CATALYST-IONOMER SYSTEMS AND METHODS FOR GAS-PHASE ELECTROLYSIS

Abstract

The disclosure provides in its first aspect a catalyst system for gas-phase electrolysis of a reactant gas to form a product in an aqueous medium, the catalyst system comprising a catalytic material; an ion-conducting polymer layer provided on the catalytic material and comprising an ion-conducting polymer that includes hydrophilic and hydrophobic groups. Said catalyst system is remarkable in that the ion-conducting polymer layer has a thickness of 2 nm to 50 nm measured by transmission-electron microscopy. In its second aspect, the disclosure provides a method of manufacturing a catalyst system for gas-phase electrolysis of reactant gas to produce a product in an aqueous medium preferably according to the first aspect. The use of the catalyst system in accordance with the first aspect in the electrochemical production of at least one multi-carbon compound from a carbon-containing gas or of at least one product from a reactant gas is also disclosed.

Claims

1-38. (canceled)

39. A catalyst system for gas-phase electrolysis of a reactant gas to form a product in an aqueous medium, the catalyst system comprising: a catalytic material comprising a catalytic metal or carbon, the catalytic material being a CO.sub.2 reduction reaction catalyst or a CO reduction reaction catalyst; and one ion-conducting polymer layer provided on the catalytic material wherein the catalytic metal comprises Cu, Ag, Pd, or Pd doped with Ag; comprising an ion-conducting polymer that includes hydrophilic and hydrophobic groups, and wherein the ion-conducting polymer layer has a thickness of 2 nm to 50 nm as measured by transmission-electron microscopy; wherein the ion-conducting polymer layer is homogeneous over the catalyst material and wherein the ion-conducting polymer comprises an ionomer with a backbone that comprises the hydrophobic groups and side chains that comprise the hydrophilic groups, wherein the hydrophilic groups comprise sulfonic acid groups.

40. The catalyst system according to claim 39, wherein the ion-conducting polymer layer is an ion-conducting polymer layer provided on the catalytic material and comprising an ion-conducting polymer that includes hydrophilic and hydrophobic groups, wherein the ion-conducting polymer layer has a morphology with separate hydrophilic and hydrophobic domains that form differentiated gas and ion transport routes.

41. The catalyst system according to claim 39, wherein the catalytic material is doped with a dopant comprising an oxide, a halide, a telluride, a chalcogenide, a hydroxide, an oxyhydroxide, a nitrate, a silicide, or a combination thereof.

42. The catalyst system according to claim 39, wherein the catalytic material comprises Bi, Al, Sn, Pb, Au, Cf, Ru, Rh, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Te, Re, Fe, Ru, Os, Co Ir, Ni, Zn, Cd, Hg, Ge, Si, or a combination thereof.

43. The catalyst system according to claim 39, wherein the catalytic material is or is comprised in a layer deposited on a gas-diffusion membrane.

44. The catalyst system according to claim 39, wherein the ion-conducting polymer comprises a perfluorinated sulfonic acid ionomer or an ionene.

45. The catalyst system according to claim 39, wherein the hydrophobic groups comprise halogenated groups.

46. The catalyst system according to claim 39, characterized in that the ion-conducting polymer layer is one or more spray-coated layers or one more coated layers formed by the ion-conducting polymer directly onto an outer surface of the catalytic material.

47. The catalyst system according to claim 39, wherein at least a part of the catalytic material is in the form of a plurality of particles and the on-conducting polymer layer is provided around the catalytic material particles, thereby providing a plurality of catalyst-polymer particles.

48. The catalyst system according to claim 47, wherein the catalyst system comprises a catalyst-ionomer bulk heterojunction (CIBH) and the CIBH is disposed on a gas-diffusion membrane.

49. The catalyst system according to claim 47, wherein a part of said catalytic material is in the form of a layer and wherein said catalyst system comprises a catalyst-ionomer bulk heterojunction (CIBH) disposed on the catalytic material layer.

50. The catalyst system according to claim 47, wherein the catalyst system comprises at least two ion-conducting polymer layers and wherein at least one ion-conducting polymer layer is comprised in the catalyst-ionomer bulk heterojunction (CIBH) and wherein the CIBH is disposed on an ion-conducting polymer layer.

51. The catalyst system according to claim 47, wherein a part of said catalytic material is in the form of a layer, wherein the catalyst system comprises at least two ion-conducting polymer layers and wherein at least one conducting polymer layer is comprised in a catalyst-ionomer bulk heterojunction (CIBH) and wherein an ion-conducting polymer layer is disposed between the CIBH and the catalytic material layer.

52. The catalyst system according to claim 47, wherein the catalyst system comprises a catalyst-ionomer bulk heterojunction (CIBH) and wherein the ion-conducting polymer comprises a perfluorinated sulfonic acid ionomer.

53. The catalyst system according to claim 47, wherein the catalyst system comprises a catalyst-ionomer bulk heterojunction (CIBH) and wherein the CIBH has a weight ratio of catalyst material to ion-conducting polymer ranging from 0.1 to 10.0.

54. The catalyst system according to claim 39, wherein the catalytic material is provided as a porous metal layer and the one or more ion-conducting polymer layers are disposed thereon to form a catalyst-ionomer planar heterojunction (CIPH).

55. A method of manufacturing a catalyst system for gas-phase electrolysis of reactant gas to produce a product in an aqueous medium, the method comprising: a) providing a catalytic material being or comprising a catalytic metal and/or carbon; wherein the catalytic metal comprises one or more selected from Cu, Ag, Pd and Pd doped with Ag; and b) disposing an ion-conducting polymer onto an outer surface of the catalytic material to form one ion-conducting polymer layer thereon, wherein the ion-conducting polymer comprises hydrophobic domains and hydrophilic domains comprising sulfonic acid groups and wherein the ion-conducting polymer layer has a thickness of 2 nm to 50 nm measured by transmission-electron microscopy; wherein the ion-conducting polymer comprises a ionomer with a backbone that comprises the hydrophobic groups and side chains that comprise the hydrophilic groups.

56. The method of claim 55, wherein the step of disposing an ion-conducting polymer onto an outer surface of the catalytic material further comprises: providing an ion-conducting polymer liquid comprising the ion-conducting polymer and a solvent; and disposing the ion-conducting polymer liquid onto the catalytic material.

57. A method of manufacturing a catalyst system for gas-phase electrolysis of reactant gas to produce a product in an aqueous medium, the method comprising: providing a catalytic material being or comprising a catalytic metal and/or carbon; wherein the catalytic metal comprises one or more selected from Cu, Ag, Pd and Pd doped with Ag; wherein at least a part of the catalytic material is in the form of a plurality of particles; and providing an ion-conducting polymer, wherein the ion-conducting polymer comprises hydrophobic domains and hydrophilic domains comprising sulfonic acid groups; wherein the ion-conducting polymer comprises an ionomer with a backbone that comprises the hydrophobic groups and side chains that comprise the hydrophilic groups; contacting the plurality of catalytic material particles with the ion-conducting polymer and at least one solvent; to dispose the ion-conducting polymer around the plurality of catalytic material particles, and to form a mixture comprising a plurality of catalyst-polymer particles; and disposing the mixture onto a substrate to form thereon a catalyst-ionomer bulk heterojunction (CIBH) that comprises the catalytic material.

58. The method according to claim 57 wherein the substrate is one selected from a gas diffusion membrane; a layer of catalytic material; a layer of ion-conducting polymer; or a layer of ion-conducting polymer disposed on a layer of catalytic material.

Description

DESCRIPTION OF DRAWINGS

[0159] FIG. 1. Techno-economic analysis (TEA) exploring the high-current (1 A/cm.sup.2) regime. Dotted line indicates an ethylene production cost of $1000/ton. (A) Contour plot of the plant-gate levelized cost of ethylene production as a function of current density and electrolyzer energy conversion. (B) Contour plot of the plant-gate levelized cost of ethylene production as a function of current density and electricity cost. Current densities in excess of 1 A/cm.sup.2 are desired to achieve levelized ethylene costs below $1000/ton

[0160] FIG. 2. Limiting current in gas-phase electrocatalysis and ionomer gas/liquid decoupled transport channels. (A) When gas and electrolyte (water and ion source) transport is decoupled, the three-phase reaction interface can be extended so all electrons participate in the desired electrochemical reaction. (B) Modelled G availability along catalyst's surface for standard (left) and decoupled (right) gas transport into a 5 M KOH electrolyte assuming an in-plane laminar gas diffusivity of D.sub.II/D.sub.KOH=1000 for the latter. Depending on the gas diffusivity within the gas transport channel, gas availability dramatically increases. (C) Modelled maximum available current density for CO.sub.2 reduction. D/D.sub.KOH manipulation enables entering into the >1 A.Math.cm.sup.−2 regime for CO.sub.2R. See methods for details on gas transport and reaction simulations.

[0161] FIG. 3. The catalyst:ionomer planar heterojunction (CIPH). (A) Schematic of metal catalyst deposited onto a polytetrafluoroethylene (PTFE) hydrophobic fiber support. A flat ionomer layer conformally coats the metal. (B) Perfluorinated ionomers such as Nafion® exhibit asymmetric hydrophilic and hydrophobic characteristics endowed by SO.sub.3.sup.− and CF.sub.3 functionalities, respectively. Depending on its assembly, hydrophobic domains could facilitate gas transport, while water/ion can be provided via hydrated hydrophilic surfaces. Laminar Nafion® arrangements have been reported depending on its thickness and substrate (see studies of Burdyny T., et al., ACS Sustain. Chem. Eng. (Supporting Info), 2017, 5, 4031-4040, entitled “Nanomorphology-Enhanced Gas-Evolution Intensifies CO.sub.2 Reduction Electrochemistry” and of Dinh C. T., et al., Science, 2018, 360, 783-787, entitled “CO.sub.2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface”). (C, D) Scanning-electron-micrographs of ionomer-coated metal catalysts. (E-G) Cryo-microtomed transmission-electron-micrograph (TEM) cross-sections of catalyst/ionomer revealing a laminar conformal overcoating. (H) Wide-angle X-Ray scattering (WAXS) spectra for reference and ionomer-modified catalysts. These reveal features at 1 Å.sup.−1, 1.28 Å.sup.−1 and 2 Å.sup.−1, associated with crystalline and amorphous PFSA phases. (I) 2D WAXS patterns of (left) reference and (right) CIPH samples. The corresponding sector cuts are shown in FIG. 3H. (111), (200), (220) and (311) lattice planes of Cu are clearly detected in both samples. PFSA features are additional present in CIPH samples at around 2.0 Å.sup.−1, and 1.2 Å.sup.−1. (J) Grazing incidence WAXS patterns of ionomer samples. (left) 2D pattern as a function of q.sub.xy and q.sub.z. The localized peaks along q.sub.xy=0 suggest a 2D lamellar orientation parallel to the substrate. Such layered structures have been reported for Nafion® films for sufficiently small thicknesses (see S. C. DeCaluwe, P. A. Kienzle, P. Bhargava, A. M. Baker, J. A. Dura, Phase segregation of sulfonate groups in Nafion interface lamellae, quantified via neutron reflectometry fitting techniques for multi-layered structures. Soft Matter. 10, 5763-5776 (2014)). (right) Cross section spectra for q.sub.xy=0 for CIPH and reference. Resonances at 0.4 Å.sup.−1 and 0.57 Å.sup.−1 are observed. These correspond to d-spacings of 17.7 Å and 11 Å, respectively. Ionomer was spray coated on cleaned Si substrates using optimum loading. (K) Raman spectra of reference and ionomer-modified catalysts revealing ionomer —CF.sub.2 and —SO.sub.3.sup.− groups distinctive features (see corresponding table).

[0162] FIG. 4. Increased limiting current and responsible mechanisms for CIPH catalysts for different reactions and materials. (A) Oxygen-reduction-reaction (ORR) showing a 30 mA/cm.sup.2 limiting current (J.sub.lim) for Ag reference catalysts as opposed to 250 mA/cm.sup.2 for a CIPH configuration. (B) For CO.sub.2RR, standard Ag catalysts yield a J.sub.lim ˜54 mA/cm.sup.2 (remaining current employed for hydrogen evolution). This is in stark contrast with CIPH samples, which retain a Faradaic efficiency (FE) above 85% for CO.sub.2 reduction to CO up to ˜500 mA/cm.sup.2. (C) This trend is maintained for Cu CIPH catalysts and hydrocarbon production: J.sub.lim towards ethylene (dominant product) is 50 mA/cm.sup.2 at −0.7 V vs RHE (Reversible Hydrogen Electrode) for bare Cu; but increases beyond 0.5 A/cm.sup.2 for CIPH (peak F.E. of 61% at 835 mA/cm.sup.2). (D) For CO reduction, J.sub.lim ˜64 mA/cm.sup.2 for standard Cu whereas CIPH achieve a maximum 340 mA/cm.sup.2 current for the same reaction —H.sub.2 byproduct generation restrained below 15% FE at all currents. Partial pressure COR studies in CO|N.sub.2 mixes for CIPH (E) and standard (F) catalyst show that only at partial pressures below 60% is Jim observed for CIPH, whereas a sharp, steady decrease, is observed for reference samples. At all partial pressures, CIPH exhibits an order of magnitude larger Jim. We note that both reference and CIPH samples exhibit comparable resistance and double layer capacitance.

[0163] FIG. 5. 3D catalyst-ionomer bulk-heterojunction (CIBH) for efficient gas-phase electrochemistry beyond 1 A/cm.sup.2. (A) Schematic representation of metal-ionomer bulk-heterojunction catalysts on top of a PTFE support. (B) Cross-sectional SEM of the BHJ catalyst (C) TEM of a cryo-microtomed BHJ and (D) elemental mapping of Cu and C revealing BHJ nanomorphology. (E) Partial current density for total CO.sub.2RR reactions, C.sub.2+ and C.sub.2H.sub.4 at maximum cathodic energy efficiency. The total CO.sub.2R current saturates at 1.3 A/cm.sup.2 for CIBH thickness beyond 6 μm. CIBH samples achieve more than a 6-fold increase in partial current density at cathodic energy efficiencies in the >40% range. (F) Performance statistics of the highest partial current configuration for 8 different samples. (G) Performance of best catalyst in an ultraslim flow-cell. A full-cell energy efficiency of 20% for C.sub.2+ products is estimated at 1.1 A/cm.sup.2 operating current.

[0164] FIG. 6. is a Knudsen diffusion schematic of gas interaction with hard cylindrical pore, where d is the pore diameter, and I is the free path.

[0165] FIG. 7. Partial current density saturation highlighting the limiting current density for increasing pH.(A) pH=14; (B) pH=14.7; (C) pH=15.

[0166] FIG. 8. Reacting gas concentration within the ionomer layer for different diffusion coefficients (D) for (A) O.sub.2 and (B) CO.sub.2.

[0167] FIG. 9. Raman spectroscopies. (A) PTFE/Ag reference and CIPH Ag samples showing the distinctive presence of sulfonate and —CF.sub.2 groups for CIPH. (B) Raman spectra of hydrated samples. Samples were excited at 738 nm and the signal was collected through air or an immersion 63× objective lens.

[0168] FIG. 10. In situ Raman spectra of Cu CIPH catalysts. The Raman spectrum of CIPH Cu samples shows the distinctive presence of sulfonate and —CF.sub.2 groups, as well as a significant amount of adsorbed CO at 280 and 350 cm.sup.−1—and hence extended CO coverage—in the case of CIPH samples. Samples were operated in a 5 M KOH electrolyte in a custom-made flow cell at −1.6 and −2 V vs Ag/AgCl for reference and CIPH samples respectively. Beyond −1.6 V vs Ag/AgCl, reference samples led to substantial H.sub.2 generation. Samples were excited was 738 nm and the signal collected through immersion 63× objective.

[0169] FIG. 11. Oxygen reduction reaction and impact of PFSA loading. Cyclic voltammograms of CIPH Ag samples for different PFSA loadings at 5 M KOH operation. The ORR limiting current increases reaching a maximum for 12 μl/cm.sup.2 loading.

[0170] FIG. 12. Raman spectrum of CIPH Ag and reference samples under ORR operation (1 M KOH −1 V vs Ag/AgCl). Reference spectra has been scaled up. CIPH shifted 0.2 a.u. for clarity. Peaks around 1150 cm.sup.−1 and 1550 cm.sup.−1 are ascribed to adsorbed O.sub.2 species (27-29), showing an increase of O.sub.2 availability for Ag CIPH catalysts. Samples were excited was 738 nm with and the signal collected through immersion 63× objective.

[0171] FIG. 13. Current-voltage characteristics of Ag/PTFE reference and CIPH samples under (A) ORR and (B) HER operation in 5 M KOH electrolyte. N.sub.2 was purged during HER operation. Reference and Ag CIPH samples exhibit a similar HER performance, revealing that the PFSA ionomer layer does not modify water or ion transport. CIPH samples show enhanced ORR current due to increased O.sub.2 availability.

[0172] FIG. 14. Current-voltage characteristics of Ag/PTFE reference and CIPH samples under (A) ORR and (B) HER operation in 1 M KHCO.sub.3 electrolyte. N.sub.2 was purged during HER operation. Reference and Ag CIPH samples exhibit a similar HER performance, revealing that the PFSA ionomer layer does not modify water or ion transport. CIPH samples show enhanced ORR current due to increased O.sub.2 availability.

[0173] FIG. 15. Current-voltage characteristics of Ag/PTFE reference and CIPH samples under (A) ORR and (B) HER operation in 0.5 M H.sub.2SO.sub.4 electrolyte. N.sub.2 was purged during HER operation. Reference and Ag CIPH samples exhibit a similar HER performance, revealing that the PFSA ionomer layer does not modify water or ion transport. CIPH samples show enhanced ORR current due to increased O.sub.2 availability.

[0174] FIG. 16. Current-voltage characteristics of Ag/PTFE reference and CIPH samples under (A) ORR and (B) HER operation in 0.7 M K.sub.2SO.sub.4 electrolyte. N.sub.2 was purged during HER operation. Reference and Ag CIPH samples exhibit a similar HER performance, revealing that the PFSA ionomer layer does not modify water or ion transport. CIPH samples show enhanced ORR current due to increased O.sub.2 availability.

[0175] FIG. 17. Product distribution of (A) Ag/PTFE reference and (B) Ag CIPH samples under CO.sub.2 reduction operation at 5 M KOH electrolyte as a function of current density. CIPH samples sustain efficient C.sub.1+ production at much higher productivities.

[0176] FIG. 18. Characterization of Ag reference and CIPH samples in a flow-cell configuration (1 M KHCO.sub.3). (A) Faradaic efficiency vs. current density and (B) current density vs. potential for Ag control samples. (C) Faradaic efficiency vs. current density and (D) current density vs. potential for Ag+ionomer (CIPH) samples. Reference samples show a CO.sub.2 to CO limiting current of 110 mA/cm.sup.2. No limiting current is observed for CIPH samples in this range.

[0177] FIG. 19. Characterization of Ag reference and CIPH samples in a flow-cell configuration (0.7 M K.sub.2SO.sub.4). (A) Faradaic efficiency vs. current density and (B) current density vs. potential for Ag control samples. (C) Faradaic efficiency vs. current density and (D) current density vs. potential for Ag+ionomer (CIPH) samples. CIPH samples evidence improved CO.sub.2 mass transport, with J.sub.CO partial current density increasing from 50 mA/cm.sup.2 to 110 mA/cm.sup.2.

[0178] FIG. 20. Product distribution of (A) Cu/PTFE reference and (B) Cu CIPH samples under CO.sub.2 reduction operation at 5 M KOH electrolyte as a function of current density. CIPH samples sustain efficient C.sub.2+ production at much higher productivities.

[0179] FIG. 21. (A-D) Scanning electron micrographs of Cu CIPH samples after CO.sub.2 reduction reaction at different magnification. The samples were operated under 5 M KOH at −3 V vs Ag/AgCl for 50 min. The PFSA layer is evident at all magnifications. Cu surface has experience surface reconstruction leading to the formation of smaller grains.

[0180] FIG. 22. Product distribution of (A) Cu/PTFE reference and (B) Cu CIPH samples under CO reduction operation at 5 M KOH electrolyte as a function of current density. CIPH samples sustain efficient C.sub.2+ production at much higher productivities.

[0181] FIG. 23. Characterization of Ag reference and CIPH samples in an h-cell configuration (0.5 M H.sub.2SO.sub.4). Ambient air is bubbled in the catholyte. In this configuration, the ORR limiting current is determined by the gas solubility in the electrolyte. Both samples exhibit a similar ORR limiting current, suggesting that the ionomer does not significantly modify local gas reactant solubility.

[0182] FIG. 24. Characterization of Ag reference and CIPH samples in an h-cell configuration (1 M KHCO.sub.3). CO.sub.2 gas is bubbled in the catholyte. In this configuration, the CO.sub.2R limiting current is determined by the gas solubility in the electrolyte. Current density for (A) Ag reference and (B) Ag CIPH. Both samples exhibit a similar CO.sub.2 limiting current, suggesting that the ionomer does not significantly modify local gas reactant solubility.

[0183] FIG. 25. Partial currents as a function of CO.sub.2|N.sub.2 partial pressure for (A) CO current Cu/PTFE reference (B) CO current Cu CIPH samples (C) C.sub.2H.sub.4 current Cu/PTFE reference (D) C.sub.2H.sub.4 current Cu CIPH.

[0184] FIG. 26. C.sub.2+ current density vs EE.sub.1/2. Partial current density towards C.sub.2 products versus cathodic energy efficiency. Dashed lines represent isosurfaces with constant current×energy efficiency. CIBH samples achieve more than a 6-fold increase in partial current density at cathodic energy efficiencies in the >40% range compared to best stable (>1 h) reported catalyst.

[0185] FIG. 27. Stability of CIBH samples integrated into a membrane-electrode-assembly (MEA) configuration. Current (top) and Faradaic Efficiency towards C.sub.2H.sub.4 (bottom) and of CIBH samples operated continuously in a 0.1 M KHCO.sub.3 electrolyte at −3.9 V full cell potential.

[0186] FIG. 28. CIPH catalyst operation at higher temperature operation. (A) FE towards ethylene for flat reference and CIPH samples show a shift towards lower voltage for a given selectivity as temperature increases from room temperature (RT) to 60° C. An opposite trend is observed for reference samples. (B) The corresponding partial current density also increases at similar potential, yielding a better C.sub.2H.sub.4 productivity at similar energy efficiency. Reference samples show a moderate increase in partial current. iR correction was applied based on EIS measurements at RT (R.sub.s=1.62) and 60° C. (R.sub.s=1.19) operating conditions and a 0.9 correction factor. Dashed lines are a linear fit to serve as a guide to the eye.

[0187] FIG. 29. CIBH catalyst operation at higher temperature operation. (A) Potential vs. applied current for CIBH samples at RT and 60° C. Improvements, up to 0.9 V are observed at large currents. Ethylene FE (B) and partial current density (C) versus voltage: a similar FE is sustained at lower potentials and higher partial current densities are obtained at lower potentials. iR correction was applied based on EIS measurements at RT (R.sub.s=1.55) and 60° C. (R.sub.s=1.15) operating conditions and a 0.9 correction factor.

[0188] FIG. 30. CIBH catalyst operation showing the ethylene partial current dependence versus cell voltage at room temperature (RT) and higher temperature (HT=60° C.-70° C.) operation.

[0189] FIG. 31. This scheme summarizes three possible arrangements of the catalyst system of the present disclosure. Panel A shows that a catalyst-ionomer bulk heterojunction (CIBH) is disposed on a gas-diffusion membrane which is a gas-diffusion electrode (GDE). B shows that a catalyst-ionomer bulk heterojunction (CIBH) is disposed on a catalyst material layer, itself disposed on a gas-diffusion membrane which is a gas-diffusion electrode (GDE). C shows that a catalyst-ionomer bulk heterojunction (CIBH) is disposed on a catalyst-ionomer planar heterojunction (CIPH), itself disposed on a gas-diffusion membrane which is a gas-diffusion electrode (GDE).

[0190] FIG. 32. This scheme details the arrangement C of the FIG. 31. A plurality of catalyst-polymer particles is represented. Electron percolation paths are thus created.

DETAILED DESCRIPTION

[0191] Techniques described herein relate to enhanced catalyst materials that can be used for electrolysis reactions, such as electrochemical CO.sub.2 or CO reduction, and the production of multi-carbon compounds. The catalyst materials include a catalytic metal (e.g., Cu or Ag other others) and an ionomer. The ionomer can be disposed as an ionomer layer provided on the catalytic metal. The present description also relates to systems and methods that use such catalyst materials, also to methods of manufacturing such catalyst materials.

[0192] In some implementations, the catalyst material includes what can be called a catalyst-ionomer bulk heterojunction (CIBH) that decouples gas, ion and electron transport, and enables thereby CO.sub.2 electrolysis at relatively elevated current densities, e.g., current densities well above 1 A/cm.sup.2. In some implementations, the CIBH comprises a catalytic metal and a superfine ionomer layer with asymmetric hydrophobic and hydrophilic functionalities that extend gas and ion transport from its range in aqueous solution, which is tens of nanometers, to the micrometer scale. The CIBH simultaneously facilitates that this range overlaps with the location of catalytically active sites. This work demonstrates this CIBH strategy using a variety of metals and target products, reporting CO.sub.2 electroreduction with a C.sub.2+ partial current density exceeding 1.3 A/cm.sup.2 at 45% cathodic energy efficiency, which is a sixfold increase over the best previously reported CO.sub.2 reduction catalysts. Because CIBH separately harnesses the benefits of accelerated reaction kinetics and extended gas transport, it facilitates high-efficiency operation at electrolyzer-relevant operating temperatures.

[0193] Broadly, the catalyst system can include a metal catalyst and a polymer or oligomer as in Formula A below, where R is an ion-conductive group leading to ion/water transport domains.

##STR00001##

[0194] Formula A

[0195] The polymer or oligomer can be an ionomer or an ionene, for example. In some implementations, an ionomer is used and can be a perfluorinated sulfonic acid (PFSA) type ionomer. Below are some properties and information regarding potential PFSA ionomer that can be used in the catalyst systems as described herein (see Kusoglu in Chem. Rev. 2017, 117, 3, 987-1104 and Sigma Aldrich website):

[0196] PSFA Ionomers: General Chemical Structure

##STR00002##

[0197] See also Trigg and Winey's paper “Nanoscale layers in polymers to promote ion transport” in Molecular Systems Design & Engineering (Issue 2, 2019) for additional example and information regarding ion-conducting polymers that could be used for the catalyst systems. It is also noted that the ion-conducting polymers (e.g., ionomers) that can be used in the catalyst systems can have one or more properties of the particular ion-conducting polymers mentioned herein, e.g., they can have “m”, “n” or “p” values within ranges that are similar to the above species (e.g., 4.5 to 6.8); they can have “x” and “y” values within the ranges mentioned above; and/or can have one or more other properties of such compounds to form the differentiated gas and ion transport routes that are at least partly defined by hydrophilic and hydrophobic domains in the one or more ion-conducting polymer layers. The side chains of ionomers that can be used may have different lengths, with shorter lengths being potentially preferred in some circumstances.

[0198] It is noted that in the present study, both Nafion® (i.e. Perfluoro(2-(2-sulfonylethoxy)propyl vinyl ether)-tetrafluoroethylene copolymer) and SSC ionomers were assessed, and it was demonstrated that performance was enhanced. Other molecules in the PFSA ionomer family are expected to work similarly, and various ion-conducting polymers and oligomers could also be used or adapted depending on the particular catalyst metal, reaction, and operating conditions of interest for a particular application such as electrolyte, pH, and temperature. Other ionomers, such as Fumion or the like, are expected to show similar transport enhancement mechanisms.

[0199] In addition, catalytic materials that were tested and demonstrated include Cu, Ag, Pd and Pt doped Ag, as well as carbon. It is noted that catalysts for which the demonstrated mechanism can also be applied include Bi, Al, Sn, Pb, Au, Cf, Ru, Rh, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Te, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Zn, Cd, Hg, C, Ge, Si, and their combinations, as well as additional dopant species (e.g., oxides, halides, tellurides, chalcogenides, hydroxides, oxyhydroxides, nitrates, silicides).

[0200] The catalyst systems were implemented onto a gas diffusion electrode (GDE). The proposed mechanism was demonstrated with porous carbon paper and PTFE as the GDE. These findings are expected to be extrapolated to any other GDE for gas electrolysis.

[0201] In terms of potential applications of the catalyst system design, the reactions over which the enhancement was demonstrated include CO.sub.2 reduction reaction (CO2RR), CORR, and Oxygen reduction reaction (ORR). It is noted that other reactions for which enhancement is expected include all gas-phase electrolysis reactions, such as Nitrogen reduction reaction (NRR), methane oxidation, ethylene oxidation, hydrogen oxidation and combinations thereof. The reactions can be provided such that a single gas reactant is converted, or such that a mixture of multiple gas reactants is present and multiple gases are converted into products. This latter possibility could be potentially relevant, for example, for flue gas (mixture of CO.sub.2, O.sub.2, sulfides, etc.) to control the gas distribution over the catalyst (e.g., use the ionomer as a filter with preferential transport of one species). For flue gas, for example, it has been found that O.sub.2 transport can be promoted with certain ionomers, but this could be engineered by exploring different ionomers to promote CO.sub.2 transport.

[0202] In terms of fabrication of the catalyst system, two main types of arrangements are described: 2D and 3D configurations.

[0203] Whatever the configurations, a gas-diffusion membrane can be used to fabricate the catalyst system of the present disclosure. Advantageously, the gas-diffusion membrane is a gas-diffusion electrode (GDE) or an ion-transport membrane or a micro-structured metal or a nano-structured metal. For example, the gas-diffusion membrane is a hydrophobic porous support and/or is electrically conductive. For example, the gas-diffusion membrane is composed of porous carbon and/or polytetrafluoroethylene (PTFE). With preference, porous carbon is selected among carbon fibers and/or carbon cloth.

[0204] For 2D samples, there is a catalyst metal layer provided on a gas-diffusion membrane and the ionomer layer is provided on top of the catalyst metal layer. In particular, the study sprayed selected molecules (Nafion® or SCC) after diluting them in alcohol solvents onto the metal layer. The study assessed methanol and isopropyl alcohol (IPA) as potential solvents, but other alcohols and polar solvents are expected to work too. The choice of solvent can be used to drive ionomer assembly (see Lin et al. “Morphology study of Nafion® membranes prepared by solutions casting”, September 2005, Journal of Polymer Science, Part B, Polymer Physics; and also Lee et al. “Morphology study of Nafion® membranes prepared by solutions casting”, July 2018, Nature, Scientific Reports 8, Article number 10739(2018), for example which provide information in this regard).

[0205] The range of ionomer that was tested was 0 wt. %-10 wt. % (around a 1.2 wt. % for 30 uL in 2.5 mL). These volumes refer to the amount of ionomer solution dissolved in the stock solution. In the case of Nafion®, the original solvent is a combination of low aliphatic alcohols and water (per Sigma Aldrich catalog). In the studies, this ratio was fixed and the amount of solution sprayed onto the final catalyst was varied. The mixed solution of solvent and ionomer was then sprayed onto the gas diffusion layer. The loading was controlled by the amount of liquid sprayed per cm.sup.2. In FIG. 11 the study refers to μL/cm.sup.2 (where μL refers to the initial Nafion® solution dispersed in the alcohol—not the final volume sprayed). The ranges explored span 1 μL/cm.sup.2 to 200 μL/cm.sup.2 and it is noted that higher loadings are possible (e.g., up to 1 mL). The optimum loading can depend on local humidity conditions (higher humidity can require higher loadings), and for the experiments was found to typically be in the range of 10-50 μl/cm.sup.2. The loadings and other conditions can be modified depending on different ionomers, metals and reaction applications that may be used.

[0206] For 3D configurations, ionomer-coated metal nanoparticles can be prepared and then applied onto the 2D configuration, onto a naked gas-diffusion membrane, or onto a gas-diffusion membrane with a metal catalyst layer. For the 3D configurations, the tested catalyst mass loadings were within a range of 0 mg/cm.sup.2-10 mg/cm.sup.2. These loadings could be extended to higher loadings depending on the 3D morphology and tortuosity, which in turn can depend on certain factors. The morphology and tortuosity of the 3D configuration can be controlled by changing the ratio of metal nanoparticles and ionomer. The best tested performance was achieved for a 4:3 Cu to PFSA w/w ratio, for that particular catalyst system and conditions. In that system, excess Cu would block gas and ion transport, while excess PFSA would preclude electron percolation. Thus, a balance between the metal catalyst and the ionomer provided good performance. This ratio can also depend on the size (and weight) of the metal nanoparticles as well as the type of ionomer, for example. The metal-to-ionomer ratio could extend from 0.1:1 to 1:0.1, for example. In terms of a weight concentration, this can be thought of like 0 mg/mL-40 mg/mL of ink.

[0207] The size of the metal nanoparticles could go from 1 nm to about 200 nm, for example, or 1-100 nm. Although it is noted that various other sizes could also be used. In some implementations, the metal nanoparticles used can be distinct composition, and include Cu nanoparticles and Ag nanoparticles, for instance.

[0208] The morphology of the final 3D architecture of the CIBH could be engineered by adjusting the solvents of the nanoparticles and the ionomer, in a similar way that bulk heterojunctions can be tuned by the choice of solvent in organic photovoltaics.

[0209] In addition, the 3D configurations could be sprayed, spin coated, blade coated, drop casted, etc. Heating can also be applied during ink drying both for 2D and 3D configurations to control sample morphology.

[0210] The following are comments on gas transport in the nanostructured polymer and the differences compared to bulk-like configurations. The study demonstrated that this mechanism is effective over ˜10 μm lengths, and enabled through ultrathin ˜10 nm laminar ionomers, explaining why these results are different to the expected gas permeability in bulky ionomer membranes (scale of hundreds of micrometers).

[0211] The study varied the thickness of the catalyst-ionomer layer (3D CIBH configuration) and measured the limiting current densities. The study found that there was an optimal thickness (˜5-6 μm) for the tested system at which the current cannot be further increased, and wherefrom lower energy efficiencies are obtained for thicker samples. This suggests that the diffusion of the gas in the catalyst-ionomer 3D layer reaches a diffusion length in this regime (slightly larger than ˜5 μm if tortuosity is considered).

[0212] It will be appreciated from the overall description and the experimentation section in particular that the catalyst materials and systems as well as the associated methods described herein can have a number of optional features, variations, and applications.

EXAMPLES & EXPERIMENTATION

[0213] The present study assessed various features and properties of catalyst system design for converting gas reactants into products. The following provides details regarding experiments and work that was performed in the context of this study.

Introduction

[0214] The electrochemical transformation of gases into value-added products using renewable energy is an attractive route to upgrade greenhouse gases such as CO.sub.2 and CO into low-carbon-footprint hydrocarbon fuels and chemical feedstocks. The viability of the approach relies on improving energy efficiency and increasing current density to minimize both operational and capital costs in gas-phase electrocatalysis. The electroreduction of CO.sub.2 and CO into multiple hydrocarbon molecules requires catalysts that facilitate the transformation of these reactant gases through the subsequent adsorption, coupling, and hydrogenation of desired reaction intermediates via proton-coupled electron transfer steps. In these reactions, water-based electrolytes act both as a proton source and as the ion conductive medium.

[0215] The solubility of gases in liquid media is limited, leading to constrained gas diffusion as gas molecules collide or react with their environment. The diffusion length of CO.sub.2 in water electrolytes can be as low as tens of nm in alkaline environments. This has limited the productivity of catalysts in aqueous cells to current densities in the range of tens of mA/cm.sup.2 due to mass transport.

[0216] In a gas-phase electrolyzer, catalyst layers are deposited onto hydrophobic gas-diffusion electrodes so that gas reactants need only diffuse short distances to reach electroactive sites on the catalyst surface. Gas reactant diffusion in the catalyst layer becomes the mass-transport-limiting step in the cathode, as observed in fuel cell oxygen reduction reactions (ORR). To improve oxygen diffusion in the fuel cell catalyst layer, its hydrophobicity is usually optimized to help extract water while maintaining sufficient ion conductivity. It is noted that “electrolyzer” in this context refers to water electrolyzers that typically operated at high current densities and are operated at higher temperatures at large scale. With the newly developed catalyst materials as described herein, which separate gas transport, one can operate at higher temperature while keeping good efficiency and performance. Both high currents and high temperatures can be used for conversion of CO.sub.2 and other gases that is normally not possible with other catalyst systems.

[0217] Unlike oxygen reduction, which generates water as a product, CO.sub.2 reduction typically requires water as a proton source for hydrocarbon production. Thus, the catalyst layer is usually hydrophilic and fully filled with water during the reaction. In this configuration, CO.sub.2 electrochemical reactions occur within a gas-liquid-solid three-phase reaction interface. This volume—in which gas reactants, electrons and electrolyte ions coexist at catalyst electroactive sites—is a very localized one, particularly at high pH used in alkaline electrolysis, and at high local current density (high local OH.sup.−) environments. A large fraction of the catalyst is in contact with gas-reactant-deficient electrolyte. Because hydrogen evolution is a competing reaction with CO.sub.2 reduction in a similar applied potential range, the large fraction of catalyst surface area exposed to CO.sub.2-depleted electrolyte promotes undesired H.sub.2 generation. While recent advances in gas-phase CO.sub.2R have led to partial current densities for CO.sub.2 reduction of ˜100 mA/cm.sup.2, other liquid-phase electrochemical technologies such as water electrolysis achieve multi Amperes/cm.sup.2.

[0218] The field of renewable fuels urgently needs improved strategies to achieve high product selectivity at a much higher current density such that capital costs can be dramatically reduced. Techno-economic analysis (TEA) shows that the time-of-day affordability of renewable electricity, which enforces a low capacity loading of the electrolyzer (e.g., 20-40% utilization factor), increases the relative importance of capital costs, and reflects the necessity to operate at high current densities. Analyses have revealed that increasing current density over 1 A/cm.sup.2 will be required to take advantage of low-cost curtailed and excess renewable electricity (FIGS. 1A and 1B).

[0219] FIG. 1A uses a constant electricity cost of 2 ¢/kWh, while FIG. 1B uses a constant energy efficiency of 60%. This analysis considers a CO.sub.2 conversion of 1 ton/day and a CO.sub.2 price of $30/ton. Both models assume a 100% utilization of CO.sub.2 and a capacity factor of 0.8. The current density is inversely proportional to the electrolyser cost, referenced according to a 1 A/cm.sup.2 electrolyser valued at $1000/kW.

[0220] High-temperature solid oxide electrolysis offers a strategy to achieve CO.sub.2 reduction with a high current density: CO.sub.2 diffuses directly to the surface of the catalyst, in the absence of liquid electrolyte, thus overcoming the gas diffusion limitations of low temperature systems. However, high temperature conditions and the absence of liquid electrolyte have thus far limited CO.sub.2 reduction to the production to CO. It is noted that CO.sub.2 and CO are two main potential gas reactants, but various other gases could be used (e.g. O.sub.2 reduction reaction which has applications, for fuel cells for instance, among others).

Study Overview Regarding Catalyst-Ionomer Design

[0221] In the present study, a hybrid catalyst design was developed and this design, by decoupling gas, ion, and electron transport, facilitates efficient CO.sub.2 and CO gas-phase electrolysis at current densities in the >1 A/cm.sup.2 regime, and with the generation of multicarbon products. The catalyst design facilitates this by exploiting an ionomer layer that, with asymmetric hydrophobic and hydrophilic functionalities, assembles into a morphology with differentiated gas and ion long-range transport routes, conformally, over the metal surface. In this context, conformally means that there is a similar thickness all over the catalyst (e.g., thickness could be within plus or minus 5-20% for example). Gas transport is promoted through a backbone of hydrophobic domains, leading to extended gas diffusion; while water uptake and ion transport occurs via hydrated hydrophilic domains (FIG. 2A). As a result, the reaction interface at which gas reactants, ions and electrons at catalytically active sites are all present is enlarged from a sub-μm regime to several μm scale over the catalyst surface.

[0222] To assess the breadth of applicability of the concept, this work studied the reduction of oxygen, CO.sub.2, and CO using different catalyst metals (Ag and Cu) to a range of different target products (CO, C.sub.2H.sub.4 and C.sub.2H.sub.5OH). This work levers differentiated transport to achieve CO.sub.2 reduction currents above 1.5 A/cm.sup.2 with curtailed H.sub.2 by-product generation. This work reports C.sub.2+ partial currents exceeding 1.3 A/cm.sup.2 at cathodic energy efficiencies above 40%—a sixfold increase relative to the best previously-reported comparable catalysts. These result in a full-cell energy efficiency towards C.sub.2 products of 20% above 1 A/cm.sup.2 operating currents. Decoupled gas transport in the new metal:ionomer hybrid catalyst overcomes prior limitations in high-temperature water-based CO.sub.2 electrolysis—curtailed by the reduced CO.sub.2 diffusion in alkaline media—and enables reporting a further 50% relative productivity improvement as operating temperature increases.

Results and Discussion

[0223] The work included modelling the available gas reactant concentration in different gas-phase electrolysis scenarios (FIG. 2B) building on previously established models (see methods for more details). This work explored how catalyst performance toward gas-electroreduction would be modified as the availability of the gas reactant varied at the gas/electrolyte interface. To do so, the work introduced an intermediate surface channel of 20-nm thickness between the catalyst and the electrolyte with an in-plane gas diffusion coefficient (D) that is appreciably different from that of bulk electrolyte (D.sub.0). As D/D.sub.0 increases, gas flow is promoted through this layer, until the gas is converted at the catalyst surface or diffuses into the electrolyte (FIG. 2B (right)), potentially enabling CO.sub.2 diffusion of a depth of several μm. For a standard catalyst configuration, CO.sub.2 is available only within <˜1 μm (FIG. 2B (left)). As the diffusion enhancement in the layer increases, so too does the current available for the electrochemical conversion of the gas reactant (FIG. 2C). A similar trend holds for other reactant gases such as O.sub.2 (FIG. 8A).

[0224] The work also sought to design and implement such an enhanced transport layer experimentally. One option was perfluorinated sulfonic acid (PFSA) ionomers, which combine asymmetric hydrophobic and hydrophilic functionalities along with ion transport. Their controlled assembly into distinct hydrophobic and hydrophilic layered domains would offer differentiated pathways wherein gas transport was promoted through the hydrophobic domains, and water/ion transport facilitated by the hydrophilic domains. The approach was designed to be compatible with ion transport, and simultaneously to allow long-range gas transport through hydrophobic channels (FIG. 3A). It is noted that the assembly of the ionomer can be controlled or promoted by selecting the structure and properties of the ionomer as well as the solvent, loading, thickness, and other factors.

[0225] The present study assessed and determined some example variables that can be used, and one can adapt the methods for determining procedures and ingredients for making various types of catalyst systems for various reactions. In addition, given catalyst systems may have particular structures in terms of how the assembled ionomer is arranged to enable the hydrophilic and hydrophobic domains for molecular transport. The precise morphology of given systems may be unknown or difficult to measure, but certain features such as hydrophilic groups being exposed to the catalyst and the electrolyte have been deduced in the present study.

[0226] PFSA ionomers such as Nafion® exhibit SO.sub.3 (hydrophilic) and CF.sub.2 (hydrophobic) differentiated groups. Nafion®, a widely used material in fuel cells as a catalyst binder and membrane material, exhibits strong structure-function dependent properties. A configuration in which SO.sub.3 was preferentially exposed to hydrophilic metal surfaces and electrolyte can help provide percolating hydrophobic paths through CF.sub.2 hydrophobic domains (FIG. 3B).

2D-Extended Reaction Interface

[0227] Seeking to promote the exposure of SO.sub.3.sup.− groups towards catalyst and electrolyte surfaces, this work prepared ionomer solutions in polar solvents, and the solutions were then spray-coated onto hydrophilic metal catalysts deposited on a porous polytetrafluoroethylene (PTFE) substrate at different loadings. Scanning-electron micrographs (SEM) reveal a homogeneous, conformal ionomer coating over the entire catalyst (FIG. 3C-D). Cryo-microtomed cross-sectional transmission-electron microscopy (TEM) images reveal the presence of a 5-10 nm continuous and conformal ionomer layer (FIG. 3F-H), establishing a catalyst-ionomer planar heterojunction (CIPH). In this regard, it is noted that the metal layer was not completed flat, as it is a porous layer sputtered onto the gas-diffusion substrate (although it could be applied using other techniques).

[0228] To characterize the CIPH structural configuration, the work carried out wide angle X-ray scattering (WAXS) measurements on PTFE/Cu/ionomer samples (FIG. 3H, FIG. 31). Both reference and CIPH samples exhibited a similar contribution of the different Cu planes and PTFE backbone support. CIPH samples, in addition, revealed strong differentiated peaks at 1 Å.sup.−1, 1.28 Å.sup.−1 and 2 Å.sup.−1. The first two peaks were associated with crystalline and amorphous PFSA phases. In some implementations, certain factors such as temperature can be modulated to influence the resulting proportion of the crystalline and amorphous PFSA phases. Resonances in the area ca. 2 Å.sup.−1 have been ascribed to overlapping diffractions from intrachain correlations associated with ionomer orientation. To gain further insight into ionomer structure, we carried out grazing-incidence WAXS measurements (FIG. 3J), revealing an oriented arrangement parallel to the substrate with d-spacings features of 17.7 Å and 11 Å. Similar arrangements have recently been reported for sufficiently thin Nafion® layers.

Seeking to characterize the CIPH and the ionomer configuration in its hydrated condition, this work designed a suite of ex situ and in situ surface-enhanced Raman spectroscopy (SERS) experiments (FIG. 3K and FIG. 9A). As-deposited ionomers on Ag catalysts exhibit strong characteristic signals at 733 cm.sup.−1 (characteristic of —CF.sub.2 and C—C vibrations, see table 1) and at 1005 cm.sup.−1 and 1130 cm.sup.−1 (associated with —SO.sub.3.sup.− modes) as well as a complex background set of features arising from other C—C(1386 cm.sup.−1), C—F (1182 cm.sup.−1, 1300 cm.sup.−1) and S═O (1446 cm.sup.−1) modes.

TABLE-US-00001 TABLE 1 Wavenumber for ionomer-relevant functional groups Wavenumber (cm.sup.−1) Group 1980 H.sub.2O 1787 OH 1610 OH 1446 S═O 1386 ν(C—C) 1300 CF.sub.3 a 1182 C—F a 1130 ν.sub.S(SO.sub.3.sub.−) 1060 S—O CCO s 1005 ν.sub.S(SO.sub.3.sub.−) 966 S—O C—S s 936 S—OH 893 ν(C—S) 802 ν(C—S) 733 CF CCC 640 CF.sub.2 rock 450 Metal-CO 388 δ(CF.sub.2)
Hydrated samples retain characteristic —CF.sub.2, C—C and —SO.sub.3.sup.− spectral features, but a notably increased relative contribution of sulfonate groups (1009 cm.sup.−1 and 1131 cm.sup.−1) compared to perfluoronate (730 cm.sup.−1). This trend is maintained during operation in 1 M KOH electrolyte at reducing potentials and with the use of other catalyst metals such as Cu, suggesting that hydrated —SO.sub.3.sup.− groups tend to face the electrocatalyst surface.

[0229] This work then sought to evaluate the electrochemical performance of the CIPH for different metals and reactions (FIG. 4). In the oxygen reduction reaction (ORR) used in fuel cells, oxygen is reduced into water. The lack of a competing reaction to the ORR at potentials more positive than HER can be used to identify gas-reactant depletion and its impact in the limiting current.

[0230] This work built CIPH structures consisting of spray-cast ionomer coatings over Ag/PTFE substrates at different loadings and monitored the ORR current (FIG. 11) using a 5 M KOH water electrolyte and air as reactant. Unmodified Ag catalysts show a current density pinned below 30 mA/cm.sup.2. CIPH catalysts, on the other hand, exhibit a significantly enlarged current density peaking at 250 mA/cm.sup.2 under the same conditions (FIG. 4A), where no H.sub.2 production was observed. This agrees with in situ Raman measurements, which show a notable increase in the presence of O.sub.2 near catalyst surface at operating conditions (FIG. 12). The observed enhancement can be explained due to ˜600× increased diffusion of O.sub.2 relative to bulk electrolyte based on Knudsen diffusion of the reacting gas via CIPH hydrophobic domains (see methods below).

[0231] To assess whether ion-transport was modified in metal-ionomer catalysts, this work compared the ORR and HER performance of standard Ag and Ag—CIPH samples for various electrolytes. Because the reactant in HER is in the aqueous phase (water or hydrated proton), the performance of the catalyst is not affected by the gas diffusion properties of the PFSA ionomer layer; instead, catalyst performance is affected only by water availability and ion transport. We found that CIPH samples exhibit similar hydrogen evolution activity to bare catalysts and increased ORR current across a wide range of electrolytes and pH (FIG. 13-16). This supports the notion that the enhanced ORR performance of the CIPH samples stems from extended gas transport.

[0232] To shed further light on the character of the ionomer modification, this work characterized the performance of CIPH samples for different reactions such as CO.sub.2 and CO reduction and investigated reduction toward different products. The work first screened Ag—CIPH samples for CO.sub.2RR targeting CO production, and observed a CO.sub.2RR partial current density of 400 mA/cm.sup.2 (FIG. 4B, FIG. 17). In contrast, Ag reference samples, limited by CO.sub.2 availability, exhibit a maximum CO.sub.2RR partial current density of ˜54 mA/cm.sup.2. This trend is maintained across different electrolytes (FIGS. 18-19).

[0233] These observations translate as well to Cu—CIPH catalysts targeting hydrocarbon generation (FIG. 4C, FIG. 20). Cu—CIPH catalysts exhibit a remarkable increase of CO.sub.2RR current. At 800 mA/cm.sup.2, H.sub.2 generation remains below 10% FE whereas the FE toward ethylene (C.sub.2H.sub.4) surpasses 60%. A CO.sub.2 partial current density of up to 510 mA/cm.sup.2 was achieved (FIG. 4C). Bare Cu catalysts, on the other hand, exhibit a limited CO.sub.2RR current of 50 mA/cm.sup.2. This performance is consistent with the increased presence of adsorbed CO intermediates, as observed using in situ Raman spectroscopy at similar conditions (FIG. 10). Based on a model that was developed, the observed enhancement can be explained due to ˜400× increased diffusion of CO.sub.2 relative to bulk electrolyte.

[0234] The electrochemical surface area (ECSA) of reference and CIPH samples, as well as cell resistance, were comparable (see methods), indicating that these were not causes of the observed enhancement. These results are further supported by the similar hydrophobicity of the catalysts before and after ionomer modification: static contact angles obtained from the reference and CIPH samples have a similar value ˜121-122°, confirming that the enhanced gas reduction in CIPH samples originates from the extended gas diffusion through the ionomer layer rather than from a redistribution of the gas/electrolyte in the PTFE substrate pores. Post-reaction SEM reveals the unmodified presence of the PFSA ionomer in the CIPH after reaction (FIG. 21).

[0235] To query the impact of CIPH when applied to other gas reactants, this study monitored the CO reduction reaction on Cu—a system with activity limited by the poor solubility of CO in the electrolyte (FIG. 4D, FIG. 22). Cu—CIPH samples yield a CORR partial current density of up to 340 mA/cm.sup.2. Bare Cu samples, in contrast, show a CORR limiting current of 64 mA/cm.sup.2.

[0236] To study the effect of the ionomer on the kinetics of the reaction, which could lead to the difference in partial current densities observed, the present study performed both ORR and CO.sub.2RR in aqueous H-cell reactors. In this configuration, gas transport to the entire surface of the catalyst takes place through the electrolyte. In ORR, the study observed a slight improvement in reaction kinetics as indicated by a higher current density at low overpotential regime for CIPH sample (FIG. 23). In CO.sub.2RR on Ag based catalyst, both bare Ag and Ag—CIPH show comparable current densities, with a slight increase in CO FE (˜5%) for Ag—CIPH samples at low current density (<40 mA/cm.sup.2)—which can be attributed to change in local environment induced by Nafion® layer. It was noted that no change in oxidation or coordination number of the metal active sites is observed during in situ X-Ray Absorption Spectroscopy (XAS). To monitor possible changes in catalyst atomic environment (oxidation state and coordination number), we carried out operando X-ray absorption spectroscopy (XAS) (fluorescence mode) for bare PTFE/Cu and PTFE/Cu/ionomer (CIPH) samples using a custom-made cell. A 5 M KOH electrolyte was circulated in both anode and cathode and CO.sub.2 gas was supplied from the backside of cathode of the in-situ XAS flow cell.

Cu K-edge XANES spectra of both reference and CIPH samples revealed a similar oxidation state of Cu (Cu°, metallic Cu), which was maintained during CO.sub.2R under the reducing potential of −2.0 V vs. Ag/AgCl.

[0237] For reference samples, Cu—Cu coordination number (CN) obtained by EXAFS analysis started to increase after CO.sub.2R (2 min), which was maintained during a 30 min initial study. CIPH samples showed a similar trend for Cu coordination before and during reaction. This reveals that the atomic local environment, coordination number and electronic structure of Cu active sites was note affected by the presence of the ionomer gas channel.

[0238] Notably, in the H-cell configuration, the study observed similar limiting current densities for bare and CIPH samples in ORR and CO.sub.2RR. These results indicate that although the presence of Nafion® on the surface can change the reaction kinetics, it is its extended gas transport properties that enable overcoming the limiting current density in gas-phase electrolysis.

[0239] To further explore the role of gas availability in the limiting current, the study varied the gas availability by tuning the partial pressure of the reactant in N.sub.2 mixes (FIG. 4E-F). A steep partial pressure dependence of limiting current density for ethylene was observed in CORR on Cu sample. Only at partial pressures below 60% was a limiting current observed for CIPH. At all CO partial pressures, Cu—CIPH exhibited an order of magnitude higher partial current density compared to bare Cu. The study observed a similar trend in CO.sub.2RR with varying CO.sub.2 partial pressure (FIG. 25). These results further confirm the role of Nafion® in enhancing reactant availability and thereby increasing current density.

CIBH and the 3D-Extended Reaction Interface

[0240] In light of these findings, the study sought to develop an additional new catalyst design that took advantage of the gas/electrolyte segregated transport beyond two dimensions. Ideally, such a catalyst would maximize the triple-phase reaction interface across an extended 3D morphology, enabling efficient operation in higher current regimes. The study implemented a 3D catalyst:ionomer bulk heterojunction (CIBH) including Cu nanoparticles and Nafion® ionomer blended and spray cast on a PTFE/Cu/ionomer (CIPH) gas diffusion layer support, forming a 3D morphology with metal and ionomer percolation paths (FIG. 5A). Cross-sectional SEM images reveal the different layers in the CIBH catalyst (FIG. 5B). High-resolution cryo-microtomed cross-section images obtained by TEM and elemental energy-dispersive X-ray spectroscopy (EDS) mapping further reveal the presence of continuous Cu NP and ionomer domains (FIG. 5C-D).

[0241] For the 3D experiments, a solution of metal particles and a solution of ionomer were prepared, and then these two solutions were mixed at desired ratio. Depending on the ratio, a different morphology can be obtained. The mixed solution was then sprayed onto the gas-diffusion membrane or spray onto the 2D version, for example. It was found that it is more robust to spray onto the 2D planar heterojunction. Results were obtained showing that the system functions whether the 3D layer is provided on the 2D layer or directly onto the gas-diffusion membrane.

[0242] The study first optimized CIBH morphology by tuning the deposition conditions as well as the Cu:ionomer blend ratio, which was found optimized for a 4:3 w/w configuration. Using this configuration, we then explored the effect of catalyst layer thickness. In an effective CIBH catalyst, CO.sub.2RR current would increase with catalyst loading, i.e., with loading of Cu:ionomer catalyst, until the length of the gas percolation paths through the ionomer phase reached the gas reactant diffusion length. This length can be thought of as the length over which the gas would be fully consumed already, and thus having a thickness greater than this length would not be accessed by any of the gas reactant. As we increase catalyst loading and its corresponding thickness, we observe a monotonic increase of total CO.sub.2RR current, which surpasses 1 A/cm.sup.2 for a nominal loading of 3.75 mg/cm.sup.2 (5.7 μm thickness), and which saturates at 1.32 A/cm.sup.2 for higher loadings (FIG. 5E). The total partial current for C.sub.2+ products (ethylene, ethanol, acetate and propane) reaches 1.21 A/cm.sup.2 (see tables 2 to 5), which is achieved in the range of 40-50% cathodic energy efficiency.

TABLE-US-00002 TABLE 2 1.67 mg/cm.sup.2, 4:3 Cu:PFSA V vs Ag/AgCl V vs RHE V vs RHE iR J (mA/cm.sup.2) 1.50 −0.43 −0.37 45 1.70 −0.63 −0.48 105 2.00 −0.93 −0.56 250 2.30 −1.23 −0.67 373 2.50 −1.43 −0.79 430 2.70 −1.63 −0.87 510 3.00 −1.93 −0.90 690 3.30 −2.23 −0.87 910 3.50 −2.43 −0.95 990 3.70 −2.63 −1.04 1060 Faradaic Efficiency (%) H.sub.2 CO COOH CH.sub.4 C.sub.2H.sub.4 EtOH Acetic Propanol total 12.2 30.5 25.6  11.1  0.3 16.7  0.1 96.4 7.4 29.4 24.9 4.5 22.6 13.1  38.1 13.4 5.3 0.2 97.1 4.0 15.6 42.4 4.9 10.1 7.1 47.8 19.0 4.8 6.4 100.3  5.2 8.9 48.5 5.9 6.0 4.8 49.9 18.3 3.2 3.9 92.0 9.7 4.3 54.2 10.8 3.1 2.6 46.5 23.2 8.2 0.2 94.5 13.3 2.2 40.4 Partial current J_X (A/cm.sup.2)* J_C.sub.2H.sub.4 J_C2 J_C EE(%)** 0.01 0.01 0.04 20.6 0.03 0.10 0.14 0.23 37.1 0.16 0.21 0.34 0.41 41.5 0.25 0.34 0.52 0.59 39.2 0.49 0.46 0.77 0.83 41.8 0.43 *Partial current of a compound X, namely J_X, equals the product of the current J by the faradaic efficiency FE of a given compound X. (J_X = J*FE_X) **EE stands for Energy Efficiency

TABLE-US-00003 TABLE 3 3.33 mg/cm.sup.2, 4:3 Cu:PFSA V vs Ag/AgCl V vs RHE V vs RHE iR J (mA/cm.sup.2) 1.50 −0.43 −0.34 60 1.70 −0.63 −0.45 122 2.00 −0.93 −0.58 238 2.30 −1.23 −0.68 368 2.50 −1.43 −0.67 510 2.70 −1.63 −0.73 600 3.00 −1.93 −0.77 775 3.30 −2.23 −0.76 980 3.50 −2.43 −0.68 1170 3.70 −2.63 −0.68 1300 4.00 −2.93 −0.88 1370 4.30 −3.23 −0.91 1550 4.50 −3.43 −0.82 1740 Faradaic Efficiency (%) H.sub.2 CO COOH CH.sub.4 C.sub.2H.sub.4 EtOH Acetic Propanol total 13.9 42.3 11.5 10.1 32.7 20.0 7.1 25.7 13.8  29.9 13.1 4.6 0.4 94.6 5.0 18.2 35.8 4.3 12.5 5.4 40.4 21.1 3.7 2.0 89.4 4.3 12.5 43.8 4.3 10.0 46.9 29.4 6.0 2.6 99.2 5.0 10.0 53.7 3.6 8.6 2.2 55.4 23.0 7.6 1.5 101.9  4.8 7.3 56.6 6.8 6.8 1.4 56.4 16.9 6.0 0.2 94.5 7.9 6.2 0.9 60.0 13.5 4.5 0.2 93.2 8.5 4.9 48.0 Partial current J_X (A/cm.sup.2)* J_C.sub.2H.sub.4 J_C2 J_C EE(%)** 0.01 0.02 0.07 0.11 0.21 30.8 0.13 0.21 0.34 0.43 40.2 0.26 0.36 0.66 0.74 48.1 0.53 0.65 1.02 1.15 52.7 0.74 0.77 1.09 1.20 44.0 0.93 1.21 1.32 42.7 0.84 *Partial current of a compound X, namely J_X, equals the product of the current J by the faradaic efficiency FE of a given compound X. (J_X = J*FE_X) **EE stands for Energy Efficiency

TABLE-US-00004 TABLE 4 5 mg/cm.sup.2, 4:3 Cu:PFSA V vs Ag/AgCl V vs RHE V vs RHE iR J (mA/cm.sup.2) 1.50 −0.43 −0.31 85 1.70 −0.63 −0.37 173 2.00 −0.93 −0.39 360 2.30 −1.23 −0.49 495 2.50 −1.43 −0.56 580 2.70 −1.63 −0.57 710 3.00 −1.93 −0.54 930 3.30 −2.23 −0.55 1120 3.50 −2.43 −0.53 1270 3.70 −2.63 −0.55 1390 4.00 −2.93 −0.80 1420 Faradaic Efficiency (%) H.sub.2 CO COOH CH.sub.4 C.sub.2H.sub.4 EtOH Acetic Propanol total 14.2 37.5 9.3 12.9 31.9 17.3 10.3 22.5 25.2  31.6  2.2 4.2 0.7  96.7 7.4 17.2 37.6 6.4 15.4 15.0  42.8 14.3 2.6 3.6 100.3 5.6 14.4 46.5 5.3 11.6 4.7 52.5 23.7 5.4 1.6 104.8 5.0 10.3 54.0 4.9 7.4 4.8 54.4 22.4 5.4 3.1 102.6 5.5 7.2 55.9 7.2 4.8 4.9 49.6 23.7 5.6 4.0  99.8 Partial current J_X (A/cm.sup.2)* J_C.sub.2H.sub.4 J_C2 J_C EE(%)** 0.01 0.03 0.11 0.14 0.31 27.4 0.19 0.25 0.37 0.54 39.0 0.33 0.49 0.77 0.93 53.9 0.61 0.69 1.08 1.24 54.7 0.78 0.70 1.18 1.31 45.4 *Partial current of a compound X, namely J_X, equals the product of the current J by the faradaic efficiency FE of a given compound X. (J_X = J*FE_X) **EE stands for Energy Efficiency

TABLE-US-00005 TABLE 5 Cu ClPH reference V vs Ag/AgCl V vs RHE V vs RHE iR J (mA/cm.sup.2) 1.70 −0.63 −0.63 0.092 2.10 −1.03 −1.03 0.335 2.50 −1.43 −1.43 0.51 2.90 −1.83 −1.83 0.72 3.30 −2.23 −2.23 0.9 Faradaic Efficiency (%) H.sub.2 CO COOH CH.sub.4 C.sub.2H.sub.4 EtOH Acetic Propanol total 3.0 24.0 6.0 0.0 25.0 4.3 3.0 12.0 77.3 4.0 8.5 3.2 0.0 50.0 15.0 3.0 3.0 86.7 13.0 6.0 1.5 0.0 69.0 8.0 5.5 0.0 103.0 14.0 10.0 1.0 0.0 53.0 17.0 6.4 0.0 101.4 21.0 3.0 2.0 0.0 42.0 17.0 11.0 0.0 96.0 Partial current J_X (A/cm.sup.2)* J_C.sub.2H.sub.4 J_C2 J_C EE(%)** 0.02 0.04 0.07 20.3 0.17 0.24 0.28 35.1 0.35 0.42 0.46 36.2 0.38 0.55 0.63 29.2 0.38 0.63 0.68 23.7 *Partial current of a compound X, namely J_X, equals the product of the current J by the faradaic efficiency FE of a given compound X. (J_X = J*FE_X) **EE stands for Energy Efficiency

[0243] The achieved C.sub.2+ partial current density represents a six-fold increase compared to previous best reports at similar energy efficiencies (FIG. 26).

[0244] The product distribution histogram for optimal CIBH catalysts at different current densities reveals that H.sub.2 generation remains below 10% from 0.2 A/cm.sup.2 above 1.5 A/cm.sup.2). At the highest current operation, optimized catalysts exhibit a maximum productivity toward ethylene with a FE in the 65-75% range, a peak partial current density of 1.34 A/cm.sup.2 at a cathodic energy efficiency of 46%. The study implemented the best CIBH catalyst in an ultra-slim flow cell (no reference electrode and minimized catholyte channel) leading to an estimated full-cell energy efficiency towards C.sub.2+ products of 20% at 1.1.A/cm.sup.2 without the benefit of IR correction (FIG. 5G). CIBH catalyst current and FE remain stable over the course of a 60 h initial study implemented in a membrane electrode assembly (MEA) configuration (FIG. 27).

[0245] Decoupling gas, ion and electron transport offers new qualities compared to conventional catalysts. While CO.sub.2 reduction kinetics improve with increasing temperature, alkaline electrolyzers see worsened CO.sub.2 diffusion at elevated temperatures—and this lower availability curtails productivity. The study explored the effect of temperature on planar CIPH metal:ionomer catalysts and observed that CIPH catalysts require lower overpotentials to attain similar FE—in contrast with planar reference catalysts (FIG. 28) —when operated at 60° C. This effect translates to 3D CIBH catalysts, which show improved performance arising from the combination of accelerated CO.sub.2 reduction kinetics and extended mass transport through the ionomer layer with increasing temperature (FIG. 29). As a result, CIBH catalysts achieve near ˜1 V reduced overpotential and more than a 50% increase in C.sub.2 productivity when operated at electrolyzer-relevant temperatures of 60° C. in a full-cell configuration (FIG. 30).

[0246] This work demonstrates metal:ionomer hybrid catalysts that exhibit phase-selective gas/electrolyte/electron transport. The new catalysts decouple gas/ion/electron transport phenomena which limited the available reaction area and subsequent activity, thus enabling gas-phase electrochemistry beyond 1 A/cm.sup.2 at cathodic energy efficiencies in the 40-50% range. The study first looked at the use of ionomers with asymmetric hydrophilic and hydrophobic functionalities and designed an ionomer coating that enables differentiated gas transport channels (through hydrophobic domains) and water uptake and cation transport via hydrophilic domains, which the study characterized with a suite of in situ and ex situ Raman and wide-angle-X-ray scattering spectroscopies and microscopies. The study showed the universal character of this approach in metal-ionomer catalysts for different electrochemical reactions of strong relevance such as oxygen, CO.sub.2 and CO reductions and for different metal catalysts and target products.

[0247] Based on these findings, the study presents a new catalyst-ionomer bulk-heterojunction design that enhances the gas-phase reaction interface across an extended 3D volume, thereby enabling efficient operation at higher reaction current regimes. Using these catalysts, the study achieves, for the first time, CO.sub.2RR currents above 1.5 Å/cm.sup.2 (a sixfold increase over previous-best reported catalysts), with minimized H.sub.2 byproduct generation and a total C.sub.2+ partial current exceeding 1.3 Å/cm.sup.2. The study reports a full-cell energy efficiency of 20% to C.sub.2 products at 1.1 Å/cm.sup.2 without IR correction. CIBH catalysts offer qualities for high-efficiency operation at electrolyzer-relevant temperatures.

[0248] The phenomena described herein open the door to new catalyst design principles that are not constrained by prior gas-ion-electron transport restrictions. The CIBH catalyst paves the way to the realization of renewable electrochemistry at operating currents needed for industrial applications.

Experimental Data

[0249] Local species concentration modeling. The system was modeled as a two-dimensional domain with a catalyst gas diffusion layer, ionomer layer, and electrolyte sub-domains, building off of previously well-established models (see studies of Burdyny T., et al., ACS Sustain. Chem. Eng. (Supporting Info), 2017, 5, 4031-4040, entitled “Nanomorphology-Enhanced Gas-Evolution Intensifies CO.sub.2 Reduction Electrochemistry” and of Dinh C. T., et al., Science, 2018, 360, 783-787, entitled “CO.sub.2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface”). The local concentrations of CO.sub.2,aq, CO.sub.3.sup.2−, HCO.sub.3.sup.−, OH.sup.−, H+, and H.sub.2O in an electrolyte solution under CO.sub.2RR conditions were modeled in COMSOL 5.4 (COMSOL Multiphysics, Stockholm, Se) using the Transport of Dilute Species physics. This model, based on previous papers, accounts for the acid-base carbonate equilibria, as well as CO.sub.2 reduction via electrocatalysis in an electrolyte solution (e.g., KOH). A time-dependent study was performed to simulate species evolution toward steady state.

Geometry

[0250] At the lower boundary, the gas-electrolyte interface, the CO.sub.2,aq concentration was specified according to Henry's Law and the Sechenov effect, with zero flux imposed for CO.sub.3.sup.2−, HCO.sub.3.sup.−, and OH.sup.−. A symmetry condition was imposed at the right boundary to model a confined pore geometry, and equilibrium concentration values were imposed at the top boundary. The left boundary contained a thin catalyst region over which the CO.sub.2,aq was reduced and OH.sup.− was produced.

CO.SUB.2 .Solubility

[0251] The quantity of dissolved CO.sub.2 in solution is determined by the temperature, pressure, and solution salinity. Assuming CO.sub.2 acts as an ideal gas, the dissolved amount is given by Henry's Law:

[CO.sub.2].sub.aq,0=K.sub.0[CO.sub.2].sub.g,  (1)

[0252] where,

[00001]$\begin{array}{cc}\ln K_0=93.4517\left(\frac{100}{T}\right)-60.2409+23.3585\ln \left(\frac{T}{100}\right),& \left(2\right)\end{array}$

[0253] where T is the temperature of the solution in K. The solubility is further diminished due to high concentration of ions in solutions according to the Sechenov Equation (3):

[00002]$\begin{array}{cc}\log \left(\frac{{\left[{\mathrm{CO}}_2\right]}_{\mathrm{aq},0}}{{\left[{\mathrm{CO}}_2\right]}_{aq}}\right)=K_s{C}_s,& \left(3\right)\end{array}$

[0254] where,

K.sub.s=Σ(h.sub.ion+h.sub.G)  (4)

h.sub.G=h.sub.G,0+h.sub.T(T−298.15),  (5)

TABLE-US-00006 TABLE 6 Sechenov constants Ion h.sub.ion K.sup.+ 0.0922 HCO.sub.3.sub.− 0.0967 CO.sub.3.sup.2− 0.1423 SO.sub.4.sup.2− 0.1117 Other parameters h.sub.G, 0, CO.sub.2 −0.0172 h.sub.T −0.000338 h.sub.G, 0, CO.sub.2 0 h.sub.G, 0, CO 0 (assumed)

Carbonate Equilibria

[0255] CO.sub.2, CO.sub.3.sup.2−, HCO.sub.3.sup.−, OH.sup.−, H.sup.+, and H.sub.2O are all in equilibrium in solution as given by:

CO.sub.2+H.sub.2O.Math.H.sup.++HCO.sub.3.sup.−(K.sub.1)  (6)

HCO.sub.3.sup.−.Math.H.sup.++CO.sub.3.sup.2−(K.sub.2)  (7)

CO.sub.2+OH.sup.−.Math.HCO.sub.3.sup.−(K.sub.3)  (8)

HCO.sub.3.sup.−+OH.sup.−.Math.CO.sub.3.sup.2−+H.sub.2O(K.sub.4)  (9)

H.sub.2O.Math.H.sup.++OH.sup.−,(K.sub.w)  (10)

[0256] where the rate constants are a function of temperature and salinity.

Species Transport

[0257] Species transport in the various layers (including electrochemistry in porous electrodes near polymer interfaces) is based on fundamentals presented by Newman and Thomas-Alyea and others, and given by the Poisson-Nernst-Planck set of equations coupled with electroreduction and acid-base equilibrium reactions:

[00003]$\begin{array}{cc}\frac{\partial c_i}{\partial t}+\frac{\partial J_i}{\partial x}=R_i,& \left(11\right)\end{array}$

[0258] where J.sub.i is the molar flux, given by:

[00004]$\begin{array}{cc}{J}_i=-\frac{D_i\partial c_i}{\partial x},& \left(12\right)\end{array}$

[0259] where D.sub.i, and is the diffusion coefficient species i (see P. Vanysek, Ionic Conductivity and Diffusion at Infinite Dilution References. CRC Handb., 77-79.):

TABLE-US-00007 TABLE 7 Infinite dilution diffusion constants Species Diffusion coefficient (10.sup.−9 m.sup.2s.sup.−1) CO.sub.2 1.91 CO.sub.3.sup.2− 0.923 HCO.sub.3.sup.− 1.185 H.sup.+ 9.31 OH.sup.− 5.273

[0260] The reaction term R.sub.i can be broken into carbonate equilibria (Equations 13-18):

R.sub.CO2=(−[CO.sub.2][H.sub.2O]k.sub.1f+[H.sup.+][HCO.sub.3.sup.−]k.sub.1r)+(−[CO.sub.2][OH.sup.−]k.sub.3f+[HCO.sub.3.sup.−]k.sub.3r)−R.sub.CO.sub.2.sub.RR  (13)

R.sub.CO.sub.3.sub.2=([HCO.sub.3.sup.−]k.sub.2f−[H.sup.+][CO.sub.3.sup.2−]k.sub.2r)+([HCO.sub.3.sup.−][OH.sup.−]k.sub.4f−[H.sub.2O][CO.sub.3.sup.2−]k.sub.4r)  (14)

R.sub.HCO.sub.3.sub.−=([CO.sub.2][H.sub.2O]k.sub.1f−[H.sup.+][HCO.sub.3.sup.−]k.sub.1r)+(−[HCO.sub.3.sup.−]k.sub.2f+[H.sup.+][CO.sub.3.sup.2−]k.sub.2r)+(−[CO.sub.2][OH.sup.−]k.sub.3f+[HCO.sub.3.sup.−]k.sub.3r)+(−[HCO.sub.3.sup.−][OH.sup.−]k.sub.4f+[H.sub.2O][CO.sub.3.sup.2−]k.sub.4r)  (15)

R.sub.H.sub.+=CO.sub.2[H.sub.2O]k.sub.1f[H.sup.+][HCO.sub.3.sup.−]k.sub.1r)+([HCO.sub.3.sup.−]k.sub.2f−[H.sup.+][CO.sub.3.sup.2−]k.sub.2r)+([H.sub.2O]k.sub.wf−[OH.sup.−][H.sup.+]k.sub.wr)  (16)

R.sub.OH.sub.−=(−[CO.sub.2][OH.sup.−]k.sub.3f+[HCO.sub.3.sup.−]k.sub.3r)+(−[HCO.sub.3.sup.−][OH.sup.−]k.sub.4f+[H.sub.2O][CO.sub.3.sup.2−]k.sub.4r)+([H.sub.2O]k.sub.wf−[OH.sup.−][H.sup.+]k.sub.wr)+R.sub.OHER  (17)

R.sub.H.sub.2.sub.O=(+[CO.sub.2][H.sub.2O]k.sub.1f+[H.sup.+][HCO.sub.3.sup.−]k.sub.1r)+([HCO.sub.3.sup.−][OH.sup.−]k.sub.4f−[H.sub.2O][CO.sub.3.sup.2−]k.sub.4r)+(−[H.sub.2O]k.sub.wf+[OH.sup.−][H.sup.+]k.sub.wr)  (18)

[0261] and into CO.sub.2 reduction and OH— evolution according to the reactions (See studies of Raciti D. et al., Nanotechnology, 2018, 29, 044001, entitled “Mass transport modelling for the electroreduction of CO.sub.2 on Cu nanowires” and of Sacco A. et al., J. Catal., 2019, 372, 39-48, entitled “Modeling of gas bubble-induced mass transport in the electrochemical reduction of carbon dioxide on nanostructured electrodes”.):

[00005]$\begin{array}{cc}{R}_{C{O}_{2}RR}=\frac{j}{F}\frac{ϵ}{L_{cat}}\frac{\Sigma {\mathrm{FE}}_{{\mathrm{CO}}_{2}RR}}{n{e}_{{\mathrm{CO}}_{2}RR}}\frac{\left[{\mathrm{CO}}_2\right]}{\left[{\mathrm{CO}}_{2,0}\right]},& \left(19\right)\end{array}$$\begin{array}{cc}{R}_{OHER}=\frac{j}{F}\frac{ϵ}{L_{cat}},& \left(20\right)\end{array}$

[0262] where j is the current density applied, F is Faraday's constant, ∈ is the catalyst porosity (0.6), and L.sub.cat is the width of the catalyst layer, FE.sub.CO.sub.2.sub.RR is the Faradaic efficiency the of a given product of CO.sub.2 reduction (based on experimental observations), ne.sub.CO.sub.2.sub.RR is number of electrons required for the reduction reaction.

[0263] The rate of CO.sub.2 reduction depends on the local concentration, [CO.sub.2], which is normalized by [CO.sub.2,0], defined as the maximum solubility concentration of CO.sub.2 based on the electrolyte concentration, pressure, and temperature (Equations 1-5); all of which is ultimately based on the Butler-Volmer relationship for concentration-dependent partial current:

[00006]$\begin{array}{cc}j=j_0\left[\frac{C_0\left(0,t\right)}{C_0^{*}}\exp \left(-\alpha f\eta \right)-\frac{C_R\left(0,t\right)}{C_R^{*}}\exp \left(\left(1-\alpha \right)f\eta \right)\right],& \left(21\right)\end{array}$

[0264] where j is the total current density, j.sub.0 is the exchange current density, C is the species concentration (normalized by a reference concentration, C*), α is the transfer coefficient, f=F/RT, and η is the overpotential.

[0265] The differential form for the diffusion-reaction equations and constants for carbonate species production are found in previous works.

Oxygen Reduction Reaction (ORR)

[0266] For the ORR simulations, the model geometry and boundaries were the same, except for the lower boundary condition for which the saturation concentration of O.sub.2 in KOH was imposed for the given O.sub.2 partial pressure. Furthermore, only diffusion and reduction of O.sub.2 were accounted for:

[00007]$\begin{array}{cc}\frac{\partial {\left[O_2\right]}_{aq}}{\partial t}=D_{O_2,\mathrm{aq}}\frac{{\partial }^2\left[O_2\right]}{\partial x^2}-\frac{\left[O_2\right]}{\left[O_{2,0}\right]}\frac{j}{F}\frac{ϵ}{L_{cat}}.& \left(22\right)\end{array}$

Porous Domain Effective Diffusion

[0267] A porous domain with Bosanquet effective diffusivity was employed for the Nafion® layer, which diminishes the effective gas diffusivity due to Knudsen diffusivity (i.e., frequent collisions with the Nafion® pore walls shown in FIG. 6).

[0268] Here, the effective diffusivity is:

[00008]$\begin{array}{cc}{D}_{\mathrm{eff}}={\left(\frac{1}{D_g}+\frac{3}{\sqrt{\frac{8\mathrm{RT}}{\pi M}}{d}_p}\right)}^{-1},& \left(23\right)\end{array}$

[0269] where D.sub.g is the bulk gas diffusivity, R is the gas constant, T is the temperature, M is the molecular mass of CO.sub.2, d.sub.p is the mean pore diameter (2 nm for Nafion® (see studies of Mauritz K. A. et al., Chem. Rev., 2004, 104, 4535-4586, entitled “State of understanding of Nafion” and of Divisek J. et al., J. Electrochem. Soc., 1998, 145, 2677, entitled “A study of capillary porous structure and sorption properties of Nafion proton-exchange membranes swollen in water”), yielding an overall diffusivity of 2.5.Math.10.sup.−7 m.sup.2 s.sup.−1. Although the effective diffusivity decreases substantially relative to the gaseous diffusivity (1.6.Math.10.sup.−5 m.sup.2 s.sup.−1), the effective diffusivity remains higher (by ˜400×) than that of CO.sub.2 in KOH since the gas travels along the hydrophobic backbone. The CO.sub.2 penetration depth into the Nafion® is further enhanced due to the partition coefficient (given by Henry's Law above) between the gas in Nafion® and the gas dissolved in electrolyte, thereby increasing the total available CO.sub.2 for the Nafion® case relative to the bare electrode case.

Limiting Current Density

[0270] To determine the limiting partial current density (FIG. 7), we first calculated the mean species concentration in the catalyst layer since the concentration determines the overall reaction rate (Equations 1,3). The partial current density is then given by:

[00009]$\begin{array}{cc}{j}_{\mathrm{partial}}=j_{\mathrm{applied}}\frac{\stackrel{\_}{\left[G\right]}}{\left[G_0\right]},& \left(23\right)\end{array}$

[0271] where the overbar denotes mean, [G] is either CO.sub.2 or O.sub.2, and [G.sub.0] is the maximum solubility concentration based on Equations 1-5. This reference concentration is the same as that chosen for Equation 19. The resulting partial current density versus applied current density was fit with a saturation function:

j.sub.partial=j.sub.lim tanh(k j.sub.applied)  (24)

[0272] where j.sub.lim and k are fitting parameters as a guide to the eye. The parameter j.sub.lim is the saturated level of the curve, thus providing the limiting current density for the given conditions modeled. Finally, we determined the species diffusivity for specific, experimental conditions by fitting the limiting current density versus diffusivity and interpolating based on the experimentally observed limiting current densities.

Temperature Effects

[0273] To model CO.sub.2 availability as temperature increases, we considered solubility and diffusion aspects. The diffusivity of CO.sub.2 will increase (see B. Jähne, G. Heinz, W. Dietrich, Measurement of the diffusion coefficients of sparingly soluble gases in water. J. Geophys. Res. 92, 10767 (1987)) according to:

[00010]$\begin{array}{cc}D=5019.Math.{10}^{-9}\exp \left(-\frac{19.51\left[\frac{\mathrm{kJ}}{\mathrm{mol}}\right]}{RT}\right),& \left(25\right)\end{array}$

[0274] where R is the universal gas constant, and T is temperature. However, the rate constants toward carbonate formation will also increase (see studies of Schulz K. G. et al., Mar. Chem., 2006, 100, 53-65 entitled “Determination of the rate constants for the carbon dioxide to bicarbonate inter-conversion in pH-buffered seawater systems”, of Mehrbach C. et al., Limnol. Oceanogr. 1973, 18, 897-907, entitled “Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure” and of Johnson K. S. et al., Limnol. Oceanogr., 1982, 27, 849-855, entitled “Carbon dioxide hydration and dehydration kinetics in seawater”), meaning that the overall CO.sub.2 availability will effectively decrease due CO.sub.2 consumption by the electrolyte in the reference case.

Materials and Methods

[0275] Materials and chemicals. Precursors for electrolyte preparation (high grade KOH, KHCO.sub.3, K.sub.2SO.sub.3, H.sub.2SO.sub.4) were purchased from Sigma-Aldrich. Electrolyte solutions were prepared from stock solutions of higher concentration in DI water, which were then diluted to the target molarity.

[0276] Sample fabrication and electroreduction reactions. The O.sub.2, CO.sub.2 and CO electroreduction characteristics of the cathode electrodes were investigated using a potentiostat (Autolab PGSTAT302N), a custom-made flow cell with a fixed 1 cm.sup.2 electrode geometric area, a digital mass flow controller (Sierra, SmartTrack 100), a current booster (Metrohm Autolab, 10 Å), and two peristaltic pumps with silicone tubing.

[0277] Sample Preparation:

[0278] Cathodic catalyst materials were deposited onto polytetrafluoroethylene (PTFE) gas diffusion layers with a 450 nm mean pore size. Approximately ˜300 nm nominal thick Ag and Cu films were sputtered onto the PTFE substrate using Ag and Cu targets (99.99%) at a sputtering rate <0.2 nm.Math.min.sup.−1 in an Angstrom Nexdep sputtering tool at a base pressure of <10.sup.−6 Torr.

[0279] Catalyst:ionomer planar heterojunctions (CIPH): The reference PTFE/metal electrodes were modified by spray-coating an ionomer layer from a solution of 700 mg ionomer (Nafion® perfluorinated resin solution, product #527084-25 mL purchased from Sigma Aldrich®) and 25 mL methanol (99.8%, anhydrous, Sigma Aldrich®) until the desired ionomer loading was achieved. Samples were dried for at least 24 h at room temperature in a vacuum chamber before operation. A single sample is typically 2 cm×2 cm in size.

[0280] Catalyst:ionomer bulk heterojunctions (CIBH): CIBH samples were fabricated by spray coating a mixture of Cu nanoparticles (25 nm diameter, Sigma Aldrich®) and ionomer solution at different ratios onto the CIPH electrodes. Samples were dried for at least 24 h at room temperature in a vacuum chamber before operation.

[0281] Flow-Cell Components:

[0282] The flow cell is comprised of three chambers: anolyte, catholyte and gas. The anolyte chamber (dimensions: 12 mm×12 mm; 9 mm depth) contains the counter electrode (nickel foam; 1.6 mm thickness). The catholyte chamber (dimensions: 12 mm×12 mm; 9 mm depth, square through hole) contains the Ag/AgCl reference electrode (CH Instruments; filled with 3M KCl solution) via a port drilled through the housing such that the frit of the reference electrode is in the center of the chamber. The anolyte and catholyte chambers are separated by the anion exchange membrane (Fumasep FAB-PK-130). The gas chamber (dimensions: 12 mm×12 mm; 9 mm depth) is used to supply the reactant gas. The gas and catholyte chambers are separated by the cathode. The catalyst side of the cathode faces into the catholyte chamber, while the PTFE gas diffusion layer faces the gas chamber. Silicone gaskets with a 1 cm.sup.2 window are placed between each layer to achieve sufficient sealing. Each chamber has an inlet and outlet connection (⅛″ OD; 1/16″ ID) to flow either electrolyte or gas.

[0283] Flow-Cell Assembly and Operation:

[0284] The designed cathode and commercially available Ni foam anodes were mounted in their respective chambers using Kapton tape for sealing and copper tape leads. Building up from the anolyte chamber, the completed assembly is sealed with even compression from four equally spaced bolts. The cathode is operated as the working electrode.

[0285] IR compensation losses between the reference and working electrodes were determined via electrochemical impedance spectroscopy (EIS) analyses. The electrode potentials upon IR compensation were scaled to the reversible hydrogen electrode (RHE) using the following expression:

E.sub.RHE=E.sub.Ag/AgCl+0.197V+0.059×pH  (26)

[0286] where E.sub.RHE is the potential of the reversible hydrogen electrode (RHE), E.sub.Ag/Agcl is the applied potential, and pH is the basicity of the catholyte. pH is calculated via a reaction-diffusion model (see study of Dinh C.-T. et al, Science, 2018, 360, 783-787, entitled “CO.sub.2 electro reduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface”). Cell resistance was measured in the 1×1 cm.sup.2 flow cell at different pH conditions (table 8). Cell resistances for reference and CIPH samples were measured to be within 10% at these configurations.

TABLE-US-00008 TABLE 8 Surface pH and cell resistance as a function of KOH concentration for a representative configuration KOH concentration (M) Surface pH Resistance (Ω) 1 12.4 5.1 5 14.5 2.0 7 14.7 1.6

[0287] Anode and cathode electrolytes of various concentrations were prepared as described above. Electrolyte solutions were supplied to the cell at a constant flow rate of 10 ml.Math.min.sup.−1 through peristaltic pumps through silicone tubing (Shore A50). CO.sub.2 and CO (Linde, 99.99%) were supplied to the gas chamber of the flow cell with a constant flow rate of 50 cm.sup.3/min, controlled by a digital mass flow controller (Sierra). For the oxygen reduction reaction (ORR), air was circulated into the gas chamber via peristaltic pumps.

[0288] For each applied potential, gas products from reduction reactions were collected in 1 mL volumes using gas-tight syringes (Hamilton chromatography syringes) at least three times with the time intervals of 200 s. This volume was injected into a gas chromatograph (PerkinElmer Clarus 680), equipped with a thermal conductivity detector (TCD), flame ionization detector (FID), and packed columns (Molecular Sieve 5A and Carboxen-1000). Argon (Linde, 99.999%) was employed as the carrier gas in the gas chromatograph.

[0289] The Faradaic Efficiencies (FEs) were determined as a function of operating current, gas chromatography and flow-rate at the outlet of the gas chamber as:

[00011]$\begin{array}{cc}FE=\frac{n.Math.F.Math.\theta .Math.f_m}{J}& \left(6\right)\end{array}$

[0290] where n is the number of electrons for a given product; F is the Faradaic constant; θ is the volume fraction of the gases; f.sub.m is the molar reacting gas flow rate; J is the current.

[0291] The combined cathodic energy efficiency (1/2) for C.sub.2 products was calculated as follows:

[00012]$\begin{array}{cc}E{E}_{\mathrm{cathodic}}=\Sigma \frac{F{E}_{C2}\times E_{C2}}{1.23-V}& \left(7\right)\end{array}$

[0292] where FE.sub.C2 is the Faradaic Efficiency of C.sub.2 products (ethylene, ethanol, acetate); E.sub.C2 is the thermodynamic cell potential for C.sub.2 prodducts (E.sub.C2=1.17 for ethylene for example); 1.23 V is the thermodynamic potential for water oxidation in the anode side; and Vis the applied potential vs. RHE after IR correction.

[0293] Liquid product analysis: liquid products were analyzed via nuclear magnetic resonance spectroscopy (NMR) from respective catholyte solutions. A new cathode, catholyte, and anolyte was used for the collection of a single liquid product distribution at a given applied potential. A constant volume of 25 mL was recirculated through anode and cathode compartments using peristaltic pumps. The flow cell was operated at the desired applied potential for at least 800 s. Cathode electrolyte was collected from the flow cell and tubing, sealed and stored in a fridge until NMR sample preparation. For NMR sample preparation, stored solutions were diluted 20 times in DI water and mixed with an internal standard, dimethyl sulfoxide (DMSO), in NMR tubes. .sup.1HNMR spectra were collected on an Agilent DD2 500 spectrometer in D.sub.2O in water suppression mode, and liquid product distributions were obtained by analyzing the resulting spectra in MestReNova. The relaxation time between the peaks was selected as 16 s to ensure complete proton relaxation.

[0294] ECSA Methods: Cyclic Voltammetry (CV) scans were recorded at five scan rates with a minimum of 3 cycles in the non-Faradaic region, specifically between −0.7 V vs. Ag/AgCl and −1.1 V vs. Ag/AgCl. Scan rates of 20 mV/s, 40 mV/s, 60 mV/s, 80 mV/s, 100 mV/s, and 200 mV/s were used. The currents at a given potential, −0.8 V vs. Ag/AgCl, were recorded from the forward and reverse scans of the third cycle. The difference between these currents was plotted against the scan rate to obtain a straight line. The slope of this line corresponds to the capacitance of the catalyst's electric double layer in Farads. The roughness factor (RF) is obtained by dividing this slope by the specific capacitance of electropolished copper. Measurements were conducted under constant CO.sub.2 flow and the recirculation of 5M KOH electrolyte.

TABLE-US-00009 TABLE 9 Double-layer capacitance determined by cyclic voltammetry Sample Double Layer Capacitance (mF) Copper on a PTFE GDL 2.5 Copper on a PTFE GDL with a 12.5 2.7 μl/cm.sup.2 ionomer overlayer Silver on a PTFE GDL 0.98 Silver on a PTFE GDL with a 12.5 0.92 μl/cm.sup.2 ionomer overlayer

[0295] Partial pressure studies: Partial pressure studies were carried out using the same configuration. The relative flows of CO.sub.2/N.sub.2 and CO/N.sub.2 gas mixtures were controlled using two mass-flow controllers (Sierra), and the total flow maintained at ˜50 cm.sup.3/min.

[0296] H-cell experiments: Experiments in the h-cell were performed by using PTFE/metal samples as working electrodes fully immersed in electrolyte solution. The area was masked using Kapton tape to be ˜1 cm.sup.2. An anion exchange membrane was used together with a Pt foil counter electrode at the anode.

Microscopies

[0297] Scanning Electron Microscopy: SEM images were acquired using a Hitachi SU-8230 apparatus at 5 keV and different magnifications. Cross-sectional elemental mapping was performed using a Hitachi CFE-TEM HF3300, the Cu coated gas diffusion layer sample was prepared using Hitachi Dual-beam FIB-SEM NB5000. Briefly, a slice (˜50-100 nm thick) of Cu coated gas diffusion layer was sectioned from its back using Ga-beam and attached to a TEM stage with tungsten deposition and lifted out for subsequent STEM-EDX analysis.

[0298] Transmission Electron Microscopy and elemental mapping: These maps and images were taken on a FEI Titan 80-300 LB TEM, operated at 200 kV. The instrument is equipped with a CEOS image corrector and a Gatan Tridiem energy filter. EELS mapping reveals the presence of copper nanoparticles and PFSA ionomer. These samples were prepared by a Zeiss NVision 40 FIB in cross-section mode.

[0299] WAXS measurements: WAXS measurements were carried out in transmission geometry at the CMS beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) office of the Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory. Samples were measured with an imaging detector at a distance of 0.153 m using an X-ray wavelength of 0.729 Å. Nika software package was used to sector average the 2D WAXS images. Data plotting was done in Igor Pro (Wavemetrics, Inc., Lake Oswego, Oreg., USA). For grazing-incidence WAXS (GIWAXS), ionomer samples were deposited by spray coating on cleaned Si substrates using a similar protocol to standard samples.

[0300] Contact angle measurements: Contact angle measurements were performed using the sessile drop method on a video-based contact angle measuring system (OCA 15EC). Briefly, a single water droplet was placed on the sample and approximately 15 seconds was given before the contact angles were measured by the computer software.

[0301] Raman measurements: In situ and ex situ Raman spectra were recorded with a Renishaw Raman spectrometer using a 785 nm excitation laser and 1200 mm.sup.−1 grating. Spectra were collected in the range of 200-3000 cm.sup.−1 over 10 acquisitions with an exposure time of 10 seconds for each acquisition. These were averaged together and analyzed using WiRE 4.4 software. The laser power was 200 ρW and a 63× magnification immersion objective was used with a custom PTFE flow cell.

[0302] The in situ flow cell had a liquid electrolyte reservoir in which the immersion objective was dipped and a gas diffusion electrode separated the electrolyte reservoir and the gas channel that continuously delivered CO.sub.2 gas to the catalyst at a flow rate of 50 cm.sup.3/min. For ORR, air was fed using peristaltic pumps. The area of the electrode in this configuration was 1 cm.sup.2. The counter electrode, a Pt wire, and the reference electrode, Ag/AgCl, were dipped in the electrolyte reservoir ˜1 cm from the cathode.

[0303] XAS measurements: in situ XAS measurements was carried out at 9BM beamline of the Advanced Photon Source (APS) in Argonne National Laboratory (Lemont, Ill.). Operando XAS experiment for CO.sub.2RR proceeded by using in situ XAS flow cell (Applied potential: −2.0 V vs. Ag/AgCl (chronoamperometry), electrolyte: 5 M KOH, CO.sub.2 flow).