RELATED APPLICATION

This application claims priority under applicable laws to United States provisional application No. 63/203.559 filed on July 27, 2021, the content of which is incorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD

The technical field generally relates to electrolytic carbon dioxide (C0 ₂ ) reduction, and more specifically to systems and methods for the production of multicarbon products via the electrocatalytic CO2 reduction reaction (C0 ₂ RR).

BACKGROUND

The electrochemical C0 ₂ RR presents the opportunity to consume C0 ₂ and produce desirable products such as multicarbon (C ₂₊ ) products. However, the alkaline conditions required for productive C0 ₂ RR result in the bulk of input C0 ₂ being lost to bicarbonate and carbonate. This loss imposes a limit of 25% conversion in the conversion of C0 ₂ to multicarbon products for systems that use anions as the charge carrier and overcoming this limit is a challenge of singular importance to the field.

Accordingly, there is a need for improved techniques and ion exchange materials for membrane electrode assembly (MEA) systems that overcome one or more of the disadvantages encountered with conventional MEA systems and methods for the conversion of C0 ₂ to multicarbon products.

SUMMARY

According to a first aspect, the present technology relates to a multilayer cathode for the electrochemical reduction of carbon dioxide comprising: a gas diffusion layer; a cathode catalyst layer disposed on the gas diffusion layer, and a permeable carbon dioxide regeneration layer comprising an anion exchange ionomer disposed on the cathode catalyst layer.

According to another aspect, the present technology relates to a method of manufacturing a multilayer cathode for the electrochemical reduction of carbon dioxide comprising: depositing a cathode catalyst material onto one side of a gas diffusion layer to provide a cathode catalyst layer thereon; and coating an anion exchange ionomer solution onto the cathode catalyst layer to provide a permeable carbon dioxide regeneration layer.

According to another aspect, the present technology relates to a membrane electrode assembly for the electrochemical reduction of carbon dioxide comprising: a multilayer cathode comprising a gas diffusion layer, a cathode catalyst layer disposed on the gas diffusion layer, and a permeable carbon dioxide regeneration layer comprising an anion exchange ionomer disposed on the cathode catalyst layer; an anode comprising an anode catalyst layer; and at least one layer of a cation exchange membrane disposed between the permeable carbon dioxide regeneration layer and the anode catalyst layer.

According to another aspect, the present technology relates to a bipolar membrane electrode assembly for the electrochemical reduction of carbon dioxide comprising: a multilayer cathode comprising a gas diffusion layer, a cathode catalyst layer disposed on the gas diffusion layer, and a permeable carbon dioxide regeneration layer comprising an anion exchange ionomer disposed on the cathode catalyst layer; an anode comprising an anode catalyst layer; and at least one layer of a cation exchange membrane and at least one layer of an anion exchange membrane disposed between the permeable carbon dioxide regeneration layer and the anode catalyst layer, wherein said at least one layer of a cation exchange membrane faces the permeable carbon dioxide regeneration layer and said at least one layer of an anion exchange membrane faces the anode catalyst layer.

According to another aspect, the present technology relates to a method of manufacturing a membrane electrode assembly for the electrochemical reduction of carbon dioxide comprising: depositing a cathode catalyst material onto one side of a gas diffusion layer to provide a cathode catalyst layer thereon; coating an anion exchange ionomer solution onto the cathode catalyst layer to provide a permeable carbon dioxide regeneration layer; placing at least one layer of a cation exchange membrane onto the permeable carbon dioxide regeneration layer; and placing an anode comprising on one side an anode catalyst material onto the at least one layer of a cation exchange membrane, said anode catalyst material facing the at least one layer of a cation exchange membrane.

According to another aspect, the present technology relates to a method of manufacturing a bipolar membrane electrode assembly for the electrochemical reduction of carbon dioxide comprising: depositing a cathode catalyst material onto one side of a gas diffusion layer to provide a cathode catalyst layer thereon; coating an anion exchange ionomer solution onto the cathode catalyst layer to provide a permeable carbon dioxide regeneration layer; placing at least one layer of a cation exchange membrane onto the permeable carbon dioxide regeneration layer; placing at least one layer of an anion exchange membrane onto the at least one layer of a cation exchange membrane; and placing an anode comprising on one side an anode catalyst material onto the at least one layer of an anion exchange membrane, said anode catalyst material facing the at least one layer of an anion exchange membrane.

According to another aspect, the present technology relates to a use of the multilayer cathode as defined herein or produced by the method as defined herein, for the production of a multicarbon product.

According to another aspect, the present technology relates to a use of the membrane electrode assembly as defined herein or produced by the method as defined herein, for the production of a multicarbon product.

According to another aspect, the present technology relates to a use of the bipolar membrane electrode assembly as defined herein or produced by the method as defined herein, for the production of a multicarbon product.

In one embodiment, the multicarbon product is ethylene or ethanol.

According to another aspect, the present technology relates to a method for electrochemical production of a multicarbon product using the bipolar membrane electrode assembly as defined herein, the method comprising the steps of: contacting carbon dioxide and an electrolyte with the multilayer cathode, such that the carbon dioxide diffuses through the gas diffusion layer and contacts the cathode catalyst layer; applying a voltage to provide a current density to cause the carbon dioxide contacting the cathode catalyst layer to be electrochemically reduced into the multicarbon product; and recovering the multicarbon product.

In one embodiment, carbonate ions are produced when applying the voltage.

In another embodiment, carbon dioxide is regenerated from the carbonate ions in the permeable carbon dioxide regeneration layer. In another embodiment, the regenerated carbon dioxide is transported to the cathode catalyst layer to be electrochemically reduced into the multicarbon product prior to the recovering step.

In another embodiment, the multicarbon product is ethylene or ethanol.

According to another aspect, the present technology relates to a method for electrochemical production of a multicarbon product using the membrane electrode assembly as defined herein, the method comprising the steps of: contacting carbon dioxide and an electrolyte with the multilayer cathode, such that the carbon dioxide diffuses through the gas diffusion layer and contacts the cathode catalyst layer; applying a voltage to provide a current density to cause the carbon dioxide contacting the cathode catalyst layer to be electrochemically reduced into the multicarbon product; and recovering the multicarbon product.

In one embodiment, carbonate ions are produced when applying the voltage.

In another embodiment, carbon dioxide is regenerated from the carbonate ions in the permeable carbon dioxide regeneration layer.

In another embodiment, the regenerated carbon dioxide is transported to the cathode catalyst layer to be electrochemically reduced into the multicarbon product prior to the recovering step.

In another embodiment, the multicarbon product is ethylene or ethanol.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a schematic representation of a MEA system according to one embodiment.

Figure 2 is a schematic representation of a bipolar membrane electrode assembly (BPMEA) system according to one embodiment. Figure 3A is a schematic representation of a MEA system with an anion exchange membrane (AEM) showing the distribution of CO2 while operating at 150 mA/cm ² and 6 seem of CO2 flow, as described in Example 6(a).

Figure 3B is a graph showing the CO2 distribution as a function of the CO2 flow rate for the MEA cell with an AEM at 150 mA/cm ² , as described in Example 6(a).

Figure 4A is a graph showing the faradaic efficiency for various multicarbon products and CO2 conversion efficiency as a function of the CO2 flow rate for the MEA cell with an AEM at 150 mA/cm ² , as described in Example 6(a).

Figure 4B is a graph showing the cell voltage as a function of the CO2 flow rate for the MEA cell with an AEM at 150 mA/cm ² , as described in Example 6(a).

Figure 4C is a graph showing the CO2 conversion efficiency with and without carbonate formation as a function of the CO2 flow rate for the MEA cell with an AEM at 150 mA/cm ² , as described in Example 6(a).

Figure 5A is a schematic representation of species transport within a MEA with a cation exchange membrane (CEM), as described in Example 6(b).

Figure 5B is a graph showing the anode gas CO2 flow rate as a function of the current density for the MEA cell with a CEM, as described in Example 6(b). Error bars represent the standard deviation of at least three measurements obtained under the same conditions.

Figure 5C is a graph showing the H faradaic efficiency as a function of the current density for the MEA cell with a CEM, as described in Example 6(b). Error bars represent the standard deviation of at least three measurements obtained under the same conditions.

Figure 6 is a graph showing the current density as a function of the cell voltage for the MEA cell with a CEM, as described in Example 6(b). The selectivity data is shown in Figure 5C, only H ₂ was detected and there are no traces of CO2RR products.

Figure 7 A is a schematic representation of species transport within a MEA with a forward bias BPM, as described in Example 6(c). Figure 7B is a graph showing the C ₂ H ₄ faradaic efficiency and voltage stability as a function of time for the MEA cell with a forward bias BPM at 50 mA/cm ² , as described in Example 6(c).

Figure 7C is a picture of the BPM after operating in the forward bias for 2 hours at 50 mA/cm ² , as described in Example 6(c). The membrane blistered at the AEM:CEM interface in the areas under the flow channels of the cell.

Figure 8 is a graph showing the electrochemical performances of the MEA cell with a forward bias BPM, effectively a graph comparing the anode gas CO2 flow rate as a function of the current density for the MEA cell with an AEM, a CEM, a permeable CO2 regeneration layer (PCRL) and a forward bias BPM, as described in Example 6(c). Error bars represent the standard deviation of at least three measurements obtained under the same conditions.

Figure 9 shows in (A) a scanning electron microscopy (SEM) image of a cathode including a copper catalyst sputtered on a polytetrafluoroethylene (PTFE) filter without a PCRL coating, in (B) a cross-section SEM image of the cathode with 1.5 mg/cm ² of PCRL coating, and in (C) an optical microscope image of the surface of the cathode with 1.5 mg/cm ² of PCRL coating at 10x magnification, as described in Example 6(d). Scale bars represent 500 nm, 5 pm and 50 pm, respectively.

Figure 10A to 10E show respectively SEM images of cathodes including a copper catalyst sputtered on a PTFE filter with PCRL loadings of 0 mg/cm ² , 0.75 mg/cm ² , 1.5 mg/cm ² , 2.25 mg/cm ² , and 3.0 mg/cm ² , as described in Example 6(d). Scale bars represent 10 pm.

Figure 11 shows in (A) polarization curves with three different PCRL loadings on a copper catalyst, in (B) a bar graph showing the highest C ₂ H ₄ faradaic efficiency and C ₂ H ₄ to H ₂ ratio for four different PCRL loadings, in (C) a bar graph showing the gas product faradaic efficiency for a copper catalyst with a 2.25 mg/cm ² PCRL coating as a function of the cell voltage, in (D) a graph comparing the anode gas C0 flow rate for the PCRL compared to the AEM and CEM cells, in (E) a graph showing the CO ₂ flow distribution with the 2.25 mg/cm ² of PCRL coating as a function of the input CO ₂ flow rate at 100 mA/cm ² and in (F) a bar graph showing the CO ₂ conversion efficiency with the 2.25 mg/cm ² of PCRL loading with varying CO ₂ flow rates at 100 mA/cm ² , as described in Example 6(d). Error bars represent the standard deviation of at least three measurements obtained under the same conditions.

Figure 12 is a graph showing the ohmic resistance as a function of PCRL loading obtained for a PCRL-coupled CEM at 100 mA/cm ² , as described in Example 6(d). Error bars represent the standard deviation of three samples measurements obtained under the same conditions.

Figure 13 is a graph showing the pH in the cell obtained with the one-dimensional multiphysics model at varying coating thicknesses and current densities, as described in Example 6(d). The pH at 30 mA/cm ² is not adequately alkaline for more than 5% faradaic efficiency to multicarbon products. The coating thicknesses are estimated from cross- sectional SEM images.

Figures 14A to 14D are respectively a graph bar showing the gas product faradaic efficiency for 0.75 mg/cm ² , 1.5 mg/cm ² , 2.25 mg/cm ² , and 3.0 mg/cm ² loadings at varying cell voltages, as described in Example 6(d).

Figures 15A to 15C are respectively a graph bar showing the CO2 conversion efficiency and the faradaic efficiency for 50 mA/cm ² , 100 mA/cm ² and 150 mA/cm ² at varying CO2 flow rates, as described in Example 6(d). A cathode with a PCRL loading of 2.25 mg/cm ² was used.

Figure 16 is a graph showing the linear gas velocity and Reynolds number of the CO2 flow in the cell, as described in Example 6(d).

Figure 17 is a graph showing the C2H4 faradaic efficiency and cell voltage as a function of time, effectively showing the stability of the PCRL-coupled CEM, as described in Example 6(d). A loading of 2.25 mg/cm ² PCRL on a copper catalyst at 100 mA/cm ² was run with 0.01 M H2SO4 anolyte. A mildly acidic anolyte was employed to maintain a constant anolyte pH over long times, accommodating any produced HCOOH or CH3COOH that diffuses into the anolyte.

Figure 18 is a graph showing the faradaic efficiency and current density for a silver catalyst with a 2.25 mg/cm ² PCRL coating at varying cell voltages, as described in Example 6(d). Figure 19 is a bar graph comparing the energy cost required for downstream CO2 separation from the cathode and anode gas product streams for a MEA with an AEM and a PCRL-coupled CEM, as described in Example 6(d). The C2H4 Gibbs free energy of reaction (1331 kJ/mol) is shown for reference. A CO2 capture energy intensity of 178.3 kJ/mol CO2 was applied, based on amine-based capture of CO2 from flue gas. ¹

DETAILED DESCRIPTION

The following detailed description and examples are illustrative and should not be interpreted as further limiting the scope of the invention. On the contrary, it is intended to cover all alternatives, modifications and equivalents that can be included as defined by the present description. The objects, advantages and other features of the methods will be more apparent and better understood upon reading the following non-restrictive description and references made to the accompanying drawings.

All technical and scientific terms and expressions used herein have the same definitions as those commonly understood by the person skilled in the art when relating to the present technology. The definition of some terms and expressions used herein is nevertheless provided below for clarity purposes.

When the term “about” are used herein, it means approximately , in the region of or around. When the term “about” is used in relation to a numerical value, it modifies it; for example, by a variation of 10% above and below its nominal value. This term can also take into account the rounding of a number or the probability of random errors in experimental measurements, for instance, due to equipment limitations.

When a range of values is mentioned in the present application, the lower and upper limits of the range are, unless otherwise indicated, always included in the definition. When a range of values is mentioned in the present application, then all intermediate ranges and subranges, as well as individual values included in the ranges, are intended to be included.

It is worth mentioning that throughout the following description when the article “a” is used to introduce an element, it does not have the meaning of “only one” and rather means “one or more”. It is to be understood that where the specification states that a step, component, feature, or characteristic “may”, “might”, “can” or “could” be included, that particular component, feature or characteristic is not required to be included in all alternatives.

When the term “comprising" or its equivalent terms “including” or “having” are used herein, it does not exclude other elements. For the purposes of the present invention, the expression “consisting of is considered to be a preferred embodiment of the term “comprising”. If a group is defined hereinafter to include at least a certain number of embodiments, it is also to be understood to disclose a group, which preferably consists only of these embodiments.

Various ion exchange materials for MEA systems and methods described herein are related to the production of multicarbon products via the electrocatalytic CO ₂ RR.

The electrochemical CO ₂ RR presents an opportunity to utilize renewable electricity to produce chemical fuels and feedstocks from CO ₂ ² ’ ³ Valuable multicarbon (C ₂₊ ) products, such as ethylene (C ₂ H ₄ ) and ethanol (C ₂ H ₅ OH), are of particular interest in view of large existing markets. ⁴ Providing reactant CO ₂ gas directly to the catalyst sites with gas diffusion electrodes enables CO ₂ RR systems to attain impressive reactions rates (» 100 mA/cm ² ). ⁵ ’ ⁶

MEA cells combine GDEs and membranes in a zero-gap fashion. ^7'10 This configuration mitigates electrolyte degradation and salt precipitation issues characteristic of alkaline flow cells. Alkaline conditions are required at the cathode ^{1 1} to suppress the hydrogen evolution reaction (HER) and enable a high faradaic efficiency towards CO ₂ RR products. ^{12 13} Locally alkaline conditions are maintained during CO ₂ RR by hydroxide anions produced at the catalyst layer (Equations 1 and 2). ¹⁴ However, these conditions result in the competing reaction of CO ₂ with hydroxide forming bicarbonate and carbonate (Equations 3 and 4). These ions electromigrate through the AEM to the anode where they combine with protons generated by the anodic oxygen evolution reaction to form CO ₂ and water. ¹⁴ Here the CO ₂ bubbles out of the locally acidic anolyte and combines with produced oxygen, rendering a gas mixture that is costly to separate. ¹⁵ This crossover of CO ₂ in MEA systems results in a low single pass conversion for CO ₂ RR. When carbonate is the dominant charge carrier through the AEM, CO ₂ conversion efficiency is limited to 50% in the production of CO. ^16'18

C0 ₂ + H ₂ 0 + 2e — > CO + 20H- (eq. 1) 2C0 ₂ + 8H ₂ 0 + 12e- ® C ₂ H ₄ + 120H (eq. 2)

C0 ₂ + OH- ® HCO3 (eq. 3)

C0 ₂ + 20tr ® col- + H ₂ 0 (eq. 4)

Compared to CO production, multicarbon production requires more electrons to be transferred through the membrane per molecule of CO ₂ converted (Equations 1 and 2): the dominant multicarbon products on a multicrystalline copper catalyst, C ₂ H ₄ and C ₂ H ₅ OH, both require 6 electrons per CO ₂ molecule converted. With a carbonate charge carrier, three molecules of CO ₂ will be transported through the membrane for each molecule of CO ₂ converted to C ₂ H ₄ or C ₂ H ₅ OH, limiting the CO ₂ conversion efficiency to a maximum of 25%. A low CO ₂ conversion efficiency necessitates energy-intensive gas separation to recover unreacted CO ₂ from both the cathodic and anodic gas product streams, ¹⁹ and the associated costs render electrocatalytic CO ₂ conversion processes unviable. Going beyond this conversion limit is a critical challenge for the field. ¹⁸

More particularly, the present technology relates to a multilayer cathode for the electrochemical reduction of CO2 including a gas diffusion layer, a cathode catalyst layer disposed on the gas diffusion layer, and a permeable CO2 regeneration layer including an anion exchange ionomer disposed on the cathode catalyst layer.

According to one example, the gas diffusion layer includes a porous material. For example, any known compatible porous material is contemplated. For example, the porous material can be a carbon paper or a porous polymer material. For instance, the porous polymer material can be a fluoropolymer such as PTFE and expanded polytetrafluoroethylene (ePTFE). For example, the gas diffusion layer can be made of a PTFE filter. Alternatively, the gas diffusion layer can be made of a carbon paper substrate with or without a PTFE treatment, preferably with a PTFE treatment. In some examples, the gas diffusion layer has a porosity with pore size in the range of from about 0.01 pm to 2 pm, limits included.

According to another example, the cathode catalyst layer includes a cathode catalyst material that promotes the electrochemical reduction of CO2. Any compatible cathode catalyst material that promotes the electrochemical reduction of CO2 is contemplated. Non-limiting examples of cathode catalyst materials include silver, copper, gold, nickel, tin, gallium, zinc, palladium, cadmium, indium, platinum, mercury, thallium, lead, bismuth, cobalt and an alloy including at least one thereof. In one variant of interest, the cathode catalyst material is copper or silver, and preferably copper. In some examples, the cathode catalyst layer has a thickness in the range of from about 50 nm to about 500 nm, limits included.

According to another example, the permeable CO2 regeneration layer can be designed to provide a substantially alkaline environment at the surface of a cathode catalyst or to enable local CO2 regeneration to the cathode catalyst. For instance, the permeable CO2 regeneration layer can be designed to simultaneously impede proton transport and to substantially facilitate the local regeneration of CO2. In some examples, the material properties of the permeable CO2 regeneration layer can be as indicated in Table 1 below.

Table 1. Material properties of the permeable CO2 regeneration layer according to some examples

According to another example, the anion exchange ionomer of the permeable CO2 regeneration layer can be selected for its high ionic conductivity or selectivity, high performances (for example, high current density and low voltages), high ion-exchange capacity, high operational efficiency and/or high stability in alkaline conditions. The anion exchange ionomer of the permeable CO2 regeneration layer can be substantially chemically and oxidatively stable across a substantially broad range of operating conditions. For instance, the anion exchange ionomer of the permeable CO2 regeneration layer can show essentially zero degradation when subjected to operating conditions such as strong alkaline conditions that would readily degrade other polymers. Any compatible anion exchange ionomer that can shield the surface of the cathode catalyst from protons and/or provide a pathway for regenerated gaseous CO2 to the cathode catalyst is contemplated. For example, the anion exchange ionomer of the permeable CO2 regeneration layer can be in solution, in dispersion, or resin form. According to another example, the anion exchange ionomers can include at least one positively charged functional group directly on the backbone chain of a polymer and/or on a side chain of the polymer. For example, these positively charged functional groups are able to transport anions and block the transport of cations.

Non-limiting examples of anion exchange ionomers include commercial Fumion™ anion ionomer solutions (Fumion™ FAA-3-SOLUT-10, Fuel Cell Store), Sustainion™ alkaline ionomers (XA-9, XB-7, XC-1 and XC-2 series, Fuel Cell Store), Orion TM1 Durion™ polymers (low or medium molecular weight anion exchange resins, Fuel Cell Store), Pention™ dispersions (D72, D35 and D18 series, Fuel Cell Store), PiperlON anion exchange dispersions or resin materials (Fuel Cell Store), Durion™ G2 (second generation) dispersions (Fuel Cell Store), Pention™ dispersions (OER and HER series, Fuel Cell Store), Dappion™ Gen1 dispersions (Fuel Cell Store), Aemion™ or Aemion ⁺ ™ membranes or ionomers (lonomr Innovations inc.). In one variant of interest, the anion exchange ionomer is a commercial Aemion™ AP1-CNN5-00-X ionomer.

According to another example, the multilayer cathode further includes a current collector adjacent to the gas diffusion layer. Any compatible current collector is contemplated.

The present technology also relates to a method of manufacturing a multilayer cathode for the electrochemical reduction of CO2 as herein defined, the method including the steps of: depositing a cathode catalyst material onto one side of a gas diffusion layer to provide a cathode catalyst layer thereon; and coating an anion exchange ionomer solution onto the cathode catalyst layer to provide a permeable CO2 regeneration layer; wherein the cathode catalyst material, the cathode catalyst layer, the gas diffusion layer, the anion exchange ionomer and the permeable CO2 regeneration layer are as defined above.

According to one example, the step of depositing the cathode catalyst material onto the gas diffusion layer can be performed by a physical vapor deposition method, for example, by sputter deposition. According to another example, the anion exchange ionomer solution can include from about 0.34 wt.% to about 0.68 wt.% of the anion exchange ionomer, limits included.

According to another example, the anion exchange ionomer solution can be obtained by dissolving an anion exchange ionomer powder in an alcohol. For example, the anion exchange ionomer powder can be dissolved in alcohol (for example, methanol) by sonication.

According to another example, the step of coating the anion exchange ionomer solution onto the cathode catalyst layer can be performed by a spray deposition method. For example, the spray deposition method can be carried out a spraying rate in the range of from about 0.4 ml_/h/cm ² to about 1.6 mL/h/cm ² , limits included.

According to another example, the method can further include affixing the other side of the gas diffusion layer on a current collector as defined above.

The present technology also relates to a MEA for the electrochemical reduction of CO2 including a multilayer cathode as defined herein or manufactured by the method as defined herein. For a more detailed understanding of the disclosure, reference is first made to Figure 1, which provides a schematic representation of a MEA system 10 in accordance with a possible embodiment.

As illustrated in Figure 1, the MEA system 10 includes a multilayer cathode 12 including a gas diffusion layer 14, a cathode catalyst layer 16 disposed on the gas diffusion layer 14, and a permeable CO2 regeneration layer 18 including an anion exchange ionomer disposed on the cathode catalyst layer 16. It is to be understood that, the gas diffusion layer 14, the cathode catalyst layer 16, the permeable CO2 regeneration layer 18 and the anion exchange ionomer are as defined above.

Still referring to Figure 1, the MEA system 10 also includes an anode 20 including an anode catalyst layer 22 and at least one layer of a CEM 24 disposed between the permeable CO2 regeneration layer 18 and the anode catalyst layer 22.

According to one example, the multilayer cathode 12 can further include a current collector as defined above (not shown in Figure 1) adjacent to the gas diffusion layer 14. As illustrated in Figure 1 , the anode catalyst layer 22 includes an anode catalyst material 26 that promotes electrochemical oxidation of water. Any compatible anode catalyst material that promotes electrochemical oxidation of water is contemplated. The anode catalyst material 26 can include a metal selected from the group consisting of iridium, nickel, iron, cobalt, ruthenium, platinum and an alloy comprising at least one thereof. For example, the anode catalyst material 26 can be a metal oxide, where the metal is as described herein. Non-limiting examples of cathode catalyst materials 26 include iridium oxide, nickel oxide, iron oxide, cobalt oxide, nickel-iron oxide, iridium-ruthenium oxide and platinum oxide. In one variant of interest, the cathode catalyst material 26 is iridium dioxide or an iron-nickel-based catalyst.

According to another example, the anode 20 can further include a current collector (not shown in Figure 1) adjacent to the anode catalyst layer 22. Any compatible current collector is contemplated.

According to another example, the CEM 24 can be used to separate the anode 20 and multilayer cathode 12 of the MEA system 10. Illustratively, the at least one layer of a CEM 24 is in contact with the permeable CO2 regeneration layer 18 of the multilayer cathode 12 and the anode catalyst layer 22 of the anode 20. For example, the CEM 24 can be used as a separator and as a solid electrolyte to selectively transport protons across the MEA system 10. Any compatible CEM is contemplated, for example, the CEM 24 can be a perfluorosulfonic acid polymer membrane or a perfluorosulfonic acid-PTFE copolymer membrane. In one variant of interest, the CEM 24 can be a Nation™ membrane such as a Nafion™ 117 membrane having a thickness of about 183 pm (Fuel Cell Store).

According to another example, the MEA system 10 is a BPMEA system, and further comprises at least one layer of an AEM (not shown in Figure 1) disposed on the CEM 24 and facing the anode catalyst layer 22 of the anode 20.

The present technology also relates to a BPMEA for the electrochemical reduction of CO2 including a multilayer cathode as defined herein or manufactured by the method as defined herein. For a more detailed understanding of the disclosure, reference is first made to Figure 2, which provides a schematic representation of a BPMEA system 110 in accordance with a possible embodiment. As illustrated in Figure 2, the BPMEA system 110 includes a multilayer cathode 112 including a gas diffusion layer 114, a cathode catalyst layer 116 disposed on the gas diffusion layer 114, and a permeable CO2 regeneration layer 118 including an anion exchange ionomer disposed on the cathode catalyst layer 116. It is to be noted that, the gas diffusion layer 114, the cathode catalyst layer 116, the permeable CO2 regeneration layer 118 and the anion exchange ionomer are as defined above.

Still referring to Figure 2, the MEA BPMEA system 110 also includes an anode 120 including an anode catalyst layer 122 including an anode catalyst material 124. the MEA BPMEA system 110 also includes at least one layer of a CEM 126 and at least one layer of an AEM 128 disposed between the permeable CO2 regeneration layer 118 and the anode catalyst layer 122, wherein said at least one layer of a CEM 126 faces the permeable CO2 regeneration layer 118 and said at least one layer of an AEM 128 faces the anode catalyst layer 122. It is to be noted that, the anode 120, the anode catalyst layer 122 and the anode catalyst material 124 are as defined above.

According to one example, the CEM 126 and the AEM 128 are provided separately. In which case the CEM 126 can be as defined above and any compatible AEM 128 is contemplated. Alternatively, the CEM 126 and the AEM 128 can be provided as a bipolar composite exchange membrane {i.e., as a single film) consisting of an anion exchange layer and a cation exchange layer. Any compatible bipolar composite exchange membrane is contemplated. In one variant of interest, the bipolar composite exchange membrane is a Fumasep™ FBM bipolar membrane (Fuel Cell Store).

Illustratively, the water is dissociated into OH- and H ⁺ ions in an intermediate layer 130 (or at the interface 130) between the CEM 126 and the AEM 128. As illustrated in Figure 2, the BPMEA system 110 can have a reverse biased mode of operation in order to promote the water dissociation reaction. Without wishing to be bound by theory, under the reverse biased operational mode, electrons are transferred from the anode 120 to the multilayer cathode 112. Water molecules readily diffuse into the intermediate layer 130 between the CEM 126 and the AEM 128 and the formation of OH and H ⁺ ions occurs as a result of the water dissociation reaction. H ⁺ ions diffuse out from the CEM 126 and migrate into the multilayer cathode 112. Contrarily, OH ions migrate the anode 120 via the AEM 128 where, under the operating conditions, the oxygen evolution reaction (Equation 5) occurs. 20H ® H ₂ 0 + ₂ 0 ₂ + 2e (eq. 5)

According to one example, the multilayer cathode 112 and the anode 120 can further include a current collector as defined above (not shown in Figure 2) respectively adjacent to the anode catalyst layer 122 and the gas diffusion layer 114.

The present technology also relates to a method of manufacturing a MEA for the electrochemical reduction of CO2 as herein defined, the method including the steps of:

- depositing a cathode catalyst material onto one side of a gas diffusion layer to provide a cathode catalyst layer thereon;

- coating an anion exchange ionomer solution onto the cathode catalyst layer to provide a permeable CO2 regeneration layer;

- placing at least one layer of a CEM onto the permeable CO2 regeneration layer; and

- placing an anode comprising on one side an anode catalyst material onto the at least one layer of a CEM, said anode catalyst material facing the at least one layer of a CEM; wherein the cathode catalyst material, the cathode catalyst layer, the gas diffusion layer, the anion exchange ionomer, the permeable CO2 regeneration layer, the CEM, the anode and the anode catalyst material are as defined above.

The present technology also relates to a method of manufacturing a BPMEA for the electrochemical reduction of CO2 as herein defined, the method including the steps of:

- depositing a cathode catalyst material onto one side of a gas diffusion layer to provide a cathode catalyst layer thereon;

- coating an anion exchange ionomer solution onto the cathode catalyst layer to provide a permeable CO2 regeneration layer;

- placing at least one layer of a CEM onto the permeable CO2 regeneration layer; placing at least one layer of an AEM onto the at least one layer of a CEM; and - placing an anode comprising on one side an anode catalyst material onto the at least one layer of an AEM, said anode catalyst material facing the at least one layer of an AEM; wherein the cathode catalyst material, the cathode catalyst layer, the gas diffusion layer, the anion exchange ionomer, the permeable CO2 regeneration layer, the CEM, the AEM, the anode and the anode catalyst material are as defined above.

According to one example, the step of depositing the cathode catalyst material onto the gas diffusion layer can be performed by a physical vapor deposition method, for example, by sputter deposition.

According to another example, the anion exchange ionomer solution can include from about 0.34 wt.% to about 0.68 wt.% of the anion exchange ionomer, limits included.

According to another example, the anion exchange ionomer solution can be obtained by dissolving an anion exchange ionomer powder in an alcohol. For example, the anion exchange ionomer powder can be dissolved in alcohol (for example, methanol) by sonication.

According to another example, the step of coating the anion exchange ionomer solution onto the cathode catalyst layer can be performed by a spray deposition method. For example, the spray deposition method can be carried out a spraying rate in the range of from about 0.4 ml_/h/cm ² to about 1.6 mL/h/cm ² , limits included.

According to another example, the method can further include affixing the other side of the gas diffusion layer on a current collector as defined above.

According to another example, the method can further include affixing the other side of the anode on a current collector as defined above.

The present technology also relates to a use of the multilayer cathode, the MEA or the BPMEA as defined herein or produced by the method as defined herein, for the production of a multicarbon product.

According to another example, the multicarbon product can be any suitable multicarbon products, for example, the multicarbon product can be ethylene or ethanol. The present technology also relates to a method for electrochemical production of a multicarbon product using the MEA or the BPMEA as defined herein, the method comprising the steps of: contacting carbon dioxide and an electrolyte with the multilayer cathode, such that the carbon dioxide diffuses through the gas diffusion layer and contacts the cathode catalyst layer; applying a voltage to provide a current density to cause the carbon dioxide contacting the cathode catalyst layer to be electrochemically reduced into the multicarbon product; and recovering the multicarbon product.

According to one example, carbonate ions are produced when applying the voltage.

According to another example, carbon dioxide is regenerated from the carbonate ions in the permeable carbon dioxide regeneration layer.

According to another example, the regenerated carbon dioxide is transported to the cathode catalyst layer to be electrochemically reduced into the multicarbon product prior to the recovering step.

According to another example, the multicarbon product is ethylene or ethanol.

The technology described herein can be applied to a wide variety of CO2 gas streams such as, for example, flue gas and air.

In some cases, where the multilayer cathode as defined herein including the permeable CO ₂ regeneration layer is coupled with a CEM cell, it can provide a multicarbon product distribution similar to one obtained with conventional AEM cells. That is, the permeable CO ₂ regeneration layer coupled CEM cells can reach about 40% faradaic efficiency towards C ₂ H ₄ and about 55% faradaic efficiency towards multicarbon products. However, using the permeable CO ₂ regeneration layer, CO ₂ crossover is limited to 15% of the amount of CO ₂ converted into products, in all cases. Substantially low crossover and low flow rates combine to enable a single pass CO ₂ conversion of 85% ± 5% (at 100 mA/cm ² ), with a multicarbon products faradaic efficiency and full-cell voltage comparable to the anion-conducting membrane electrode assembly.

According to some example, the multilayer cathode as defined herein can be designed to block the transport of protons while providing a pathway for regenerated water and gaseous CO2. This can be achieved via the permeable CO2 regeneration layer which acts as a permeable anion-selective CO2 regeneration layer that provides an alkaline condition at the surface of the cathode catalyst layer, amid acidic conditions are provided by the CEM. In this configuration the CC>2-crossover blocking capability of a BPM is substantially retained, with the distinction that evolved CO2 remains substantially available for reaction. Reactant CO2 lost to bicarbonate and carbonate can be regenerated locally, and the permeability of the layer allows for the transport of regenerated CO2 to the cathode catalyst layer for subsequent reactions (Figure 1).

EXAMPLES

The following non-limiting examples are illustrative embodiments and should not be construed as limiting the scope of the present invention. These examples will be better understood with reference to the accompanying Figures.

Example 1: Electrodes preparation and MEA configurations

(a) AEM cell ( for comparative purposes )

A conventional AEM cell was assembled for comparative purposes. The cathode was prepared by sputtering 250 nm of copper (99.99%) onto a porous PTFE filter. A stabilizing carbon layer and a conductive graphite layer were then applied on the cathode. ⁸¹³ The anode was prepared by etching a titanium mesh (0.002” thickness), or titanium felt (0.3 mm thickness) with boiling 0.5 M oxalic acid for about 10 minutes. The etched titanium mesh or felt was then dip coated in an IrC x H2O (30 mg) in isopropanol (10 mL) solution and then calcinated at a temperature of about 500°C for about 10 minutes. This dip coating process was repeated until a loading of about 1 mg/cm ² was obtained. ⁸ ’ ²⁰²¹ The AEM cell was assembled by placing an AEM (Sustainion™ X37-50) between the cathode and the anode described in the present example.

(b) CEM cell (for comparative purposes ) A conventional CEM cell was assembled for comparative purposes. The cathode was prepared by sputtering 250 nm of copper (99.99%) onto a porous PTFE filter. The anode was prepared by etching a titanium mesh (0.002” thickness), or titanium felt (0.3 mm thickness) with boiling 0.5 M oxalic acid for about 10 minutes. The etched titanium mesh or felt was then dip coated in an IrCb x H2O (30 mg) in isopropanol (10 mL) solution and then calcinated at a temperature of about 500°C for about 10 minutes. This dip coating process was repeated until a loading of about 1 mg/cm ² was obtained. ⁸²⁰²¹ The CEM cell was assembled by placing a CEM (Nafion™ 117) between the cathode and the anode described in the present example.

(c) BPMEA cell ( for comparative purposes )

A conventional BPM cell was assembled for comparative purposes. The cathode was prepared by sputtering 200 nm of copper (99.99%) onto a PTFE filter. The anode was prepared by etching a titanium mesh (0.002” thickness), or titanium felt (0.3 mm thickness) with boiling 0.5 M oxalic acid for about 10 minutes. The etched titanium mesh or felt was then dip coated in an IrCb x H ₂ 0 (30 mg) in isopropanol (10 mL) solution and then calcinated at a temperature of about 500°C for about 10 minutes. This dip coating process was repeated until a loading of about 1 mg/cm ² was obtained. ⁸ ’ ²⁰²¹ The BPM cell was assembled by placing a BPM (Fumasep™ FBM) between the cathode and the anode described in the present example. id) Permeable CO? regeneration layer cell (PCRL-coupled CEM cell)

A MEA cell comprising the permeable CO2 regeneration layer of the present application and a CEM was assembled (PCRL-coupled CEM cell). The cathode was prepared by sputtering 200 nm of copper (99.99%) onto a PTFE filter. The cathode was then sprayed with a dilute anion exchange ionomer solution to achieve the desired loading. The anion exchange ionomer solution was prepared by adding 175 mg of ionomer powder (Aemion ^™ AP1-CNN5-00-X, lonomr) to 25 g of methanol and sonicating until fully dissolved. The anode was prepared by etching a titanium mesh (0.002” thickness), or titanium felt (0.3 mm thickness) with boiling 0.5 M oxalic acid for about 10 minutes. The etched titanium mesh or felt was then dip coated in an IrCb x H2O (30 mg) in isopropanol (10 mL) solution and then calcinated at a temperature of about 500°C for about 10 minutes. This dip coating process was repeated until a loading of about 1 mg/cm ² was obtained. ⁸²⁰²¹ The PCRL- coupled CEM cell was assembled by placing a CEM (Nafion™ 117) between the cathode and the anode described in the present example.

Example 2: Electrochemical measurements

The CO2RR experiments were performed in a 5 cm ² cell with 316L stainless steel cathode flow field and a grade 2 titanium anode with matching serpentine flow fields. Throughout all experiments, unless otherwise specified, CO2 was flowed at 80 seem using a mass flow controller, while the anode side was fed with 100 mM KHCO3 with the AEM and deionised (Dl) water with the CEM, BPM and PCRL-coupled CEM cells at 10 mL/min with a peristaltic pump. The electrochemical measurements were performed with a potentiostat (Autolab PGSTAT204 equipped with 10A booster). The cell voltages are reported in all figures without iR correction (or iR compensation).

Example 3: Product analysis

The CO2RR gas products, oxygen and CO2 were analyzed by sampling the gas outlet stream with a gas chromatograph (Perkin Elmer Clarus™ 590) coupled with a thermal conductivity detector (TCD) and flame ionization detector (FID). The gas chromatograph was equipped with a Molecular Sieve 5A Capillary Column and a packed Carboxen-1000 Column with argon as the carrier gas. The volumetric gas flow rates in and out of the cells were measured with a bubble column.

The liquid products were quantified by proton nuclear magnetic resonance ( ¹ H NMR) spectroscopy using an Agilent DD2500 MHz NMR Spectrometer in deuterium oxide (D 0) using water suppression mode, with dimethyl sulfoxide (DMSO) as the internal standard.

Example 4: Electrode characterization

The copper catalyst and the permeable CO2 regeneration layer were characterized by scanning electron microscopy (SEM) using a FEI Quanta™ FEG 250 environmental SEM at low vacuum or under ESEM mode. The optical microscope images were taken using a LEICA DMC 2900 microscope with a 10x magnification objective.

Example 5: One-dimensional modeling A one-dimensional system was modeled using COMSOL Multiphysics™ version 5.5, building upon the previous modeling works. ^22-27 The pH and species concentrations of different PCRL coating thicknesses (2 pm, 5 pm, and 10 pm) were compared. The detailed simulation consisted of a copper cathode catalyst, a permeable CO2 regeneration layer, a CEM, and an iridium anode catalyst. The secondary current distribution and transport of diluted species physics modules were applied for the numerical models. The simulation assumed a constant concentration supply of CO2 at the left boundary of the cathode catalyst layer and constant species concentrations at the right boundary of the anode layer.

The CO2 solubility was calculated based on Henry’s Law and sets of Sechenov Equations. ²⁷ The temperature and pressure affect CO2 solubility. The corresponding Sechenov constants are listed in Table 2 below. ²⁸

Table 2. Sechenov constants

Diffusion and electromigration were considered for all species, and they are governed by the Nernst-Planck set of equations. The porosity coefficient was of 0.9, 0.1 for the permeable CO2 regeneration layer and the CEM layer, respectively. The transportation of species was calculated in the same manner as the previous work. ²⁷ The corresponding diffusion coefficients and charge numbers are listed in Table 3 below. ^29-31

Table 3. Diffusion coefficients and charge numbers

Ohm’s Law was applied to determine the electrode and electrolyte potentials, and the Poisson Equations were considered for the electromigration of the charged species (H ⁺ , OH , HCO3 , CO3 ² ). The electromigration effect was calculated in the same manner as the previous work. ²⁷ The corresponding electrical conductivities of different layers are listed in Table 4 below. ^32-33

Table 4. Diffusion coefficients and charge numbers

Five electrochemical catalyst reactions were considered in this simulation. More particularly, the CO2RR produces H ₂ , CO, C2H4, and C2H5OH at the cathode catalyst layer. The oxygen evolution reaction occurs at the anode catalyst layer. The electrochemical reactions were calculated in the same manner as the previous work. ²⁷ The corresponding CO2 reduction reactions and oxygen evolution reaction are listed in Tables 5 and 6 below.

Table 5. CO2RR

Table 6. Oxygen evolution reaction

The steady-state equilibrium between H ⁺ , OH-, HCO3 , CO3 ² , and CO2 were determined by the sets of carbonate equilibrium equations. Water dissociation was also considered in this simulation. The carbonate equilibrium equations were calculated in the same manner as the previous work. ²⁷ The corresponding carbonate equilibrium equations and water dissociation equation are listed in Tables 7 and 8 below. Table 7. Carbonate equilibrium equations

Table 8. Water dissociation equation

Example 6: Results and discussion

/a) Electrochemical performances in an AEM cell (for comparative purposes)

Figure 3 shows CO2 reactant loss in the conventional MEA with an AEM prepared in Example 1(a). As illustrated in Figure 3A, the AEM cell while operating at 150 mA/cm ² and a CO2 flow rate of 6 seem the distribution of CO2 obtained was 5% of unreacted CO2, 25% of utilized CO2 and 70% of crossover of CO2 and liquid products from the cathode to the anode. Figure 3B shows a graph of the composition of the cathode and anode gas and liquid products or CO2 distribution as a function of the CO2 flow rate obtained for the AEM cell operating at 150 mA/cm ² while the flow rate of the CO2 fed into the cell was varied. At flow rates of 20 and 40 seem, there was sufficient mass transport of CO2 to the catalyst, evidenced by the low 7% H ₂ faradaic efficiency (Figures 4A and 4B). However, the total amount of input CO2 converted to products was less than 15%. At flow rates between 6 seem and 10 seem, CO2 mass transport became limiting and the hydrogen evolution reaction increased from 8% faradaic efficiency at 10 seem to 20% faradaic efficiency at 6 seem. The unreacted CO2 in the outlet stream reached a minimum value of 1 % that of inlet CO2 (8 seem). The CO2 conversion reached its maximum between 25 and 30% (exceeding the established conversion limit for multicarbon production due to a small amount of Ci production).

The CO2 transported through the membrane matched that predicted for the case of carbonate as the sole charge carrier. The resulting anode head gas contained a mixture of 60-70 vol. % CO2 and 30-40 vol. % O2. Regenerating a reactable CO2 stream from this mixture would require an energy-intensive chemical absorption separation process (e.g. monoethanolamine CO2 absorption). ³⁴

The CO2 conversion efficiency (%) with no carbonate formation in the MEA with an AEM was also calculated. Figure 4C shows the conversion efficiency when carbonate formation was subtracted in the MEA with an AEM at 150 mA/cm ² at varying CO2 flow rate. The anode gas flow rate and composition were measured to determine the amount of CO2 that was transported through the membrane via carbonate. The conversion with no carbonate increased as the CO2 flow rate was decreased from 40 seem to 8 seem because there was a smaller quantity of unreacted CO2. As the CO2 flow rate was decreased from 8 seem to 2 seem, the CO2 mass transport to the catalyst was insufficient and the amount of CO2 that left the cell unreacted increased. Of the CO2 that did not react to form carbonate, 94% could be reacted to form products at 8 seem.

(b) Electrochemical performances in a CEM cell (for comparative purposes )

Figure 5 shows the CO2RR electrochemical performance using the conventional MEA with a CEM prepared in Example 1(b). Figure 5A shows a schematic representation of species transport within the MEA with a CEM. As can be observed, incorporating a CEM in place of the AEM blocks carbonate transport to the anode. ³⁵ The CO2RR performance was measured. Deionized water was employed as the anolyte to ensure that protons were the sole charge carrier. The loss of CO2 was avoided at all current density (Figure 5B), but the cathode environment was too acidic for efficient CO2RR at current density greater than 25 mA/cm ² {i.e., no CO2RR products were detected) (Figure 5C). ^36-40 The acidic cathode environment can improve hydrogen evolution reaction kinetics and can deteriorate CO2RR kinetics (Figure 6); therefore, hydrogen evolution reaction dominates in the CEM configuration. fc) Electrochemical performances in a BPM cell ( for comparative purposes )

Pairing anion and cation selective membrane layers in a BPM is another approach to block reactant and product crossover in electrolyzers. ¹⁶³⁵ With the CEM adjacent to the cathode (in a conventional reverse-bias BPM configuration) the cathode becomes acidic due to the influx of protons and, as in the CEM electrolyzer, is not productive in the CO2RR without an additional buffer layer. ^41-42 An alkaline environment at the cathode can be achieved in a conventional forward-bias BPM configuration, with the AEM layer adjacent to the cathode.

Figure 7 shows the CO2RR electrochemical performance using the conventional MEA with a BPM prepared in Example 1(c) in the forward bias at a low current density (50 mA/cm ² ). Figure 7A shows a schematic representation of species transport within the MEA with a BPM. More CO2 in the anode gas was observed compared to the CEM cell (Figure 8) due to the accumulation and pressure build-up of water and gaseous CO2 at the membrane junction and subsequent migration to both the cathode and anode sides. The formation of the CO2 and water at the membrane junction caused the AEM and CEM to delaminate (Figure 7C), and resulted in loss of ethylene faradaic efficiency within 0.5 hours (Figure 7B). ¹⁶ The conventional BPM does not provide a solution to the CO2 conversion challenge because reactant CO2 is lost to the membrane junction and the system is unstable even at a low current density (50 mA/cm ² ).

(d) Electrode characterization and electrochemical performances in a PCRL-couoled CEM cell

The copper catalyst and permeable CO2 regeneration layer of the cathode prepared in Example 1(d) were characterized by SEM. As described in Example 1(d), the cathode was prepared by first sputtering copper on a porous PTFE filter (Figure 9A), then a PCRL coating was deposited onto the copper layer (Figures 9B and 9C). Figures 10A to 10E show SEM images obtained for cathodes with different PCRL loadings. It is to be noted that since the PCRL is not electrically conductive, any exposed copper catalyst would be bright in comparison. As can be seen in Figures 10B to 10E, the PCRL coatings appear to be substantially uniform.

Without wishing to be bound by theory, the functional groups of the anion exchange polymer (Aemion ^™ AP1-CNN5-00-X) can create a positive space charge, enabling the transport of anions and impeding the transport of cations. The polymer coating on the cathode can allow for CO2 transport to the catalyst via diffusion through the water-filled hydrated ionic domains in the polymer matrix. ⁴³ · ⁴⁴ For example, the PCRL coating can be substantially thin, less than about 10 pm (Figure 9B), to substantially minimize the obstruction of water and CO2 from the membrane junction to the catalyst surface. ⁴⁵ The CO2RR typically requires the presence of alkali metal cations in the Outer Helmholtz Plane to create a reaction environment suitable for efficient conversion. ^46-48 However, within the PCRL, the positively charged functional groups can also act as a fixed positive charge near the catalyst surface that can stabilize CO2RR intermediates to promote C-C coupling on copper catalysts. The quaternary ammonium and heterocyclic (including imidazolium and benzimidazolium) functional groups that are commonly used as the positive charge in anion exchange ionomers ⁴³ have been shown to allow for the intermolecular interaction of water with surface adsorbed CO and promote the hydrogenation of surface bound CO to ethylene. ^49-54 In some example, the cations contained within the polymer structure of the PCRL can eliminate the need for alkali metal cations in the electrolyte.

The electrochemical performances in the MEA cell with a Nafion™ 117 CEM, an Ir0 ₂ anode, and Dl water anolyte prepared in Example 1(d) were evaluated (Figure 11) to assess the impact of the PCRL on the cathode pH and CO2 conversion efficiency. The Nafion™ 117 CEM was selected to provide a substantially greater thickness compared to more commonly applied Nafion™ XL and Nafion™ 211 membranes. For example, a thicker CEM can provide a larger diffusion barrier to minimize transport of CO2 through the hydrophobic domains of the Nafion™ polymer. ⁵⁵⁵⁶ The use of Dl water anolyte can ensure that protons are the only cations that can transport charge through the CEM. If any salts were to be present in the anolyte, the associated cations would be transported through the CEM and react with carbonate and bicarbonate to form salts at the junction of the PCRL and CEM, thus preventing CO2 from being regenerated and recycled to the cathode catalyst. Cathodes with PCRL coatings of different loadings were prepared and their performances were assessed in an electrolyzer in terms of current, faradaic efficiency, CO2 crossover and overall CO2 conversion efficiency.

The current-voltage response with loadings of PCRL coating between 1.5 mg/cm ² and 3 mg/cm ² were characterized (Figure 11A). The voltage was varied from 3 to 5 V and the samples with lower loadings reached higher currents. The observed differences in current density among the samples are not due to changes in the ionic conduction because of the relatively constant ohmic resistance (Figure 12). Figure 12 presents ohmic resistance results obtained with a PCRL-coupled CEM. The ohmic resistance was measured using electrochemical impedance spectroscopy (EIS) in a 5 cm ² cell. The loading of 0 represents a cell assembled with a cathode, but without the PCRL. Differences in the thickness of the PCRL coating could not explain the observed changes in current density because of the relatively high ionic conductivity of the PCRL coating (> 10 mS/cm). The current response was instead attributed to changes in the local pH at the cathode; a 3 mg/cm ² loading provided a higher pH, and thus a larger Nernstian pH voltage loss, compared to a 1.5 mg/cm ² loading. Nernstian loss increased cell voltage by 0.059 V per unit difference in pH between the cathode and anode. For each PCRL loading, a substantially large change in current density was observed once about 40 mA/cm ² was reached, which can correspond to a change in the reaction mechanism. At current density less than about 40 mA/cm ² , the potential required for protons to pass through the PCRL and be consumed directly in the CO2RR and hydrogen evolution reaction was less than the potential required to form alkaline conditions at the cathode. At current density greater than about 40 mA/cm ² , the PCRL was not adequately conductive for protons to pass through at a sufficient rate, so it became kinetically favourable for water near the catalyst to become the proton donor leading to a further increase in the pH from the produced hydroxide ions. This effect is confirmed by a one-dimensional multiphysics model that estimated the pH at the cathode as a function of the coating thickness and the current density (Figure 13). This shift was reflected in the current-voltage response (Figure 11A) and corresponded to a higher cathode pH and an increase in C2H4 selectivity (Figure 14).

Increasing the PCRL loading from 0.75 mg/cm ² to 2.25 mg/cm ² caused the hydrogen evolution reaction to decrease from 54% to 23%, faradaic efficiency and the CO2RR towards C2H4 to increase from 8% to 40% faradaic efficiency (Figure 11B). The increased PCRL thickness can create a substantially more effective proton transport barrier, leading to a higher pH at the cathode. Increasing the PCRL loading to 3.0 mg/cm ² did not further increase the faradaic efficiency for H ₂ and C2H4 compared to the 2.25 mg/cm ² layer which suggests that the local pH at the cathode is not the limiting factor beyond a threshold alkaline pH. The 2.25 mg/cm ² case exhibited similar currents to the 3 mg/cm ² layer while showing similar product selectivity. As the voltage was increased from 3.0 V to 3.6 V, the faradaic efficiency for H ₂ decreased and CO became the major product at 28% faradaic efficiency (Figure 11 C). Once the voltage was increased from 3.8 V to 4.2 V, the pH at the cathode became high enough for significant multicarbon production and the maximum C2H4 faradaic efficiency of 40% was reached. Increasing the voltage beyond 4.2 V increased the faradaic efficiency for H2due to CO2 mass transport limitations in the PCRL, an effect observed previously for hydrophilic cathode layers. ⁵⁵ The 2.25 mg/cm ² PCRL loading case provided steady selectivity and cell voltage for 8 hours of continuous operation at 100 mA/cm ² (Figure 17).

To measure the effectiveness of the PCRL-coupled CEM in preventing CO2 loss, the concentration and flow rate of CO2 in the anode gas (Figure 11D) were measured. With the PCRL layer, CO2 outflow from the anode gas was less than 4% that of the AEM comparative case (i.e., 0.2 seem with the PCRL, versus > 5 seem with the AEM at the same reaction rate of 150 mA/cm ² ). Some CO2 in the anode gas can be attributed to liquid product crossover and subsequent oxidation (further supported by the 5-10% missing faradaic efficiency (Figure 15). Depending on the liquid product oxidized to CO2 ( e.g . ethanol vs. formate), this route could account for 30-100% of the 0.2 seem of CO2 measured in the anode tail gas. For all input CO2 flow rates, the amount of CO2 that crossed over (Figure 11 E) was less than 15% of the amount of CO2 converted into products (e.g. 0.2 seem crossover, compared to 1.4 seem converted). Low crossover enables high CO2 conversion at flow rates less than 2 seem. Selectivity was relatively constant at high input CO2 flow rates (Figure 11F), but below 4 seem CO2 mass transport limitations were reached and the faradaic efficiency for H2 increased. At 1 seem, a CO2 conversion efficiency of 85% ± 5% was achieved (with a faradaic efficiency of 53% toward CO2RR products), representing the highest CO2 conversion efficiency reported in the literature to date, regardless of the targeted product. ⁵⁷

Figure 16 presents the linear gas velocity and Reynolds Number of the CO2 flow in the cell. The linear gas velocity and Reynolds number use the average flow velocity in a 0.8 mm by 0.8 mm channel.

To challenge the general applicability of the PCRL strategy this approach was applied with a CO-producing sputtered silver catalyst (Figure 18). The current densities in the silver catalyst case were lower than those with the copper catalyst because the production of CO requires three times more CO2 per unit of current. The PCRL strategy resulted in selective production of CO with over 75% faradaic efficiency for all current densities up to 100 mA/cm ² . This result demonstrates that PCRL-coupled CEM configuration provides a locally alkaline cathodic environment that is applicable to CO2RR catalysts, generally.

The high CO2 conversion achieved with the PCRL approach does not come at the cost of other performance metrics. The cell voltage and faradaic efficiency with the PCRL were similar to those achieved with the conventional AEM cell (Figures 4A and 4B), and advances in AEMs are applicable to the PCRL. The major sources of voltage loss for both cells are the thermodynamic potential, the catalyst overpotentials, and the Nernstian pH loss. ⁸⁵⁸ The faradaic efficiency toward C ₂ H ₄ in particular could be improved further by incorporating specialized catalysts, such as polyamine incorporated Cu. ^59-61 The energy efficiency of the PCRL system may also be increased further with advances in the CO2 permeability of the anion exchange ionomers, an active area of research. ⁴⁴

A major benefit of high CO ₂ conversion is the avoidance of gas separation costs. After passing through the electrolyzer, any substantial CO ₂ content in the anode tail gas must be separated and recirculated, and any unreacted CO ₂ in the cathode tail gas must be separated from desired gas products. While membrane-based and pressure-swing separation approaches are emerging for C ₂ H ₄ /CO ₂ separation, ⁶²⁶³ typical C02 removal processes currently rely on a chemical absorption unit, such as monoethanolamine absorption. ³⁴ In the best-case conversion scenarios achieved here, the molar ratio of output CO ₂ to C ₂ H ₄ produced streams was 0.6 in the PCRL case, compared to 12 with the AEM. The 20-fold reduction in CO ₂ content of the cell output, most of which was achieved on the anode side, results in dramatic savings in CO ₂ separation energy costs (Figure 19). The energy cost of CO ₂ separation from the AEM electrolyzer anode output stream dominates, at 2067 kJ/mol of produced C ₂ H ₄ . The energy intensity of this peripheral separation process surpasses the Gibbs free energy of the reaction, rendering the conventional AEM approach untenable. Therefore, the PCRL approach can provide a solution to the CO ₂ conversion challenge and a way forward for the electrocatalytic conversion of CO ₂ .

The following documents and any others mentioned herein are incorporated herein by reference in their entirety.

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