ELECTRODES USEFUL FOR MEMBRANE-FREE ELECTRODE ASSEMBLIES, MEMBRANE-FREE ELECTRODE ASSEMBLIES, METHODS, AND USES THEREOF

CROSS REFERENCE TO RELATED APPLICATION

[0001]This application claims priority to United States Provisional Patent Application No. U.S. 63/419,062, filed Oct. 25, 2022 the entire contents of which are hereby incorporated by reference.

FIELD

[0002]The present disclosure relates generally to electrodes useful for membrane-free electrode assemblies, membrane-free electrode assemblies, methods of making, and uses thereof.

BACKGROUND

[0003]Advances have been made in CO₂ electroreduction (CO₂R) and CO electroreduction (COR) technology to produce various feedstock chemicals and fuel.^1,2 Inspired by the commercial water electrolyzer, low-temperature CO₂ electrolysis has transitioned from H-cell type configuration to flow-cell electrolyzers wherein the use of gas diffusion layer (GDL) has enabled industrially relevant current density.³ SUMMARY

[0009]In one or more embodiments, the solid polymer electrolyte layer has a thickness that is sufficient to support the standard and/or expected operation of an electrode assembly.

[0012]In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the solid polymer electrolyte layer is prepared by the methods as herein.

[0013]In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the solid polymer electrolyte layer is directly deposited on an electrode or catalyst layer by the methods described herein.

[0014]In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations act as a catalyst.

[0015]In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations act as an ionomer-dispersed catalyst layer.

[0016]In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the resultant electrode is suitable for use in one or more of the electroreduction reactions as described herein.

[0017]In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations facilitate catalysis of one or more of the electroreduction reactions as described herein.

[0018]In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations facilitate catalysis of one or more of the gaseous components introduced into the one or more electroreduction reactions as described herein.

[0022]An ionomer is a polymer where at least some of the monomer units comprises an ionic functionality. In one or more embodiments, the ionomer is any ionomer acceptable for use in an ion exchange membrane. In one or more embodiments, the ionomer is any ionomer acceptable for use in a cation exchange membrane. In one or more embodiments, the ionomer is any ionomer acceptable for use in an anion exchange membrane. In one or more embodiments, the ionomer comprises anion exchange ionomer; cation exchange ionomer; or a combination thereof.

[0023]In one or more embodiments, the ionomer comprises a perfluorosulfonic acid polymer, a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, or a combination thereof (e.g., Nafion.)

[0024]In one or more embodiments, the ionomer comprises a 1H-Imidazole, 1,2,4,5-tetramethyl-, compound with 1-(chloromethyl)-4-ethenylbenzene polymer with ethenylbenzene (e.g., Sustainion XA9™). In one or more embodiments, the ionomer is an alkaline ionomer comprising hydrophilic poly (4-vinylbenzyl alkyl-imidazolium chloride) (e.g., Sustainion XA9™)

[0034]In one or more embodiments, the solid polymer electrolyte layer single-directionally conducts cations and/or anions when the solid polymer electrolyte layer has a thickness approaching that of a standalone membrane. In one or more embodiments, the solid polymer electrolyte layer single-directionally conducts cations and/or anions when the solid polymer electrolyte layer has a thickness approaching that of a standalone membrane, where a standalone membrane has a thickness of about 25 μm to about 200 μm.

[0035]In one or more embodiments, the solid polymer electrolyte layer single-directionally conducts cations and anions when the solid polymer electrolyte layer has a loading on the catalyst layer that is ≥100 μL/cm².

[0049]In one or more embodiments, the anode comprises any metal or metal catalyst suitable for an electrode assembly. In one or more embodiments, the anode comprises any metal or metal catalyst suitable and/or stable for use in alkaline conditions. In one or more embodiments, the anode comprises any metal or metal catalyst suitable for minimizing overpotential. In one or more embodiments, the anode comprises any metal or metal catalyst suitable for and/or stable at higher current densities. In one or more embodiments, the anode comprises a transition metal catalyst. In one or more embodiments, the anode comprises a water oxidation catalyst; an organic oxidation catalyst; an oxidation catalyst, or a combination thereof.

[0050]In one or more embodiments, the assembly as described herein can operate and is stable at higher current densities, and thus exhibits better performance relative to assemblies that cannot operate or are not stable at higher current densities.

BRIEF DESCRIPTION OF THE FIGURES

[0065]Embodiments of the present disclosure will now be described, by way of example only, with reference to the attached Figures.

[0066]FIG. 1 depicts (a) illustration of AEM incorporated MEA for CO₂ electrolysis, (b) Faradaic efficiency of electrolysis products and cell voltage at different current density with AEM (Sustainion® X37-50 Grade RT) and 1 M KOH as anolyte, (c) concentration of OH⁻, CO₃²⁻ and HCO₃⁻ in the anolyte at 50 mA/cm² (initial volume 20 mL), (d) Faradaic efficiency of CO₂ electrolysis products and cell voltage at 50 mA/cm² over time.

[0067]FIG. 2 depicts (a) illustration of CEM incorporated MEA for CO₂ electrolysis, (b) Faradaic efficiency of electrolysis products and cell voltage at different current density with cation exchange membrane (Nafion 117) using 1 M KOH as anolyte, (c) Faradaic efficiency of electrolysis products and cell voltage at 50 mA/cm² over time, and (d) salt blockage in the serpentine channel and collected salt slurry draining from cathode outlet after 10 mins of CO₂R.

[0068]FIG. 3 depicts (a) schematic of MEA using USPE, (b) Faradaic efficiency of electrolysis products and cell voltage at different current density with USPE-100 using 1 M KOH as anolyte, (c) cross-sectional SEM image of USPE-50, Faradaic efficiency of electrolysis products and cell voltage at different current density with (d) USPE-50 using 1 M KOH as anolyte, and (e) USPE-50 using 3 M KOH as anolyte, (f) Faradaic efficiency of electrolysis products and cell voltage with USPE-50 at 50 mA/cm² over time (initial anolyte was 20 mL of 1 M KOH).

[0069]FIG. 4 depicts (a) SEM image of CISPE-50-0.15M, (b) Faradaic efficiency of electrolysis products and cell voltage at different current density with CISPE-50-0.15 M, (c) CISPE infused with different K⁺ concentration at 100 mA/cm², (d) CISPE-50-1M at different current density. The anolyte was 1 M KOH.

[0070]FIG. 5 depicts (a) Cell voltage and faradaic efficiency using CISPE-50-0.15M at 100 mA/cm² during reaction for 110 hours. Flowrate of humidified CO₂ was c.a. 6 sccm. (b) Summary of two performance metrics (stability and energy consumption for C₂H₄ production) of one-step CO₂ electrolysis in MEA setup. For depicted Ref [8, 9, 12, 14, 15, 17, 19, 21, 24, 63, 64 and 65] wherein a total product analysis was not reported and a high CO₂ flowrate was used, an achievable value of cathode separation was assumed to calculate the total energy consumption. Details of the performance metrics are provided in Table S.3 and the calculation is provided at Example 2, Section H.

[0071]FIG. 6 depicts illustration of anolyte titration using a strong acid as a titrant. Phenolphthalein and methyl orange were used as pH indicators.²

[0072]FIG. 7 depicts a schematic diagram of ion transport in (a) CEM and (b) AEM.

[0073]FIG. 8 depicts electrochemical impedance results taken at OCP in a three-electrode assembly. Nyquist plots of (a) pristine PTFE, (b) Cu sputtered PTFE (Cu/PTFE) and (c) USPE-50 respectively.

[0074]FIG. 9 depicts (a) Electrochemical impedance experimental setup taken at OCP in a Flow cell system. Air was circulated in the cathode, while 1 M KOH was circulated in the anode. (b) Nyquist plots of USPE-50 and CEM (Nafion™ 117). (c) The comparison of corresponding R_s values of the catalytic systems.

[0075]FIG. 10 depicts (a) Schematic diagram of proton conductivity experiment. The proton conductivity measurements for (b) the CEM and (c) USPE-50.

[0076]FIG. 11 depicts schematic charge and mass flow: (a) Anion exchange membrane (Sustainion® X37-50 Grade RT), (b) Cation exchange membrane (Nafion™ 117), (c) USPE-50 (d) CISPE-50-0.15M. Numbers are in millimole (mM) unit for 60 minutes basis at 50 mA/cm². Initial anolyte was 100 mL of 1 M KOH. The arrows indicate mass diffusion of compounds, or migration of ions. Diffusion mechanism is related to water uptake. KOH_(aq) diffusion indicates transport of hydrated K⁺ and its associated OH⁻. Similar explanation also stands for KHCO₃(a_q) transport. In case of commercial thick AEM and CEM, the charge balance is primarily governed by unidirectional ion migration (a and b). In contrast, the charge balance is governed by bidirectional ion migration mechanism (c and d) in case of directly deposited USPE and CISPE. The anion such as OH⁻, HCO₃⁻ and CO₃²⁻ are possible to migrate in AEM. To simplify the quantification, ion migration is written as CO₃²⁻ (final product in the anolyte). The aqueous K₂CO_3(aq) and KHCO_3(aq) are possibly diffuse in AEM and CEM. For simplicity, the carbonate diffuse is assumed as KHCO_3(aq)³.

[0077]FIG. 12 depicts the concentration of OH⁻ and CO₃ as measured and calculated. The measured OH⁻ concentration was lower than the concentration would be if the OH⁻ drop was only due to OH⁻ consumption for OER reaction. The extra OH⁻ drop can be related to OH⁻ diffusion and OH⁻ reaction with KHCO₃ from the anode. The measured CO₃²⁻ concentration was higher than the CO₃²⁻ concentration would be if the increase of CO₃²⁻ concentration was only due to the CO₃²⁻ migration. The extra CO₃²⁻ concentration can be related to the diffusion of KHCO_3(aq)³.

[0078]FIG. 13 depicts the faradaic efficiency and cell voltage of Nafion™ 211 using 1 M KOH as anolyte.

[0079]FIG. 14 depicts the salt precipitated in the serpentine channel after 1 hour using USPE-100 at 20 mA/cm².

[0080]FIG. 15 depicts SEM images of the USPE-50 surface. USPE-50 contains 50 mL/cm² of Nafion coating on sputtered Cu catalyst. The top-view SEM with high magnification (left) and low magnification (right) indicated the conformal coating of the gas diffusion layer by the USPE-50.

[0081]FIG. 16 depicts optical profilometry on the USPE coated catalyst surface. The studied area was 1100 μm×1000 μm. The maximum height (thickness) was 3.15 μm with the arithmetic mean height of 0.1284 μm and the root mean square height of 0.2048 (ISO 25178).

[0082]FIG. 17 depicts the faradaic efficiency and cell voltage of USPE-50 using 0.5 M K₂CO₃ as anolyte.

[0083]FIG. 18 depicts (a) FTIR spectra of USPE-50, CISPE-50-0.15M, and CISPE-50-1M. (b) Relative ratio of integrated area of —SO₃H⁺ to C—F as the representation of proton exchange with cation in the sulfonic acid groups in the Nafion structure.

[0084]FIG. 19 depicts the SEM (left) and EDX elemental mapping (right) of the cross-section of as-prepared CISPE-50-0.15M.

[0085]FIG. 20 depicts the faradaic efficiency of CO₂R using CISPE-50-0.15M and 1 M KOH anolyte at current density of 100 and different CO₂ flowrate.

[0086]FIG. 21 depicts the faradaic efficiency and cell voltage of (a) CISPE-50 with 0.15 M LiOH and (b) CISPE-50 with 0.15 M CsOH with 1 M KOH as anolyte.

[0087]FIG. 22 depicts the faradaic efficiency of CO₂R using CISPE-50-1M and 1 M KOH anolyte at current density of 100, 150 and 200 mA/cm².

[0088]FIG. 23 depicts schematic mass transport in different system as estimated in Section 1F: (a) Nafion™ 117 membrane with 3 M KOH as anolyte, (b) Nafion™ 211 membrane with 1 M KOH as anolyte, (c) USPE-25 with 1 M KOH as anolyte, and (d) USPE-100 with 1 M KOH as anolyte. The anion such as OH⁻, HCO₃⁻ and CO₃²⁻ are possible to transport in AEM. To simplify the quantification, ion migration was written as CO₃²⁻ (final product in the anolyte).

[0089]FIG. 24 depicts the concentration of hydroxide ion and carbonate ion in the anolyte at 50 mA/cm²: Initial volume of KOH anolyte was 30 mL.

[0090]FIG. 25 depicts the effect of current density and energy consumption on the levelized cost of C₂H₄. The methodology of the economic evaluation was adapted from Jouny et al.¹¹ with addition on carbonate recovery system. All parameter except CO₂ conversion (27%), C₂H₄ selectivity (65%) and electricity price ($0.02/kWh) was considered for the base scenario from Jouny et al.¹¹.

[0091]FIG. 26 depicts the summary of two performance matrices (current density and electrolyzer efficiency for C₂H₄) of one-step CO₂ electrolysis in MEA setup.

[0092]FIG. 27 depicts (a) a schematic diagram of AEM-based MEA configuration. (b) Faradaic efficiency and cell voltage of CO electrolysis using commercial AEM (Sustainion® X37-50 Grade RT Membrane) at different current densities and (c) over time at 50 mA/cm² using 1 M KOH. (d) The ion transport within the AEM is in mmol basis after 1 hour of operation at 50 mA/cm² using 1 M KOH. Bottom arrow and top arrow represent the electromigration (bottom) and diffusion (top), respectively. See Example 4—section G for the details of the ion transport experiment.

[0093]FIG. 28 depicts (a) schematic diagram of USPE. Faradaic efficiency and cell voltage of CO electrolysis at different current densities using (b) USPE-50; (c) USPE-12 with 1 M KOH anolyte; (d) cross-sectional SEM image of USPE-12. The dotted lines indicate the thickness of the ionomer layer. (e) Faradaic efficiency and cell voltage using USPE-12 at 50 mA/cm² with 1 M KOH anolyte for 120 minutes. (f) The electromigration of K⁺ and Faradaic efficiency of C₂H₄ using different loading of USPE with 1 M KOH as anolyte at 50 mA/cm².

[0094]FIG. 29 depicts (a) SEM image of pristine CISPE-12-0.15MKOH. The dotted lines indicate the thickness of the ionomer layer. (b) The anolyte flux to the cathode for Nafion™-117, USPE-50, USPE-12, and CISPE-12-0.15MCsOH under open circuit potential. The product distribution and cell voltage of COR with (c) CISPE-12-0.15M CsOH using different concentrations of anolyte (Ni foam as anode catalyst) at 150 mA/cm² and (d) CISPE-12-0.15 M CsOH with 0.5 M KOH anolyte and NiFe-LDH anode catalyst.

[0095]FIG. 30 depicts (a) The cell voltage and Faradaic efficiency using CISPE-12-0.15M CsOH at 100 mA cm⁻² during the reaction for 206 hours. (b) Summary of two performance matrices (current density and electrolyzer efficiency for C₂H₅OH production) of COR in MEA setup using alkaline anolyte.^{14, 16, 24, 40-44}

[0096]FIG. 31 depicts an illustration of anolyte titration using a strong acid as a titrant. Phenolphthalein and methyl orange were used as pH indicators 4.

[0097]FIG. 32 depicts a schematic diagram of ion transport in (a) CEM and (b) AEM based MEA.

[0098]FIG. 33 depicts OER studies at 1 M KOH. a) LSV at a scan rate of 5 mV/sec, b) Corresponding Tafel slopes, c) EIS at 1.6 V vs RHE, d-e) LSV and the overpotential difference at a defined current densities, f) GSTAT at 50 mA cm⁻² for 16 hr.

[0099]FIG. 34 depicts EIS studies in a flow-cell system at 1 M KOH. a) a typical flow-cell setup used for EIS study, b) Corresponding Nyquist plots at OCP (square=Cu/PTFE; circle=Cu/PTFE/Nafion-111; triangle=USPE-12), c) The observed R_s and R_ct values from the EIS results of USPE-12, Cu/PTFE, and Cu/PTFE/Nafion™-117(CEM).

[0100]FIG. 35 depicts the surface SEM image and the EDS analysis of the used sample for COR using AEM with 1 M KOH for 2 hours.

[0101]FIG. 36 depicts the selectivity and cell voltage using AEM MEA at 50 mA/cm² over time using different anolytes: (a) 0.2 M KOH anolyte (C₂H₄ FE drop=0.14%/minute, cell voltage drop=1.0 mV/minute) and (b) 1 M KOH+1 M KHCO₃ anolyte (C₂H₄ FE drop=0.22%/minute, cell voltage drop=3.2 mV/minute).

[0102]FIG. 37 depicts the product distribution and cell voltage of CO electrolysis using commercial AEM with different anolytes: (a) 0.2 M KOH and (b) 2 M KOH.

[0103]FIG. 38 depicts the effect of cation size on (a) CO electrolysis and (b) hydrogen evolution reaction. The electrolyte was 1 M KOH for 1 M [K⁺], and 1 K KOH+1 M K₂CO₃ for 3 M [K⁺].

[0104]FIG. 39 depicts (a) The schematic diagram of CEM MEA. (b) The Faradaic efficiency and cell voltage of CO electrolysis using commercial CEM (Nafion™-117) at different current densities and (c) over time at 50 mA/cm² using 1 M KOH (d) The ion transport within the CEM in mmol basis for 1 hour operation at 50 mA/cm² using 1 M KOH. Top arrow and bottom arrow represent the electromigration and diffusion, respectively. See Example 4—S1G for the details of ion transport experiment.

[0105]FIG. 40 depicts the surface SEM image and the EDX analysis of the used sample for COR using CEM with 1 M KOH for 2 hours.

[0106]FIG. 41 depicts the selectivity and cell voltage using Nafion-117 at 50 mA/cm² over time using 1 M LiOH.

[0107]FIG. 42 depicts ion transport within the membrane at 50 mA/cm² for 1 hours in different configuration: (a) USPE-50, (b) USPE-25, (c) CISPE-12-0.15MKOH, and (d) CISPE-12-0.15MCsOH.

[0108]FIG. 43 depicts the top view SEM image of fresh sample of USPE-12, after Cu sputtering and direct deposition of ionomer; and before any experiments were conducted.

[0109]FIG. 44 depicts (a) The FTIR spectra of USPE-12, CISPE-12-0.15MKOH, and CISPE-12-0.15MCsOH. (b) The relative ratio of integrated area of —SO₃H to C—F as the representation of proton exchange with cation in the sulfonic acid groups in the Nafion structure.

[0110]FIG. 45 depicts the SEM and EDX images of fresh CISPE-12-0.15MKOH.

[0111]FIG. 46 depicts the selectivity and cell voltage using (a) CISPE-12-0.15 M KOH and (b) CISPE-12-0.15 M CsOH at different current densities using 1 M KOH.

[0112]FIG. 47 depicts the product distribution and cell voltage of CO electrolysis using CISPE-12-0.15MCsOH at different current densities with (a) 3 M KOH anolyte, (b) 0.5 M KOH anolyte, and (c) 0.2 M KOH anolyte using bare Ni anode catalyst.

[0113]FIG. 48 depicts the product distribution of CO electrolysis using CISPE-12-0.15M CsOH with 0.5 M KOH anolyte at different current densities. Anode catalyst:NiFe LDH. CO flowrate was 50 standard mL/min.

[0114]FIG. 49 depicts the product distribution of CO electrolysis using CISPE-12-0.15M CsOH at 100 mA/cm² with different CO flowrates. Anolyte: 0.5 M KOH; anode catalyst:NiFe LDH. For 1.2 standard mL/min, the actual flowrate was fluctuating from 0.8 to 1.4 standard mL/min.

[0115]FIG. 50 depicts the CO conversion of CISPE-12-0.15MCsOH using 0.5 M KOH anolyte with NiFe LDH at different CO flowrate.

[0116]FIG. 51 depicts the electrolyzer energy efficiency for COR-to-C₂H₄ at different current densities using pristine Ni foam (bottom curve) and NiFe LDH (top curve) as anode catalysts using 0.5 M KOH anolyte. CO flowrate was 50 standard mL min⁻¹.

[0117]FIG. 52 depicts the product distribution and cell voltage of CO electrolysis using CISPE-12-0.15MCsOH with 0.2 M KOH anolyte. Anode catalyst:NiFe LDH.

[0118]FIG. 53 depicts the product distribution of CO electrolysis using CISPE-12-0.15M CsOH at 100 mA/cm² with different anolyte concentrations. Anode catalyst:NiFe LDH.

[0119]FIG. 54 depicts a summary of performance matrices (a) current density, electrolyzer efficiency for C₂H₄ production and stability of CO electrolysis in MEA setup; (b) current density and energy efficiency for C₂H₄ production; (c) stability and energy efficiency for C₂H₄ production; (d) current density and energy efficiency for C₂H₄ and C₂H₅OH production. Data for our work was obtained using CISPE-12-0.15MCsOH with 0.5 M KOH anolyte.

[0120]FIG. 55 depicts (a) Top view appearance. SEM images of CISPE-12-0.15MCsOH after 206 hours of reaction (b) top-view and (c) cross section.

[0121]FIG. 56 depicts the SEM and EDX images of CISPE-12-0.15MCsOH after 206 hours of reaction.

[0122]FIG. 57 depicts the effect of current density and energy consumption on the levelized cost of C₂H₄.

[0123]FIG. 58 depicts (a) the schematic diagram of the MEA system; (b) partial current density of products C₂H₄ and H₂ and cell voltage at different current density; (c) schematic mass and ion transport through the membrane at 50 mA/cm². The dotted arrow and the solid arrow represent the diffusion and the electromigration of ion, respectively; and (d) change in faradaic efficiency of gas products and cell voltage at 150 mA/cm² overtime; using Sustainion® X37-50 Grade RT membrane and using 100 mL of 1 M KOH as anolyte in b-d.

[0124]FIG. 59 depicts cross sectional SEM image of 25 μL/cm² of Sustainion XA9/Cu; using solvent (a) methanol and (b) combination mixture of isopropyl alcohol and DI water (1:1 by mass). Faradaic efficiency of electrolysis gas products and cell voltage at different current density using 25 μL/cm² of Sustainion XA9/Cu; using solvent (c) methanol and (d) combination mixture of isopropyl alcohol and DI water (1:1 by mass); using 100 mL of 1 M KOH as anolyte in c and d.

[0125]FIG. 60 depicts (a) partial current density of products C₂H₄ and H₂ and cell voltage at different current density and (b) mass transport using 12.5 μL/cm² of Sustainion XA9/Cu, (c) partial current density of products C₂+ and H₂ and cell voltage at different current density and (d) mass transport using 25 μL/cm² of Sustainion XA9/Cu and (e) partial current density of products C₂H₄ and H₂ and cell voltage at different current density and (f) mass transport using 50 μL/cm² of Sustainion XA9/Cu; using 100 mL of 1 M KOH as anolyte. The dotted arrow and the solid arrow represent the diffusion and the electromigration of ion, respectively in b, d and f.

[0126]FIG. 61 depicts (a) Schematic diagram of the MEA system using Sustainion XA-9 ionomer coating on Cu catalyst; (b) cross sectional SEM image of 25 μL/cm² of Sustainion XA9/Cu; (c) partial current density towards C₂H₄ and H₂ and cell voltage at three different loadings of ionomer at 350 mA/cm²; (d) total products distribution and cell voltage at different current density using 25 μL/cm² of Sustainion XA9/Cu; using 100 mL of 1 M KOH as anolyte in c and d.

[0127]FIG. 62 depicts (a) Schematic diagram of the MEA system using bilayer ionomer coating on Cu catalyst; (b) cross sectional SEM image of 25 μL Sustainion XA9/25 μL Nafion/Cu; (c) partial current density towards C₂H₄ and H₂ and cell voltage at different current density using 25 μL/cm² of Sustainion XA9/25 μL/cm² Nafion/Cu; (d) change in faradaic efficiency of gas products and cell voltage over time at 150 mA/cm²; using 100 mL of 1 M KOH as anolyte in c and d.

[0128]FIG. 63 depicts (a) cross sectional SEM image of 25 μL/cm² of Sustainion XA9/25 μL/cm² Nafion+1 M KOH/Cu; (b) gas products distribution and cell voltage at 250 mA/cm² using different concentration of KOH infusion; (c) partial current density towards C₂₊ and H₂ products at different current density; using 100 mL of 1 M KOH as anolyte in b and c; (d) change in cell voltage and faradic efficiency of C₂₊ and H₂ at 350 mA/cm² current density over 24 hours period of time.

[0129]FIG. 64 depicts mass transport in bilayer ionomers with different cation concentration infused in Nafion layer; (a) 0 M, (b) 0.15 M, (c) 0.5 M and (d) 1 M. The dotted arrow and the solid arrow represent the diffusion and the electromigration of ion, respectively.

[0130]FIG. 65 depicts water diffusion through different ionomer samples from anode to cathode without any applied potential.

[0131]FIG. 66 depicts CO₂ mass transfer flux through different ionomer samples from cathode to anode.

DETAILED DESCRIPTION

[0132]Unless defined otherwise, all technical and scientific terms used herein have the meaning as commonly understood in the art.

[0133]As used in the specification and claims, the singular forms “a”, “an” and “the” include plural references unless the context dictates otherwise.

Carbon Dioxide Electroreduction (CO 2 R)

[0134]Economic viability of carbon dioxide electroreduction (CO₂R) relies on improved performance accompanied with scalable system design. Membranes are commonly used for the separation of reduction and oxidation products, as well as to provide a suitable micro-environment for CO₂R. Commercial membranes often address only one of the key challenges in CO₂R: either they offer suitable micro-environment for CO₂R (e.g., anion exchange membrane) or suppress carbonate cross-over (e.g., cation exchange membrane and bipolar membrane).

[0135]Inspired by the proton exchange membrane (PEM) water electrolyzers, CO₂ electrolysis in a membrane electrode assembly (MEA) has gained momentum.⁴ The use of solid polymer electrolyte in a MEA offers several advantages over liquid catholyte in a flow-type cell, including significant reduction in ohmic overpotential due to thin polymer electrolyte, protection of the catalysts from delamination and poisoning, avoidance of cell leakage, as well as ease of scale up and handling.^4,5

[0136]Three types of membranes have been reported for CO₂R: namely (i) anion exchange membrane (AEM), (ii) cation exchange membrane (CEM) and (iii) bipolar membrane (BPM).⁶ When AEM is used, the charge balance is primarily governed by the transport of anionic species (e.g., carbonate (CO₃²⁻), hydroxide (OH⁻) and bicarbonate (HCO₃⁻))^{4, 7}, and the high pH local environment at the cathode surface improves the activity of CO₂R^8-10. Nevertheless, significant loss of CO₂, due to crossover in the form of anionic species across the AEM, has been recognized as a key challenge of AEM-based CO₂ electrolyzers^7,11,12. This carbonate crossover leads to the neutralization of the alkaline anolyte over the course of reaction, which results in dissolution of Ni-based low-cost catalyst.¹³ Moreover, this sets a requirement to find active and stable catalyst for the oxygen evolution reaction (OER) in near-neutral electrolyte. Consequently, most of the long-term CO₂ electrolysis experiments in MEA cell are performed in neutral electrolyte with precious Ir-based catalyst as anode (see Table S.3)^{8, 9, 12, 14-24}.

[0137]Alternatively, a bipolar membrane (BPM) can be operated in reverse bias mode (i.e., cation exchange layer facing the cathode) to suppress undesired CO₃²⁻ crossover with the benefit of using non-noble metal catalyst (e.g., Ni) in the alkaline anolyte.^25,26 Despite these advantages and encouraging results, BPM-based configurations have their own limitations, including excessive hydrogen evolution reaction (HER) due to high H⁺ flux at the cathode, delamination of cationic and anionic layers, and high operating voltage associated with water dissociation (thermodynamic voltage ˜0.83 V) at the bipolar interface.^27,28 There are additional challenges with BPM, including the voltage drop associated with ohmic losses and low hydration of the membrane.^26,29

[0138]Another MEA cell configuration to suppress CO₃²⁻ crossover is to use a cation exchange membrane (CEM) with neutral or acidic anolyte.^12,30,31 Perfluorinated sulfonic acid (PFSA) ionomer such as Nafion-based CEM can be used in CO₂ electrolysis wherein H⁺ as well as alkali metal cations electro-migrate via ion exchange mechanism, and negatively charged sulfonic acid groups suppress anion transport (i.e., OH⁻, CO₃²⁻).³² The high mobility of H⁺ together with well-developed and robust CEM offers lowest cell resistance. The use of CEM with acidic or neutral anolyte, however, creates a highly acidic reaction environment at the cathode which favors the HER¹². It has been demonstrated that with proper interface engineering, CO₂ regeneration can be achieved at the cathode by the incoming H⁺ instead of undergoing HER.¹² However, the stability of this approach has not been demonstrated for extended experimental period (>8 hours).¹²

[0139]In all these MEA configurations for CO₂ electrolysis, the use of alkaline anolyte offers multiple benefits. Apart from permitting the use of Ni-based catalyst (as opposed to precious Ir-based ones in neutral anolyte), it supports higher conductivity, faster OER kinetics and low cell resistance, resulting in low cell voltage^{30, 33}. Additionally, alkali metal cations from the anolyte can electro-migrate to the cathode instead of H⁺ under the operating conditions. It has been demonstrated that excessive cation migration towards the cathode leads to precipitation of carbonate salts which impedes cell performance^{19, 31}. However, slow crossing of cation towards the cathode as well as the presence of a small amount of cation is necessary to achieve high CO₂ activity^34-38.

[0140]Combining the benefits of the CEM and alkaline anolyte, it was thought that alkali cation migration from the anode could be tuned by reducing the thickness of the CEM, wherein charge balance can be governed by a bidirectional flow of cations and anions. Thus, if slower migration of alkali metal cation can be achieved simultaneously with slow CO₃²⁻ cross-over through the CEM, high stability and CO₂ utilization efficiencies may result.

[0141]Described herein is an electrode useful for membrane-free electrode assemblies, the electrode comprising: a catalyst layer; and a solid polymer electrolyte layer for ion-conduction supported on the catalyst layer. As described herein, the solid polymer electrolyte layer for ion-conduction supported on the catalyst layer is directly deposited on the catalyst layer, such that is it not a standalone membrane. As used herein, ‘membrane-free’ refers to a lack of a stand-alone membrane that is introduced as a separate component into electrode assemblies. A standalone membrane is one that is not directly deposited onto an electrode, or the catalyst layer of an electrode; and/or a standalone membrane is a pre-made component that is then used in an electrode assembly.

[0142]In an example, it is described and/or demonstrated that substituting commonly used thick and standalone AEM or CEM with a directly-deposited ultrathin (˜3 μm) PFSA ionomer (e.g., nafion) enabled a high electrolysis efficiency of 27% at 100 mA/cm². This translated into lower energy costs (including CO₂ conversion and carbonate regeneration) in a one-step CO₂ electrolysis to ethylene (e.g., about 296, or about 290 GJ/ton) with 110 hours of stable operation in a MEA CO₂R with alkaline anolyte. Detailed analysis indicated that such performance may have been achieved due to effects of i) bidirectional ion transport through the ultrathin ionomer layer limiting salt precipitation and carbonate cross-over, ii) low water uptake of the ultrathin ionomer layer limiting cathode flooding and improving CO₂R selectivity and stability, and iii) low cell voltage due to use of ultrathin ionomer layer and alkaline anolyte with Ni based anode. By infusing K⁺ into the ultrathin ionomer layer, further demonstrated was a ˜90% selectivity towards CO₂R products at 200 mA/cm² with a C₂₊ partial current density of 144 mA/cm².

Carbon Monoxide Electroreduction (COR)

[0143]Power-to-chemical routes have received growing attention in the recent past to generate multi-carbon (C₂₊) products. Carbonate formation has been a challenge on direct CO₂ electroreduction (CO₂R) to C₂₊ products for deployment to the commercial scale.¹ Fortunately, high selectivity and stability have been demonstrated for CO₂ electroreduction (CO₂R) to CO using high temperature electrolyzers (solid oxide systems).^{2, 3} The high system performance of CO₂R-to-CO amplifies the opportunity of CO electroreduction (COR) to be a downstream step of a two-step decarbonization strategy (i.e., CO₂→CO→C₂₊) with higher energy efficiency, thanks to the use of CO feed in COR which can avoid carbonate formation and consequently enable the use of alkaline electrolyte. Considering the low solubility of CO (i.e., approximately 2.89×10−5 gram CO/gram H₂O at 25° C.) in aqueous electrolytes,⁴ COR has transitioned from H-cell configuration to membrane electrode assembly (MEA) electrolyzers enabling the use of gas diffusion layer (GDL) to achieve industrially applicable current density (>300 mA/cm²).⁵ The presence of solid polymer electrolyte (SPE) in MEA (as opposed to liquid electrolyte in flow-cell configuration) leads to multiple benefits, such as less tendency to cell flooding, lower ohmic overpotential and catalyst protection from degradation.^{6, 7}

[0144]The membrane governs ion transport (e.g., conductivity and selectivity) and water transport (e.g., diffusion) in MEA.³ Membranes which primarily facilitate anion transport, namely anion exchange membrane (AEM) has been recently studied for COR.^5,8 COR can be operated using AEM in MEA using alkaline anolyte without sacrificing anolyte pH.⁸ In this configuration, the charge is mainly balanced by the electromigration of hydroxide ion (OH⁻) from the cathode to the anode which recovers the consumed OH⁻ during oxygen evolution reaction (OER). Water transport also plays a role in COR. When alkaline anolyte (e.g., potassium hydroxide, KOH) is used, the concentration gradient between the anode and the cathode promotes water diffusion across the AEM along with the hydrated cations and its corresponding anion (it is called aqueous KOH (KOH_(aq))). Wheeler et al. quantified the water transport across the AEM in MEA using 1 M KOH anolyte.⁹ The same group synthesized thin (˜25 μm) AEM and reported that thinner membrane suppresses water flux to the cathode due to the water repulsion by the hydrophobic cathode (back convection).¹⁰

[0145]It is widely accepted that OH⁻ promotes selectivity for C₂₊ by enhancing the dimerization of adsorbed CO at high pH.^{11, 12} One can exploit the benefits of OH⁻ by using a cation exchange membrane (CEM) which primarily allows cation transport from the anode and suppresses the transport of locally generated OH⁻ towards the anode. This in turn maintains high local pH at the cathode and intercepts the protons before it can be consumed for hydrogen evolution reaction (HER).¹³ However, in practice, the use of CEM for COR is challenging. Apart from water diffusion, the electroosmotic drag due to the migration of hydrated cation tends to magnify the water transport to the cathode. The excessive water transport to the cathode (cathode flooding) causes pore blockage by water.¹⁰ As a result, a high-pressure flow of CO (>4 bar) tends to be required to minimize the water accumulation to maintain the COR selectivity.¹⁴

[0146]Alkaline anolyte offers lower cell voltage due to lower overpotential for OER, resulting in high electrolyzer energy efficiency, and enables the use of non-noble anode catalyst (e.g., nickel).¹⁵ It has been reported that the use of alkaline anolyte (e.g., 1 M KOH) can exhibit a C₂₊ selectivity of up to ˜91%.¹⁶ However, a study by Ozden et al. showed that excessive concentration of alkaline anolyte (>3 M KOH) also leads to lower C₂₊ selectivity due to low CO availability at the catalyst layer, which is caused by a poisoning effect of incoming cations from the anolyte.¹⁶ By using a covalent organic framework (COF) that can offer controlled cation diffusion functionalities, Ozden et al. reported high energy efficiency (towards C₂₊) and single pass conversion efficiency of 41% and 91%, respectively. Another strategy to overcome this challenge is adding a thin coating of ionomer (e.g., Nafion at a thickness of about 10 nm) on top of the catalyst layer to act as a bridge between a standalone ion exchange membrane and an electrode (e.g., to improve dispersion of gaseous components between the electrode and standalone membrane). It is reported that the Nafion layer enhances CO availability at the catalyst interface.¹⁷ Nafion is a perfluorosulfonic acid (PFSA) polymer which consists of the sulfonic acid group (SO₃⁻) as hydrophilic side-chains over the hydrophobic polytetrafluoroethylene (PTFE) backbone. The SO₃⁻ facilitates electromigration of cation, while the PTFE backbone makes Nafion a mechanically robust and chemically inert cation transport ionomer.^{18, 19} The cation conducting nature (or ionic selectivity) of Nafion can be tuned into anion conducting by modifying the SO₃⁻ with proazaphosphatranium²⁰ and dimethylpiperazinium.^{21, 22} As described herein, another strategy to adjust the ionic conductivity of an ionomer such as Nafion is by controlling the thickness using a direct deposition method.²³ Previous studies reported that controlling the thickness enables the adjustment of the charge balance²³ and lessened the cathode flooding.¹⁰ As described herein, the ionomer is deposited directly onto a catalyst layer of an electrode to act as a solid polymer electrolyte, thereby replacing and negating the need of a standalone membrane. As described herein, the ionomer is deposited at a thickness on a scale that is greater than nanometers, such as on the scale of μm. Thicknesses on the scale of nanometer could cause an electrode assembly, such as a MEA, to short-circuit, and thus would not be suitable to replace a standalone membrane.

[0147]Combining these benefits, it was hypothesized that the electromigration of cation, as well as the water transport towards the cathode, may be optimized by changing the thickness of a directly deposited PFSA layer. If the charge balance is governed by electromigration of OH⁻, water transport can also be suppressed, thereby a stable and high selectivity COR would be possible as a result.

[0148]Described herein is an electrode useful for membrane-free electrode assemblies, the electrode comprising: a catalyst layer; and a solid polymer electrolyte layer for ion-conduction supported on the catalyst layer. In an example, use of a directly-deposited cation-infused ultrathin Nafion ionomer, which substituted the need for a standalone membrane, is described and/or demonstrated for enhanced CO electrolysis as indicated by high electrolysis energy efficiency (21%) toward ethylene (C₂H₄) at 100 mA/cm² for over 200 hours of operation in MEA configuration. In a broader perspective, this performance may enable a lower energy cost of a two-step CO₂-to-C₂H₄ reaction (e.g., about 218 GJ/ton-C₂H₄) using an alkaline anolyte. This performance was accomplished using an ultrathin Nafion layer which could suppress electromigration of K⁺ and water diffusion. Selectivity was further enhanced towards COR products at higher current densities by implanting Cs⁺ cation in the directly deposited Nafion layer. In contrast to the cation that electromigrates from the anode and accumulates at the cathode which subsequently impedes CO availability, the infused cation in the directly deposited solid polymer electrolyte (SPE) was found to maintain just the stoichiometric amount to enhance C₂₊ selectivity. In an example, C₂₊ selectivity refers to selectively forming carbon products comprising at least two carbons (e.g., C₂CH₄, C₂H₅OH, etc).

[0152]In one or more examples, the solid polymer electrolyte layer is deposited directly on the catalyst layer.

[0155]In one or more examples, the solid polymer electrolyte layer has a thickness that is sufficient to support the standard and/or expected operation of an electrode assembly. In one or more embodiments, the solid polymer electrolyte layer has a thickness that is sufficient to reduce, inhibit, or prevent short-circuiting of any electrode assembly is it used with.

[0156]In one or more examples, the solid polymer electrolyte layer has a thickness that is on the order of micrometers (μm).

[0159]In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the solid polymer electrolyte layer is prepared by the methods as herein.

[0160]In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the solid polymer electrolyte layer is directly deposited on an electrode or catalyst layer by the methods described herein.

[0161]In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations act as a catalyst.

[0162]In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations act as an ionomer-dispersed catalyst layer.

[0163]In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the resultant electrode is suitable for use in one or more of the electroreduction reactions as described herein.

[0164]In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations facilitate catalysis of one or more of the electroreduction reactions as described herein.

[0165]In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations facilitate catalysis of one or more of the gaseous components introduced into the one or more electroreduction reactions as described herein.

[0169]An ionomer is a polymer where at least some of the monomer units comprises an ionic functionality. In one or more examples, the ionomer is any ionomer acceptable for use in an ion exchange membrane.

[0170]In one or more examples, the ionomer is any ionomer acceptable for use in a cation exchange membrane. In one or more examples, the ionomer is any ionomer acceptable for use in an anion exchange membrane. In one or more embodiments, the ionomer comprises anion exchange ionomer; cation exchange ionomer; or a combination thereof.

[0171]In one or more examples, the ionomer comprises a perfluorosulfonic acid polymer, a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, or a combination thereof (e.g., Nafion.)

[0172]In one or more examples, the ionomer comprises a 1H-Imidazole, 1,2,4,5-tetramethyl-, compound with 1-(chloromethyl)-4-ethenylbenzene polymer with ethenylbenzene (e.g., Sustainion XA9™) In one or more embodiments, the ionomer is an alkaline ionomer comprising hydrophilic poly (4-vinylbenzyl alkyl-imidazolium chloride) (e.g., Sustainion XA9™)

[0182]In one or more examples, the solid polymer electrolyte layer single-directionally conducts cations and/or anions when the solid polymer electrolyte layer has a thickness approaching that of a standalone membrane. In one or more examples, the solid polymer electrolyte layer single-directionally conducts cations and/or anions when the solid polymer electrolyte layer has a thickness approaching that of a standalone membrane, where a standalone membrane has a thickness of about 25 μm to about 200 μm.

[0183]In one or more examples, the solid polymer electrolyte layer single-directionally conducts cations and anions when the solid polymer electrolyte layer has a loading on the catalyst layer that is ≥100 μL/cm².

[0197]In one or more examples, the anode comprises any metal or metal catalyst suitable for an electrode assembly.

[0198]In one or more examples, the anode comprises any metal or metal catalyst suitable and/or stable for use in alkaline conditions.

[0199]In one or more examples, the anode comprises any metal or metal catalyst suitable for minimizing overpotential.

[0200]In one or more examples, the anode comprises any metal or metal catalyst suitable for and/or stable at higher current densities.

[0201]In one or more examples, the anode comprises a transition metal catalyst. In one or more examples, the anode comprises a water oxidation catalyst; an organic oxidation catalyst; an oxidation catalyst, or a combination thereof.

[0202]In one or more examples, the assembly as described herein can operate and is stable at higher current densities, and thus exhibits better performance relative to assemblies that cannot operate or are not stable at higher current densities.

[0242]In one or more embodiments, the solid polymer electrolyte layer is deposited directly on the catalyst layer. In one or more embodiments, the solid polymer electrolyte layer is deposited directly on the catalyst layer such that is it is not a standalone membrane. In one or more embodiments, the solid polymer electrolyte layer has a thickness that is sufficient to support the standard and/or expected operation of an electrode assembly. In one or more embodiments, the solid polymer electrolyte layer has a thickness that is sufficient to reduce, inhibit, or prevent short-circuiting of any electrode assembly is it used with. In one or more embodiments, the solid polymer electrolyte layer has a thickness that is on the order of micrometers (μm). In one or more embodiments, the solid polymer electrolyte layer has a thickness that is not on the order of nanometers (nm), as such a thickness may prevent an assembly from functioning; such as, by short circuiting.

[0243]In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the solid polymer electrolyte layer is prepared by the methods as herein. In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the solid polymer electrolyte layer is directly deposited on an electrode or catalyst layer by the methods described herein. In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations act as a catalyst. In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations act as an ionomer-dispersed catalyst layer. In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the resultant electrode is suitable for use in one or more of the electroreduction reactions as described herein. In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations facilitate catalysis of one or more of the electroreduction reactions as described herein. In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations facilitate catalysis of one or more of the gaseous components introduced into the one or more electroreduction reactions as described herein. In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the resultant electrode provides C₂₊ selectivity in one or more of the electroreduction reactions as described herein.

[0244]An ionomer is a polymer where at least some of the monomer units comprises an ionic functionality. In one or more embodiments, the ionomer is any ionomer acceptable for use in an ion exchange membrane. In one or more embodiments, the ionomer is any ionomer acceptable for use in a cation exchange membrane. In one or more embodiments, the ionomer is any ionomer acceptable for use in an anion exchange membrane. In one or more embodiments, the ionomer comprises anion exchange ionomer; cation exchange ionomer; or a combination thereof. In one or more embodiments, the ionomer comprises a perfluorosulfonic acid polymer, a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, or a combination thereof (e.g., Nafion.) In one or more embodiments, the ionomer comprises a 1H-Imidazole, 1,2,4,5-tetramethyl-, compound with 1-(chloromethyl)-4-ethenylbenzene polymer with ethenylbenzene (e.g., Sustainion XA9™). In one or more embodiments, the ionomer is an alkaline ionomer comprising hydrophilic poly (4-vinylbenzyl alkyl-imidazolium chloride) (e.g., Sustainion XA9™). In one or more embodiments, the ionomer comprises a hydrocarbon backbone (e.g., AEMION™). In one or more embodiments, the ionomer is an alkaline ionomer comprising hydrophilic poly (4-vinylbenzyl alkyl-imidazolium chloride) (e.g., AEMION™).

[0245]In one or more embodiments, the solid polymer electrolyte layer single-directionally conducts cations and/or anions when the solid polymer electrolyte layer has a thickness approaching that of a standalone membrane. In one or more embodiments, the solid polymer electrolyte layer single-directionally conducts cations and/or anions when the solid polymer electrolyte layer has a thickness approaching that of a standalone membrane, where a standalone membrane has a thickness of about 25 μm to about 200 μm. In one or more embodiments, the solid polymer electrolyte layer single-directionally conducts cations and anions when the solid polymer electrolyte layer has a loading on the catalyst layer that is 100 μL/cm².

[0270]In one or more embodiments, the anode comprises any metal or metal catalyst suitable for an electrode assembly. In one or more embodiments, the anode comprises any metal or metal catalyst suitable and/or stable for use in alkaline conditions. In one or more embodiments, the anode comprises any metal or metal catalyst suitable for minimizing overpotential. In one or more embodiments, the anode comprises any metal or metal catalyst suitable for and/or stable at higher current densities. In one or more embodiments, the anode comprises a transition metal catalyst. In one or more embodiments, the anode comprises a water oxidation catalyst; an organic oxidation catalyst; an oxidation catalyst, or a combination thereof.

[0271]In one or more embodiments, the assembly as described herein can operate and is stable at higher current densities, and thus exhibits better performance relative to assemblies that cannot operate or are not stable at higher current densities.

[0300]In one or more embodiments, the solvent is used for dispersing the ionomer, otherwise referred to as ionomer resin. In one or more embodiments, the solvent comprises organic solvents, volatile organic solvents, water, aqueous solutions, or a combination thereof. In one or more embodiments, the solvent comprise methanol, ethanol, isopropanol, acetone, dichloromethane, THF, water, or a combination thereof.

[0304]To gain a better understanding of the invention described herein, the following examples are set forth. It should be understood that these examples are for illustrative purposes only. Therefore, they should not limit the scope of this invention in anyway.

EXAMPLES

Example 1—Directly-Deposited Ultrathin Solid Polymer Electrolyte for CO 2 Electrolysis, Such as Enhanced CO 2 Electrolysis

[0305]Commercial membranes often address only one of the challenges in CO₂R: either they offer suitable micro-environment for carbon dioxide electroreduction (CO₂R) (e.g., anion exchange membrane) or suppress carbonate cross-over (e.g., cation exchange membrane and bipolar membrane). Herein described is a cation-infused bifunctional ultrathin (˜3 μm) solid polymer electrolyte (CISPE) that, in one or more embodiments or examples, may address both of these challenges via bidirectional ion transport mechanism and suppressed anolyte diffusion or cathode flooding. An example of this directly-deposited ultrathin CISPE (that substitutes commonly used pre-made membrane), as prepared and tested, offered 110 hours of stable operation with relatively low energy cost (e.g., about 296, or about 290 GJ/ton) and/or full-cell energy efficiency of 27% for a one-step CO₂ electrolysis to C₂H₄ at 100 mA/cm² (e.g., for the end-to-end process of CO₂ capture and electro-reduction, carbonate regeneration, CO₂ separation from anode and cathode streams in a membrane electrode assembly (MEA) CO₂R cell.) When compared to commercial anion exchange membrane, the CISPE demonstrated 40% reduction of carbonate crossover. By applying this design strategy, a 90% selectivity was achieved towards CO₂R products at 200 mA/cm² with C₂+ partial current density of 142 mA/cm². The herein described CISPE may offer stability, and efficient electrolysis of high-value feedstock chemicals and fuels using low-cost catalysts.

[0306]Performance was evaluated relative to a reference AEM in a MEA cell with a cathode consisting of a porous polytetrafluoroetheylene (PTFE) gas diffusion layer (GDL) sputtered with a ˜300 nm Cu catalyst layer (FIG. 1a) (see Example 2 for more details). The Faradaic efficiency (FE) of CO₂R in this configuration in 1 M potassium hydroxide (KOH) anolyte and at different current densities is demonstrated in FIG. 1b. A continuous drop of the selectivity for H₂ and CO with an increase of the C₂H₄ selectivity was noted as the current density was increased from 25 mA/cm² to 100 mA/cm²; this was found to be consistent with literature reports.³³ The highest partial current density towards C₂H₄ using AEM was 45 mA/cm².

[0307]The concentration of OH⁻, HCO₃⁻ and CO₃²⁻ in the anolyte was measured over time at 50 mA/cm² to assess the CO₃²⁻ and HCO₃⁻ cross-over through the AEM (FIG. 1c) (see Example 2, section F). Initially, the oxygen evolution reaction (OER, R.1 below) occurred in the anode given that plenty of OH⁻ were available in the anolyte as indicated by high concentration of OH⁻. As the OH⁻ was continuously consumed in OER, the OH⁻ concentration decreased with time (FIG. 1c). In addition, higher water uptake of the thick AEM contributed to the decline of OH⁻ concentration (OH⁻ was hydrated)³⁹. In the cathode, CO₂ electrolysis along with hydrogen evolution reaction (HER) occurred with OH⁻ generation (see reactions R.6 and R.7 below). The OH⁻ instantaneously reacted with unreacted CO₂ and HCO leading to CO₃ formation via reactions R.8 and R.9 below. The formed CO₃²⁻ also possibly reacted with excess CO₂ and H₂O to form HCO₃⁻ (R.10 below). This enabled anions (OH⁻, HCO₃⁻, CO₃²⁻) migration and increase of CO₃²⁻ concentration (R.5 below) in the anolyte (FIG. 1c). This observation was consistent with previous studies using AEMs, wherein the high pH local environment during CO₂R lead to formation of carbonates (CO₃²⁻ and HCO₃⁻ with OH⁻) which then crossed-over to the anolyte^7,40. The present analysis suggested that when using an AEM, the charge was balanced by the CO₃²⁻ migration along with possible OH⁻ and KHCO₃ migration to the anode (FIG. 11a), which was consistent with previous computational and experimental studies^{4, 7, 41, 42}. Due to the higher water uptake of the thick membrane, the diffusion of KOH and KHCO₃ or K₂CO₃ through the AEM was also observed in the present experiments, which was consistent with earlier reports^39,43. Both KHCO₃ and K₂CO₃ diffusion as well as CO₃²⁻ and HCO₃⁻ migration were responsible for the increase of CO₃²⁻ concentration in the anolyte as shown in FIG. 12. After 90 minutes of reaction, the OER (R.2) occurred in the anode due to the absence of OH⁻. The proton (H⁺) from the OER reacted with CO₃²⁻ to form HCO ₃⁻, CO₂ and H₂O via the reactions R.3 and R.4. This explained the increase of HCO₃⁻ and the decrease of CO₃²⁻ after 120 minutes as shown in FIG. 1c.

Anode:

Cathode:

[0308]The change of the OH⁻ concentration hampered the steady electrolysis operation (FIG. 1d). The decrease of OH⁻ lead to a rise of the overall cell voltage, which was attributed to the sluggish OER kinetics and higher ohmic resistance due to lower ion conductivity of HCO₃⁻ and CO₃²⁻ compared to OH⁻.^{33, 44} A gradual drop of the CO₂R product selectivity and increased HER was noted, as shown in FIG. 1d. This observation was consistent with a report by Endrödi et al. who demonstrated that the use of alkaline anolyte with an AEM leads to salt precipitation at the cathode, which erodes the selectivity towards CO₂R products.³¹ These results indicated that continuous operation in this mode would therefore lead to a decline of the CO₂ product selectivity as well as near-neutral anolyte (due to CO₃²⁻ crossover) with consequent Ni catalyst degradation. In other words, the AEM in the MEA cell with alkaline anolyte and Ni based OER catalyst was not stable for extended operation.

[0309]Use of a CEM was considered to be a possible solution to suppress CO₃²⁻ crossover to the anode. However, Colin et al. demonstrated that the CEM incorporated in a MEA cell with deionized (DI) water as anolyte primarily drives the HER at the cathode (˜100% selectivity).¹² To avoid such high proton flux from OER, 1 M KOH as the anolyte was used and CO₂R performance measured. A MEA was assembled using a CEM (Nafion 117 membrane), 300 nm sputtered Cu catalyst on PTFE GDL (FIG. 2a) (see Example 2 for more details). The faradaic efficiency (FE) of CO₂R in this configuration is shown in FIG. 2b at different current densities. A maximum selectivity of 40% towards C₂H₄ was obtained at 50 mA/cm². Further increase of the current shifted the selectivity primarily towards the HER. The CO₂ electrolysis activity at low current densities can be explained by the basic local environment of the cathode for CO₂ electrolysis due to OH⁻ diffusion from anode. However, the selectivity and the cell voltage was not stable even at 50 mA/cm² (FIG. 2c). A salt slurry was observed at the cathode outlet within few minutes (FIG. 2d). In contrast to stable operation using the CEM in the MEA with neutral anolyte as reported in literature¹², the charge was mainly balanced by alkali cation (K⁺) migration to the cathode when KOH was used as anolyte. Additionally, CO₃²⁻ was not detected in the anolyte when using a CEM in the titration experiment (FIG. 11b). The combination of K⁺ migration and negligible CO₃²⁻ crossover to the anolyte resulted in an accumulation of K⁺ and CO₃²⁻ in the cathode. This then lead to salt formation at the cathode, loss in selectivity, and higher cell voltage¹⁹.

[0310]The above results using the AEM and CEM in a MEA cell with alkaline anolyte signify some challenges and benefits. The ease of anion transport through the AEM compromises the stability of the cell, as demonstrated by the loss of selectivity and anolyte alkalinity. In the case of the CEM, charge balance by the K⁺ migration to the cathode resulted in failure of the cell, as evident from CO₂R selectivity drop, salt accumulation, and rapid increase in cell voltage. Despite these challenges, one of the key benefits of CEM that was desirable to exploit was the ability to migrate K⁺ from the anode to cathode and to block CO₃²⁻ transport across the membrane. Studies have demonstrated a 3-fold increase in CO₂ reduction selectivity due to crossover of K⁺ from the anode to the cathode in a MEA with BPM³⁸. However, the results described above suggested that excessive K⁺ migration was detrimental to stable cell operation when commercial thick AEM and CEM (FIG. 1d and FIG. 2c) were used. Based on these observations, it was hypothesized that the anion transport may be controlled by tuning the total negative charge of sulfonic acid (e.g., fixed —SO₃⁻) sites within a PFSA; which may be achieved by reducing the thickness of the PFSA. Charge balance may be achieved via bidirectional migration of K⁺ and CO₃²⁻ ions. As a result, suppressed migration of K⁺ to the cathode may enhance CO₂R selectivity and reduce CO₃²⁻ crossover (as compared to the AEM).

[0311]To investigate this, CO₂R was carried out using a CEM (Nafion 211 membrane, 25 μm; CEM-211), which was substantially thinner than Nafion 117 (183 μm; CEM-117) in 1 M KOH as anolyte. While ˜0.5 V lower cell voltage was observed at similar current densities with Nafion 211 due to the lower resistance, no noticeable difference was observed of the CO₂ electrolysis selectivity compared to the Nafion 117 membrane (FIG. 2b and FIG. 13). Similar to the MEA with Nafion 117 system, a reduction of the CO₂ product selectivity and a rise in cell voltage was observed when using Nafion 211 due to salt accumulation at the cathode GDL. This result indicated that the electro-migration of K⁺ through the 25 μm thick Nafion 211 membrane was similar to that of the 183 μm thick Nafion 117 membrane.

[0312]Limited by the availability of Nafion membrane with thickness below 25 μm, PFSA ionomer such as Nafion was spray coated to act as an ultrathin solid polymer electrolyte (USPE) on the sputtered Cu surface (see Example 2 for details) (FIG. 3a). The use of USPE may substitute the need for commonly used thick pre-made commercial membranes; which may lead to a reduction of the cell voltage. It was hypothesized that apart from the ease and flexibility of tuning the thickness of the USPE, the direct coating approach^46-49 may protect the catalyst from commonly reported degradation mechanism (e.g., dissolution, aggregation etc.) due to the intimate contact between the catalyst and ionomer.^50-53 Additionally, intimate contact between the catalyst and ionomer may allow enhanced CO₂ transport across the catalyst surface⁹ and it may also minimize anolyte diffusion.

[0313]The USPE was developed by spraying ionomer with a loading of 100 μL/cm² (denoted as USPE-100) on the surface of Cu catalyst and performed CO₂R in 1 M KOH as anolyte (see Example 2 for more details). While no noticeable improvement in selectivity was observed, better cell stability (e.g., the salt accumulation was significantly lower) was observed over commercially available Nafion 117 and Nafion 211 (FIG. 3b and FIG. 14). This result provided preliminary evidence that salt precipitation due to K⁺ transport towards cathode may be suppressed by using a USPE. Next, a USPE was developed with ionomer loading of 50 μL/cm² (denoted as USPE-50) and CO₂R was performed in 1 M KOH as anolyte. Cross sectional SEM indicates a USPE-50 layer with ˜3 μm thickness (FIG. 3c). The uniformity and conformal coating of the USPE-50 was further confirmed using top-view SEM and optical profilometry analysis (FIG. 15 and FIG. 16). With 1 M KOH anolyte, a reduction of the HER selectivity (˜9%) and enhanced selectivity towards C₂H₄(˜38%) was found at the current densities of 50 and 75 mA/cm² (FIG. 3d) as compared to USPE-100 (FIG. 3b). The highest C₂H₄ partial current density that was observed and measured was 28 mA/cm². At current densities beyond 75 mA/cm², a drop in C₂H₄ selectivity was measured, with enhanced selectivity towards the HER and methane.

[0314]It was believed that the use of 1 M KOH anolyte would limit the CO₂ availability at the cathode at higher current densities due to OH⁻ diffusion and consequent carbonate formation.³⁹ To investigate this, the concentration difference was tripled by using 3 M KOH as anolyte and CO₂R performance was measured. As shown in FIG. 3e, the CO₂R selectivity decreased when using 3 M KOH as compared to 1 M KOH, which supported the hypothesis that OH⁻ diffusion is faster and consequently limits CO₂ availability at the cathode. To further investigate this, 0.5 M K₂CO₃ anolyte was used. The absence of OH⁻ diffusion (since replaced with carbonate) deteriorated CO₂R activity (see Example 2, FIG. 17). This was attributed to the absence of OH⁻ in the anolyte (e.g., 0.5 M K₂CO₃): the OER lead to the generation of H⁺ at the anode (2H₂O→O₂+4H⁺+4e⁻), which in turn acidified the cathode and promoted HER.

[0315]It was hypothesized that the improved selectivity of USPE-50 as compared to CEM-117, CEM-211, and USPE-100 may be attributed to the reduced migration and deposition of K⁺ at the cathode. The presence of K⁺ in the cathode may suppress HER and enhance CO₂R. To investigate this, the stability of CO₂R using USPE-50 (FIG. 3f) was tested. In contrast to CEM-117, CEM-211, and USPE-100, no salt was observed on the cathode and in the serpentine channel during 5 hours run when using USPE-50 (FIG. 3f, inset). The C₂H₄ and H₂ selectivity gradually improved and declined over the first 3 hours, respectively; which indicated that the reduced migration of K⁺ plays an important role to improve the selectivity over time. On the other hand, the direct deposition of USPE may provide intimate contact between PFSA ionomer and the catalyst surface, as opposed to the possibility of having an air-gap between the pre-made thick membrane (Nafion 117 and Nafion 211) and catalyst surface. Despite the ultrathin ionomer layer, the intimate contact may play an important role to suppress KOH diffusion induced salt formation at the cathode; leading to longer effective CO₂R. KOH loss from the anolyte without applied potential was compared for Nafion-117 and USPE-50, which should be primarily governed by KOH diffusion (see Example 2, Section G). An estimate of nearly twice the mass loss when using Nafion-117 as compared to USPE-50 was made. Despite the ultrathin layer of USPE, the intimate contact was important to suppress KOH diffusion and salt precipitation. Due to the gradual loss of OH⁻ (due to OER induced loss and HCO₃⁻ diffusion), a gradual increase of the cell voltage over time was noted (FIG. 3f). It was considered that the lower cell voltage of USPE-50 (FIG. 3d) versus Nafion™ 117 (FIG. 2b) may be attributed to the ultrathin layer with enhanced ionic conductivity. Electrochemical impedance spectroscopy (EIS) measurements (see Example 2, Section I) revealed lower resistance with facile ion transport in the USPE as compared to Nafion™ 117 (FIG. 9).

[0316]The presence of alkali metal cations such as Cs⁺, K⁺, Na⁺, Li⁺ in the liquid catholyte have been reported to have positive effect on the CO₂ electrolysis activity and selectivity in a flow-type CO₂ electrolysis cell.^{31, 37, 55-58} When using a solid polymer electrolyte (SPE) such as in a MEA cell, the metal cations can be transported from alkaline anolyte³¹, which can have a promotional effect on CO₂R at low current density (˜50 mA/cm²) (FIG. 3d). However, at higher current density, the electro-migrated K⁺ was not sufficient to suppress HER. Increasing the concentration of KOH to enhance K⁺ transport did not suppress HER, instead it had a detrimental effect on CO₂R (FIG. 3e). It was considered that the presence of cations in the local environment may be important to improve CO₂R selectivity at higher current density. It was considered that by infusing alkali metal cation into the USPE during the spray coating, a cation-infused USPE (denoted as CISPE) may be developed to maintain low activity of HER and enhance partial current density towards C₂H₄.

[0317]First, a Nafion solution was mixed with 0.15 M KOH solution and then the mixture was sprayed on the electrode with a loading of 50 μL/cm² (see Example 2, Section C) (this configuration is noted as CISPE-50-0.15M). Initially, 0.15 M KOH was selected to be infused to the Nafion since the H⁺ of the —SO₃⁻ group can be stoichiometrically exchanged with K⁺ at ˜0.13 M KOH (see Example Section E). The ion exchange between H⁺ in the —SO₃⁻ group with K⁺ was further indicated by using FTIR (FIG. 18). The thickness of CISPE-50-0.15M was similar to that of USPE-50 (FIG. 4a). The presence of K⁺ in the Nafion was further indicated by elemental mapping using Energy Dispersive X-Rays (FIG. 19).

[0318]It was noted that CISPE-50-0.15M coating suppressed HER activity at higher current density as compared to USPE-50 (FIG. 4b, FIG. 20, and FIG. 3d, respectively). The partial current density towards C₂H₄ reached 52 mA/cm² when using CISPE-50-0.15M as compared to 28 mA/cm² for USPE-50. These results indicated a positive role of alkali metal cation in promoting the activity of CO₂ electrolysis when infused/implanted in the polymer metrics of Nafion. Experiments were carried out by infusing smaller cation (Li⁺) and larger cation (e.g., Cs⁺) in CISPE (FIG. 21). Cs⁺ infusion led to a higher partial current density of 60 mA/cm². This indicated that the promotion effect of larger cations in liquid electrolyte on CO₂ electrolysis performance can be extended towards solid polymer electrolytes.^{37, 55, 59, 60} The implanted cations (that is the case with CISPE-50-0.15M) were found to be more effective than the ones transported from the anolyte (i.e., USPE-50) to improve the CO₂R partial current density. In another words, increasing the concentration of cation in the local environment of CO₂R is an important factor to improve C₂₊ selectivity. To test this, experiments using the CISPE-50 with higher loading of K⁺ were carried out (FIG. 4c). At current density of 100 mA/cm², increasing the K⁺ loading in the Nafion (from 0.15 M to 1 M) increased the faradaic efficiency of gas products (e.g., CO and C₂H₄ combined) from 47% to 57%, while the faradaic efficiency of H₂ remained the same. At 200 mA/cm², 90% selectivity towards CO₂R products was achieved using CISPE-50-1.0M and 1 M KOH anolyte, with 100 mA/cm² partial current density towards C₂H₄; the latter is >2-fold and >5-fold higher than that of commercial AEM and Nafion-117 in the same testing condition, respectively (FIG. 1b, FIG. 2b, and FIG. 4d). The partial current density towards total C₂₊ products was estimated to be 144 mA/cm² (FIG. 22).

[0319]To investigate the underlining cause of enhanced CO₂R performance of CISPE as opposed to the AEM (Sustainion® X37-50 Grade RT), CEM (Nafion 117), the mass transport (both charged and neutral) through ultrathin solid polymer electrolytes were investigated (see Example 2 Section F). In the case of USPE-50, the charge was balanced by bidirectional ion migration including the migration of K⁺ to the cathode and migration of CO₃²⁻ to the anode (FIG. 11). The bidirectional ion migration provided: (1) limited crossover of CO₃²⁻, its measured concentration was 40% lower than the case with AEM (FIG. 11a); and (2) limited K⁺ migration, 74% lower than the case with CEM, which resulted in minimum salt formation (FIG. 11a and FIG. 24). With further control experiments using thinner USPE-25 (25 μL/cm² Nafion) and thicker USPE-100 (100 μL/cm² Nafion) (FIG. 23c and FIG. 23d, respectively), it was considered that the thickness of the USPE can be fine-tuned to control the bidirectional ion transport mechanism between anode and cathode. On the other hand, the present mass balance analysis indicated that the water flux (as observed in case of AEM and Nafion™ 117 as in FIG. 11a and FIG. 11b) to the cathode (and associated salt diffusion) was suppressed in the presence of USPE (FIG. 11c, FIG. 23c and FIG. 23d). This further supported the hypothesis that direct deposition of USPE ensured lower water uptake and intimate contact at the catalyst/ionomer interface, which suppressed cathode flooding, and salt precipitation (see Example 2, Section G).

[0320]Ion transport in CISPE-50-0.15M was further investigated (FIG. 11d). The transport properties of Nafion ionomer depend on its water content, which is governed by the interaction between the cation and fixed —SO₃⁻ sites. Shouwen et al. reported that the water uptake (A) capacity of Nafion membrane decreased upon replacing H⁺ with monovalent cations in the order of increasing ionic radius: λ(H⁺)>λ(Li⁺)>λ(Na⁺)>λ(K⁺)>λ(Cs⁺).⁶¹ Lower water flux also resulted in lower diffusion of KHCO₃ from the cathode to the anode in CISPE-50-0.15M as compared to the case of USPE-50.

[0321]At 100 mA/cm², further demonstrated was over 110 hours of stable operation using CISPE-50-0.15 (FIG. 5a) with 18% single pass conversion efficiency. It was noted that such high stability was achieved while using alkaline anolyte to reduce the cell voltage, which led to a demonstrated record high one-step CO₂-to-C₂H₄ electrolysis efficiency of 27% at 100 mA/cm² (Example 2, Section H, and FIG. 26). This demonstrated that use of a directly-deposited ionomer (e.g., that substituted a pre-made commercial membrane) MEA wherein ˜3 μm coating of cation-infused USPE enhanced CO₂ electrolysis performance. An energy calculation for the end-to-end process (including CO₂ capture, conversion and carbonate regeneration, see Example 2, Section H) indicated that the herein described CISPE enables low energy cost (e.g., about 296, or about 290 GJ/ton) for one-step CO₂ electrolysis to C₂H₄as compared to prior reports (FIG. 5b, Table S.3, FIG. 26). While the energy for carbonate regeneration from an alkaline anolyte was comparable to that of the energy required for CO₂ separation from produced O₂ in a neutral anolyte system (Example 2, section H); in an example, the overall low energy cost of the herein described system appeared to be from the use of alkaline anolyte which offers higher electrolysis efficiency.

[0322]Relative to prior systems, the herein described system appears to offer: 1) use of ultrathin ionomer with bidirectional ion transport as well as the use of alkaline anolyte, leading to lower cell voltage that can compensate the energy requirement for carbonate and CO₂ regeneration, 2) lower water flux by the ultrathin ionomer that limits cathode flooding with improved selectivity and stability, 3) direct deposition method that offers intimate contact between the catalyst and ionomer, resulting in better cathode stability by protecting the copper from dissolution, physical detachment and possible poisoning⁶², and/or 4) infused K⁺ in the solid polymer electrolyte that suppresses water flux and HER. Technoeconomic analysis suggests that lowering the energy cost may be important to driving down the economics of ethylene electrosynthesis for practical applications (FIG. 25).

[0323]The development of an ultrathin solid polymer electrolyte (USPE) using a facile spray coating approach to improve the stability and reduce the crossover in CO₂ electrolysis using MEA with alkaline anolyte is described herein. The use of commercial thick AEM and CEM with alkaline anolyte lead to high crossover and low stability in a MEA, respectively. The use of an ultrathin (˜3 μm) USPE directly coated onto a polycrystalline Cu cathode suppressed K+(74% less as compared to commercial CEM) or CO₃²⁻ crossover (40% less as compared to commercial AEM) and cathode flooding, which enabled enhanced MEA stability over commercial thick CEM and AEM. This improved performance was achieved by the directly-deposited ultrathin (˜3 μm) ionomer layer, which suppressed water uptake and cathode flooding as well as allowed good migration of K⁺ and CO₃²⁻ between the electrodes.

[0324]It was further demonstrated that by implanting K⁺ within the USPE (e.g., CISPE) during the spray-coating process, the water flux or cathode flooding could be suppressed with good K⁺ migration to cathode. This then enabled the demonstration of a high electrolyzer energy efficiency of 27% in one-step CO₂ conversion to C₂H₄, along with low energy cost of about 296, or about 290 GJ/ton C₂H₄, as well as 110 hours of stable operation at 100 mA/cm². This facile, low-cost and scalable approach to directly coat the cathode with an ultrathin polymer eliminated the need for a commonly used pre-made membrane (and associated cost of production, pre-treatment with acid for purity, hot pressing and assembling). Additionally, the use of a cation-infused USPE (CISPE) eliminated the need of commonly used precious metal anodes (e.g., IrO₂) and allowed CO₂ electrolysis with alkaline anolyte.

Example 2—Directly-Deposited Ultrathin Solid Polymer Electrolyte for CO 2 Electrolysis—Supplementary Information

S1. Experimental Information

A. Materials

[0325]Potassium hydroxide (KOH), lithium hydroxide (LiOH), cesium hydroxide (CsOH), and methanol were purchased from Sigma Aldrich (ACS reagent). Sustainion® XA-9 solution (5% in ethanol) and Nafion™ perfluorinated resin solution (5 wt. % in mixture of lower aliphatic alcohol and water) were received from Dioxide Materials and Sigma Aldrich, respectively. The membranes such as Sustainion® X37-50 Grade RT, Nafion 117 and Nafion 211 membranes were received from Fumasep.

B. Experimental Setup

[0326]A membrane electrode assembly (MEA), consisting of anode (grade 2 titanium) and cathode (904L stainless steel) was made by Dioxide materials. A humidified CO₂ and 1 M KOH (unless mentioned otherwise) with a flowrate of ˜60 sccm was fed to the cathode and anode sides, respectively using a flow meter (Cole-parmer 39067) and peristaltic pump (Fisherbrand™ Variable-Flow Peristaltic Pumps), respectively. BioLogic potentiostats with 10 A booster was used to obtain the electrochemical response without iR correction. Parsche Airbrush set was used to spray the solution on the target sheet. The gas products were analyzed using Perkin Elmer Gas Chromatography with flame ionization detector (FID) and thermal conductivity detector (TCD). The liquid products were identified using BrukerAVANCE III 600 MHz nuclear magnetic resonance spectroscopy equipped with pulsed-field gradient probes.

C. Cathode Preparation

[0327]The cathode catalyst was approximately 300 nm sputtered copper on polytetrafluoroethylene (PTFE). The Nafion spray solution was prepared by diluting Nafion™ perfluorinated resin solution (Sigma Aldrich) in methanol (Sigma Aldrich) with a ratio of 1:5 by volume. The ultra-thin solid polymer electrolyte (USPE) was fabricated by spray-coating (Paasche airbrush) of the desired quantity of Nafion spray solution on top of the cathode catalyst and dried for overnight under atmospheric condition. For the sample with the area of ˜4 cm², USPE-50 (contains 50 μL/cm² of Nafion™ perfluorinated resin solution) was synthesized by spray coating 1200 μL of the Nafion spray solution stock.

[0328]The cation-infused solid polymer electrolyte (CISPE) was prepared by spray-coating of the desired quantity of a Nafion-cation spray solution and dried overnight. The Nafion-cation spray solution was prepared by mixing Nafion spray solution stock with cation solution at the desired concentration with a ratio of 1:9 by volume. Cation solution could be LiOH, KOH, or CsOH. For example, to synthesize ˜4 cm² of CISPE-50-0.15M (contains 50 μm/cm² of Nafion™ perfluorinated resin solution with 0.15 M KOH), the Nafion-cation spray solution was prepared by adding 0.15 M KOH solution to the Nafion spray solution. Then, 1333 μL of the Nafion-KOH spray solution was spray coated to the cathode catalyst and dried overnight.

[0329]On the anode side, a Ni-foam sheet (0.08 mm, MTI Corporation) was used as catalyst to avoid damage on the Nafion layer which may result in short circuit.

D. Characterization

[0330]The morphology and structure of USPE and CISPE were characterized by scanning electron microscopy (SEM). SEM observation was performed using a FEI Quanta 250 FEG field emission scanning electron microscope which was equipped with EDS analysis.

[0331]Fourier Transform Infrared (FTIR) spectra were recorded by Perkin Elmer Frontier FT-IR spectrometer in the range of 4000 to 400 cm- to study the chemical structure of ionomer before and after adding KOH.

E. Mass of Nafion in the Solution

[0336]The density of the diluted Nafion solution (ρ_sol-A) was calculated as follow:

$\begin{array}{c}{\rho }_{\mathrm{sol}·A}=\frac{V_{\mathrm{naf}}\times {\rho }_{\mathrm{naf}}\times x_{\mathrm{naf}}}{V_{\mathrm{naf}}\times V_{\mathrm{meoh}}}\\ =\frac{1\mathrm{mL}\times 0.93\frac{g_{\mathrm{sol}}}{\mathrm{mL}}\times \frac{5}{100}\frac{g_{\mathrm{naf}}}{g_{\mathrm{sol}}}}{1\mathrm{mL}+5\mathrm{mL}}\\ =\frac{0.0465g_{\mathrm{naf}}}{6\mathrm{mL}}\\ =0.00775g_{\mathrm{naf}}/\mathrm{mL}\end{array}$

V_naf, V_meoh and x_naf are the volume of Nafion™ perfluorinated resin, volume of methanol and mass fraction of Nafion in the Nafion™ perfluorinated resin, respectively.

[0337]The mass of Nafion in the cathode with Nafion loading of 50 μL/cm² was estimated as follows:

$\begin{array}{c}{m}_{naf}=0.05\frac{\mathrm{mL}}{{\mathrm{cm}}^2}\times 0.93\frac{g_{\mathrm{sol}}}{\mathrm{mL}}\times \frac{5}{100}\frac{g_{naf}}{g_{\mathrm{sol}}}\\ =2.325\mathrm{mg}/{\mathrm{cm}}^2\end{array}$

[0338]The modified Nafion was synthesized by adding a certain concentration of cation hydroxide solution in the Nafion solution with a volumetric ratio of Nafion solution: cation hydroxide solution of 9:1. Firstly, the mass of Nafion in 9 mL of Nafion solution was calculated.

$\begin{array}{c}{m}_{\mathrm{naf}}=9\mathrm{mL}\times 0.00775g_{\mathrm{naf}}/\mathrm{mL}\\ =0.0698g\end{array}$

[0339]The mass fraction of sulfonic acid group (x_sa) in the Nafion was estimated as follows (assume 1 mol Nafion as a basis):

$\begin{array}{c}{x}_{sa}=\frac{81g}{554g}\\ =0.15\end{array}$

[0340]The mole of sulfonic acid was calculated as follow:

$\begin{array}{c}{n}_{sa}=\frac{0.0698g\times 0.15}{81g/\mathrm{mol}}\\ =0.000129\mathrm{mol}\end{array}$

The required concentration of 1 mL cation hydroxide solution to stoichiometrically neutralize 1.29×10⁻⁴ mol of sulfonic acid was 0.129 M.

F. Measurement of Hydroxide and Carbonate Concentrations Using Total Alkalinity Method

[0341]The mass flow for a constant charge transfer was measured by operating at constant current density (50 mA/cm²) for 60 minutes in 1 M KOH anolyte. A 50 mA/cm² current density was selected because at this current density was observed considerable CO₂R activity on all studied samples. The experiments were carried out for 60 minutes to ensure the change of the anolyte was observable and the system was still under alkaline condition to avoid proton generation in the anode. Measured was the concentration of OH⁻ and CO₃²⁻ in the anolyte before and after the reaction using total alkalinity method to estimate molar mass of ion transport via migration and diffusion mechanism.

[0342]The concentrations of hydroxide and carbonate was measured by a practical method from industry.^{1, 2} On each measurement, 1 mL of the sample was taken and poured in a transparent beaker. Then, one drop of phenolphthalein (ACS reagent, Sigma Aldrich) was added to the beaker. 0.1 M HCl (ACS reagent, Sigma Aldrich) was gradually added to the beaker using 20 μL-scale pipets. The total volume of HCl was measured until the color of phenolphthalein indicator changed from pink to transparent (phenolphthalein alkalinity/PA). The phenolphthalein alkalinity represented the titration of OH⁻ and 1/2 of CO₃²⁻ present in the electrolyte. Then, one drop of methyl orange (ACS reagent, Sigma Aldrich) was added to the beaker. The HCl was again added and its total volume was measured until the color of methyl orange indicator changed from yellow to light orange (total alkalinity/TA). This titration step represented the neutralization of the other half of CO₃²⁻ present in the electrolyte (FIG. 6). The results of titration were calculated using Table S1. Control samples were analyzed and verified the accuracy of the method.

[0343]From the titration data, the following information was estimated: charge transferred from anode to the cathode, ion migration and ion diffusion. To interpret the data (material balance), four basic rules were assumed: (1) cation exchange membrane facilitates cation migration, (2) anion exchange membrane facilitates anion migration, (3) ion movement disobey rule (#1) and (#2) was assumed as diffusion of hydrated cation and its corresponding anion, (4) remaining materials after applying rule (#1) to (#3) was assumed as non-ideality of the membrane (e.g., cation exchange membrane allows anion to move). The ion transport in cation exchange membrane (CEM) includes migration of K⁺ (charge difference) and diffusion of KOH_(aq) and KHCO_3(aq) (concentration difference), as shown in FIG. 7a. The later transport mechanism was a result of water uptake properties of Nafion™ 117. In AEM, the ion transport consisted of migration of CO₃²⁻ and diffusion of KOH_(aq) and KHCO_3(aq) (concentration difference), as shown in FIG. 7b. Salt was the bicarbonate which resided in the cathode (Table S.2).

[0344]For instance, on the experiment using the Nafion 117 membrane using 1 M KOH as anolyte for ˜1 hour operation:

$\begin{array}{c}q=-741\mathrm{sA}\\ F=-96485\mathrm{sA}/\mathrm{mol}\\ n_e=\frac{-731\mathrm{sA}}{96485\frac{\mathrm{sA}}{\mathrm{mol}}}\times \frac{1000\mathrm{mmol}}{1\mathrm{mol}}=-7.7\mathrm{mmol}\end{array}$

Where q, F and n_e are the total charge (from the potentiostat), Faraday constant and number of moles of charge, respectively. The minus sign indicates the negative charge flows from the anode to the cathode.

[0345]In the anolyte:

$\begin{array}{c}{c}_{\mathrm{OH},in}^{-}=0.989M❘\\ v_{,in}=12\mathrm{mL}\\ n_{\mathrm{OH},in}^{-}=0.989\frac{\mathrm{mol}}{L}\frac{1L}{1000\mathrm{mL}}102\mathrm{mL}\times \frac{1000\mathrm{mmol}}{1\mathrm{mol}}=100.8\mathrm{mmol}\\ c_{{\mathrm{CO}}_3^{2-},in}=0.007M\\ n_{{\mathrm{CO}}_3^{2-},in}=0.007\frac{\mathrm{mol}}{L}\frac{1L}{1000\mathrm{mL}}102\mathrm{mL}\times \frac{1000\mathrm{mmol}}{1\mathrm{mol}}=0.7\mathrm{mmol}\end{array}$

Where c_{OH-, in}; v, _in, and n_OH⁻, in; c_CO3²⁻_{, in}; and n_CO3²⁻, in are the initial concentration, initial volume, and initial number of moles of OH⁻; initial concentration and initial number of moles of carbonate, respectively. After reaction:

$\begin{array}{c}{c}_{\mathrm{OH},\mathrm{fn}}^{-}=0.926M\\ v_{,fn}=98\mathrm{mL}\\ n_{\mathrm{OH},fn}^{-}=0.926\frac{\mathrm{mol}}{L}\frac{1L}{1000\mathrm{mL}}98\mathrm{mL}\times \frac{1000\mathrm{mmol}}{1\mathrm{mol}}=90.7\mathrm{mmol}\\ c_{{\mathrm{CO}}_3^{2-},\mathrm{fn}}=0.014M\\ n_{{\mathrm{CO}}_3^{2-},\mathrm{fn}}=0.014\frac{\mathrm{mol}}{L}\frac{1L}{1000\mathrm{mL}}98\mathrm{mL}\times \frac{1000\mathrm{mmol}}{1\mathrm{mol}}=1.2\mathrm{mmol}\end{array}$

Where C_OH⁻_{, fn}; v_{, fn}; and n_OH⁻_{, fn}; c_CO3²⁻_{, fn}; and n_CO3²⁻_{, fn} are the final concentration, final volume, and final number of moles of OH⁻; final concentration and final number of moles of carbonate, respectively.

[0346]Under the alkaline condition, the oxygen evolution reaction (OER) obeys the following reaction:

[0347]The generation of one mole of electron consumes one mole of OH⁻:

$\begin{array}{c}{n}_{\mathrm{OH}-,\mathrm{cons}}=n_e\\ =-7.7\mathrm{mmol}\end{array}$

[0348]The measured OH⁻ loss was calculated as follows:

$n_{\mathrm{OH},ms}^{-}=n_{\mathrm{OH},\mathrm{fn}}^{-}-n_{\mathrm{OH},in}^{-}=90.7\mathrm{mmol}-100.8\mathrm{mmol}=-10.1\mathrm{mmol}$

[0349]Then, the difference between the OH⁻ for OER and the hydroxide ions loss (n_OH⁻_{, dif}) was calculated:

$\begin{array}{c}{n}_{{\mathrm{OH}}^{-},\mathrm{dif}}=n_{{\mathrm{OH}}^{-},ms}-n_{\mathrm{OH}-,\mathrm{cons}}\\ =-10.1\mathrm{mmol}-(-7.7\mathrm{mmol})\\ =-2.4\mathrm{mmol}\end{array}$

The above value indicated that the actual OH⁻ consumption was 2.4 mmol larger than the one that was used for OER. The extra OH⁻ consumption on top of OER could be due to the water uptake (KOH_(aq) diffusion) or carbonation reaction to convert bicarbonate (HCO₃⁻) into carbonate (CO₃²⁻) via thefollowing reaction:

[0350]The nature of CEM does not allow migration of anion from the anode to the cathode. However, from the experiment, the increase of CO₃²⁻ concentration was observed, as an indication of HCO₃⁻/CO₃²⁻ crossover via diffusion mechanism. For simplicity, it was assumed that KHCO_3(aq) (hydrated K⁺ and its corresponding HCO₃⁻) transported from the cathode to the anode, as proposed earlier³. Please note that one mole of CO₃²⁻ is equal to one mole of HCO₃⁻ according to the carbonation reaction of HCO₃⁻. The mole of HCO₃⁻ crossover

$\left(n_{HC{O}_3^{2-},\mathrm{xover}}\right)$

can be calculated as follow:

$n_{HC{O}_3^{2-},\mathrm{xover}}=n_{C{O}_3^{2-},\mathrm{gen}}$

[0351]Where the generation of

${\mathrm{CO}}_3^{2-}\left(n_{C{O}_3^{2-},\mathrm{gen}}\right)$

in the anolyte can be calculated as follow:

$\begin{array}{c}{n}_{{\mathrm{CO}}_3^{2-},\mathrm{gen}}=n_{{\mathrm{CO}}_3^{2-},\mathrm{fn}}-n_{{\mathrm{CO}}_3^{2-},in}\\ =1.2\mathrm{mmol}-0.7\mathrm{mmol}\\ =0.5\mathrm{mmol}\end{array}$

[0352]Recalling the HCO₃⁻ crossover equation:

$n_{HC{O}_3^{2-},\mathrm{xover}}=n_{C{O}_3^{2-},\mathrm{gen}}$$n_{HC{O}_3^{2-},\mathrm{xover}}=0.5\mathrm{mmol}$

[0353]The occurrence of KHCO_3(aq) transport can be explained by the reaction between CO₂ and KOH in the cathode

[0354]The CO₂ was supplied to the cathode as the feed for CO₂R, while the presence of KOH in the cathode was due to the water uptake properties of CEM which allowed KOH_(aq) (hydrated K⁺ and its corresponding OH⁻) to transport from the anode to the cathode via diffusion mechanism⁴. The rate of OH⁻ that diffuse from anode to cathode (n_OH⁻_{, d-carb}) can be estimated as follow:

$\begin{array}{c}{n}_{{\mathrm{OH}}^{-},d-\mathrm{carb}}=n_{HC{O}_3^{2-},\mathrm{xover}}\\ =0.5\mathrm{mmol}\end{array}$

[0355]Recalling that KHCO_3(aq) diffuse from the cathode to the anode

$\left(n_{HC{O}_3^{2-},\mathrm{xover}}=0.5\mathrm{mmol}\right).$

Once HCO₃⁻ reaches the anode, it reacts with OH⁻ to form CO₃²⁻ and H₂O:

[0356]From the above reaction, it is known that to one mole of OH⁻ is required to convert HCO₃⁻ into CO₃²⁻. As the reactions occur in the anolyte, the amount of OH⁻ that was consumed in the anolyte (n_OH⁻_{, carb}) is estimated as follow:

$\begin{array}{c}{n}_{{\mathrm{OH}}^{-},\mathrm{carb}}=-\left(1\times n_{HC{O}_3^{2-},\mathrm{xover}}\right)\\ =-\left(1\times 0.5\mathrm{mmol}\right)\\ =-0.5\mathrm{mmol}\end{array}$

[0360]Recall that there are an extra 2.4 mmol of OH⁻ consumption on top of OH⁻ consumption for OER. It was considered that the remaining OH⁻ loss in the anode was due to OH⁻ diffusion from the anode to the cathode.

$\begin{array}{c}{n}_{{\mathrm{OH}}^{-},d-\mathrm{lost}}=n_{O{H}^{-},\mathrm{dif}}-n_{O{H}^{-},d-\mathrm{carb}}-n_{O{H}^{-},\mathrm{carb}}\\ =-2.4\mathrm{mmol}-\left(-0.5\mathrm{mmol}\right)-\left(-0.5\mathrm{mmol}\right)\\ =1.4\mathrm{mmol}\end{array}$

[0361]The transport mechanism can be migration of ion and diffusion. The migration mechanism is the transport of ion due to the charge difference between two spots. The diffusion mechanism is the transport of ion due to the concentration difference. The diffusion of anion species is to be followed by the diffusion of the corresponding cation. For example, the diffusion of OH⁻ is followed by the diffusion of K⁺ as the corresponding counterion of KOH_(aq)⁴. Similarly, the diffusion of HCO₃⁻ is followed by the diffusion of K⁺. Hence, the material balance of K⁺ can be calculated.

[0362]Begin with the amount of K⁺ before reaction (n_{K+, in}):

$\begin{array}{c}{n}_{K^{+},\mathrm{in}}=n_{{\mathrm{OH}}^{-},\mathrm{in}}+\left(2\times n_{{\mathrm{CO}}_3^{2-},\mathrm{in}}\right)\\ =100.8\mathrm{mmol}+0.7\mathrm{mmol}\\ =102.2\mathrm{mmol}\end{array}$

[0363]Then, estimate the amount of K⁺ after reaction:

$\begin{array}{c}{n}_{K^{+},\mathrm{fn}}=n_{O{H}^{-},\mathrm{fn}}+\left(2\times n_{C{O}_3^{2-},\mathrm{xover}}\right)\\ =90.7\mathrm{mmol}+1.2\mathrm{mmol}\\ =91.9\mathrm{mmol}\end{array}$

[0364]The mole of K⁺ consumption can be calculated as follow:

$\begin{array}{c}{n}_{K^{+},\mathrm{djff}}=n_{K^{+},\mathrm{fn}}-n_{K^{+},\mathrm{in}}\\ =102.2\mathrm{mmol}-91.9\mathrm{mmol}\\ =-9.1\mathrm{mmol}\end{array}$

[0365]In the CEM, K⁺ migrated from the anode to the cathode to balance the charge. The amount of K⁺ migration (n_K+mgr) can be estimated as follows:

$\begin{array}{c}{n}_{K^{+},\mathrm{mgr}}=n_e\\ =-7.7\mathrm{mmol}\end{array}$

[0366]The amount of K⁺ consumption was higher than its counterpart of K⁺ migration.

$\begin{array}{c}{n}_{K^{+},\mathrm{djf}}=n_{K^{+},\mathrm{mgr}}-n_{K^{+},ms}\\ =-7.7\mathrm{mmol}-\left(-9.1\mathrm{mmol}\right)\\ =-1.4\mathrm{mmol}\end{array}$

[0367]From the above calculation, it is shown that an extra 1.4 mmol of K⁺ is consumed on top of the K⁺ consumption due to migration mechanism. Thus, the extra K⁺ loss can be related to the K⁺ transport via diffusion mechanism. Remember that diffusion of K⁺ is strongly related to the diffusion of its corresponding anion. For example, the K⁺ diffusion along with OH⁻ diffusion from the anode to the cathode which results in the HCO₃⁻ formation (n_{K+, d-carb}) can be calculated as follow:

$\begin{array}{c}{n}_{K^{+},d-\mathrm{carb}-\mathrm{ct}}=n_{{\mathrm{OH}}^{-},d-\mathrm{carb}}\\ =-0.5\mathrm{mmol}\end{array}$

[0368]The K⁺ diffusion along with HCO₃⁻ from the cathode to the anode can be calculated as follow:

$\begin{array}{c}{n}_{K^{+},d-\mathrm{carb}-\mathrm{an}}=n_{HC{O}_3^{2-},\mathrm{xover}}\\ =0.5\mathrm{mmol}\end{array}$

[0369]The K⁺ loss after considering the diffusion of KOH_(aq) for KHCO_3(aq) formation and the diffusion of KHCO_3(aq) can be considered as diffusion of KOH which lost (remain) in the cathode.

$\begin{array}{c}{n}_{K^{+},d-\mathrm{lost}}=n_{K^{+},\mathrm{djf}}-\left(n_{K^{+},mgr}+n_{K^{+},d-carb-ct}+n_{K^{+},d-carb-an}\right)\\ =-9.1\mathrm{mmol}-\left(-7.7\mathrm{mmol}+\left(-0.5\mathrm{mmol}\right)+0.5\mathrm{mmol}\right)\\ =-1.4\mathrm{mmol}\end{array}$

[0370]The experiments were conducted for 1.1 hours. The value as calculated above is normalized for 1 hour experiment. The final value is presented in Figure S.5. The solubility of KOH, K₂CO₃ and KHCO₃ in water is summarized in Table S.2.

G. Measurement of Mass Diffusion

[0371]The mass transport experiments (diffusion experiment) were carried out using a similar setup with CO₂ electrolysis using Nafion-117 or CISPE in MEA without applied potential. For instance, the transport flux of the Nafion-117 system was executed with sputtered copper in the cathode, Ni foam in the anode and Nafion-117 between the anode and the cathode. Similarly, the transport flux of the CISPE system was carried out with CISPE in the cathode and Ni foam in the anode. No standalone membrane was used in CISPE system. The humidified N₂ at 60 sccm was directed to the cathode to mimic the cathode condition in CO₂ electrolysis and to avoid salt formation. 1 M KOH solution at 60 sccm was circulated in the anode. The convection effect due to pressure difference that is generated by the peristaltic pump can be neglected due to the dihedral angle of 90°. The experiment was terminated when the change of anolyte level become observable.

$\mathrm{Nafion}-117$$\mathrm{Initial}\mathrm{anolyte}\mathrm{volume}\left(V_{\mathrm{in}}\right)=20.\mathrm{mL}$$\mathrm{Final}\mathrm{anolyte}\mathrm{volume}\left(V_{\mathrm{fn}}\right)=19.1\mathrm{mL}$$\begin{array}{c}\mathrm{Volume}\mathrm{difference}\left(V_{\mathrm{diff}}\right)=V_{\mathrm{in}}-V_{\mathrm{fn}}\\ =20.\mathrm{mL}-19.1\mathrm{mL}=0.9\mathrm{mL}\end{array}$$\mathrm{Density}\mathrm{of}1M\mathrm{KOH}\left({\rho }_{an}\right)=1.05g/\mathrm{mL}$$\begin{array}{c}\mathrm{Mass}\mathrm{difference}\left(m_{\mathrm{diff}}\right)=0.9\mathrm{mL}\times 1.05g/\mathrm{mL}\\ =0.945g\end{array}$$\mathrm{Duration}\left(t\right)=69,780s$$\mathrm{Area}\left(A\right)=3.99{\mathrm{cm}}^2$$\begin{array}{c}\mathrm{Flux}\left(N\right)=\frac{m_{\mathrm{diff}}}{A\times t}\\ =\frac{0.945g}{3.99{\mathrm{cm}}^2\times 69,780s}\\ =3.39\times 10^{-6}\frac{g}{c{m}^{2}s}\end{array}$$\mathrm{USPE}-50$$\mathrm{Initial}\mathrm{anolyte}\mathrm{volume}\left(V_{\mathrm{in}}\right)=20.\mathrm{mL}$$\mathrm{Final}\mathrm{anolyte}\mathrm{volume}\left(V_{\mathrm{fn}}\right)=19.5\mathrm{mL}$$\begin{array}{c}\mathrm{Volume}\mathrm{difference}\left(V_{\mathrm{diff}}\right)=V_{\mathrm{in}}-V_{\mathrm{fn}}\\ =20.\mathrm{mL}-19.5\mathrm{mL}=0.5\mathrm{mL}\end{array}$$\mathrm{Density}\mathrm{of}1M\mathrm{KOH}\left({\rho }_{an}\right)=1.05g/\mathrm{mL}$$\begin{array}{c}\mathrm{Mass}\mathrm{difference}\left(m_{\mathrm{diff}}\right)=0.5\mathrm{mL}\times 1.05g/\mathrm{mL}\\ =0.525g\end{array}$$\mathrm{Duration}\left(t\right)=77,100s$$\mathrm{Area}\left(A\right)=3.96{\mathrm{cm}}^2$$\begin{array}{c}\mathrm{Flux}\left(N\right)=\frac{m_{\mathrm{diff}}}{A\times t}\\ =\frac{0.525g}{3.96{\mathrm{cm}}^2\times 77,100s}\\ =1.72\times {10}^{-6}\frac{g}{c{m}^{2}s}\end{array}$

H. Performance Calculation

[0372]The CO₂R experiment was carried out at current density (j) of 100 mA/cm² using with a flowrate humidified CO₂ inlet (v) of 6 sccm (standard cm³ per minute).

$v=6{\mathrm{cm}}^3/\min$$R=82.06\frac{\mathrm{atm}{\mathrm{cm}}^3}{\mathrm{mol}K}$$T=298.15K\left(25°C.\right)$$P=1\mathrm{atm}$$\begin{array}{c}{n}_{\mathrm{CO}2}=V\frac{P}{\mathrm{RT}}\\ =6\frac{{\mathrm{cm}}^3}{\min }\frac{1\mathrm{atm}}{82.06\frac{\mathrm{atm}{\mathrm{cm}}^3}{\mathrm{mol}K}298.15K}\frac{60\min }{1\mathrm{hour}}\\ =1.47\times 10^{-2}\frac{\mathrm{mol}}{\mathrm{hour}}\end{array}$

[0373]Where R, T, P and n represent gas constant, temperature, pressure and mole flow, respectively.

$\begin{array}{c}i=\text{}100\mathrm{mA}/{\mathrm{cm}}^2\\ A=4{\mathrm{cm}}^2\\ j=i\times A\\ =100\frac{\mathrm{mA}}{{\mathrm{cm}}^2}\times 4{\mathrm{cm}}^2\times \frac{1A}{1000\mathrm{mA}}\\ =0.4A\end{array}$

[0374]Where i, A and j are current density, electrode area and total current, respectively. Then, the energy supplied to the electrolyzer (E_el) was calculated for 1 hour basis.

$\begin{array}{c}{E}_{\mathrm{el}}=j\times V\times 1\mathrm{hour}\\ =0.4A\times 2.8V\times \frac{1\mathrm{hour}}{1\mathrm{hour}}\\ =1.14\frac{\mathrm{Whour}}{\mathrm{hour}}\times \frac{3.6\mathrm{kJ}}{1\mathrm{Whour}}\\ =4.1\frac{\mathrm{kJ}}{\mathrm{hour}}\times \frac{1\mathrm{GJ}}{{10}^6\mathrm{kJ}}\\ =4.1\times {10}^{-6}\frac{\mathrm{GJ}}{\mathrm{hour}}\end{array}$

[0375]From the Faradaic efficiency (FE), the flowrate of products (n_prod) was calculated:

$n_{\mathrm{prod}}=\frac{\mathrm{FE}\times j}{n_e\times F}$

F is the Faraday constant 96485 sA/mol. n_e is the number of electrons involved in the reaction.

[0376]The flowrate of CO₂ converted into product in the electrolyzer can be estimated from the product flowrate.

[0378]The single pass conversion (X) can be calculated as follow:

$X=\frac{\sum n_{n-{\mathrm{CO}}_2}}{n_{\mathrm{CO}2}}$

[0379]The energy efficiency of electrolyzer for C₂H₄ production (E_el-C2H4) can be calculated as follow [8]:

$E_{\mathrm{el}-C_2{H}_4}=\frac{V_{C_2{H}_4}\times j}{V_{\mathrm{cell}}\times j}$

Where

$V_{C_2{H}_4}^o$

is defined as follow:

$V_{C_2{H}_4}=V_{C_2{H}_4}^O\times {\mathrm{FE}}_{C_2{H}_4}$

and FE_C2H4 are the standard potential and Faradaic efficiency of C₂H₄, respectively.

[0380]The energy required for CO₂ gas separation (E_CO2) in the cathode is estimated as follow:

[0382]The energy required for carbonate recovery (E_carb) in the anode is estimated as follow:

[0384]The flowrate of carbonate was calculated based on the experimental data in FIG. 11 which consider both migration and diffusion of carbonate ions (CO₃²⁻ and HCO₃⁻). For example, the flowrate of carbonate ion in the case of CISPE-50-0.15M with 1 M KOH anolyte was 1.03 times the carbonate migration. Similarly, for the AEM with 1 M KOH anolyte, the flowrate of carbonate was 1.74 times the carbonate migration.

[0385]The calculation and the data for calculation is shown above in the Example 2, Section H.

I. Electrochemical Impedance Spectroscopy (EIS)

EIS Study in Three-Electrode Cell and Flow Cell Systems

[0386]The observed CO₂R studies suggested that the USPE (3 mm), devoid of the standalone CEM (180 mm), endorsed high selective CO₂ reduction at the electrode/electrolyte interface (FIG. 3d). To further inquire about the nature of ionic conductivity, EIS were carried out in a three-electrode set up for the (a) PTFE, (b) Cu sputtered PTFE (Cu/PTFE) and (c) USPE-50 at open circuit potential (OCP) in 1 M KOH from 10 kHz to 0.1 Hz with an amplitude of 5 mV. From FIG. 8a-c, the PTFE delivered high R_s (solution resistance, 17,025Ω) due to hydrophobic behaviour from polymer backbones. Once Cu was sputtered, R_s was decreased up to 6 folds (3,045Ω). Further addition of directly deposited ultrathin ionomer layer (USPE-50) resulted in reduced R_s to 30Ω indicating facile ion transport. This facile ion transport resulted in lower cell voltage as was observed in performing CO₂ electrolysis experiment.

[0387]To specifically unveil the ionic resistivity in a flow cell system, it was then opted to perform EIS study in a representative cell as shown in FIG. 9a. From the corresponding R_s values examined from Nyquist plots given in FIGS. 9b and c, mainly when comparing USPE-50 and CEM, the resistance at the interface was higher in the latter one with 6-fold increment (32.4Ω). The presence of directly deposited ultrathin Nafion coating that strongly adhered to the Cu sputtered surface supports facile ion transport, which resulted in lower cell voltage. In case of CEM, because of high thickness (180 μm), resistance was increased for the ionic transport.

Proton Conductivity Studies

[0388]Also performed were proton conductivity study for directly deposited ultrathin ionomer, that was compared with the commercial thick stand-alone Nafion™ 117 membrane. Please note that the experimental setup for CO₂R was different than the setup for proton conductivity experiment. In CO₂R experiment 1 M KOH was used as cation source, while in the proton conductivity experiment humidified environment was used as proton source. Nafion consists of hydrophobic polymeric backbone and hydrophilic sulfonic acid regions. These hydrophilic groups allow proton transport from the hydrated ionic clusters. This nature of proton transport is affected via many factors such as temperature, humidity, and thickness of the membrane. Based on this, proton conductivity measurements were carried out in a conductivity cell that consisted of Pt electrodes connected via Au wire, and the temperature was around 25° C. with the relative humidity of 3%. The studied samples were CEM and the USPE-50. The results of EIS measurement at open circuit potential are given in FIG. 10.

[0389]From the R_s values obtained, conductivity(s) was calculated. A conductivity of 0.01035 S/m² was calculated for CEM, and 0.01951 S/m² for USPE-50, indicating that the latter offers improved proton conductivity. These results suggested that the presence of directly deposited ultrathin Nafion improved the proton conductivity. This could be attributed to more active sites to occupy hydrated ionic clusters with the modified microstructural features in USPE-50. As shown from SEM and optical profilometry (FIG. 3c, FIG. 15, and FIG. 12), USPE-50 had Nafion sprayed over the surface of sputtered copper that helps to keep the migrated ions in close vicinity of CO₂. However, as the membrane environment was different from CO₂R in MEA assembly, where USPE-50 offered good migration of (K⁺), this may be correlated to more available SO₃ groups to ensure transport of ions. These findings suggested that the directly deposited ultrathin Nafion offered improved performance versus commercially available thick Nafion membrane with improved ion conductivities, low cell voltage and stable CO₂ electroreduction.

Background, Detailed Description—Carbon Dioxide Electroreduction (CO 2 R), and Example 1 References

EXAMPLE 2 AND FIGS. 6 - 26 REFERENCES

Example 3—Modulating Cation and Water Transports for CO Electrolysis, Such as Enhanced CO Electrolysis, Via Lonomer Coating

[0478]Electrification of the chemical industry has been considered an enabler for energy transition on a massive scale. In this context, carbon monoxide electroreduction (COR) to produce multi-carbon (C₂₊) products is considered one of the forefront of emerging technologies. A challenge in COR comes from cation crossover to the cathode via electromigration and water diffusion, which limits CO availability and impedes product selectivity. Commercial anion exchange membrane (AEM) suppress the electromigration of cations, however, tends to suffer from water diffusion which facilitates cation crossover. As described herein, these challenged due to cation crossover and water diffusion may be addressed by directly depositing an ultrathin nafion ionomer on a cathode (sputtered Cu) surface. The approach may enable full-cell energy efficiency of 21% for converting CO into ethylene (C₂H₄) at 100 mA/cm² with over 200 hours of stable operation. Also observed and exhibited were high energy efficiency for ethanol (C₂H₅OH) production with CO-to-C₂H₅OH electrolysis efficiency of 17%. The herein described approach to directly deposit ultrathin ionomer on a cathode to enhance system performance may benefit other electrochemical systems to overcome challenges associated with scalability, stability, and efficiency to produce high-value chemicals.

Results and Discussion

[0479]Performance of commercial AEM (Sustainion® X37-50 Grade RT Membrane) in MEA was first investigated. The cathode used was ˜300 nm copper catalyst sputtered on a PTFE gas diffusion layer (GDL) (FIG. 27a). Cell voltage and Faradaic efficiency (FE) of COR using 1 M KOH as anolyte at various current densities are depicted in FIG. 27b. Observed was a continuous drop of C₂H₄ selectivity when current density was increased from 25 to 200 mA/cm², which is in line with the literature.²⁴ While using AEM, the highest partial current density for C₂H₄ was found to be only 24 mA/cm².

[0480]Next investigated was the selectivity and cell voltage over time at 50 mA/cm² (FIG. 27c). A continuous increase of HER activity was observed, and consequently, a continuous decrease of COR selectivity during 2 hours of reaction. This was associated with the accumulation of cation (e.g., K⁺) which blocks the catalyst surface and thus limits the CO availability.¹⁶ Accumulation of K⁺ was confirmed using energy-dispersive X-ray (EDX) analysis (FIG. 35). A possible mechanism of K⁺ crossover is water transport that brings KOH_(aq) from the anolyte. To investigate this, COR experiments were carried out using different anolyte. It was observed that the COR selectivity and stability varied with the change in anolyte concentrations which indicated water transport within AEM (FIGS. 36 and 37). An ion transport experiment (Example 4—Section S1G) further confirmed this, elucidating the diffusion of water (with KOH_(aq)) to the cathode, as well as the electromigration of OH⁻ to the anode (FIG. 27d). This observation was consistent with literature, wherein the AEM facilitated OH⁻ transport to the anode as well as water diffusion to the cathode.^{8, 9, 23} Another possible reason for the depletion of COR selectivity was the promotion effect of cation on HER.²⁵ To investigate this, linear sweep voltammetry (LSV) was carried out using different anolytes with different K⁺ concentrations. Observed was lower COR and HER activities on higher K⁺ concentrations when using Cu catalysts (FIG. 38). This ruled out the possibility of the promotion effect of cation on HER activity. Instead, the negative impact of cations on COR selectivity could be attributed to catalyst poisoning due to cation accumulation.¹⁶

[0481]Looking at the adverse effect of K⁺ crossover via water diffusion on CO availability in AEM, it was considered that the degradation (gradual loss of selectivity) for COR in CEM would be faster than in AEM due to the electromigration of cation to the cathode. To investigate this a commercial CEM (Nafion™ 117 membrane) in MEA using 1 M KOH anolyte (FIG. 39a) was assembled. It was noticed that the highest partial current density for C₂H₄ was 11 mA/cm², which was lower than its counterpart using AEM (FIG. 39b and FIG. 27b). A stability experiment at 50 mA/cm² showed that the selectivity for C₂H₄ drops within 5 minutes of reaction, while the cell voltage increased by ˜350 mV due to cation crossover (FIG. 39c). K⁺ accumulation was observed on the used sample surface using EDX analysis (FIG. 40). To further confirm the cathode poisoning due to cation accumulation, similar experiments were performed using 1 M LiOH anolyte in which Li⁺ has a smaller size than K⁺. The degradation of C₂H₄ selectivity was lower when using 1 M LiOH anolyte as compared to its counterpart when using 1 M KOH, because with the same rate of diffusion, the small cation covers less cathode area (reduced cathode poisoning) as compared to the large cation (FIG. 41 and FIG. 39c). From the ion transport experiment (Example 4—section S1G), it was observed that the charge was majorly balanced by the electromigration of K⁺ to the cathode, while electromigration of OH⁻ was not observed (FIG. 39d). All these observations suggested that cation accumulation and subsequent loss of CO in the cathode were responsible for the lower C₂H₄ selectivity, which was consistent with literature.¹⁶

[0482]Limited by the availability of commercial membrane that is relatively thinner (<30 μm), it was sought to directly deposit hydrophobic PFSA ionomer (e.g., Nafion) on a cathode catalyst (Example 4—section S1C). This ultrathin solid polymer electrolyte (USPE) served as a separator that modulated ion transport as well as water transport between anode and cathode in a MEA configuration and substituted use of a standalone membrane. It was considered that a direct deposition method could protect a cathode catalyst from physical degradation (e.g., dissolution, detachment, poisoning) due to the direct contact between ionomer and catalyst.^{14, 26, 27} Furthermore, direct contact between the ionomer (e.g., PFSA) and catalyst may enhance CO availability on the catalyst surface.¹⁷ The thin layer of the membrane may also minimize the water transport from the anode.^{10, 23}

[0483]Based on this, 50 μL/cm² of Nafion was spray coated onto a cathode catalyst surface (denoted as USPE-50, FIG. 28a) and carried out COR experiments with 1 M KOH anolyte (Example 4—section S1B). Although lower COR selectivity was observed at lower current densities (e.g., 25 mA/cm² and 50 mA/cm²), an improved selectivity was observed at higher current densities (e.g., 75 mA/cm² and 100 mA/cm²) as compared to Nafion™ 117 CEM (FIG. 28b and FIG. 39b). This finding provided initial support that electromigration of K+ could be adjusted by changing the thickness of a SPE. To measure the K⁺ transport, an ion transport experiment was carried out using USPE-50 at 50 mA/cm² (Example 4—section S1G). It was found that the charge balance was governed by bidirectional electromigration of K⁺ and OH⁻ (FIG. 42a). While water transport (e.g., KOH_(aq) diffusion) to the cathode was not observed in a 1-hour ion transport experiment, it was considered that water transport occurred during the reaction. To investigate this, a diffusion experiment was performed for 18 hours with 1 M KOH anolyte under open circuit potential (OCP) in a MEA configuration (Example 4—section S1H). Without applying potential, it was observed that the anolyte transported to the cathode, which was considered primarily due to the concentration gradient of water between the anode and the cathode. It was found that the water flux in USPE-50 was half of the water flux compared to Nafion™-117 (Example 4), which supported that the directly deposited USPE suppressed water transport and consequently provided higher CO availability.

[0484]To further reduce the thickness of the USPE, 12 μL/cm² PFSA was spray coated onto a sputtered Cu (USPE-12), then performed COR with 1 M KOH anolyte. Enhanced C₂H₄ selectivity was observed at all the studied current densities using USPE-12 (FIG. 28c). When using USPE-12, the calculated highest partial current density towards C₂H₄ was 11 mA/cm² at 50 mA/cm², which was higher than the partial current density towards C₂H₄ using USPE-50 (4 mA/cm² at 50 mA/cm²). USPE-12 delivered a minimum K⁺ electromigration and the charge was mainly balanced by the electromigration of OH⁻ to the anode (FIG. 42b). It was observed that the cell voltage of USPE-12 was lower than the cell voltage of the Nafion™_117 membrane. This could be attributed to the lower resistance in USPE-12 as compared to the Nafion™_117 (FIG. 34). It was found that the thickness of USPE-12 was ˜0.7 μm as measured using cross-sectional SEM (FIG. 28d). Uniformity and consistency of the PFSA coating was investigated using SEM (FIG. 43). COR experiments were carried using USPE-12 at 50 mA/cm² for 2 hours (FIG. 28e). The stable cell voltage and C₂H₄ selectivity may be attributed to the direct coating of the ionomer layer which can protect the catalyst surface from degradation (i.e., surface poisoning by the cation accumulation).^{14, 26, 27}

[0485]From the previous observations using different USPE, it was noticed that C₂H₄ selectivity can be increased by suppressing the electromigration of K⁺ to the cathode (FIG. 28f). One strategy to further suppress the electromigration of K⁺ may be done by replacing the H⁺ with larger cations (e.g., Li⁺, Na⁺, Cs⁺) within PFSA to suppress ionic conductivity.^{28, 29} A recent study reported that K⁺ crossover has a detrimental impact on the COR selectivity due to limited CO availability.¹⁶ However, there are also studies which report that to some extent alkali metal cations enhance the activity of COR by stabilizing the dimer intermediates.^30-33 While the positive effect of hydrated cations (from catholyte) affects the selectivity in the flow cell and H-cell configurations, ^30-32 a similar effect in MEA might not be consistently present due to the thin and stagnant interfacial layer between the membrane and cathode surface.^{16, 34} Examples 1 and 2 herein described that implanted cations in a Nafion layer enhance CO₂R selectivity, despite the fact that bulk cations crossing over from an anode offered a detrimental effect on CO₂R selectivity.²³ This suggested that the role of mobilized cation from the anode is different than the role of an immobilized implanted cation. Therefore, it was considered that the presence of cation in the USPE could also enhance CO availability due to lower electromigration of K⁺ to the cathode. This was explored by adding 0.15 M KOH to the PFSA solution and then depositing the mixed KOH and PFSA solution with a loading of 12 μL/cm² to the sputtered copper by spray coating (Example 4—Section S1C). This was denoted as CISPE-12-0.15MKOH. 0.15 M KOH solution was selected to be added to the PFSA solution because H⁺ of the SO₃⁻ group can be stoichiometrically and fully exchanged with K⁺ at ˜0.13 M KOH. The transformation of H⁺-form into K⁺-form within PFSA is confirmed using Fourier-transformed infrared spectroscopy (FTIR) (FIG. 44). The SEM analysis showed the thickness of CISPE-12-0.15MKOH to be ˜0.7 μm (FIG. 29a). The presence of implanted K⁺ was also confirmed by the EDX analysis (FIG. 45).

[0486]K⁺ infusion within the PFSA (CISPE-12-0.15MKOH) was observed to suppress the activity of HER as indicated by the lower Faradaic efficiency toward H₂ as compared to USPE-12 (FIG. 46a and FIG. 28c). The highest partial current density toward C₂H₄ on CISPE-12-0.15MKOH was 17 mA/cm² (at a current density of 75 mA/cm²) which was higher than 11 mA/cm² (at a current density of 50 mA/cm²) on USPE-12. This result supported that the presence of implanted K⁺ in the PFSA (as opposed to bulk cation which tends to excessively accumulate at the catalyst surface) has a beneficial effect on overall COR activity. The ion transport experiment also indicated that the K⁺ infusion into the Nafion layer suppressed electromigration of K⁺ to the cathode (FIG. 42c).

[0487]COR experiments were then carried out using a larger cation by introducing 0.15 M CsOH to the PFSA solution and spray-coating the mixed solution to the sputtered copper (denoted as CISPE-12-0.15MCsOH). A larger cation on the catalyst surface has been reported to stabilize the dimer intermediate as well as minimize water at the surface, which can be beneficial for enhanced C₂₊ formation and COR selectivity.^{23, 33, 35} The highest partial current density towards C₂H₄ reached 40 mA/cm² when using CISPE-12-0.15MCsOH at 100 mA/cm², which was two folds higher compared to CISPE-12-0.15MKOH (FIGS. 46a and 46b). From the ion transport experiment, it was observed that in both cases (CISPE-12-0.15M KOH and CISPE-12-0.15McsOH), the charge was majorly balanced by the electromigration of OH⁻ to the anode, while electromigration of K⁺ to the cathode was not observed (FIGS. 42c and 42d).

[0488]This finding indicated that the enhanced COR activity and C₂H₄ selectivity could be attributed to the larger cations in the PFSA structure which stabilizes dimer intermediates.^{23, 33, 35} FIG. 29b summarizes the anolyte flux to the cathode under open circuit potential when using Nafion™-117, USPE-50, USPE-12, and CISPE-12-0.15MCsOH from the anolyte diffusion experiments. The low anolyte flux on both CISPE-12-0.15MKOH and CISPE-12-0.15MCsOH as compared to the Nafion™-117 also provides further evidence that the directly deposited SPE with implanted cation plays a role in COR selectivity by suppressing water transport to the cathode, thus minimizing the GDL pore blockage.¹⁰

[0489]It was sought to optimize COR selectivity using CISPE-12-0.15MCsOH by changing the anolyte concentrations and measuring the COR selectivity (FIGS. 29c, 47a, 47b, and 47c). Lower HER selectivity was found on lower anolyte concentration, which may be attributed to the lower K⁺ crossover due to water diffusion. Although 0.2 M KOH anolyte exhibits lower HER activity, higher C₂H₄ selectivity was observed when using 0.5 M KOH anolyte, with the highest partial current density towards C₂H₄ of 47 mA/cm² at 150 mA/cm², which can be attributed to the local pH at the cathode.^{12, 36} While high C₂₊ selectivity was achieved using 0.5 M or 0.2 M KOH anolytes, observed was a higher overpotential for OER as compared to 1 M or 3 M KOH anolytes. To suppress the anodic overpotential, nickel iron-layered double hydroxide (NiFe LDH) was synthesized as the anode catalyst via a chloride corrosion treatment as opposed to pristine nickel foam.³⁷ A a lower cell voltage was noticed (around 0.4 V) when using NiFe LDH as compared to pristine nickel foam, while the COR selectivity in both cases was identical (FIGS. 29d, 47b, 48 and 49). This enables high CO conversion (up to 90% CO single pass conversion) and more energetically efficient COR-to-C₂H₄(up to 21% full-cell energy efficiency) using 0.5 M KOH anolyte (FIGS. 50 and 51). Also observed was a lower cell voltage when using NiFe LDH with 0.2 M KOH anolyte, with liquid products like C₂H₅OH as a major product at 100 mA/cm² (FIGS. 47c, 52, and 53). To support the highly active nature of NiFe LDH, Tafel, Electrochemical Impedance Spectroscopy (EIS), and stability studies (FIG. 33) were conducted to showcase its improvement towards OER as compared to the bare Ni foam as an anode (Example 4—section S2A).^{38, 39}

[0490]A long-term COR experiment was carried out to understand the stability of CISPE-12-0.15MCsOH in a MEA configuration. The ultrathin PFSA layer suppressed K⁺ crossover to the cathode due to electromigration as well as water diffusion. The presence of implanted Cs⁺ further enhanced C₂H₄ selectivity at higher current densities, resulting in higher partial current densities toward C₂H₄. Over 200 hours of stable operation was demonstrated using CISPE-12-0.15MCsOH at the cathode and NiFe LDH at the anode; using 0.5 M KOH anolyte with the CO-to-C₂H₄ electrolyzer efficiency (EE) of 21% at 100 mA/cm² (FIGS. 30a, 54a, 54b, 54c and 54d). The image of the post-reaction catalyst after the stability experiment provides evidence of the stable nature of the Nafion layer (FIGS. 55 and 56). With the same configuration using 0.2 M KOH anolyte, a CO-to-C₂H₅OH electrolysis efficiency of 17% was exhibited (FIG. 30) using an ultrathin PFSA layer (0.7 μm) for enhanced CO-to-C₂H₅OH electrolysis (Table S6).

[0491]Considering the motivation of CO electrolysis as a part of the carbon utilization strategy, an energy calculation for two-step CO₂-to-C₂H₄(CO₂ capture, CO₂-to-CO conversion, CO-to-C₂H₄ conversion and separation, see Example 4, Section I) and found that the herein described USPE/CISPE in MEA configuration enables lower energy consumption (e.g., 218 GJ/ton-C₂H₄) in alkaline anolyte (Table S6). In regard to CO electrolysis, the herein described USPE/CISPE in MEA configurations may provide: 1) use of ultrathin PFSA layer (as opposed to the thick standalone membrane) that can suppress K⁺ crossover to the cathode by controlling electromigration and water transport, 2) direct contact between the PFSA layer and copper surface that may protect the catalyst surface from physical degradation,⁴⁵ 3) implanted Cs⁺ in the PFSA layer that may further suppress water flux and HER activity; 4) use of highly active NiFe LDH anode by replacing pristine Ni foam for improved energy efficiency. From the technoeconomic analysis, it was highlighted that with further development, COR may be a promising intermediate step for producing low-cost C₂H₄, particularly when the sustainable electricity price reach $0.02 per kWh (FIG. 57).

Non-Binding Conclusion

[0492]Herein described and/or developed is an ultrathin solid polymer electrolyte (USPE) to augment the energy efficiency of CO electrolysis in MEA. Investigations began with a standalone AEM and CEM using alkaline anolyte and it was observed that the challenge on both standalone membranes comes from K⁺ crossover to the cathode which results in the cathode flooding. Herein, K⁺ crossover was suppressed by using a cation infused USPE (CISPE) which was directly deposited to copper surface of a catalyst layer, resulting in stable and highly efficient CO electrolysis. The crossover of K⁺ was suppressed by reducing the thickness of the ionomer layer, which also enabled controlling of water transport. Further utilized was NiFe LDH to suppress anodic overpotential. Then demonstrated was a higher electrolysis energy efficiency of 21% and 17% in C₂H₄ and C₂H₅OH production, respectively, from CO using alkaline anolyte at 100 mA/cm² with over 200 hours of stable operation. From the CO₂-to-C₂H₄ perspective, this result can be translated into a lower energy cost of about 218 GJ/ton-C₂H₄. The direct ionomer coating on the cathode catalyst offered a facile and scalable approach to eliminate the use of a standalone membrane to enhance the selectivity, stability, and efficiency of electrochemical systems.

Example 3, and Detailed Description—Carbon Monoxide Electroreduction (COR) References