CARBON DIOXIDE ELECTROLYZER AND METHODS FOR MAKING THE SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application No. 63/403,338, inventors Tianyu Zhang et al., filed September 2, 2022, the disclosure of which is incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under DE-EE0009421 awarded by the Department of Energy, Energy Efficiency and Renewable Energy (DOE EERE). The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates generally to electrochemical cells, such as electrolyzers and fuel cells, and relates more particularly to a novel gas diffusion electrode suitable for use in a carbon dioxide electrolyzer, to a membrane electrode assembly comprising said gas diffusion electrode, to a carbon dioxide electrolyzer comprising said membrane electrode assembly, and to methods for fabricating said gas diffusion electrode, said membrane electrode assembly, and said carbon dioxide electrolyzer.

The efficient electrochemical conversion of carbon dioxide into valuable carbonbased fuels and chemicals is desirable both for reducing carbon emissions and for storage of renewable electricity. In many instances, the electrochemical conversion of carbon dioxide into valuable products is accomplished using a carbon dioxide electrolyzer. Typically, such a carbon dioxide electrolyzer includes a membrane electrode assembly. The membrane electrode assembly, in turn, often includes an ion exchange membrane positioned between a cathode and an anode. In order to promote a reaction rate that is suitable for carbon dioxide conversion on an industrial scale, the cathode is typically in the form of a gas diffusion electrode. Such a gas diffusion electrode typically includes a catalyst layer and a gas diffusion layer. The catalyst layer functions as the reaction zone (i.e., where the electrochemical reduction of carbon dioxide actually occurs). By contrast, the gas diffusion layer, which may be, for example, a filamentary structure defining a plurality of pores, functions as the mass transport pathway for supplying carbon dioxide reactant to the catalyst layer. In many cases, the gas diffusion layer is made of a carbon material in order to provide high electron conductivity, which is necessary for the electrochemical reaction to proceed. However, because carbon materials typically tend to be hydrophilic, which, as discussed below, is undesirable, the surface of the carbon material making up the gas diffusion layer is commonly coated with a thin layer of a fluorinated material, for example, 5-30 wt% of one or more fluorinated polymers, such as polytetrafluoroethylene (PTFE), to increase the hydrophobicity and gas transport efficiency of the gas diffusion layer. Therefore, gas diffusion layers of the type described above operate by having electrons conducted through the solid carbon material of the gas diffusion layer and by having gas phase reactants and products transported through the pores defined by the hydrophobically-coated surface of the carbon material making up the gas diffusion layer.

Ideally, a membrane electrode assembly comprising a gas diffusion electrode is designed to minimize electrolyzer resistance and to increase energy efficiency. However, a membrane electrode assembly comprising a gas diffusion electrode as described above is susceptible to low carbon dioxide transport efficiency during long-term operation due to one or more of the following reasons: (1) carbon-based gas diffusion layers that are coated with a hydrophobic fluorinated polymer gradually lose hydrophobicity over time due to an erosion of the hydrophobic coating and, thus, become deficient in preventing flooding of the cathode catalyst layer with water; (2) the carbon material providing the structure of the gas diffusion layer tends to oxidize over time and, thus, to wear down; (3) water pressure at the cathode catalyst layer gradually builds up as water migrates from the anode side to the cathode side, eventually breaking the pores of the gas diffusion electrode; and (4) the hydrogen evolution reaction, which competes with the reduction of carbon dioxide at the cathode catalyst layer, turns some of the liquid water present at the cathode into hydrogen gas, resulting in a net fluid flux from the catalyst layer to the flow channels of the gas diffusion layer, thereby hindering the diffusion of carbon dioxide towards the catalyst layer. As can be appreciated, when flooding of the cathode with water occurs, the availability of carbon dioxide at the cathode decreases, thereby resulting in low carbon dioxide reduction selectivity.

One approach that has been taken to addressing the above-described gas transport efficiency problem has been to provide a gas diffusion electrode that comprises an ePTFE (expanded polytetrafluoroethylene) membrane, which is a porous hydrophobic structure. In such a gas diffusion electrode, a catalyst layer is loaded on top of the ePTFE membrane, followed by coating a layer of conductive carbon material for electron conduction. Consequently, as can be appreciated, a gas diffusion electrode comprising an ePTFE membrane of the above-described type decouples the gas transport and electron conduction functions since the carbon dioxide reactant diffuses through the ePTFE membrane whereas electrons are conducted through the conductive carbon material coating. Because, unlike conventional carbon-based gas diffusion membranes, the hydrophobicity of an ePTFE membrane does not wane over time, a gas diffusion electrode employing an ePTFE membrane intrinsically solves the hydrophobicity degradation problem of conventional gas diffusion electrodes. However, the problems of water pressure building up in the catalyst layer and of net gas flux from the cathode layer to the flow channel due to the hydrogen evolution reaction have not been solved. These problems can have a significant impact on the amount of carbon dioxide that reaches the cathode layer and, thus, can significantly limit the amount of carbon dioxide that is electrochemically converted. Accordingly, there is a need for an alternative gas diffusion electrode that addresses the issue of flooding.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel gas diffusion electrode that is suitable for use in a carbon dioxide electrolyzer.

It is another object of the present invention to provide a gas diffusion electrode as described above that overcomes at least some of the shortcomings associated with at least some existing gas diffusion electrodes.

It is still another object of the present invention to provide a gas diffusion electrode as described above that is easy to manufacture and easy to use.

Therefore, according to one aspect of the invention, there is provided a gas diffusion electrode suitable for use in a carbon dioxide electrolyzer, the gas diffusion electrode comprising (a) a gas diffusion layer, the gas diffusion layer comprising (i) an electron- conductive domain, the electron-conductive domain having a thickness and comprising a plurality of pores, and (ii) a non-conductive hydrophobic domain, the non-conductive hydrophobic domain having a thickness ranging from about 10% to about 100% of the thickness of the electron-conductive domain, the non-conductive hydrophobic domain randomly occupying a portion of the volume of the pores of the electron-conductive domain, the non-conductive hydrophobic domain being porous to permit gas transport through the gas diffusion layer; and (b) a catalyst layer, the catalyst layer being coupled to the gas diffusion layer.

In a more detailed feature of the invention, the non-conductive hydrophobic domain may comprise at least one of an ether polymer, an acrylic polymer, a fluorocarbon polymer, a polystyrene polymer, a poly(vinyl chloride) polymer, and a poly(N-vinylpyrrolidone) polymer.

In a more detailed feature of the invention, the non-conductive hydrophobic domain may comprise at least one of a fiber, a particle, a pellet, and a rod.

In a more detailed feature of the invention, the non-conductive hydrophobic domain may comprise a particle, and the particle may have a size ranging from about 1 nm to about 1 mm.

In a more detailed feature of the invention, the non-conductive hydrophobic domain may have a size ranging from about 5 nm to about 100 pm.

In a more detailed feature of the invention, the non-conductive hydrophobic domain may comprise at least one of a fiber and a rod, and the non-conductive hydrophobic domain may have a diameter ranging from about 1 nm to about 1 mm and a length ranging from nanometer to centimeter scale.

In a more detailed feature of the invention, the non-conductive hydrophobic domain may have a diameter ranging from about 5 nm to about 100 pm.

In a more detailed feature of the invention, the electron-conductive domain may comprise carbon, the non-conductive hydrophobic domain may have a mass, the electron- conductive domain may have a mass, and the mass of the non-conductive hydrophobic domain may range from about 30 wt% to about 600 wt% of the mass of the electron- conductive domain.

In a more detailed feature of the invention, the electron-conductive domain may comprise at least one of carbon, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, and brass.

In a more detailed feature of the invention, the electron-conductive domain may comprise at least one of a paper, a foam, a felt, a sinter, a cloth, and a mesh.

In a more detailed feature of the invention, the electron-conductive domain may be coated with at least one hydrophobic polymer with a loading ranging from about 0.1 wt% to about 40 wt% of the electron-conductive domain.

In a more detailed feature of the invention, the electron-conductive domain may have a thickness ranging from about 0.5 pm to about 2 mm.

In a more detailed feature of the invention, the gas diffusion electrode may further comprise an interlayer, the interlayer may be disposed between and in direct contact with each of the gas diffusion layer and the catalyst layer, and the interlayer may be porous and electron-conductive.

In a more detailed feature of the invention, the interlayer may have a thickness ranging from about 5 nm to about 30 pm.

In a more detailed feature of the invention, the interlayer may comprise at least one of carbon, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, and brass.

In a more detailed feature of the invention, the interlayer may be coated with one or more hydrophobic materials with a weight ratio of about 5 wt% to about 40 wt% of the interlayer.

In a more detailed feature of the invention, the interlayer may comprise a mixture of electron-conductive particles and hydrophobic polymer particles. In a more detailed feature of the invention, the hydrophobic polymer particles may be present in the interlayer at a mass ratio ranging from about 5 wt% to about 200 wt% of the electron-conductive material particles.

In a more detailed feature of the invention, the catalyst layer may comprise at least one of copper, silver, gold, carbon, bismuth, iron, boron, nitrogen, oxygen, fluorine, aluminum, silicon, phosphorus, sulfur, chromium, manganese, cobalt, nickel, zinc, gallium, selenium, molybdenum, ruthenium, rhodium, palladium, iridium, and platinum.

In a more detailed feature of the invention, the non-conductive hydrophobic domain may comprise a porous hydrophobic particle.

In a more detailed feature of the invention, the porous hydrophobic particle may have a particle size of about 1 nm to about 1 mm.

In a more detailed feature of the invention, the porous hydrophobic particle may have at least one internal pore, with a pore diameter ranging from about 50 nm to about 5 pm.

In a more detailed feature of the invention, the porous hydrophobic particle may be made by cryo-milling a bulk quantity of a porous material into powder.

In a more detailed feature of the invention, the porous hydrophobic particle may comprise at least one of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and polyvinyl chloride (PVC).

According to another aspect of the invention, there is provided a method of fabricating a gas diffusion electrode, the method comprising the steps of (a) forming a gas diffusion layer, wherein said gas diffusion layer forming comprises (i) providing a slurry, the slurry comprising at least one hydrophobic material and at least one organic solvent; (ii) providing an electron-conductive substrate, the electron-conductive substrate having a thickness and comprising a plurality of pores; (iii) loading the slurry into the pores of the electron-conductive substrate; and (iv) sintering the slurry, whereby the solvent evaporates, forming a non-conductive hydrophobic domain within the pores of the electron-conductive substrate, the non-conductive hydrophobic domain having a thickness ranging from about 50% to about 100% of the thickness of the electron-conductive domain, the non-conductive hydrophobic domain configured to permit gas transport through the electron-conductive substrate; and (b) positioning a catalyst layer over the gas diffusion layer.

In a more detailed feature of the invention, the loading step may comprise one of screen printing, doctor blade coating, slot-die coating, vacuum filtration coating, and airbrush coating. In a more detailed feature of the invention, the slurry may further comprise at least one of water and one or more binders.

In a more detailed feature of the invention, the loading step may comprise airbrush coating, and the airbrush coating may be conducted at a pressure ranging from about 0.5 bar to about 15 bar.

In a more detailed feature of the invention, the pressure may range from about 2 bar to about 6 bar.

In a more detailed feature of the invention, the sintering step may be performed at a temperature no greater than the melting point of the at least one hydrophobic material.

In a more detailed feature of the invention, the temperature may be about 0°C to about 50°C below the melting point temperature of the at least one hydrophobic material.

In a more detailed feature of the invention, the sintering step may be performed at a pressure ranging from about 0 lb. to about 5000 lbs.

In a more detailed feature of the invention, the method may further comprise the step of positioning an interlayer directly between the catalyst layer and the electron-conductive substrate.

In a more detailed feature of the invention, the interlayer positioning step may comprise at least one of spraying, screen printing, doctor blade coating, vacuum filtration coating, compression coating, and slot-die coating.

According to yet another aspect of the invention, there is provided a gas diffusion electrode made by the above method.

According to still yet another aspect of the invention, there is provided a method of fabricating a gas diffusion electrode, the method comprising the steps of (a) forming a gas diffusion layer, wherein said gas diffusion layer forming comprises (i) providing at least one non-conductive hydrophobic polymer; (ii) providing at least one electron-conductive material; (iii) mixing the at least one non-conductive hydrophobic polymer and the at least one electron-conductive material to form a mixture; (iv) extruding the mixture to form a sheet; (v) stretching the sheet at a temperature below the melting point of the at least one non- conductive hydrophobic polymer to form pores for gas transport through the sheet; and (b) positioning a catalyst layer over the gas diffusion layer.

In a more detailed feature of the invention, the stretching may comprise at least one of uniaxial stretching and biaxial stretching of the sheet. In a more detailed feature of the invention, a mass ratio of the at least one electron- conductive material to the at least one non-conductive hydrophobic polymer may range from about 15 wt% to about 300 wt%.

In a more detailed feature of the invention, the mass ratio may be from about 50 wt% to about 200 wt%.

In a more detailed feature of the invention, the at least one electron-conductive material may be one or more of carbon, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, and brass.

In a more detailed feature of the invention, the at least one electron-conductive material may be one or more of a fiber, a particle, a pellet, and a rod.

In a more detailed feature of the invention, the at least one electron-conductive material may have a particle size ranging from about 1 nm to about 1 mm.

In a more detailed feature of the invention, the at least one electron-conductive material may be at least one of a fiber and a rod, and the at least one electron-conductive material may have a diameter ranging from about 1 nm to about 1 mm and a length ranging from the nanometer scale to the centimeter scale.

In a more detailed feature of the invention, the temperature may be the melting point temperature of the at least one non-conductive hydrophobic polymer.

In a more detailed feature of the invention, the mixture may further comprise a lubricating agent and a pore former.

In a more detailed feature of the invention, the stretching may comprise stretching at a ratio ranging from about 2: 1 to about 40:1.

According to a further aspect of the invention, there is provided a gas diffusion electrode made by the above method.

According to yet a further aspect of the invention, there is provided a method of fabricating a gas diffusion electrode, the method comprising the steps of (a) forming a gas diffusion layer, wherein said gas diffusion layer forming comprises (i) providing a first gas diffusion layer piece, the first gas diffusion layer piece being porous and electron-conductive; (ii) providing a second gas diffusion layer piece, the second gas diffusion layer piece being porous, non-conductive and hydrophobic, wherein the first gas diffusion layer piece and the second gas diffusion layer piece are complementarity shaped to mate with one another in the same plane; (iii) mating together the first gas diffusion layer piece and the second gas diffusion layer piece; and (b) positioning a catalyst layer over the gas diffusion layer. In a more detailed feature of the invention, the first gas diffusion layer piece and the second gas diffusion layer piece may form an interdigitated construction.

In a more detailed feature of the invention, the first gas diffusion layer piece may comprise a transverse opening, and the second gas diffusion layer piece may be dimensioned to mate with the transverse opening.

In a more detailed feature of the invention, the gas diffusion layer may have a uniform distribution of one or more electron-conductive areas and one or more non-conductive, hydrophobic areas.

In a more detailed feature of the invention, each of the first gas diffusion layer piece and the second gas diffusion layer piece may have an area, and a ratio of the areas of the first and second gas diffusion layer pieces may range from about 1:2, respectively, to about 2: 1, respectively.

In a more detailed feature of the invention, the first gas diffusion layer piece may comprise at least one of carbon paper, metal foam, metal mesh, and metal paper.

In a more detailed feature of the invention, the second gas diffusion layer piece may comprise at least one of a fluoropolymer film and a polyvinyl chloride film.

In a more detailed feature of the invention, the first and second gas diffusion layer pieces may have similar thicknesses.

According to still a further aspect of the invention, there is provided a gas diffusion electrode made by the above method.

According to still yet another aspect of the invention, there is provided a method for fabricating a gas diffusion electrode, the method comprising the steps of (a) forming a gas diffusion layer, wherein said gas diffusion layer forming comprises (i) providing a porous electron-conductive substrate, the porous electron-conductive substrate having an outer surface; (ii) preparing a slurry, the slurry comprising a random mixture of at least one electron-conductive material, at least one non-conductive hydrophobic polymer material, and at least one solvent; and (iii) coating the outer surface of the porous electron-conductive substrate with the slurry, which, upon drying, forms a porous coating on the outer surface of the porous electron-conductive substrate; and (b) positioning a catalyst layer over the porous coating of the gas diffusion layer.

In a more detailed feature of the invention, a volume ratio of the at least one electron- conductive material to the at least one non-conductive hydrophobic polymer material may range from about 9:1, respectively, to about 1:3, respectively. In a more detailed feature of the invention, the porous coating may have a thickness ranging from about 1 pm to about 200 pm.

In a more detailed feature of the invention, the at least one electron-conductive material may have a particle size from about 10 nm to about 5 pm.

In a more detailed feature of the invention, the at least one non-conductive hydrophobic polymer material may have a particle size of from about 20 nm to about 200 pm.

In a more detailed feature of the invention, the coating step may comprise at least one of doctor blade coating, slot-die coating, bar-coating, and screen printing.

In a more detailed feature of the invention, the method may further comprise, prior to the coating step, pre-coating the electron-conductive substrate with at least one hydrophobic material to a weight percentage of about 0.1 wt% to about 40 wt%.

According to still yet a further aspect of the invention, there is provided a gas diffusion electrode made by the above method.

The present invention is also directed at novel methods for fabricating a gas diffusion layer, a membrane electrode assembly, and an electrochemical cell, such as an electrolyzer or fuel cell.

For purposes of the present specification and claims, various relational terms like “top,” “bottom,” “proximal,” “distal,” “upper,” “lower,” “front,” and “rear” may be used to describe the present invention when said invention is positioned in or viewed from a given orientation. It is to be understood that, by altering the orientation of the invention, certain relational terms may need to be adjusted accordingly.

Additional objects, as well as aspects, features, and advantages, of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. In the description, reference is made to the accompanying drawings which form a part thereof and in which is shown by way of illustration various embodiments for practicing the invention. The embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are hereby incorporated into and constitute a part of this specification, illustrate various embodiments of the invention and, together with the description, serve to explain the principles of the invention. The drawings are not necessarily drawing to scale, and certain components may have undersized and/or oversized dimensions for purposes of explication. In the drawings wherein like reference numeral represents like parts:

FIG. 1 is a simplified side view of a first embodiment of an electrochemical cell constructed according to the present invention;

FIG. 2 is a simplified section view of the membrane electrode assembly shown in FIG. 1;

FIG. 3 is a simplified section view of the cathode shown in FIG. 2;

FIG. 4 is a simplified schematic view of a first embodiment of a system constructed according to the present invention for converting carbon dioxide into products like ethylene, the system including the electrochemical cell of FIG. 1 ;

FIG. 5 is a partly exploded simplified section view of a second embodiment of an electrochemical cell constructed according to the present invention;

FIG. 6 is a simplified section view of an alternative membrane electrode assembly to the membrane electrode assembly of FIG. 2;

FIG. 7 is a flowchart depicting a first embodiment of a method according to the present invention for fabricating a gas diffusion layer of the type shown in FIG. 3;

FIG. 8 is a schematic depiction of a gas diffusion layer being made by the slot-die technique of FIG. 7 ;

FIG. 9 is a flowchart depicting a second embodiment of a method according to the present invention for fabricating a gas diffusion layer of the type shown in FIG. 3;

FIG. 10 is a schematic depiction of a gas diffusion layer being made by the doctor blade technique of FIG. 9;

FIG. 11 is a flowchart depicting a third embodiment of a method according to the present invention for fabricating a gas diffusion layer of the type shown in FIG. 3;

FIG. 12 is a schematic depiction of a gas diffusion layer being made by the airbrush technique of FIG. 11 ;

FIG. 13 is a flowchart depicting a fourth embodiment of a method according to the present invention for fabricating a gas diffusion layer of the type shown in FIG. 3; FIG. 14 is a schematic depiction of a gas diffusion layer made by the stretching technique of FIG. 13;

FIG. 15 is a schematic depiction of an alternative gas diffusion layer made by the stretching technique of FIG. 13;

FIG. 16 is a top view of a first alternative gas diffusion layer to the gas diffusion layer shown in FIG. 3, the first alternative gas diffusion layer being constructed according to the present invention;

FIG. 17 is a top view of a second alternative gas diffusion layer to the gas diffusion layer shown in FIG. 3, the second alternative gas diffusion layer being constructed according to the present invention;

FIG. 18 is a simplified section view of a third alternative gas diffusion layer to the gas diffusion layer shown in FIG. 3, the third alternative gas diffusion layer being constructed according to the present invention;

FIG. 19 is a graph depicting the Faradaic efficiencies of carbon dioxide reduction products and corresponding cell voltages versus the applied total current densities for the test described in Example 1 ; and

FIG. 20 is a graph depicting the cell voltages and corresponding Faradaic efficiencies of carbon dioxide reduction products versus time for the test described in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

As noted above, the presence of excess water at the cathode catalyst layer of a carbon dioxide electrolyzer can significantly limit the amount of carbon dioxide that reaches the cathode catalyst layer and, thus, can significantly reduce the amount of carbon dioxide that is reacted by the carbon dioxide electrolyzer. The present invention is directed, at least in part, at a novel gas diffusion electrode that may be suitable for ameliorating the foregoing problem. In particular, as will be discussed further below, the gas diffusion electrode of the present invention may comprise a novel gas diffusion layer and a catalyst layer coupled to the novel gas diffusion layer. More specifically, in at least one embodiment, the novel gas diffusion layer may comprise (i) an electron-conductive domain, the electron-conductive domain comprising a plurality of pores, and (ii) a non-conductive (i.e., electron non-conductive) hydrophobic domain, the non-conductive hydrophobic domain randomly (i.e., non-uniformly) occupying a portion of the volume of the pores of the electron-conductive domain, the non- conductive hydrophobic domain, itself, being porous to permit gas transport through the gas diffusion layer. Alternatively, in another embodiment, the novel gas diffusion layer may comprise an electron-conductive domain and a non-conductive hydrophobic domain, wherein each of the electron-conductive domain and the non-conductive hydrophobic domain is porous and wherein the electron-conductive domain and the non-conductive hydrophobic domain are complementarity shaped to collectively form a planar structure. Alternatively, in another embodiment, the novel gas diffusion layer may comprise (i) an electron-conductive domain, the electron-conductive domain comprising a pair of opposing surfaces and a plurality of pores designed for gas transport between the pair of opposing surfaces; and (ii) a porous layer positioned over one of the opposing surfaces of the electron-conductive domain, the porous layer comprising a random mixture of electron-conductive materials and non- conductive hydrophobic materials.

The present invention is also directed, at least in part, at a membrane electrode assembly that includes a gas diffusion electrode as described above and is further directed, at least in part, at an electrochemical cell, such as a fuel cell or electrolyzer (e.g., carbon dioxide electrolyzer), that includes such a membrane electrode assembly. Moreover, the present invention is additionally directed, at least in part, at methods for fabricating the gas diffusion layer of such a gas diffusion electrode, as well as to methods for fabricating the gas diffusion electrode, itself, to methods for fabricating a membrane electrode assembly comprising the aforementioned gas diffusion electrode, and to methods for fabricating an electrochemical cell, such as a fuel cell or electrolyzer, comprising the aforementioned membrane electrode assembly.

Referring now to FIG. 1, there is shown a simplified side view of a first embodiment of an electrochemical cell constructed according to the present invention, the electrochemical cell being represented generally by reference numeral 11. Details of electrochemical cell 11 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 1 and/or from the accompanying description herein or may be shown in FIG. 1 and/or described herein in a simplified manner.

Electrochemical cell 11 may be a fuel cell or electrolytic cell but is preferably a carbon dioxide electrolyzer designed, for example, to convert carbon dioxide into carbon- containing materials, such as, but not limited to, ethylene. Electrochemical cell 11 may comprise a membrane electrode assembly 13, which is discussed in greater detail below. Electrochemical cell 11 may further comprise a first current collector 14, a second current collector 16, a first endplate 18, a second endplate 20, and a voltage source 22, all of which may be conventional. First current collector 14 and second current collector 16 may be coupled to membrane electrode assembly 13 on opposite sides thereof. In addition, first current collector 14 and second current collector 16 may be electrically coupled to voltage source 22 through leads 24-1 and 24-2, respectively. First endplate 18 may be positioned above and coupled to first current collector 14, and second endplate 20 may be positioned below and coupled to second current collector 16.

Although not shown, electrochemical cell 11 may further comprise various gaskets, frames, and gas transport structures of the type conventionally used in electrochemical cells, particularly carbon dioxide electrolyzers.

Where, for example, electrochemical cell 11 is a carbon dioxide electrolyzer, carbon dioxide may be delivered to the cathode side of membrane electrode assembly 13, and a suitable reductant, such as a potassium hydroxide solution, may be delivered to the anode side of membrane electrode assembly 13. As a result of electrolysis, the carbon dioxide supplied to electrochemical cell 11 may be reduced to ethylene or the like.

Referring now to FIG. 2, there is shown a simplified section view of membrane electrode assembly 13. Details of membrane electrode assembly 13 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 2 and/or from the accompanying description herein or may be shown in FIG. 2 and/or described herein in a simplified manner. Membrane electrode assembly (MEA) 13, which may be a zero-gap membrane electrode assembly, may comprise an ion exchange membrane 15, an anode 17 positioned on one side of, and in direct contact with, ion exchange membrane 15, and a cathode 19 positioned on the opposite side of, and in direct contact with, ion exchange membrane 15. Ion exchange membrane 15, which functions as a separator and ion conductor between anode 17 and cathode 19, may be an ion exchange membrane of the type typically used in a carbon dioxide electrolyzer. More specifically, ion exchange membrane 15 may be, for example, a proton exchange membrane, an anion exchange membrane, or a bipolar membrane, with anion exchange membranes and bipolar membranes being preferred and with anion exchange membranes being particularly preferred.

Anode 17 may comprise an anode of the type conventionally used in a carbon dioxide electrolyzer or otherwise used to target an electrochemical oxidation reaction, such as the oxygen evolution reaction (OER). For example, the anode may be a piece of metal foam that is catalytically active for anode reactions, or the anode may be a piece of metal foam having a surface on which active catalysts are deposited, or the anode may be a gas diffusion electrode comprising a porous transport layer and a catalyst layer.

Referring now to FIG. 3, cathode 19, which may be a gas diffusion electrode, may comprise a gas diffusion layer 21 and a catalyst layer 23. Gas diffusion layer 21, in turn, may comprise an electron-conductive domain 25 and an electron non-conductive (i.e., non- conductive) hydrophobic domain 27. Electron-conductive domain 25 may comprise any type of porous, electron-conductive structure and, thus, may be similar to a conventional or non- conventional electron-conductive gas diffusion layer. As such, examples of structures suitable for use as electron-conductive domain 25 may comprise, for example, an electron- conductive mesh, woven (e.g., cloth) or felt material, paper, or disc, a porous graphite film, a metal foam, a metal sinter, or combinations thereof. Materials suitable for use in making the above types of structures may include carbon and/or one or more metals including, but not limited to, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, and brass.

Electron-conductive domain 25 may include a plurality of intrinsic pores or channels 26 suitable for gas transport therethrough. In the present embodiment, electron-conductive domain 25 may be a unitary member consisting of a single piece of material, such as a single sheet of carbon paper or a single piece of a metal mesh; however, it is to be understood that, alternatively, electron-conductive domain 25 could be made by joining together a plurality of separate pieces of material, for example, by lamination or otherwise, to form a coherent structure. Also, it is to be understood that, although, in the embodiment shown, electron- conductive domain 25 is shown as a network of filamentary structures, electron-conductive domain 25 need not be formed from filamentary structures.

Electron-conductive domain 25 may range in thickness from about 0.5 pm to about 2 mm, with a preferred thickness ranging from about 1 pm to about 200 pm.

Electron-conductive domain 25 may optionally be coated with one or more hydrophilic polymers, with a loading that may range from about 0.1 wt% to about 40 wt% of electron-conductive domain 25, and preferably from about 5 wt% to about 30 wt% of electron-conductive domain 25.

Non-conductive hydrophobic domain 27 may comprise one or more hydrophobic materials randomly occupying a portion of the volume of the pores 26 of electron-conductive domain 25. Non-conductive hydrophobic domain 27 may, itself, be sufficiently porous to permit gas transport through gas diffusion layer 21, by virtue of non-conductive hydrophobic domain 27 incompletely filling pores 26, thereby leaving spaces 28 in pores 26, and/or by virtue of non-conductive hydrophobic domain 27 being made of an inherently porous material. The porous and hydrophobic features of non-conductive hydrophobic domain 27 may allow for high gas permeability and may repel water by capillary action. Moreover, the comparatively large quantity of material used to form non-conductive hydrophobic domain 27, as opposed to the amount of hydrophobic material conventionally used to provide a thin film on electron-conductive domain 25, may help to ensure that gas diffusion layer 21 can function in the long term with a minimal decrease in hydrophobicity. In addition, the non- conductive feature of non-conductive hydrophobic domain 27 may counter the electrowetting phenomenon encountered with conventional gas diffusion electrodes.

Materials suitable for use in forming non-conductive hydrophobic domain 27 may include, for example, one or more hydrophobic polymers, such as, but not limited to, one or more of ether polymers, acrylic polymers, fluorocarbon polymers, polystyrene polymers, poly(vinyl chloride) polymers, and poly(N-vinylpyrrolidone) (PVP) polymers. Such materials may take the physical form of, for example, one or more of a fiber, a particle, a pellet, a rod, and the like. When, for example, the hydrophobic material used to form non- conductive hydrophobic domain 27 is in the form of a particle, the particle size may range from about 1 nm to about 1 mm, with a preferred particle size ranging from about 5 nm to about 200 nm. On the other hand, when the hydrophobic material used to form non- conductive hydrophobic domain 27 is in the form of a fiber or a rod, the fiber or rod may have a diameter ranging from about 1 nm to about 1 mm, preferably from about 5 nm to about 30 pm, and may have a length ranging from the nanometer to centimeter scale.

Where, for example, electron-conductive domain 25 is made of carbon, the mass of the hydrophobic polymer making up non-conductive hydrophobic domain 27 versus the mass of electron-conductive domain 25 may range from about 30 wt% to about 600 wt%, with a preferred range being from about 200 wt% to about 400 wt%.

Non-conductive hydrophobic domain 27 may have a thickness that is equal to or less than that of electron-conductive domain 25, with the thickness of non-conductive hydrophobic domain 27 ranging from about 5% to about 100% of that of electron-conductive domain 25, preferably about 10% to about 100% of that of electron-conductive domain 25, and more preferably about 50% to about 100% of that of electron-conductive domain 25. In this manner, by having the thickness of electron-conductive domain 25 be at least as great as the thickness of non-conductive hydrophobic domain 27, one may ensure that gas diffusion layer 21 may conduct electrons across its entire thickness

It is to be understood that, although, in the embodiment shown in FIG. 3, the hydrophobic material used to form non-conductive hydrophobic domain 27 is randomly positioned within pores 26 of electron-conductive domain 25 and is not additionally used to uniformly coat the filamentary structures of electron-conductive domain 25, one could position hydrophobic material within pores 26 of electron-conductive domain 25 and could additionally uniformly coat the filamentary structures of electron-conductive domain 25 with a thin film of hydrophobic material. For example, electron-conductive domain 25 may be coated with one or more hydrophobic polymers, with a loading ranging from about 0.1 wt% to about 40 wt%, preferably from about 5 wt% to about 30 wt%.

After the hydrophobic material used to form non-conductive hydrophobic domain 27 is loaded into pores 26 of electron-conductive domain 25, the hydrophobic polymer may be sintered. The sinter temperature may be around the melting point of the hydrophobic polymer, preferably about 0°C to about 50°C below the melting point of the hydrophobic polymer, more preferably about 10°C to about 30°C below the melting point of the hydrophobic polymer. The sinter pressure applied may range from about 0 lb. to about 5000 lbs.

Cathode 19 may further comprise an interlayer 29, which may be one or more layers positioned between gas diffusion layer 21 and catalyst layer 23 and which may serve to increase the electron-conductivity of cathode 19. Interlayer 29 may comprise one or more electron-conductive materials including, but not limited to, carbon, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, and brass. Interlayer 29 may additionally comprise one or more hydrophobic materials, preferably one or more hydrophobic polymer materials of the types described above. In one embodiment, interlayer

29 may comprise a first layer of electron-conductive materials coated with a second layer of hydrophobic materials, with the hydrophobic materials constituting about 5 wt% to about 40 wt%, preferably about 5 wt% to about 30 wt% of the electron-conductive materials. In another embodiment, interlayer 29 may comprise a single layer that may be a mixture of electron-conductive material particles and hydrophobic polymer particles, and the mass ratio of hydrophobic polymer particles to electron-conductive material particles may range from about 5 wt% to about 200 wt%.

Interlayer 29, which may be formed by one or more deposition techniques including spraying, screen printing, doctor blade coating, vacuum filtration coating, compression coating, and slot-die coating, may have a thickness that may range from about 5 nm to about

30 pm, preferably from about 5 nm to about 100 nm.

Catalyst layer 23, which may be conventional in composition and structure and which may be formed using conventional techniques, may be deposited or otherwise positioned directly over interlayer 29. For example, catalyst layer 23 may be a layer of catalyst particles coated on top of interlayer 29 to a thickness ranging from 100 nm to 10 pm. Alternatively, catalyst layer 23 may comprise, for example, catalyst particles loaded onto a substrate layer, which are then coated together onto interlayer 29. The catalyst layer may comprise one or more of copper, silver, gold, carbon, bismuth, iron, boron, nitrogen, oxygen, fluorine, aluminum, silicon, phosphorus, sulfur, chromium, manganese, cobalt, nickel, zinc, gallium, selenium, molybdenum, ruthenium, rhodium, palladium, iridium, and platinum.

It is to be understood that interlayer 29 could be omitted (depending, for example, on the electron conductivity of electron-conductive domain 25) and that, in such a case, catalyst layer 23 could be positioned directly on top of gas diffusion layer 21.

Referring now to FIG. 4, there is schematically shown one embodiment of a system for converting carbon dioxide into products like ethylene, the system being constructed according to the present invention and being represented generally by reference numeral 31. Details of system 31 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 4 and/or from the accompanying description herein or may be shown in FIG. 4 and/or described herein in a simplified manner.

System 31 may comprise electrochemical cell 11. As can be seen, water and a dry or humidified stream of carbon dioxide may be delivered to the cathode of electrochemical cell 11 , and a suitable reductant, such as a potassium hydroxide solution, may be delivered to the anode of electrochemical cell 11. The resulting products may then flow from electrochemical cell 11 to a condenser 33 for separation or purification of the products.

Referring now to FIG. 5, there is shown a partly exploded simplified section view of a second embodiment of an electrochemical cell constructed according to the present invention, the electrochemical cell being represented generally by reference numeral 41. Details of electrochemical cell 41 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 5 and/or from the accompanying description herein or may be shown in FIG. 5 and/or described herein in a simplified manner.

Electrochemical cell 41 may be a fuel cell or electrolytic cell but is preferably a carbon dioxide electrolyzer designed, for example, to convert carbon dioxide into carbon- containing materials, such as, but not limited to, ethylene. Electrochemical cell 41 may be similar in many respects to electrochemical cell 11 and may comprise a membrane electrode assembly 43, which may be similar or identical to membrane electrode assembly 13. To this end membrane electrode assembly 43 may comprise an ion exchange membrane 45, which may be similar or identical to ion exchange membrane 15, an anode 47, which may be similar or identical to anode 17, and a cathode 49, which may be similar or identical to cathode 19.

Electrochemical cell 41 may further comprise a first current collector 44, which may be similar or identical to first current collector 14, and a second current collector 46, which may be similar or identical to second current collector 16. First current collector 44 and second current collector 46 may be electrically coupled to a voltage source (not shown). In addition, electrochemical cell 41 may further comprise a first endplate 48, which may be similar or identical to first endplate 18, and a second endplate 50, which may be similar or identical to second endplate 20. First endplate 48 may be positioned directly above and coupled to first current collector 44, and second endplate 50 may be positioned directly below and coupled to second current collector 46.

Electrochemical cell 41 may further comprise a first bipolar plate 52 and a second bipolar plate 54, both of which may be conventional. First bipolar plate 52 may be positioned between, and in direct contact with each of, first current collector 44 and anode 47 of membrane electrode assembly 43. Second bipolar plate 54 may be positioned between, and in direct contact with each of, second current collector 46 and cathode 49 of membrane electrode assembly 43. Electrochemical cell 41 may further comprise a first gasket 56 and a second gasket 58, both of which may be conventional. First gasket 56 may be positioned on the anode side of membrane electrode assembly 43, and second gasket 58 may be positioned on the cathode side of membrane electrode assembly 43.

Electrochemical cell 41 may be used in a fashion analogous to that for electrochemical cell 11.

Referring now to FIG. 6, there is shown a simplified section view of an alternative membrane electrode assembly to membrane electrode assembly 13, the alternative membrane electrode assembly being constructed according to the present invention and being represented generally by reference numeral 71. Details of membrane electrode assembly 71 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 6 and/or from the accompanying description herein or may be shown in FIG. 6 and/or described herein in a simplified manner.

Membrane electrode assembly 71 may be similar in certain respects to membrane electrode assembly 13. To this end, membrane electrode assembly 71 may comprise an ion exchange membrane 75, which may be similar or identical to ion exchange membrane 15, an anode 77, which may be similar or identical to anode 17, and a cathode 79, which may he similar or identical to cathode 19.

On the other hand, whereas membrane electrode assembly 13 is designed for use in a zero-gap electrolyzer, membrane electrode assembly 71 is designed for use in a flow electrolyzer. Accordingly, membrane electrode assembly 71 may further comprise an anode buffer layer 81 and a cathode buffer layer 83, each of which may be used to flow electrolyte therethrough. Anode buffer layer 81, which may be conventional, may be positioned between and in direct contact with each of ion exchange membrane 75 and anode 77, and cathode buffer layer 83, which may be conventional, may be positioned between and in direct contact with each of ion exchange membrane 75 and cathode 79.

Gas diffusion layer 21, which may be used in the construction of cathode 19, cathode 49, and cathode 79, may be made by a number of different techniques. Some of these techniques are discussed below.

For example, referring now to FIG. 7, there is shown a flowchart depicting a first embodiment of a method according to the present invention for fabricating a gas diffusion layer like gas diffusion layer 21, the method being represented generally by reference numeral 101. Details of method 101 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 7 and/or from the accompanying description herein or may be shown in FIG. 7 and/or described herein in a simplified manner.

Method 101 may comprise a step 103 of providing a slurry or ink comprising non- conductive hydrophobic polymer particles. Such a slurry/ink may comprise one or more types of non-conductive hydrophobic polymer particles dispersed in an organic solvent. Examples of non-conductive hydrophobic polymers that can be used for such particles may include ether polymers, acrylic polymers, fluorocarbon polymers, polystyrene polymers, poly(vinyl chloride) polymers, and poly(N-vinylpyrrolidone) (PVP) polymers. The particle size may range from about 1 nm to about 1 mm. If the non-conductive hydrophobic polymer is in fiber or rod form, the diameter of the fiber or rod may range from about 1 nm to about 1 mm whereas the length may range from the nanometer to centimeter scale. Where the polymer particle size is beyond 10 um, the particle, itself, may include internal pores. A pore former and lubricant agent may be applied.

Method 101 may further comprise a step 105 of providing a porous electron- conductive substrate. (It is to be understood that, although step 105 is shown in the present embodiment as occurring after step 103, step 105 may occur before step 103 or concurrently with step 103.) The porous electron -conductive substrate may comprise a material, such as, but not limited to, carbon, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, and brass. The porous electron-conductive substrate may be in the form of a structure, such as, but not limited to, paper, foam, felt, sinter, cloth, and mesh. The thickness of the porous electron-conductive substrate may range from about 0.5 pm to about 2 mm and may be, for example, 1 pm, 90 pm, 200 pm, 300 pm, and 1 mm. A hydrophobic treatment may be applied to the surface of the porous electron-conductive substrate to increase the water breakthrough pressure. In addition to, or independently of, the foregoing, a hydrophilic treatment may be applied to some area of the porous electron- conductive substrate to facilitate water flow along the substrate material.

Method 101 may further comprise a step 107 of coating the slurry/ink onto the porous electron-conductive substrate using a slot-die method.

Method 101 may further comprise a step 109 of drawing the slurry/ink into the pores of the porous electron-conductive substrate using a strong vacuum. (A vacuum filtration method may alternatively be used to draw the slurry/ink into the pores of the porous electron- conductive substrate when preparing the gas diffusion layer on a small scale.)

Method 101 may further comprise a step 111 of removing any excess slurry/ink from the top surface of the porous electron-conductive substrate. The thickness of the porous hydrophobic polymer domain formed by the slurry /ink may range from 5 to 100% of the thickness of the porous electron-conductive substrate. The mass of the hydrophobic polymer versus the mass of the porous electron-conductive substrate may range from about 30 wt% to about 600 wt%.

Method 101 may further comprise a step 113 of sintering the slurry/ink within the porous electron-conductive substrate and evaporating the solvent in the slurry/ink, whereby spaces are left in the pores of the electron-conductive substrate for gas transport. The sintering temperature is preferably about 0°C to about 50°C below the melting point of the hydrophobic polymer, more preferably about 10°C to about 30°C below the melting point of the hydrophobic polymer. A pressure of about 0 to about 5000 lbs. may be applied.

Referring now to FIG. 8, there is schematically shown a gas diffusion layer being made according to method 101. More specifically, slot die 121 can be seen depositing a layer 123 of slurry/ink on top of porous electron-conductive substrate 125, and a vacuum force is being used to draw layer 123 into the pores (not shown) of porous electron-conductive substrate 125.

Referring now to FIG. 9, there is shown a flowchart depicting a second embodiment of a method according to the present invention for fabricating a gas diffusion layer like gas diffusion layer 21, the method being represented generally by reference numeral 151. Details of method 151 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 9 and/or from the accompanying description herein or may be shown in FIG. 9 and/or described herein in a simplified manner.

Method 151 may be similar in many respects to method 101 and, in this regard, may comprise a step 153, which may be similar or identical to step 103 of method 101, a step 155, which may be similar or identical to step 105 of method 101, a step 159, which may be similar or identical to step 109 of method 101, a step 161, which may be similar or identical to step 111 of method 101, and a step 163, which may be similar or identical to step 113 of method 101. By contrast, method 151 may differ from method 101 in that method 151 may comprise, instead of step 107 of method 101, a step 157 in which the slurry/ink may be coated onto the porous electron-conductive substrate using a doctor blade coating technique, instead of slot-die coating technique.

Referring now to FIG. 10, there is schematically shown a gas diffusion layer being made according to method 151. More specifically, doctor blade 161 can be seen being used to coat a layer 163 of slurry/ink on top of porous electron-conductive substrate 165, and a vacuum force is being used to draw layer 163 into the pores (not shown) of porous electron- conductive substrate 165.

It is to be understood that techniques similar to slot-die coating and doctor-blade coating, such as, but not limited to, screen printing, may alternatively be used to impregnate the pores of a porous electron-conductive substrate with a slurry of a non-conductive polymer.

Referring now to FIG. 11 , there is shown a flowchart depicting a third embodiment of a method according to the present invention for fabricating a gas diffusion layer like gas diffusion layer 21, the method being represented generally by reference numeral 201. Details of method 201 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 11 and/or from the accompanying description herein or may be shown in FIG. 11 and/or described herein in a simplified manner.

Method 201 may comprise a step 203 of providing an ink comprising non-conductive hydrophobic polymer particles. Such an ink may comprise one or more types of non- conductive hydrophobic polymer particles in a suitable solvent. Examples of non-conductive hydrophobic polymers that can be used for such particles may include ether polymers, acrylic polymers, fluorocarbon polymers, polystyrene polymers, poly(vinyl chloride) polymers, and poly(N-vinylpyrrolidone) (PVP) polymers. The particle size may range from about 1 nm to about 1 mm. If the non-conductive hydrophobic polymer is in fiber or rod form, the diameter of the fiber or rod may range from about 1 nm to about 1 mm whereas the length may range from the nanometer to centimeter scale. One or more binders may also be included in the ink.

Method 201 may further comprise a step 205 of providing a porous electron- conductive substrate. (It is to be understood that, although step 205 is shown in the present embodiment as occurring after step 203, step 205 may occur before step 203 or concurrently with step 203.) The porous electron-conductive substrate may comprise a material, such as, but not limited to, carbon, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, and brass. The porous electron-conductive substrate may be in the form of a structure, such as, but not limited to, paper, foam, felt, sinter, cloth, and mesh. The thickness of the porous electron-conductive substrate may range from about 0.5 pm to about 2 mm and may be, for example, 1 pm, 90 pm, 200 pm, 300 pm, and 1 mm. A hydrophobic treatment may be applied to the surface of the porous electron-conductive substrate to increase the water breakthrough pressure. In addition to, or independently of, the foregoing, a hydrophilic treatment may be applied to some area of the porous electron- conductive substrate to facilitate water flow along the substrate material. Method 201 may further comprise a step 207 of spraying the ink into the pores of the porous electron-conductive substrate using an airbrush printing method. Strong gas pressure may be used for the aforementioned airbrush printing method. Such gas pressure may range from about 0.5 bar to about 15 bar, depending on the strength of the porous electron- conductive substrate, and, in some instances, may range from about 2 bar to about 6 bar. In addition, a vacuum may also be applied in a step 209. The mass of the hydrophobic polymer applied versus the mass of the porous electron-conductive substrate may range from about 50 wt% to about 600 wt%.

Method 201 may further comprise a step 213 of sintering the ink within the porous electron-conductive substrate and evaporating the solvent in the slurry/ink, whereby spaces are left in the pores of the electron-conductive substrate for gas transport. The sintering temperature is preferably about 0°C to about 50°C below the glass transition temperature of the hydrophobic polymer, more preferably about 10°C to about 30°C below the glass transition temperature of the hydrophobic polymer. A pressure of about 0 to about 5000 lbs. may be applied.

Referring now to FIG. 12, there is schematically shown a gas diffusion layer being made according to method 201. More specifically, air brush 221 can be seen depositing a layer 223 of ink onto porous electron-conductive substrate 225, and a vacuum force is being used to draw layer 223 into the pores (not shown) of porous electron-conductive substrate 225.

Referring now to FIG. 13, there is shown a flowchart depicting a fourth embodiment of a method according to the present invention for fabricating a gas diffusion layer like gas diffusion layer 21, the method being represented generally by reference numeral 251. Details of method 251 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 13 and/or from the accompanying description herein or may be shown in FIG. 13 and/or described herein in a simplified manner.

Method 251 may comprise a step 253 of providing one or more non-conductive hydrophobic polymers. Examples of the one or more non-conductive hydrophobic polymers may include ether polymers, acrylic polymers, fluorocarbon polymers, polystyrene polymers, poly (vinyl chloride) polymers, and poly (N- vinylpyrrolidone) (PVP) polymers.

Method 251 may further comprise a step 255 of providing one or more electron- conductive materials. (It is to be understood that, although step 255 is shown in the present embodiment as occurring after step 253, step 255 may occur before step 253 or concurrently with step 253.) The one or more electron-conductive materials may be selected from, for example, one or more of carbon, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, and brass, and may be in the form of, for example, one or more of fibers, particles, pellets, rods, and the like. The particle size of the one or more electron- conductive materials may range from about 1 nm to about 1 mm. If the electron-conductive materials are in fiber or rod form, the diameter of the fiber or rod may range from about 1 nm to about 1 mm whereas the length may range from the nanometer to centimeter scale.

Method 251 may further comprise a step 257 of mixing the one or more non- conductive hydrophobic polymers and the one or more electron-conductive materials. The mass ratio of the one or more electron-conductive materials versus the one or more non- conductive hydrophobic polymers may range from about 15% to about 300%, preferably from about 50% to about 200%.

A lubricating agent may be added so that the mixture forms a paste that can then be extruded into a sheet at a temperature beyond the melting point of the one or more non- conductive hydrophobic polymers.

Method 251 may further comprise a step 259 of extruding a sheet of the mixture formed in step 257.

Method 251 may further comprise a step 261 of uniaxially or biaxially stretching the sheet at a temperature that is below the melting point of the one or more non-conductive hydrophobic polymers, preferably about 20°C lower than the melting point of the one or more hydrophobic polymers. The stretching ratio may range between 2:1 and 40:1. The lubricating agent will be volatilized during the stretching process.

Referring now to FIG. 14, there is schematically shown a gas diffusion layer made according to method 251, the gas diffusion layer being represented generally by reference numeral 281. Details of gas diffusion layer 281 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 14 and/or from the accompanying description herein or may be shown in FIG. 14 and/or described herein in a simplified manner.

As can be seen, gas diffusion layer 281 may comprise a non-conductive hydrophobic component, which, after stretching, forms a porous hydrophobic membrane structure 283. Gas diffusion layer 281 may additionally comprise a conductive material 285 in fiber form, which spans the thickness of the aforementioned porous hydrophobic membrane structure 283. In this manner, gas may be transported through the porous hydrophobic membrane structure 283 whereas electrons may be conducted through the thickness of conductive material 285. An alternative embodiment to gas diffusion layer 281 is shown in FIG. 15 and is represented generally by reference numeral 291. Details of gas diffusion layer 291 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 15 and/or from the accompanying description herein or may be shown in FIG. 15 and/or described herein in a simplified manner.

Gas diffusion layer 291 may be similar to gas diffusion layer 281 in that gas diffusion layer 291 may comprise a porous hydrophobic membrane structure 293, which may be similar or identical to porous hydrophobic membrane structure 283 of gas diffusion layer 281. On the other hand, gas diffusion layer 291 may differ from gas diffusion layer 281 in that gas diffusion layer 291 may comprise a conductive material 295 in pellet form whereas, as noted above, gas diffusion layer 281 may comprise a conductive material 285 in fiber form.

Referring now to FIG. 16, there is shown a top view of a first alternative gas diffusion layer to gas diffusion layer 21, the first alternative gas diffusion layer being constructed according to the present invention and being represented generally by reference numeral 311. Details of gas diffusion layer 311 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 16 and/or from the accompanying description herein or may be shown in FIG. 16 and/or described herein in a simplified manner.

Gas diffusion layer 311 may comprise a first gas diffusion layer piece 313 and a second gas diffusion layer piece 315. First gas diffusion layer piece 313 and second gas diffusion layer piece 315 may be complementarily shaped to mate with one another in the same plane so as to form a complete gas diffusion layer structure. First gas diffusion layer piece 313 may comprise a porous electron-conductive gas diffusion material, such as, but not limited to, carbon paper, metal foam, metal mesh, or metal paper. Second gas diffusion layer piece 315 may comprise a porous non-conductive hydrophobic gas diffusion material, such as, but not limited to, an expanded polytetrafluoroethylene (e-PTFE) membrane, an expanded polyvinylidene fluoride (e-PVDF) membrane, or other porous hydrophobic non-conductive materials such as polymer films (e.g., fluoropolymer film, polyvinyl chloride film), meshes, woven structures, packed particles and so on. First gas diffusion layer piece 313 and second gas diffusion layer piece 315 preferably have a similar thickness. As can be seen, in the present embodiment, gas diffusion layer 311 has an interdigitated construction, with first gas diffusion layer piece 313 and second gas diffusion layer piece 315 having mating pluralities of fingers 317 and 319, respectively. Preferably, when first gas diffusion layer piece 313 and second gas diffusion layer piece 315 are assembled, the porous electron-conductive and porous non-conductive hydrophobic areas are distributed relatively evenly such that the area ratio between the electron-conductive area and the non-conductive area may vary from about 1:2 to about 2:1. In the present embodiment, first gas diffusion layer piece 313 and second gas diffusion layer piece 315 may alternate at a 1 mm interval. Based on the accuracy of manufacturing, the interval can be as low as 300 pm.

Referring now to FIG. 17, there is shown a top view of a second alternative gas diffusion layer to gas diffusion layer 21, the second alternative gas diffusion layer being constructed according to the present invention and being represented generally by reference numeral 331. Details of gas diffusion layer 331 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 17 and/or from the accompanying description herein or may be shown in FIG. 17 and/or described herein in a simplified manner.

Gas diffusion layer 331 may comprise a first gas diffusion layer piece 333. First gas diffusion layer piece 333 may comprise a porous electron-conductive gas diffusion material, such as, but not limited to, carbon paper, metal foam, metal mesh, or metal paper. One or more transverse pinholes or openings, which in the present embodiment are circular, are provided in first gas diffusion layer piece 333.

Gas diffusion layer 331 may further comprise a plurality of second gas diffusion layer pieces 335, each of which may comprise a porous non-conductive hydrophobic gas diffusion material, such as, but not limited to, an expanded polytetrafluoroethylene (e-PTFE) membrane, an expanded polyvinylidene fluoride (e-PVDF) membrane or other porous hydrophobic non-conductive materials such as polymer films (e.g., fluoropolymer film, polyvinyl chloride film), meshes, woven structures, packed particles and so on. Second gas diffusion layer pieces 335 may be appropriately sized and shaped to mate with, or fill, the transverse pinholes of first gas diffusion layer piece 333 so as to form a complete gas diffusion layer structure. First gas diffusion layer piece 333 and second gas diffusion layer pieces 335 preferably have similar thicknesses.

Referring now to FIG. 18, there is shown a simplified section view of a third alternative gas diffusion layer to gas diffusion layer 21, the third alternative gas diffusion layer being constructed according to the present invention and being represented generally by reference numeral 351. Details of gas diffusion layer 351 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 18 and/or from the accompanying description herein or may be shown in FIG. 18 and/or described herein in a simplified manner.

Gas diffusion layer 351 may comprise a porous electron-conductive substrate 353. Porous electron-conductive substrate 353 may comprise any of the porous electron- conductive materials discussed above including, but not limited to, an electron-conductive mesh, woven (e.g., cloth) or felt material, paper, or disc, a porous graphite film, a metal foam, a metal sinter, or combinations thereof.

Gas diffusion layer 351 may further comprise a microporous layer 355 positioned directly on top of porous electron-conductive substrate 353. Microporous layer 355, which may have a thickness of about 1 pm to about 200 pm, may comprise a random mixture of one or more electron-conductive materials 357 and one or more non-conductive hydrophobic polymer materials 359. Materials suitable for use as the one or more electron-conductive materials 357 and the one or more non-conductive hydrophobic polymer materials 359 may include any of the respective materials described above. The volume ratio between the one or more electron-conductive materials and the one or more non-conductive hydrophobic polymer materials may vary from about 9:1 to about 1:3. The particle size of the one or more electron -conductive materials may vary from about 10 nm to about 5 pm, and the particle size of the one or more non-conductive hydrophobic materials may vary from about 20 nm to about 200 pm.

To fabricate gas diffusion layer 351, one may prepare a slurry/ink comprising the one or more electron-conductive materials, the one or more non-conductive hydrophobic polymer materials, and one or more solvents. A binder may also be included in the slurry/ink. After the slurry/ink is uniformly mixed, the slurry/ink may be coated on top of porous electron- conductive substrate 353 by bar-coating, slot-die coating, doctor blade coating, or screen printing and then may be allowed to dry. Prior to such coating, porous electron-conductive substrate 353 may be pretreated with a hydrophobic material. In contrast with some of the techniques discussed above, the present fabricating technique does not involve applying a vacuum to draw the slurry/ink into porous electron-conductive substrate 353. Instead, the slurry/ink forms a microporous layer on top of the porous electron-conductive substrate, thereby forming a two-layer structure.

As noted above, in some instances, the non-conductive hydrophobic polymer used in the present invention may, itself, be inherently porous. The particle size of such a porous non-conductive hydrophobic polymer particle may vary from about 1 pm to about 1 mm and may have internal pores with a pore diameter that may vary from about 50 nm to about 5 pm. Preferably, the particle size of such a porous non-conductive hydrophobic polymer particle is about 100 pm and its pore diameter varies from about 200 nm to about 5 pm. Such a porous non-conductive hydrophobic polymer particle may be prepared by cryo-milling a piece of porous non-conductive hydrophobic polymer film (e.g. polytetrafluoroethylene film, polyvinylidene fluoride film, polyvinyl chloride film, etc.) into a powder.

It is to be understood that the gas diffusion electrode of the present invention is not limited to use in carbon dioxide electrolyzers and may also be suitable for use in other electrochemical energy conversion reactions involving reactants and products in multiple phases, such as, but not limited to, N2 reduction reaction, O2 reduction reaction, H ₂ oxidation reaction, O2 evolution reaction, and H2 evolution reaction.

It is also to be understood that the gas diffusion electrode of the present invention is not limited to use in a cathode and may also be used in anode. However, where the gas diffusion electrode of the present invention is used as an anode, the electron-conductive material cannot be carbon.

The following examples are given for illustrative purposes only and are not meant to be a limitation on the invention described herein or on the claims appended hereto.

Example 1:

An electrochemical carbon dioxide reduction performance test was conducted using an MEA electrolyzer comprising a gas diffusion electrode of the present invention. More specifically, the gas diffusion electrode was made by airbrushing a hydrophobic polymer slurry into a carbon-based gas diffusion layer, followed by spraying a layer of copper to form a catalyst layer. The gas diffusion electrode was then assembled with a VERSOGEN anion exchange membrane (Versogen, Newark, DE) and a stainless-steel paper-based Ni-Fe catalyst electrode to make the MEA. During electrolysis, a dry carbon dioxide stream with a flow rate of 100 seem was fed to the cathode chamber, while a 1 mL min" ¹ of 1 M KOH electrolyte was introduced to the anode chamber. The electrolyzer was tested in galvanostatic mode. A constant current density ranging from 100 to 400 mA cm" was applied to the electrolyzer while the voltage and flue gas component was monitored online. As shown in FIG. 19, the present gas diffusion electrode achieved 42% C2H4 selectivity at a total current density of 200 mA cm' ² and a cell voltage of 2.44 V.

Example 2:

A stability test was conducted under a total current density of 100 mA cm" . The test condition was the same as the one described above in Example 1. As shown in FIG. 20, the gas diffusion electrode maintained over 40% C2H4 selectivity for over 24 h without observable selectivity degradation. Moreover, the cell voltage maintained at 2.19 V for over 24 h, and no observable increase was detected.

Some additional comments, features, aspects and/or observations relating to one or more embodiments of the present invention are provided below.

• In one embodiment, the gas diffusion electrode (sometimes alternatively referred to as a water management gas diffusion electrode) may comprise (1) a gas diffusion domain comprising a hydrophobic, and non-conductive polymer, (2) an electron-conductive material network, (3) an optional, thin, porous, electron-conductive interlayer, and (4) a catalyst layer.

• The gas diffusion domain may comprise, for example, one or more of ether polymers, acrylic polymers, fluorocarbon polymers, polystyrene polymers, poly(vinyl chloride) polymers, and poly(N-vinylpyrrolidone) (PVP) polymers.

• The hydrophobic material may be in the form of one or more of a fiber, a particle, a pellet, a rod, and so on.

• When the hydrophobic material is in the form of a particle, the particle size may range from about 1 nm to about 1 mm, with a preferred particle size ranging from about 5 nm to about 200 nm.

• When the hydrophobic material is in the form of a fiber or a rod, its diameter may range from about 1 nm to about 1 mm whereas its length may range from nanometer to centimeter scale. A preferred diameter may range from about 5 nm to about 30 pm.

• The thickness of the porous, hydrophobic, polymer domain may range from about 5% to about 100% of the electron-conductive material network thickness, with a preferred range for the thickness of the porous, hydrophobic, polymer domain being from about 50% to about 100% of the electron-conductive material network thickness.

• When the electron-conductive material is carbon, the mass of the hydrophobic polymer versus the mass of the electron-conductive material network may range from about 30 wt% to about 600 wt%, with a preferred range being from about 200 wt% to about 400 wt%.

• The electron-conductive material network may comprise one or more of carbon, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, and brass.

•The electron-conductive material network, which is porous, may be in the form of one or more materials like paper, foam, felt, sinter, cloth, mesh, and so on.

• The electron-conductive material network may be coated with one or more hydrophobic polymers with a loading ranging from about 0.1 wt% to about 40 wt%, with a preferred range being from about 5 wt% to about 30 wt%. • The electron-conductive material network may be coated with one or more hydrophilic polymers with a loading ranging from about 0.1 wt% to about 40 wt%, with a preferred range being from about 5 wt% to about 30 wt%.

• The electron-conductive material network may range in thickness from about 0.5 pm to about 2 mm, with a preferred range for the thickness of the electron-conductive material network being from about 1 pm to about 200 pm.

• The thickness of the electron-conductive material network may be equal to or greater than the thickness of the porous hydrophobic gas diffusion domain.

• The thickness of the optional interlayer, when present, may range from about 5 nm to about 30 pm, with a preferred range for the optional interlayer, when present, being from about 5 nm to about 100 nm.

• The optional interlayer may comprise one or more of carbon, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, and brass.

• The optional interlayer may be coated with one or more hydrophobic materials with a weight ratio of about 5 wt% to about 40 wt%, with a preferred range being from about 5 wt% to about 30 wt%.

• The optional interlayer may comprise a mixture of electron -conductive material particles and hydrophobic polymer particles. The mass ratio of the hydrophobic polymer particles to electron-conductive material particles may range from about 5 wt% to about 200 wt%.

• The catalyst layer may comprise one or more of copper, silver, gold, carbon, bismuth, iron, boron, nitrogen, oxygen, fluorine, aluminum, silicon, phosphorus, sulfur, chromium, manganese, cobalt, nickel, zinc, gallium, selenium, molybdenum, ruthenium, rhodium, palladium, iridium, and platinum.

• When using a screen printing, doctor blade, slot-die, vacuum filtration, or airbrush technique to fabricate the water management gas diffusion electrode, a hydrophobic material slurry may comprise one or more hydrophobic materials, water, one or more organic solvents, and one or more binders.

• When using an airbrush technique to fabricate the water management gas diffusion electrode, the airbrush pressure may range from about 0.5 bar to about 15 bar, with a preferred pressure ranging from about 2 bar to about 6 bar.

• When fabricating the water management gas diffusion electrode, the hydrophobic polymer may be sintered after being loaded into the electron-conductive material network. • The sinter temperature may be around the glass transition temperature of the hydrophobic polymer, with a preferred range being about 0°C to about 50°C below the glass transition temperature of the hydrophobic polymer, and more preferably about 10°C to about 30°C below the glass transition temperature of the hydrophobic polymer.

• The sinter pressure may range from about 0 lb. to about 5000 lbs.

• The interlayer may be loaded by one or more techniques including spraying, screen printing, doctor blade, vacuum filtration, compression, and slot-die.

• The water management gas diffusion electrode may alternatively be fabricated by uniaxial or biaxial stretching of a hydrophobic polymer membrane embedded with electron- conductive material.

• When fabricating the water management gas diffusion electrode by uniaxial or biaxial stretching of a hydrophobic polymer membrane embedded with electron-conductive material, the mass ratio of electron-conductive material to hydrophobic polymer may range from about 15 wt% to about 300 wt%, with a preferred range being from about 50 wt% to about 200 wt%.

• When fabricating the water management gas diffusion electrode by uniaxial or biaxial stretching of a hydrophobic polymer membrane embedded with electron-conductive material, the electron-conductive material may be one or more of carbon, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, and brass.

• When fabricating the water management gas diffusion electrode by uniaxial or biaxial stretching of a hydrophobic polymer membrane embedded with electron-conductive material, the electron-conductive material may be in the form of one or more of a fiber, a particle, a pellet, a rod, and so on.

• When fabricating the water management gas diffusion electrode by uniaxial or biaxial stretching of a hydrophobic polymer membrane embedded with electron-conductive material, the particle size of the electron-conductive material may range from about 1 nm to about 1 mm. If the electron-conductive materials are in fiber or rod form, the diameter may range from about 1 nm to about 1 mm whereas the length may range from the nanometer scale to the centimeter scale.

• When fabricating the water management gas diffusion electrode by uniaxial or biaxial stretching of a hydrophobic polymer membrane embedded with electron-conductive material, the hydrophobic polymer membrane may be uniaxially or biaxially stretched at a temperature below the melting point of hydrophobic polymer, with a preferred temperature being the glass transition temperature of the hydrophobic polymer. • When fabricating the water management gas diffusion electrode by uniaxial or biaxial stretching of a hydrophobic polymer membrane embedded with electron-conductive material, a lubricating agent and a pore former may be added.

• When fabricating the water management gas diffusion electrode by uniaxial or biaxial stretching of a hydrophobic polymer membrane embedded with electron-conductive material, the stretching ratio may range from about 2:1 to about 40:1.

• The water management gas diffusion electrode may be used as a gas diffusion electrode for a CO2-reduction reaction, an ^-reduction reaction, an Ch-reduction reaction, an Opevolution reaction, an H ₂ -evolution reaction, and an H ₂ -reduction reaction.

• The water management gas diffusion electrode may be fabricated by matching or mating together a piece of an electron-conductive substrate and a piece of a non-conductive, porous, hydrophobic substrate.

• When fabricating the water management gas diffusion electrode by matching or mating together a piece of an electron-conductive substrate and a piece of a non-conductive, porous, hydrophobic substrate, the two pieces of substrate may be cut into a patterned shape.

• When fabricating the water management gas diffusion electrode by matching or mating together a piece of an electron-conductive substrate and a piece of a non-conductive, porous, hydrophobic substrate, the distribution of the electron-conductive area and the non- conductive, hydrophobic area should be uniform.

• When fabricating the water management gas diffusion electrode by matching or mating together a piece of an electron-conductive substrate and a piece of a non-conductive, porous, hydrophobic substrate, the ratio of the electron-conductive area to the non-conductive area may vary from about 1:2, respectively, to about 2:1, respectively.

• When fabricating the water management gas diffusion electrode by matching or mating together a piece of an electron-conductive substrate and a piece of a non-conductive, porous, hydrophobic substrate, the electron-conductive substrate may comprise one or more of carbon paper, metal foam, metal mesh, and metal paper.

• When fabricating the water management gas diffusion electrode by matching or mating together a piece of an electron-conductive substrate and a piece of a non-conductive, porous, hydrophobic substrate, the non-conductive porous hydrophobic substrate may comprise one or more of a fluoropolymer film and a PVC film.

• When fabricating the water management gas diffusion electrode by matching or mating together a piece of an electron-conductive substrate and a piece of a non-conductive, porous, hydrophobic substrate, the electron-conductive and non-conductive materials preferably have similar thicknesses.

• The water management gas diffusion electrode may alternatively be fabricated by cocoating a random mixture comprising (i) hydrophobic, non-conductive particles/materials; and (ii) electron-conductive particles/materials on top of an electron-conductive substrate.

• When fabricating the water management gas diffusion electrode by co-coating a random mixture comprising hydrophobic, non-conductive particles/materials and electron-conductive particles/materials on top of an electron-conductive substrate, the volume ratio of the electron-conductive particles to the non-conductive hydrophobic particles may vary from about 9:1, respectively, to about 1:3, respectively.

• The thickness of the random mixture layer comprising hydrophobic, non-conductive particles/materials and electron-conductive particles/materials may vary from about 1 pm to about 200 pm.

• The particle size of the electron-conductive materials in the random mixture layer comprising hydrophobic, non-conductive particles/materials and electron-conductive particles/materials may vary from about 10 nm to about 5 pm.

• The particle size of the non-conductive hydrophobic materials in the random mixture layer comprising hydrophobic, non-conductive particles/materials and electron-conductive particles/materials may vary from about 20 nm to about 200 pm.

• When fabricating a water management gas diffusion electrode by co-coating a random mixture comprising hydrophobic, non-conductive particles/materials and electron-conductive particles/materials on top of an electron-conductive substrate, techniques including one or more of doctor blade, slot-die, bar-coating, and screen printing may be used to apply the random mixture layer.

• When fabricating a water management gas diffusion electrode by co-coating a random mixture comprising hydrophobic, non-conductive particles/materials and electron-conductive particles/materials on top of an electron-conductive substrate, the substrate may be pre-coated with one or more hydrophobic materials with a weight percentage of about 0.1 wt% to about 40 wt%.

• With respect to the various water management gas diffusion electrodes discussed above, the hydrophobic particle/material, itself, may be porous.

• The particle size of the aforementioned porous hydrophobic particle may vary from about 1 pm to about 1 mm. Such a particle may have internal pores, with a pore diameter that may vary from about 50 nm to about 5 pm. • The aforementioned porous hydrophobic particle may be made by cryo-milling a bulk quantity of porous material into powder.

• Materials that may be used for the porous hydrophobic particles may include polytetrafluoroethylene (PTFE), poly vinylidene fluoride (PVDF), and polyvinyl chloride (PVC).

The embodiments of the present invention described above are intended to be merely exemplary and those skilled in the art shall be able to make numerous variations and modifications to it without departing from the spirit of the present invention. All such variations and modifications are intended to be within the scope of the present invention.