: supplying a catholyte solution and CO, CO 2 or any mixture thereof to a cathodic compartment comprising a catalyst layer in contact with the catholyte solution: flowing an anolyte solution through an anodic compartment having a product outlet to release the C2+ products, the anodic compartment comprising an anode; providing a bipolar membrane between the cathodic compartment and the anodic compartment, the bipolar membrane comprising: a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution; an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions into the anolyte solution; and an interfacial layer defined between the cation-exchange layer and the anion-exchange layer for splitting water into the protons and the hydroxide ions; and retaining a portion of the catholyte solution as a stationary catholyt
• the process can include maintaining an operating temperature between 20°C and 50°C, and optionally about 35°C. In some implementations, supplying CO, CO2 or any mixture thereof to the cathodic compartment is performed at an inlet flowrate between 1 sccm and 15 sccm. In some implementations, the process can include providing the cathode with an applied current density between 100 and 400 mA.cm -2 . In some implementations, the process can include forming the stationary catholyte layer by providing a solid porous support between the cathode and the CEL, and saturating the solid porous support with the catholyte solution.
• the saturating can be performed to reach a liquid content of the stationary catholyte layer between 5 and 50 ⁇ L.cm-2, optionally between 10 and 20 ⁇ L.cm-2, and further optionally about 10 ⁇ L.cm-2 when the solid porous support is saturated with the catholyte solution.
• the catholyte solution can be supplied with a concentration of cations between 0.25 M and 3 M, and optionally between 0.5 M and 2 M, and further optionally about 1M.
• the stationary catholyte layer can be formed with a thickness between 20 ⁇ m and 250 ⁇ m as measured by a spiral micrometer; preferably between 40 ⁇ m and 200 ⁇ m, more preferably between 50 ⁇ m and 150 ⁇ m.
• the process can include utilizing CO, CO 2 or any mixture thereof according to a single-pass utilization of the stream of CO, CO2 or any mixture thereof of at least 50% for an inlet flowrate between 1 sccm and 15 sccm.
• the process can include utilizing CO, CO2 or any mixture thereof according to a single-pass utilization of the stream of CO, CO 2 or any mixture thereof of at least 60% for an inlet flowrate between 1 sccm and 8 sccm.
• the process can include producing the C 2+ products according to a Faradeic Efficiency (FE) for conversion into the C2+ products that is of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 mA.cm-2.
• FE Faradeic Efficiency
• the process can include producing the C2+ products according to the FE for conversion into the C2+ products that is of at least 25% during at 30 hours of operation and the applied current density of 350 mA.cm -2 .
• the process can include using the system according to all implementations as defined herein. The inventors have thus discovered that a cation effect can be allowed at the cathode surface to enable carbon oxide reduction in the acidified catholyte solution.
• the electroreduction system includes a bipolar membrane and a stationary catholyte layer that maintains a catholyte solution within a cathodic compartment, to facilitate the participation of regenerated CO 2 in CO 2 RR reactions.
• the presently designed electroreduction system showed a single-pass CO 2 utilization of more than 60%, representing twice the previously reported state-of-art designs that produced C2+. Owing to its high single-pass CO 2 utilization (SPU), the presently proposed electroreduction system minimizes the energy input associated with CO 2 recovery, while enabling comparable performance and stability to the benchmark alkaline and neutral media AEM- based flow cell or MEA electrolyzers.
• the invention provides an electroreduction system for converting carbon oxides selected from CO, CO 2 or any mixture thereof into multicarbon (C 2 +) products, the system comprising: a cathodic compartment having a reactant inlet for receiving a stream of CO, CO2 or any mixture thereof and comprising a cathode, the cathode comprising a catalyst layer that is contactable with a catholyte solution: an anodic compartment having a product outlet to release the C 2+ products, the anodic compartment comprising an anode and being configured to accommodate a flowing anolyte solution; a bipolar membrane being positioned between the cathodic compartment and the anodic compartment, the bipolar membrane comprising: a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution; an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions at a surface of the an electroreduction
• the cathodic compartment further comprises a solid porous support in between the CEL and the catalyst layer, the solid porous support being configured to be saturated with the catholyte solution to form the stationary catholyte layer.
• the solid porous support comprises polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polycarbonate, nylon, cellulose acetate, cellulose nitrate, polypropylene, alumina, or any combinations thereof.
• the thickness of the stationary catholyte layer is between 20 ⁇ m and 400 ⁇ m, optionally between 20 ⁇ m and 300 ⁇ m, further optionally between 20 ⁇ m and125 ⁇ m.
• the system according to any one of embodiments 1 to 10, wherein the cations in the catholyte solution are one or more selected from K + , Na + , Cs + , Rb + , NH 4+ , Mg 2+ , Ca 2+ , Al 3+ .
• the system according to any one of embodiments 1 to 11, wherein the catholyte solution is a buffered solution.
• the buffered solution is a solution comprising one or more selected from KHCO3, K3PO4, K2HPO4, KH 2 PO 4
• the buffered solution is a mixture of glycine and sodium hydroxide, or a mixture of H3BO3 and sodium hydroxide.
• the system according to embodiment 14, wherein the non-buffered solution is or comprises K 2 SO 4 , KCl or any other combinations of the Cl- anions or SO 4 2- anions with Na + , Cs + , Rb + , NH 4+ , Mg 2+ , Ca 2+ , or Al 3+ cations.
• the system according to any one of embodiments 1 to 15, wherein the anolyte solution is a neutral solution.
• the system according to embodiment 16 wherein the anolyte solution has a pH between 7 and 10.
• the system according to embodiment 16 or 17, wherein the neutral solution is a KHCO 3, K 2 SO 4 , or K 2 HPO 4 solution.
• the system according to any one of embodiments 1 to 22, wherein the catalyst layer of the cathode comprises copper (Cu), silver (Ag), platinum (Pt), carbon (C), or any combination thereof.
• the cathode further comprises a gas diffusion layer for contacting the stream of CO, CO2 or any mixture thereof, and the catalyst layer is deposited onto the gas diffusion layer.
• the gas diffusion layer is a hydrophobic carbon paper, or a copper sputtered hydrophobic PTFE layer.
• the system according to any one of embodiments 1 to 25, wherein the anode comprises an anodic catalyst layer and an anodic current collector layer.
• the system according to embodiment 26, wherein the anodic catalyst layer comprises one or more selected from IrO 2 , Pt, Pd, Ni, NiOx, CoOx.
• the system according to embodiment 26 or 27, wherein the current collector layer comprises Ti felt, hydrophilic carbon paper, or Ni foam.
• the system according to any one of embodiments 1 to 28, wherein the interfacial layer of the bipolar membrane comprises a water dissociation catalyst.
• the system according to embodiment 29 or 30, wherein the water dissociation catalyst comprises one or more selected from TiO 2 , IrO 2 , NiO, SnO 2 , graphene oxide, CoOx, ZrO2, Al2O3, Fe(OH)3, MnO2, Ru, Rh, RuPt alloy, PtIr alloy, Ir, Pt.
• the system according to any one of embodiments 1 to 34 having a single-pass utilization of the stream of CO, CO 2 or any mixture thereof of at least 60% for an inlet flowrate between 1 sccm and 8 sccm.
• the system according to any one of embodiments 1 to 36 having a Faradeic Efficiency (FE) for conversion into the C2+ products of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 mA.cm- 2 .
• the system according to embodiment 37, wherein the FE for conversion into the C 2+ products is of at least 25% during at 30 hours of operation and the applied current density of 350 mA.cm -2 .
• the invention provides a carbon oxides electroreduction process for converting CO, CO 2 or any mixture thereof into C 2+ products, the process comprising: supplying a catholyte solution and a stream of CO, CO2 or any mixture thereof to a cathodic compartment comprising a catalyst layer in contact with the catholyte solution: flowing an anolyte solution through an anodic compartment having a product outlet to release the C2+ products, the anodic compartment comprising an anode; providing a bipolar membrane between the cathodic compartment and the anodic compartment, the bipolar membrane comprising: a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution; an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions into the anolyte solution; and an interfacial layer defined between the cation-exchange layer and the anion-exchange
• the process of embodiment 39 comprising maintaining an operating temperature between 20°C and 50°C, and optionally about 35°C.
• the process of embodiment 39 or 40 wherein supplying the stream of CO, CO2 or any mixture thereof to the cathodic compartment is performed at an inlet flowrate between 1 sccm and 15 sccm.
• the process of any one of embodiments 39 to 41 comprising providing the cathode with an applied current density between 100 and 400 mA.cm-2 .
• the process of any one of embodiments 39 to 42 comprising forming the stationary catholyte layer by providing a solid porous support between the cathode and the CEL, and saturating the solid porous support with the catholyte solution.
• the process of embodiment 43 wherein the saturating is performed to reach a liquid content of the stationary catholyte layer between 5 and 50 ⁇ L.cm-2, optionally between 10 and 20 ⁇ L.cm-2, and further optionally about 10 ⁇ L.cm-2 when the solid porous support is saturated with the catholyte solution.
• the process of any one of embodiments 39 to 46 comprising utilizing the stream of CO, CO 2 or any mixture thereof according to a single-pass utilization of at least 50% for an inlet flowrate between 1 sccm and 15 sccm.
• the process of any one of embodiments 39 to 46 comprising utilizing the stream of CO, CO 2 or any mixture thereof according to a single-pass utilization of at least 60% for an inlet flowrate between 1 sccm and 8 sccm.
• the process according to any one of embodiments 39 to 48 comprising producing the C2+ products according to a Faradeic Efficiency (FE) for conversion into the C 2+ products that is of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 mA.cm -2 .
• FE Faradeic Efficiency
• the process according to embodiment 49 comprising producing the C2+ products according to the FE for conversion into the C2+ products that is of at least 25% during at 30 hours of operation and the applied current density of 350 mA.cm -2 .
• the process of any one of embodiments 39 to 50 further comprising using the system as defined in any one of embodiments 2 to 38.
• Figures 1A and 1B schematically illustrate carbonate formation and CO2 crossover mechanisms in known alkaline and neutral media CO 2 RR electrolyzers (prior art):
• Figure 1A conventional AEM-based flowing-electrolyte electrolyzer
• Figure1B zero- gap gas-phase electrolyzer
• Figures 1C and 1D schematically illustrate carbonate formation and CO 2 crossover mechanisms in the electroreduction system that is proposed herein:
• Figure 1C bipolar membrane (BPM)-based stationary catholyte layer (SC)-MEA
• Figure 1D mechanism of cation effects: potassium ions form an electrochemical double layer on the surface of a Cu catalyst, which modulates the local pH and prohibits the proton adsorption, and thereby enhances the selectivity of CO 2 RR and suppress that of HER (see references 1 and 33).
• FIGS. 1E and 1F are scanning electron microscopy (SEM) images of the cathode electrode (1E) and a cation exchange layer (CEL) / anion exchange layer (AEL) interface of the customized BPM used for SC-MEA in neutral anolyte (1F).
• SEM scanning electron microscopy
• FIGS. 2A to 2F are graphs resulting from experimental investigations on the CO2RR performance of the SC-MEA using neutral anolyte:
• Figure 2A is a graph showing gas product FEs (%) for the SC-MEA for zero gap (direct contact) and four catholyte solutions based on an applied current density producing maximum ethylene FE (marked on the top of each column) and at an operating temperature of 20oC.
• FIG. 7A to 7C The dependence of cell voltage and gas FE on the current density can be found in Figures 7A to 7C;
• Figure 2B is a graph showing the dependence of cell voltage on current density at various operating temperatures;
• Figure 2C is a graph showing the dependence of cathode gas products FEs on current density at an operating temperature of 35 °C;
• Figure 2D is a graph showing CO 2 /O 2 ratio in an anode gas for a conventional electrolyzer (black squares) and the present SC-MEA (grey squares) at various current densities. O 2 and CO 2 flow rates in the present SC-MEA are also indicated.
• FIG. 2E is a graph showing the dependence of the cathode products FEs on an inlet CO2 flow rate (sccm);
• Figure 2F is a graph showing an SPU of CO2 versus the inlet CO2 flow rate.
• the ideal SPU values of the conventional electrolyzers are marked for comparison.
• the ' Simulated ideal' refers to the simulated upper limit of SPU in an AEM-based MEA, assuming the AEM- based MEA has the same product distribution to the present BPM-based SC-MEA operating at a CO2 flow rate of 1 sccm, and based on the assumption that one extra CO2 is lost to carbonate per two OH- generated.
• the justification of the ideal SPU simulation can be found in SI1 of Supplemental Information.
• Figures 14A, 14B and 15 The CO 2 RR performance at lower pHs and various temperatures are shown in Figures 14A, 14B and 15 in Supplemental Information. Under all the acidic conditions, the CO 2 concentration in the anode gas was below the detection limit.
• Figure 3A is a graph showing the dependence of cell voltage and cathode gas products FEs on current density
• Figure 3B is a graph showing the dependence of the cathode products FEs on the inlet CO2 flow rate
• Figure 3C is a graph showing the SPU of CO2 versus the inlet CO2 flow rate.
• 'CO ideal' and 'C2+ (excl.
• the ' Simulated ideal' refers to the simulated upper limit of SPU in AEM-based MEA electrolyzers, assuming it has the same product distribution to the present BPM-based SC-MEA operating at an inlet CO2 flow rate of 1 sccm, and based on the assumption that one extra CO 2 is lost to carbonate per two OH- generated.
• Figure 4A to 4C are graphs showing an extended CO2RR performance of the present BPM-based SC-MEA:
• Figure 4A is a graph of ethylene FEs and cell voltages versus operating time using a neutral anolyte at an operating temperature of 35°C, an inlet CO 2 flow rate of 9.0 sccm, and an applied current density of 250 mA.cm-2;
• Figure 4B is a graph of ethylene FEs and cell voltages versus operating time using an acidic anolyte at an operating temperature of 35°C, an inlet CO2 flow rate of 9.0 sccm and an applied current density of 350 mA.cm -2 ;
• Figure 4C is a graph of the FE distribution and SPU versus operating time for the present BPM-based SC-MEA using
• the extent of the gradients in pH are not precisely known and are not drawn to scale.
• Figure 6 is a schematic illustration of CO2 regeneration in a stationary catholyte layer for a rough estimation of the impacts of a thickness of the stationary catholyte layer on the present BPM-based SC-MEA performance.
• Figures 7A to 7C are graphs showing the FE and full cell voltage for the SC-MEA for various catholyte solution comprised in the stationary catholyte layer: ( Figure 7A) DI- water; ( Figure 7B) 0.25M K2SO4; ( Figure 7C) 2M KCl. In all cases, the anolytes are 0.1M KHCO3. The measurements were performed at 20 o C with a CO2 inlet flow rate of 15 sccm.
• Figure 8 is a graph showing ethylene FE and cell voltage with extended operating hours for the present BPM-based SC-MEA with a catholyte solution of 1M KHCO3 in the stationary catholyte layer and an anolyte solution of 1M KHCO 3 as the anolyte.
• the measurement was carried out at 20 ° C with a CO2 flow of 9 sccm and a current density of 100 mA cm -2 .
• CEL Nafion XL
• Figure 9A is a schematic representation of the present stationary catholyte layer in combination with an MEA
• Figure 9B is a graph showing the full cell voltage of the CEM-based SC-MEA at various current densities
• Figure 9C is a graph showing gas products FEs of the CEM-based SC-MEA at the same various current densities.
• Figure 10 is a graph showing full cell voltage of a BPM-based SC-MEA at various current densities, for a water-splitting BPM including a layer of TiO 2 nanoparticles as water dissociation catalyst sandwiched by CEM and AEM (customized BPM), a commercially available BPM (Fumasep), and a membrane with CEM and AEM simply compressed together.
• a 5 cm 2 Pt/C loaded hydrophilic carbon paper and a 5 cm 2 IrO 2 loaded Ti felt were used as cathode and anode of the MEA, respectively.
• Figure 11 is a graph showing a gas product FE and full cell voltage for the SC- MEA based on commercially available BPM (Fumasep) and 0.1 M KHCO3 neutral anolyte.
• Figure 12A is a graph showing the gas products FE and full cell voltage for the SC-MEA based on customized BPM and 0.1 M H3PO4 + 0.5 M K2SO4 acidic anolyte, and
• Figure 12B is a graph showing the cell voltage versus operating time diagram of the BPM- based SC-MEA.
• Figure 16 is a graph showing a CO2: O2 ratio in an anode gas phase for the SC- MEA using the presently customized BPM with a 0.1 M K2SO4 anolyte solution (black), or using a commercially available BPM (Fumasep) and a 0.1 M KHCO 3 anolyte solution (red).
• the catholyte solution is 1M K+ in the stationary catholyte layer in both cases. All the gas samples were recorded after the cell operating for 1 hour at each current density.
• Figure 21 The exploration of the CO2RR performance and energy intensity of SC- BPMEA with restricted reactant availability. All the measurements were conducted at 35 o C and 200 mA cm -2 , and the data were collected after 2 h of continuous operation. The total CO 2 single-pass utilization (the CO 2 -to-ethylene single-pass conversion see Fig.18) for the SC-BPMEAs with different catholyte thickness and input CO2 flow rates.
• Figure 22 The exploration of the CO2RR performance and energy intensity of SC- BPMEA with restricted reactant availability. All the measurements were conducted at 35 o C and 200 mA cm -2 , and the data were collected after 2 h of continuous operation.
• Figure 24 Measurements of CO2 SPU of the SC-BPMEAs with 125 ⁇ m 0.5 M K 2 SO 4 , operating at 300 mA cm-2 (a, b) or 200 mA cm-2 (c, d).
• (a, c) The FE distributions at different input CO 2 flow rates.
• (b, d) The total CO2 SPU and CO2-to-ethylene conversion at different input flow rates.
• the present electroreduction system particularly includes a stationary catholyte layer being configured to facilitate mass transfer via diffusion of regenerated CO 2 or CO across the stationary catholyte layer and back to an adjacent cathode of the system.
• the system and related process implementations that are described herein in relation to CO2 electroreduction can be applied to CO electroreduction, or the electroreduction of a mixture of CO 2 and CO, without departing from the scope of the present techniques.
• the catholyte solution of the present electroreduction system is not provided flowing in and out of a cathodic compartment but rather remains within the cathodic compartment as a stationary catholyte layer between a cathode and a membrane separating the cathodic compartment from an adjacent anodic compartment.
• the electroreduction system includes a bipolar membrane (e.g., including a cation-exchange layer (CEL), an interfacial layer comprising a water dissociation catalyst, and anion-exchange layer (AEL)) that separates an anodic compartment from a cathodic compartment.
• the bipolar membrane is used to dissociate water, thereby providing hydroxide ions to the anodic compartment side and protons to the cathodic compartment side.
• the electroreduction system further includes a cathode (e.g., gas diffusion layer plated with Cu catalyst) that is positioned in the cathodic compartment, and an anode (e.g.
• hydrophilic electrode such as IrO2 catalyst coated on a support of Ti felt
• CO2 can be provided to the cathode via an inlet of the cathodic compartment so as to be converted into gas products (C2H4, CO, H2) at a surface of the cathode.
• C2H4, CO, H2 gas products
• a portion of the CO 2 can be lost to carbonate formation.
• the present electroreduction system further includes the stationary catholyte layer sandwiched between the bipolar membrane and the cathode, with the stationary catholyte layer comprising a catholyte solution that receives the carbonate/bicarbonate ions derived from the lost CO2 portion.
• protons are provided from the bipolar membrane into the catholyte solution, these protons can combine with the carbonate/bicarbonate ions in the catholyte solution to regenerate the portion of CO 2 that did not serve to form gas products.
• the regenerated CO 2 can then diffuse back to the cathode surface to form the gas products that will be recovered in a cathode gas mixture.
• the bipolar membrane can favor the conversion of carbonate/bicarbonate ions back to CO 2 and further prevent ions crossover between the anodic and cathodic compartments.
• the stationary catholyte layer further enables local alkalinity to promote CO2RR in (bulk) acidified catholyte solution, and facilitates thereby that the regenerated CO2 participates in CO2RR reactions.
• the catalyst layer of the cathode comprises copper (Cu), silver (Ag), platinum (Pt), carbon (C), or any combination thereof.
• the cathode further comprises a gas diffusion layer for contacting the CO 2 stream, and the catalyst layer is deposited onto the gas diffusion layer.
• the gas diffusion layer is hydrophobic.
• the gas diffusion layer is a hydrophobic carbon paper, or a copper sputtered hydrophobic PTFE layer.
• hydrophobic means a water contact angle following ISO 19403-6:2017 of at least 30°.
• the anode comprises an anodic catalyst layer and an anodic current collector layer.
• the anodic catalyst layer comprises one or more selected from IrO 2 , Pt, Pd, Ni, NiOx, CoOx.
• the current collector layer comprises Ti felt, hydrophilic carbon paper, or Ni foam.
• hydrophilicme ans a water contact angle following ISO 19403-6:2017 below 30°.
• the thickness of the stationary catholyte layer can be selected to enhance the mass transport via diffusion of the regenerated CO2 to the cathode surface.
• the stationary catholyte layer can be characterized as including a boundary where CO 2 is regenerated and from which regenerated CO2 can diffuse towards the cathode surface (Cu) or the CEL, this diffusional phenomenon being driven by a concentration gradient.
• the distances from this boundary to the cathode surface and CEL are noted as L1 and L2, respectively.
• the portion of the stationary catholyte layer between the boundary and the cathode surface can be defined as a diffusion layer having a diffusion layer thickness L1.
• the thickness of the stationary catholyte layer of the present system is selected to minimize the diffusion layer thickness, L1, and facilitate regenerated CO2 mass transport while providing a mechanically robust stationary catholyte layer.
• the diffusion layer thickness L1 can be estimated based on physical properties of protons and carbonates. For example, one can determine the position where protons and carbonates meet each other by estimation of encounter problems, and considering that the speed is proportional to their mobility. More precise determination can be via a cross-platform finite element analysis, solver and multiphysics simulation software such as COMSOL®.
• the stationarity of the catholyte layer as defined herein thus prevents the regenerated CO2 to be flushed away from the cathodic compartment and allows the CO 2 to diffuse back into the cathode for conversion thereof into value-added products.
• being stationary means that the catholyte is not flowing out of the cathodic compartment during CO2 conversion and that the volume of the catholyte solution contained in the stationary catholyte layer remains between the bipolar membrane and the cathode during operation of the electroreduction system.
• the catholyte solution can flow/move within the stationary catholyte layer according to various mass transport mechanisms (diffusion, migration, convection if any).
• the nature of the catholyte solution of the stationary catholyte layer can be further selected to reduce the diffusion layer thickness.
• the stationary catholyte layer can include a solid porous support having pores that are saturated with the catholyte solution.
• the porous solid support is or comprises polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polycarbonate, nylon, cellulose acetate, cellulose nitrate, polypropylene, alumina, or any combinations thereof.
• the solid porous support has a mean pore diameter (i.e., a mean pore size) between 0.05 and 50 ⁇ m as determined by scanning electron microscopy; for example, between 0.08 and 25 ⁇ m; for example, between 0.10 and 10 ⁇ m; for example, between 0.15 and 5 ⁇ m; for example, between 0.20 and 1.0 ⁇ m; for example, between 0.30 and 0.80 ⁇ m.
• the solid porous support has a mean pore size of 0.45 ⁇ m.
• the porous solid support can be provided to contact the cathode surface at one side and the cation exchange layer of the bipolar membrane at another side thereof.
• the thickness of the stationary catholyte layer can thus be equal to the thickness of the solid porous support.
• the thickness of the stationary catholyte layer can be at most 280 ⁇ m as measured by a spiral micrometer.; preferably, at most 250 ⁇ m; preferably, at most 220 ⁇ m; preferably at most 200 ⁇ m; preferably at most 180 ⁇ m; preferably, at most 150 ⁇ m; preferably at most 140 ⁇ m; preferably, at most 130 ⁇ m; and more preferably at most 125 ⁇ m.
• the thickness of the stationary catholyte layer can be at least 20 ⁇ m as measured by a spiral micrometer.; preferably, at least 25 ⁇ m; preferably, at least 30 ⁇ m; preferably at least 40 ⁇ m; preferably at least 45 ⁇ m; preferably, at least 50 ⁇ m; preferably at least 55 ⁇ m; preferably, at least 60 ⁇ m; and more preferably at least 65 ⁇ m.
• the thickness of the stationary catholyte layer can be ranging from 20 to 280 ⁇ m as measured by a spiral micrometer.; for example, from 20 to 250 ⁇ m; for example, from 30 to 220 ⁇ m; for example, from 40 to 200 ⁇ m; for example, from 45 to 180 ⁇ m; for example, from 50 to 150 ⁇ m; for example, from 55 to 140 ⁇ m; for example, from 60 to 130 ⁇ m; for example, from 65 to 125 ⁇ m.
• the thickness of the stationary catholyte layer is measured by a spiral micrometer.
• the thickness of the stationary catholyte layer can be about 125 ⁇ m, when measured by a spiral micrometer.
• the thickness of the stationary catholyte layer is not to be bound to these values and is selected to minimize the diffusion layer thickness while maintaining the mechanical robustness of the stationary catholyte layer.
• the mechanical robustness can refer herein to a resistance to compression that can be estimated for example via a compression- stress test.
• the stationary catholyte layer is configured to resist compression an maintain sufficient thickness to avoid direct contact of the BPM with the cathode.
• the thickness of the stationary catholyte layer could be inferior to 125 ⁇ m if a solid porous support having such thickness is used and yet maintain robustness.
• the stationary catholyte layer When the solid porous support is saturated with the catholyte solution, the stationary catholyte layer has a liquid content between 5 and 50 ⁇ L.cm -2 ; for example, between 8 and 35 ⁇ L.cm-2; for example, between 10 and 20 ⁇ L.cm-2.
• the stationary catholyte layer has a liquid content of about 10 ⁇ L.cm-2 .
• BPMs e.g. Fumasep FBM
• a conventional BPM can consist of a cation-exchange layer (CEL) laminated with an anion-exchange layer (AEL). With the CEL facing the cathode side, the concentration (and hence conductivity) of carbonate/bicarbonate anions in the BPM is substantially reduced due to the Donnan effect (see reference 17).
• the BPM also generates protons and hydroxide ions via water dissociation at the junction between the CEL and AEL(see references 18 and 19) under appropriate external potential.
• the protons are driven to the cathode surface, where, with judicious device engineering, they can intercept carbonate/bicarbonate, reverting it to CO 2 .
• the catholyte reacts with and absorbs CO2.
• the presently described system can be referred to as a stationary-catholyte MEA (SC-MEA) or a BPM-based SC-MEA.
• CO2RR on Cu catalyst with an SPU of at least 60% (C 2+ FE of 26%) was achieved, which twice the value of the best prior known electrolyzers producing C2+ (see references 2, 4, 15 and 24) and, as a result, the theoretical upper limit of SPU for neutral/alkaline C 2+ systems previously demonstrated was also surpassed.
• the present system maintains an ethylene FE at a steady rate of above 30% for more than 30 hours of continuous operation.
• the BPM of the present disclosure comprises a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution; an anion-exchange layer (AEL) in anion communication with the anolyte solution to provide hydroxide ions at a surface of the anode; and an interfacial layer defined between the cation-exchange layer and the anion-exchange layer for splitting water into the protons and the hydroxide ions.
• CEL cation-exchange layer
• AEL anion-exchange layer
• the interfacial layer can comprise a water dissociation catalyst; with preference that the water dissociation catalyst comprises one or more selected from TiO 2 , IrO 2 , NiO, SnO 2 , graphene oxide, CoOx, ZrO 2 , Al 2 O 3 , Fe(OH) 3 , MnO 2 , Ru, Rh, RuPt alloy, PtIr alloy, Ir, Pt. More preferably, the water dissociation catalyst can be a combination of IrO2 on the AEL side) and NiO on the CEL side. In an embodiment, the water dissociation catalyst is present as nanoparticles.
• the AEL is a membrane comprising poly(aryl piperidinium), polystyrene methyl methylimidazolium, or polystyrene tetramethyl methylimidazolium.
• the CEL is a NafionTMmembrane (CAS number 31175-20-9)
• the system further comprises a temperature controller configured to maintain an operating temperature between 20°C and 50°C, for example, 25°C and 45°C; for example, 30°C and 40°C optionally about 35°C.
• the system is having a single-pass utilization of the CO2 stream of at least 50% for a CO 2 inlet flowrate between 1 sccm and 15 sccm.
• the system is having a single-pass utilization of the CO2 stream of at least 60% for a CO2 inlet flowrate between 1 sccm and 8 sccm.
• the system is having a Faradeic Efficiency (FE) for conversion into the C2+ products of at least 20% during at least 20 hours of operation and under an applied current density between 100 and 400 mA.cm-2.
• the FE for conversion into the C2+ products is of at least 25% during 30 hours of operation and the applied current density of 350 mA.cm-2.
• the present disclosure also relates to a CO2 electroreduction process for converting CO2 into C 2+ products, the process comprising: supplying a catholyte solution and CO2 to a cathodic compartment comprising a catalyst layer in contact with the catholyte solution: flowing an anolyte solution through an anodic compartment having a product outlet to release the C2+ products, the anodic compartment comprising an anode; providing a bipolar membrane between the cathodic compartment and the anodic compartment, the bipolar membrane comprising: a cation-exchange layer (CEL) in cation communication with the catholyte solution to provide protons into the catholyte solution; an anion-exchange layer (AEL) in anion communication with the anolyte solution/anode?
• CEL cation-exchange layer
• AEL anion-exchange layer
• Figure 1E is an SEM photograph of a cathode that was prepared by spraying Cu nanoparticles onto a hydrophobic carbon paper (gas diffusion layer) for CO 2 RR.
• the anode can be IrO 2 supported on titanium felt to support the oxygen evolution reaction (OER).
• Figure 1F is an SEM photograph of a customized BPM under reverse bias that was installed with the anion-exchange layer (AEL) contacting the anode, and the cation-exchange layer (CEL) contacting the stationary catholyte layer. The cathode was then compressed onto the solid porous support of the stationary catholyte layer, and anodic and cathodic flow-field plates sandwiched the system.
• AEL anion-exchange layer
• CEL cation-exchange layer
• the present system can include a porous stationary catholyte layer having a thickness that is below 400 ⁇ m, e.g. about 125 ⁇ m and including a solid porous support that is configured to be saturated with the catholyte solution.
• the solid porous support can be, for example, a PVDF filter membrane having a mean pore size (i.e. diameter) of 0.45 ⁇ m and configured to receive a liquid content of about 10 ⁇ L.cm -2 .
• the lower thickness of this porous stationary layer allows for improving the mass transport efficiency of the in-situ recovered CO2 to catalyst (quantitatively simulated in section SI3 of Supplemental Information provided further below) in comparison to known systems. It was further discovered that the composition of the stationary catholyte layer greatly impacts CO2RR performance in SC-MEA (analyzed and rationalized in sections SI2 and SI3 of Supplemental Information provided further below).
• the catholyte solution can be designed as a non-buffered catholyte solution, e.g.
• K 2 SO 4 in order to introduce cation effects as a means to promote selectivity to CO2RR over HER (see references 25 and 26).
• cations such as K+ can form an electrochemical double layer on the catalyst surface (see Figure 1D), introducing changes in polarity, absorption preference, local pH, and local CO2 concentration, as observed and modeled before (see references 25 and 26).
• Increasing K+ concentration from 0 to 2 M in the stationary catholyte layer was seen to enhance CO2RR selectivity.
• the ethylene FE was observed to increase from 0.5% (0 M) and 2.5% (0.5 M) to 25% (1 M) and 27% (2 M).
• the cation concentration of the catholyte solution is ranging from 0.25 M to 6.00 M; for example, from 0.50 M to 3 M; for example, from 0.50 M to 2.80 M, for example from 0.75 M to 2.50 M, for example from 1.00 M to 2 M; for example, the cation concentration of the catholyte solution is about 1 M.
• the cations in the catholyte solution are one or more selected from K + , Na + , Cs + , Rb + , NH 4+ , Mg 2+ , Ca 2+ , Al 3+ .
• the catholyte solution can be a solution of K 2 SO 4 having a K+ concentration equal to or greater than 0.5M; preferably equal to or greater than 1.0 M, more preferably equal to or greater than 1.5 M.
• the catholyte solution can be a solution of K 2 SO 4 having a K+ concentration of at most 3.00 M.
• the catholyte solution is buffered.
• the buffered solution is a solution comprising one or more selected from deionized (DI) water, KHCO 3 , K 3 PO 4 , K2HPO4, KH2PO4 or the buffered solution is a mixture of glycine- and sodium hydroxide, or a mixture of H 3 BO 3 and sodium hydroxide.
• the catholyte solution is non-buffered.
• the non- buffered solution is or comprises K 2 SO 4 , KCl or any other combinations of the Cl- anions or SO4 2- anions with Na + , Cs + , Rb + , NH 4+ , Mg 2+ , Ca 2+ , or Al 3+ cations.
• the anolyte solution has an anolyte concentration between 2.0 M and 0.01M; for example, between 1.5 M and 0.05M; for example, between 1.0 M and 0.08 M; for example, between 0.5 M and 0.1 M.
• the anolyte solution has an anolyte concentration of about 0.1 M.
• the anolyte solution is neutral.
• the anolyte solution has a pH between 7.0 and 10.0; preferably a pH between 7.5 and 9.5.
• the anolyte (neutral) solution is a KHCO 3, K 2 SO 4 , or K 2 HPO 4 solution.
• the anolyte solution is acidic.
• the anolyte solution has a pH between 1.0 and 4.0; for example, between 1.5 and 3.5; for example, between 2.0 and 3.0.
• the acidic solution is an H3PO4 solution, H2SO4 solution or a combination thereof.
• the mechanism of preventing CO2 crossover in the SC-MEA is depicted in Figure 1C. Under applied potential, the protons generated at the CEL/AEL interface of the BPM migrate to the cathode, while the carbonate/bicarbonate ions generated at the cathode migrate to the CEL side of the BPM.
• the carbonate/bicarbonate ions are reverted to CO2 when being intercepted by the protons near the interface between the stationary catholyte layer and CEL (at the boundary of the diffusion layer) and subsequently diffuse back to the cathode to participate in CO 2 RR.
• Proton-induced CO 2 regeneration could also be accomplished by coupling a cation- exchange membrane and acidic anolyte since the anodic OER can supply protons. It was observed that the CO2 crossover in this system is below the detection limits. However, the experimental and theoretical analyzes showed that this system does not operate continuously because of co-ion transport and water balance issues (discussed in SI3 of Supplemental Information).
• BPM prevents CO 2 crossover in SC-MEA using neutral anolyte
• the CO2RR performance of the present BPM-based SC-MEA was firstly measured, using a flowing neutral anolyte solution (0.1 M KHCO 3 , pH ⁇ 8.2), and the results are summarized in Figures 2A to 2F.
• the operating temperature on CO 2 RR performance was then optimized and it was discovered that 35 ° C can be the optimal temperature in the presently designed BPM- based SC-MEA (see Figures 2B, 2C, 13A and section SI5 of Supplemental Information).
• a CO 2 /O 2 ratio of an anodic gas mixture (also referred to as an anode gas) was further studied to evaluate the capability to prohibit CO2 crossover in the BPM-based SC-MEA, the key to achieving a high SPU (SI1) (see references 13 and 14).
• SI1 high SPU
• the AEM-based MEA showed CO2/O2 ratios very close to 2 for current densities from 100 to 300 mA.cm-2 ( Figure 2D), indicating that the majority of the anionic charge carrier in AEM-based MEA is CO 3 2- , which causes ca. one molecule of CO2 loss per two electrons transferred.
• the CO2/O2 ratio in the anode gas produced in the present BPM-based SC- MEA is one order of magnitude lower than that in AEM-based MEA, evidencing the prevention of CO2 crossover.
• the detected anode CO2 flow is not ascribed to the acidification of KHCO 3 , as supported by the performed control experiments ( Figure 16 in SI6 of Supplemental Information).
• the CO2/O2 ratio of the anode gas decreased as the operating current density increased, which was assigned to the fact that higher current density (also higher cell voltage) decreases the pH at the CEL surface and lowers the effective diffusion coefficient of CO2 and HCO3-/CO3- in the CEL where they must move against the outward flow of hydrated H + (see references 19 and 28).
• the present BPM- based SC-MEA is shown to present high CO 2 SPU at a low inlet CO 2 flow rate.
• Figure 2E shows FEs of gas products at the cathode within a given range of CO2 feed rates (Figure 2C).
• Lowering the CO2 flow rate from 15 (Figure 3a) to 8 sccm ( Figure 3b) was shown not to cause a significant change in the gas product distribution.
• a further decrease in CO2 flow rate led to the domination of HER over the CO 2 RR, likely due to the limited CO 2 mass transport (see reference 15).
• 10% of the CO2RR FE was ‘missing’, which can be attributable to the liquid products being oxidized at the anode and/or being trapped in the stationary catholyte layer and thus not found in the analysis.
• the SPU was calculated by substituting the products' FE values into Equation (3) in section SI1 of Supplemental Information and is reported in Figure 2F.
• the SPU of SC-MEA increases from 23% (8 sccm) to 61% (1 sccm), exceeding the upper limit of the SPU for the ordinary electrolyzers producing fully CO (50%) or fully C2+ (excluding acetate) products (25%), and is also higher than the state-of- art reported SPU (30%) for producing C 2+ products (see reference 15).
• the upper limit of SPU for an AEM-based electrolyzer was also simulated.
• the upper limit of the resulting SPU should be in the range of 13% to 15% (red zone in Figure 2F, simulation details in section SI1 of Supplemental Information). Acidic anolyte further suppresses CO2 crossover
• the CO2 crossover ⁇ despite being significantly reduced ⁇ was not down to zero.
• some of the CO2 generated in the stationary catholyte layer may diffuse through the BPM's CEL, combining with hydroxide ions in the AEL and migrate to the anode.
• Table 1 Summary of energy penalty associated with CO 2 recovery energy consumption for SC-MEA, simulated AEM-based MEA ( Figure 3C), and benchmark neutral media AEM- based MEA electrolyzer from literature. Table 1 summarizes and compares the CO 2 recovery energy consumptions of the SC- MEA (acidic anolyte), and the literature benchmark neutral media AEM-based MEA electrolyzer (see reference 15).
• the present BPM-based SC-MEA can allow a 86% and 72% reduction in energy penalty associated with CO2 recovery compared to the simulated MEA electrolyzer and literature benchmark neutral media electrolyzer 15 , respectively.
• These results highlight the need for high-SPU CO2RR devices, i.e., lower energy consumption.
• CO2RR stability of the present BPM-based SC-MEA The stability of the proposed BPM-based SC-MEA operating under optimized conditions (in terms of ethylene FE) ( Figures 4A and 4B) was then investigated.
• the cathode of the system could be further designed to include a macroporous gas diffusion layer, microporous gas diffusion layer, a metallic layer containing Cu with/without Al and/or Zn and/or Mg in any form (ionic or reduced or nanoparticles), short-side-chain ionomers (e.g., Nafion or similar ionic polymers), an organic molecular compound (e.g., pyridine) either in free form or grafted into any of the above layer.
• a macroporous gas diffusion layer e.g., a metallic layer containing Cu with/without Al and/or Zn and/or Mg in any form (ionic or reduced or nanoparticles)
• short-side-chain ionomers e.g., Nafion or similar ionic polymers
• an organic molecular compound e.g., pyridine
• NafionTM 212, NafionTM XL, Fumasep (FAS-PET-130) and titanium (Ti) felt were purchased from Fuel Cell Store.
• Iridium(IV) chloride hydrate (Premion®, 99.99%, metals basis, Ir 73% min) was purchased from Alfa Aesar. The water used in this study was 18 M ⁇ Milli-Q deionized- (DI-) water.
• Nafion membranes were activated through the following procedure: 1 hour in 80 °C 1M H 2 SO 4 – 1 hour in 80 °C H2O2 – 1 hour in 1M H2SO4 – stored in DI-water. Fumasep was used as received and stored in 1M KCl.
• Piperion (40 ⁇ m) was purchased from W7Energy and stored in 0.5M KOH. Fabrication of a water dissociation catalyst layer of the customized bipolar membrane (BPM) The water dissociation catalyst layer was fabricated following a similar procedure in a previous report (see reference 18). TiO 2 nanoparticles ink were prepared by sonicating the mixture of TiO2, DI-water, and IPA with the weight ratio of 1: 833: 2833 for 30 minutes. TiO 2 nanoparticle ink was spray-coated onto a Nafion 212 membrane, of which the edges were sealed by Kapton tapes. The exposed membrane dimension was 2.2 cm ⁇ 2.2 cm. The nominal loading of TiO 2 is 0.2 mg cm-2.
• cathode gas diffusion electrodes were prepared by spray- depositing a catalyst ink dispersing 1 mg mL -1 of Cu nanoparticles and 0.25 mg mL -1 of NafionTM 1100W in methanol onto a hydrophobic carbon paper.
• the mass loading of Cu NPs in the GDE was kept at 1.5 mg/cm2. The GDEs were dried in the air overnight before experiments.
• the anode electrodes were prepared following a recipe described in Ozden, A., Li, F., Garc ⁇ a De Arquer, F.P., Rosas-Hernández, A., Thevenon, A., Wang, Y., Hung, S.F., Wang, X., Chen, B., Li, J., et al. (2020). High-rate and efficient ethylene electrosynthesis using a catalyst/promoter/transport layer. ACS Energy Lett.
• the electrode preparation procedure involves: etching the Ti felt in hydrochloric acid at 70 oC for 40 min; rinsing the etched Ti felt with DI water; immersing the Ti felt into an Ir(IV) chloride hydrate solution; drying and sintering the Ir(IV) loaded Ti felt. The loading, drying, and sintering steps were repeated until a final Ir loading of 1.5 mg cm-2 was achieved.
• Assembly of the stationary catholyte membrane electrode assembly (SC-MEA) The MEA set (5 cm2) was purchased from Dioxide Materials. A cathode was cut into a 2.1 cm ⁇ 2.1 cm piece and placed onto the MEA cathode plate with a flow window with a dimension of 2.23 cm ⁇ 2.23 cm.
• the four edges of the cathode were sealed by Kapton tapes, which also made the flow window fully covered.
• the exposed cathode area was measured every time before the electrochemical tests, which was in the range of 3.1 to 4.2 cm 2 .
• a PVFD filter membrane (2 cm ⁇ 2 cm) saturated with desirable electrolyte (sonicate in electrolyte for 15 minutes to degas) was carefully placed.
• This PVDF layer serves as the ‘stationary catholyte layer.’
• the BPMs used in neutral and acidic conditions were a customized one and a commercially available one (Fumasep), respectively, to achieve a better compromise of CO 2 RR performance and stability.
• the considerations of membrane selection can be found in SI4 of Supplemental Information.
• the electrochemical measurements were performed with a potentiostat (Autolab PGSTAT204 with 10A booster). The cell voltages reported in this work are not iR corrected.
• the exemplified flow rate of anolyte should not be taken as a limitation and different values (other than 10 mL/min) would provide an SPU of at least 60% as encompassed herein.
• the CO 2 RR gas products, oxygen, and CO 2 were analyzed by injecting the gas samples into a gas chromatograph (Perkin Elmer Clarus 590) coupled with a thermal conductivity detector (TCD) and a flame ionization detector (FID).
• the gas chromatograph was equipped with a Molecular Sieve 5A Capillary Column and a packed Carboxen-1000 Column with argon as the carrier gas. The volumetric gas flow rates in and out of the cell were measured with a bubble column.
• the FE of a gas product is calculated as follows: Where x i is the volume fraction of the gas product i, V is the outlet gas flow rate in L s -1 , P is atmosphere pressure 101.325 kPa, R is the ideal gas constant 8.314 J mol -1 K -1 , T is the room temperature in K, n i is the number of electrons required to produce one molecule of product F is the Faraday Constant 96485 C mol-1, and J is the total current in A.
• the liquid products from the cathode side of the SC-MEA were collected using a cold trap cooled to 0 ° C.
• the collected liquid was combined with anolyte (some crossover liquid product) for quantifying by the proton nuclear magnetic resonance spectroscopy (1H NMR) on an Agilent DD2500 spectrometer in D 2 O using water suppression mode and dimethyl sulfoxide (DMSO) as the internal standard.
• anolyte most crossover liquid product
• DMSO dimethyl sulfoxide
• the major cathode reactions in neutral or alkaline media include:
• the CO 2 /O 2 ratio in anode gas provides insight into the identity of the anionic charge carrier(s) that combine with the H + generated on the anode (see reference 14).
• the charge carrier is HCO 3 - or CO 3 2-
• the CO 2 /O 2 ratio in the anode gas stream is 4 or 2, respectively (see reference 14). While the other charge carriers like OH-, HCOO- or CH 3 COO- do not release CO 2 by acidification at the anode.
• the inlet CO 2 (C in ) is balanced by four parts: the CO 2 in outstream (C1), the electrochemically reduced CO 2 (C 2 ), the absorbed CO 2 (C 3 ), and the crossover CO 2 (C4).
• the mass balance of CO 2 (in mole per second) is:
• the carbon utilization efficiency is evaluated by single-pass utilization (SPU):
• HER does not contribute to C 2 but still generates hydroxide that can drive CO2 crossover.
• the CO2RR performances of the electrolyzers that show state-of-art SPU in the references were identified and summarized in Table S1. None of the reported electrolyzers can achieve an SPU exceeding 30% for C2+ production, and 44% for CO production.
• the missing FE can be ascribed to three groups of liquid products, i.e., formate, acetate, and ethanol/propanol.
• This deviation is typically called the water dissociation overpotential and represents the losses associated with generating H + and OH- and transporting it out of the interfacial layer between CEL and AEL and out of the BPM.
• a BPM induces a “thermodynamic” voltage loss of 0.83 V - however as discussed above, this is incorrect - the losses can be quite small.
• the cell voltage of a BPM- based water electrolyzer can be lower than that of an AEM-based electrolyzer at the current density up to 500 mA cm -2 (see reference 18).
• BPM electrolyzers can begin to split water with a total voltage of ⁇ 2 V, which would be impossible if there were an intrinsic 830 mV penalty for using the BPM.
• the 0.2 to 0.3 V cell voltage gap is likely ascribed to two factors: the ohmic loss due to thicker BPM (ca. 50 ⁇ m CEL + 40 ⁇ m AEL) than AEM (ca. 40 ⁇ m); the cathode pH gradient (see Figures 5A to 5C).
• the stationary catholyte layer is a 125 ⁇ m-thick 0.5 M K 2 SO 4 solution (conductivity 0.15 S cm -1 ). Although the total ionic conductivity of this catholyte is large, the H + / OH- / HCO 3 - / CO3 2- conductivity is very small. Because these are the relevant ionic charge carriers in carbon dioxide electrolysis at steady state, a large pH gradient between Cu and the bulk catholyte is induced. Establishing this pH gradient is a source of an additional concentration overpotential. As shown in Figure 5A, there is no cathode pH gradient in AEM-based MEA as long as fresh base (e.g.
• SC-MEA is likely to save energy compared to ex-situ CO 2 capture (3.5 to 4.7 GJ per ton -see reference 29), especially as the costs of renewable electricity decrease, and if cross-over of the buffer ions can be minimized or eliminated.
• the migration of protons from the CEL to Cu acidifies the stationary catholyte, making the diffusion path shorter than that in a buffer catholyte.
• this phenomenon also creates a larger pH gradient than that in buffer catholyte SC-MEA.
• the pH gradient is greater at higher current densities, as needed to drive the larger proton/hydroxide fluxes.
• the catholyte thickness can be reduced to below 10 ⁇ m and a buffer catholyte can be used with minimum cross-over that will minimize the pH gradients developed within the system and thus the concentration overpotential losses associated with them.
• the SC-MEA may be a high-total-energy-efficient system for CO2-to-C2+ production.
• SI3 Additional information for the catholyte engineering towards high SPU CO 2 RR
• the CO2 consumed for CO2RR is provided by two sources: the inlet CO 2 flow (gas), and the regenerated CO 2 (dissolved form) in the stationary catholyte layer.
• the CO2 regeneration procedure in the SC-MEA will supply 75% of the CO2 consumption if the target is to achieve 100% SPU.
• the mass transport effectiveness of the regenerated CO2 is an important consideration in the SC-MEA, which is determined by the thickness of the stationary catholyte layer because a too thick catholyte layer cannot effectively deliver the regenerated CO 2 to Cu catalyst, as analyzed below.
• the net current in the stationary catholyte layer of the SC-MEA should be primarily driven by the electromigration of protons/hydroxide and carbonate/bicarbonate ions simultaneously generated from water dissociation of BPM and reactions [1]/[2], respectively.
• the protons and carbonate/bicarbonate ions combine somewhere in the stationary catholyte layer, forming a virtual boundary where CO 2 is regenerated and diffuses towards Cu and CEL driven by the concentration gradient.
• the distances from this boundary to the Cu and CEL are noted as L 1 and L 2 , respectively.
• the zones between the boundary and Cu/CEL are noted as Zone 1 and Zone 2 for convenient discussion.
• the CO2 concentration is zero everywhere in the stationary catholyte layer. After the electrolysis starts and proceeds, the CO 2 concentration at the boundary gradually arises, driving the generated CO2 (dissolved form) to diffuse towards Cu and CEL.
• Zone 1 The CO 2 (dissolved form) that diffuses deeper into the stationary catholyte layer towards the CEL is not consumed by CO 2 RR but accumulates in Zone 2 until reaching a concentration close to that at the boundary (then no driving force). In the other direction, the CO2 diffuses towards Cu is consumed for CO 2 RR, forming a concentration gradient in Zone 1, which creates a continuous CO2 diffusion flux from boundary to Cu. Note that the real CO2 (dissolved form) concentration distribution in Zone 1 deviates from linear due to the local pH gradient. Zone 1 can be described as a diffusion layer (see reference 40). The diffusion layer thickness, L1, has great impacts on the CO2 mass transport (see reference 40).
• L1 and L2 are known to be proportional to the mobility of the corresponding ions.
• L 1 +L 2 is the total thickness of the stationary catholyte layer
• a thinner stationary catholyte layer has a thinner CO2 diffusion layer (L1), which is beneficial for CO2 mass transport from bulk to catalyst (see reference 40).
• L1 CO2 diffusion layer
• thinner, yet mechanically robust porous layers can be employed to improve the mass transport of the regenerated CO 2 , as discussed in SI2.
• the K2SO4 catholyte in the stationary catholyte layer will gradually be partially transformed to KHCO 3 over time owing to reactions [1] and [2] as well as the slow leakage of SO 4 2- to anolyte.
• the CO2RR performance of SC-MEA was measured when a 1 M KHCO 3 buffer electrolyte is used as catholyte at the beginning. It was found that using 1 M KHCO3, the SC-MEA shows an expectable C2H4 FE of ca.23% at the current density of 100 mA cm-2. However, its stability is poor in that the FE gradually decreases by > 50% after the initial 5 hours.
• HCO 3 - has a higher buffer capacity than SO 4 2- , and the pH gradient built up in KHCO 3 is thus expected to be smaller than that in K2SO4.
• the CO2 diffusion layer thickness in KHCO3 is thus likely greater than that in 0.5 M K 2 SO 4 catholyte (analyzed above).
• the regenerated CO 2 may gradually accumulate at the boundary ( Figure 6) because the concentration gradient-driven CO2 diffusion flux is lower than the CO 2 generation rate. When the accumulated aq. CO 2 reaches the saturated concentration, it could bubble out periodically and physically damage the catalyst layer and/or stationary catholyte layer.
• BPM BPM-MEA
• a customized BPM was adopted, which consisted of a layer of TiO2 nanoparticles as a water dissociation catalyst sandwiched by a CEM and an AEM.
• a commercially available BPM Framasep
• Water splitting measurement was firstly conducted to compare the resistance of customized BPM and Fumasep.
• Figure 10 shows that the BPM with TiO2 water dissociation catalyst (black plots) has lower resistance than the one without water dissociation catalyst (blue plots) and commercially available Fumasep BPM.
• a one-dimensional multiphysics model in COMSOL® was applied to investigate the catholyte layer in BPM-based CO2RR electrolyzers.
• the CO 2 reactant is provided by two sources: the inlet CO 2 flow (gas) and the regenerated CO2 (dissolved form, aq.) in the catholyte.
• the inlet CO 2 flow gas
• the regenerated CO2 dissolved form, aq.
• the cathode CO 2 supply relies more on regeneration: in an ideal case with 100% SPU and 100% C2+ selectivity, regeneration contributes 75% of the consumed CO 2 .
• the mass transport of regenerated CO2 is most critical, and that transport is governed by catholyte composition and thickness.
• electrolysis creates a pH gradient through the catholyte layer: the pH is high near the cathode and low near the CEL.
• the protons and (bi)carbonate ions recombine in the catholyte, forming CO2 (aq.) that diffuses, in response to a concentration gradient, to the Cu catalyst.
• Simulations resolve the local cathode environment as a function of dimensions, electrolyte and running conditions.
• the modeled thicknesses 250, 125, 65 and 16 ⁇ m were selected to correspond to commercially available materials.
• Use of a buffering catholyte e.g.
• Fig.1c and 1d show the simulated concentration profiles of CO 2 (aq.) in the non-buffering SC-layer.
• the CO2 (aq.) is continuously supplied to the cathode to participate in CO 2 RR, forming a concentration gradient (the boundary was defined here as the position where CO2 concentration is 1% lower than the saturated concentration) to the cathode surface.
• Prior studies have termed the zone between the cathode and this boundary the diffusion layer (see reference 40).
• the thickness of the diffusion layer controls the efficiency of CO2 (aq.) mass transport (see reference 40).
• the thicknesses of the diffusion layers are 75, 35, 12 and 5 ⁇ m for the catholyte layers with the thicknesses of 250, 125, 65 and 16 ⁇ m, respectively.
• the CO 2 (aq.) diffusion layer thickness in H-cells is typically 40-100 ⁇ m, and this does not support current densities exceeding 100 mA cm-2. It was expected that diffusion layers ⁇ 40 ⁇ m, and a corresponding catholyte thickness ⁇ 150 ⁇ m, are required for sufficient mass transport in a non-buffering catholyte.
• the total thickness could not exceed 12 ⁇ m, and the cathodic pH would not be sufficiently alkaline for selective CO 2 RR.
• the simulation results suggest the following design principles for the catholyte layer in a BPM-based electrolyzer: the local cathode pH and the diffusion layer thickness of the regenerated CO2 increase as the catholyte thickness increases; the buffering capacity of the catholyte increases the diffusion layer thickness and reduces transport. Precise control of the thickness of a non-buffering catholyte should thus offer a route to high SPU, CO2RR selectivity and reaction rate.
• the BPM employed in this work sandwiched TiO 2 nanoparticles as the water dissociation catalyst (see reference 18).
• This custom BPM can lower the cell voltage by ⁇ 1 V compared with commercial BPMs (e.g. Fumasep).
• the full cell voltage of such custom BPM-based electrolyzers is close to that of AEM systems.
• the anode CO 2 /O 2 ratio decreases as the operating current density increases, an effect that was ascribed to an increased flux of protons toward the cathode. This flux decreases the pH at the CEL surface and reduces the diffusion of CO 2 and HCO 3 7CO 3 - in the CEL (see references 19 and 28).
• the thickness of the stationary catholyte was found to have a major impact on cell voltage.
• the cell voltage of the SC-BPMEA decreases as the thickness of the SC-layer decreases (Fig. 17) from 250 ⁇ m (5.1 V, 200 mA cm -2 ) to a minimum at 65 ⁇ m (3.8 V, 200 mA cm -2 ). Further thinning the catholyte to 16 ⁇ m resulted in higher voltage (4.4 V, 200 mA cm -2 ) - an effect of the lower-porosity support layer used in the 16 ⁇ m case ( ⁇ 20% vs. > 70% for the thicker layers).
• the CO2 regeneration rate inside the SC-layer also depends on the current density, and for thicker SC-layers (e.g. > 125 ⁇ m), CO 2 bubbles are more prone to form near the CEL. These bubbles obstruct ion migration, increasing the ohmic resistance of the SC-BPMEA. Electrochemical impedance spectroscopy measurements also support this finding.
• the cell voltage of a BPM-based CO2RR electrolyzer can be as low as that of an AEM-based electrolyzer with a current density of up to 200 mA cm -2 , while suppressing unwanted crossover and providing high SPU.
• the thickness of the SC-layer also affects selectivity towards CO 2 RR. With thicknesses of 65, 125 and 250 ⁇ m, the H2 Faraday efficiencies (FEs) are consistent ( ⁇ 20% at 200 mA cm-2, Fig. 19a-19c), confirming that high local pH conditions are maintained the cathode in these cases.
• the SC-BPMEA surpasses the SPU of conventional CO2-to-C2+ electrolyzers, in which carbonate is the dominant charge carrier. Measuring the CO 2 SPUs with a restricted CO 2 flow rate is a direct approach to determining the upper bound of SPU in the CO 2 RR electrolyzers.
• the stationary catholyte thickness affects the SPU of the SC-BPMEA.
• the SPU gradually increases up to 21 , 61 and 78% for the SC-BPMEAs with SC-layer thicknesses of 250, 125 and 65 ⁇ m, respectively (Fig. 21).
• the experimental trends are generally consistent with those of the simulations.
• the SC- BPMEA with a dissolved CO 2 diffusion layer thicker than 75 ⁇ m fails to surpass the SPU limit because of insufficient mass transfer.
• a 65- ⁇ m SC-layer facilitates efficient mass transport of the regenerated CO 2 (diffusion layer thickness of 12 ⁇ m) and simultaneously promotes high local cathode pH.
• the protons also drag water molecules ( ⁇ 1 per proton) by electro-osmosis. It was accordingly calculated the water balance for different cathode products as listed in Table 2.
• the water generated and transported to the cathode appears to dilute and push out the electrolyte in the stationary catholyte layer, of which the volume is small (ca.10 ⁇ L per cm2 electrode area). This phenomenon results in flooding of the cathode (as confirmed experimentally) and loss of supporting electrolyte, thus degrading performance.
• the BPM slows co-ion transit across the membrane, compared to the CEM, by the large outward flux of OH- and H + from the water dissociating junction. Table 2.
• the cathode-anode water balance in an SC-CEMEA The cathode-anode water balance in an SC-CEMEA.
• Energy assessment of the SC-BPMEA with optimal SC-layer The energy costs (measured in gigajoules per tonne of the target product, GJ/t) for a CO 2 - to-C2+ electrolyzer include the electrolysis electrical energy, cathodic stream separation, and anodic stream separation.
• CO 2 RR performance metrics of importance include cell voltage, target product FE, SPU and CO2 crossover (see reference 12). High SPU and high energy efficiency have not been accomplished simultaneously in C 2+ electroproduction.
• SC-BPMEAs a higher SPU reduces the energy required for cathode separation, but the accompanying decrease in the ethylene selectivity (Fig.22c) elevates the specific energy requirement.
• the acidic MEA used an anion-exchange ionomer coating on the catalyst layer to promote CO2RR over HER.
• the modification of the surface with the anion exchange ionomer resulted in a higher ohmic loss, and thus the cell required potentials of 3.8 V and 4.4 V at 100 mA cm-2 and 200 mA cm-2, respectively.
• These devices thus eliminated the anodic CO 2 /O 2 separation energy but at the penalty of larger cell voltages and/or lower ethylene FEs.
• SC-BPMEA shows a cell voltage of 3.8 V at 200 mA cm -2 with an ethylene FE of 42% - voltages and selectivities comparable to the best conventional neutral-electrolyte CO 2 -to-ethylene MEAs (see reference 15).
• the energy intensity of the SC-BPMEA is 36% and 12% lower than acidic flow cell and acidic MEA, respectively (Table 3).
• the inventors have demonstrated a BPM-based CO2-to-C2+ MEA, with a judiciously- designed SC-layer between catalyst and BPM, that overcomes the (bi)carbonate- formation reactant loss issue without compromising performance.
• the composition and thickness of the SC-layer determine the CO 2 RR performance and SPU via a strong influence on the local pH and the chemistry and transport of CO2.
• the buffering capacity and the thickness of the SC-layer determine the efficiency of the regeneration, the transport, and the availability of reactant CO2. These effects were predicted in simulations and supported by experiments.
• the SC-BPMEA design largely eliminates the energy penalty associated with the CO2 loss in electrochemical CO2 reduction.
• the performance of the SC-BPMEA might be further improved using, for example, ionic liquid or other organic salts as the catholyte, and by optimizing the porosity, structure and hydrophobicity of the porous support layers.
• the CO 2 RR performance of the SC-BPMEA might be improved with new cathodic catalysts, optimizing the loading and processing of the catalyst layer, and by implementing BPMs with further-lowered water-dissociation voltage loss.
• the SC-BPMEA is a useful platform for evaluating CO2RR catalysts operating with high CO 2 utilization.
• the strategy and findings presented here are also relevant to the electrochemical systems such as nitrate reduction and (bi)carbonate reduction, where controlling dissimilar microenvironments near each electrode is useful, and the exchange/transport of species (other than OH- or H+) between cathode and anode is problematic.
• porous supports were also purchased from Fisher Scientific: 125 ⁇ m PVDF (0.45 ⁇ m pore size), 65 ⁇ m PTFE (0.44 ⁇ m pore size) and 16 ⁇ m PC (0.4 ⁇ m pore size).
• NafionTM 212, NafionTM XL, Fumasep (FAS-PET-130) and titanium (Ti) felt were purchased from Fuel Cell Store.
• Iridium(IV) chloride hydrate (Premion®, 99.99%, metals basis, Ir 73% min) was purchased from Alfa Aesar. The water used in this study was 18 M ⁇ Milli-Q deionized- (DI-) water.
• Nafion membranes were activated through the following procedure: 1 h in 80 o C 1M H2SO4 – 1 h in 80 o C H2O2 – 1 h in 1 M H 2 SO 4 – stored in DI-water. Fumasep was used as received and stored in 1 M KCl. Piperion (40 ⁇ m) was purchased from W7Energy and stored in 0.5 M KOH. Fabrication of water dissociation catalyst layer of the custom bipolar membrane (BPM) The water dissociation catalyst layer was fabricated following a similar procedure in a previous report17.
• BPM custom bipolar membrane
• TiO 2 nanoparticles inks were prepared by sonicating the mixture of TiO 2 , DI-water, and IPA with the weight ratio of 1: 833: 2833 for 30 min.
• TiO2 nanoparticle ink was spray-coated onto a Nafion 212 membrane, of which the edges were sealed by Kapton tape.
• the exposed membrane dimension was 2.2 cm ⁇ 2.2 cm.
• the nominal loading of TiO2 is 0.2 mg cm -2 .
• the TiO2-coated Nafion TM was immediately used for assembling electrolyzers once prepared.
• Electrode preparation For the CO2RR, we prepared the gas diffusion electrodes (GDEs) by spray-depositing a catalyst ink dispersing 1 mg mL-1 of Cu nanoparticles and 0.25 mg mL-1 of NafionTM 1100W in methanol onto a hydrophobic carbon paper. The mass loading of Cu NPs in the GDE was kept at 1.5 mg/cm2. The GDEs were dried in the air overnight prior to experiments.
• the OER electrode preparation procedure involves: etching the Ti felt in hydrochloric acid at 70 o C for 40 min; rinsing the etched Ti felt with DI water; immersing the Ti felt into an Ir(IV) chloride hydrate solution; drying and sintering the Ir-loaded Ti felt.
• the loading, drying, and sintering steps were repeated until a final Ir loading of 1.5 mg cm -2 was achieved.
• Assembly of the stationary catholyte membrane electrode assembly (SC-BPMEA)
• the MEA set (5 cm 2 ) was purchased from Dioxide Materials.
• a cathode was cut into a 2.1 cm ⁇ 2.1 cm piece and placed onto the MEA cathode plate with a flow window with a dimension of 2.2 cm ⁇ 2.2 cm.
• the four edges of the cathode were sealed by Kapton tape, which also made the flow window fully covered.
• the exposed cathode area was measured every time before the electrochemical tests, in the range of 3.1 to 4.2 cm2.
• a porous support layer (2 cm ⁇ 2 cm with various thicknesses, 250 ⁇ m was stacking two 125 ⁇ m-thick PVDF) saturated with desirable electrolyte (sonicated in electrolyte for 15 min to degas) was carefully placed.
• This porous support layer serves as the ‘stationary catholyte layer (SC-layer).’
• SC-layer stationary catholyte layer
• a TiO 2 -coated Nafion membrane was placed onto the SC-layer with the TiO 2 layer facing up, then covered by a Piperion (5 cm ⁇ 5 cm) membrane.
• the electrochemical measurements were performed with a potentiostat (Autolab PGSTAT204 with 10A booster). The cell voltages reported in this work are not iR corrected. The system was allowed to stabilize at the specific conditions for > 1000 seconds before recording the results. All the error bars represent standard deviations based on three measurements.
• Product analysis The CO 2 RR gas products, oxygen, and CO 2 were analyzed by injecting the gas samples into a gas chromatograph (Perkin Elmer Clarus 590) coupled with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The gas chromatograph was equipped with a Molecular Sieve 5A Capillary Column and a packed Carboxen-1000 Column with argon as the carrier gas.
• the volumetric gas flow rates in and out of the cell were measured with a bubble column.
• the FE of a gas product is calculated as follows: Where x i is the volume fraction of the gas product i, V is the outlet gas flow rate in L s-1, P is atmosphere pressure 101.325 kPa, R is the ideal gas constant 8.314 J mol-1 K-1, T is the room temperature in K, n i is the number of electrons required to produce one molecule of product F is the Faraday Constant 96485 C mol-1, and J is the total current in A.
• the liquid products from the cathode side of the SC-BPMEA were collected using a cold trap cooled to 0 o C.
• the collected liquid was combined with anolyte (some crossover liquid product) for quantifying by the proton nuclear magnetic resonance spectroscopy ( 1 H NMR) on an Agilent DD2500 spectrometer in D2O using water suppression mode and dimethyl sulfoxide (DMSO) as the internal standard.
• 1 H NMR proton nuclear magnetic resonance spectroscopy
• DMSO dimethyl sulfoxide
• the local pH and different species concentrations were simulated for different catholyte thicknesses (16 ⁇ m, 65 ⁇ m, 125 ⁇ m, and 250 ⁇ m).
• Two different catholytes K 2 SO 4 and KHCO 3 ) were used in the simulation. All the chemical reactions between species were considered in this one-dimensional modeling.
• the simulation included a 50 ⁇ m thick gas diffusion layer (GDL), a 0.1 ⁇ m thick Cu cathode catalyst (CL), a catholyte region with various thicknesses indicated above, and a cation exchange layer (CEL) boundary. Constant concentration (Dirichlet) boundary conditions were used.
• a constant concentration 37.8 mM of CO2 was assumed within the GDL layer, as this region is in direct contact with the input CO 2 flow and thus assumed to be at equilibrium with gas phase CO 2 over this region for the purposes of the simulation.
• the BPM was interpreted as a boundary with a constant species concentration (1 M H3O+ at the CEL surface), because it was assumed to generate protons as the dominant ionic charge carrier at a constant rate under constant current density (200 mA cm-2).
• a user-controlled mesh is employed in the COMSOL simulation. Edge type of mesh is used for GDL, CL, catholytes, respectively. Specifically, the mesh distribution is predefined with an interval of 500 nm for GDL and catholytes, and an interval of 5nm for CL.
• the electrochemical reactions at cathode catalyst layer are determined by the following equations: Where I i represents the partial current density for CO, CH 4 , C 2 H 4, and C2H5OH occurred at the cathode catalyst layer, respectively. n L represents the number of electrons transferred per mole reactant. F represents faraday’s constant. I totQl represents the total current density. The FEs for the specific product is determined by the experimental results. ⁇ represents the catalyst porosity value. L catatyst represents the cathode catalyst length.
• the Transport of Diluted Species physics model was used.
• the Nemst-Planck set of equations governed the species diffusion, and they were calculated in the same manner as previous work. 13,14 Migration was ignored for simplicity as the experiments were performed in the concentrated electrolyte.
• the ion species transport is thus calculated by solving the two equations below.
• J i is the molar flux
• r i represents the heterogeneous electrode reactions for CO 2 reduction that were modelled at the cathode catalyst layer.
• R i represents the rates of the homogeneous reactions indicated above.
• the Millington and Quirk model is used to determine the effective diffusivity, D i .
• ⁇ p represents porosity coefficient.
• ⁇ F,i represents tortuosity coefficient.
• the porosity value of 0.6 was used for the cathode catalyst and the porosity value of 1 for the catholyte region.
• the species diffusion coefficients are listed below. Henry’s law and sets of Sechenov equation are applied to calculate the CO2 concentration. The concentration of CO 2 in electrolytes depends on temperature and pressure. It is estimated in the same manner as previous work. The Sechenov coefficients are listed below.37 Energy assessment We evaluated the energy consumptions for electrolyzer electricity, cathodic separation, and anodic separation in the context of ethylene. We consider the state-of-the-art CO2RR systems from the literature, including alkaline flow-cell electrolyzers, neutral MEA electrolyzers, acidic flow-cells and MEAs.
• the anodic separation (for neutral MEA electrolyzer) is modelled based on an alkaline capture solvent.
• the amount of CO2 crossover to the anode is calculated for one tonne of ethylene produced.
• the energy required to separate the CO2/O2 mixture is calculated based on a recent report by Carbon Engineering, in which 5.25 GJ/tonne CO 2 thermal energy and 77 kWh/tonne CO 2 are reported to be required to capture CO2 and release at 1 bar. This energy consumption is a typical value for the alkaline capture process.

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