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. 63/352,040, inventors Tianyu Zhang et al., filed June 14, 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 carbon dioxide electrolyzers 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 controlling 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 where the electrochemical reduction of carbon dioxide actually occurs whereas the gas diffusion layer, which is typically a porous structure, functions as the mass transport pathway for supplying the carbon dioxide reactant to the catalyst layer. In many cases, the gas diffusion layer is made of a carbon material in order to provide a high electron conductivity, which is necessary for the electrochemical reaction to proceed. Because the carbon material typically tends to be hydrophilic, which, as discussed below, is undesirable, the surface of the carbon material that is used to make the gas diffusion layer is commonly coated with a fluorinated material to increase the hydrophobicity and gas transport efficiency of 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 conventional 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) conventional carbon-based gas diffusion layers gradually lose hydrophobicity over time, thereby becoming deficient in preventing flooding of the cathode catalyst layer with water; (2) 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 (3) 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 channel of the gas diffusion layer, thereby hindering the diffusion of carbon dioxide towards the catalyst layer.

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. 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 a porous hydrophobic structure having a pair of opposing sides, the gas diffusion layer comprising a set of one or more pores designed for gas transport through the gas diffusion layer and a set of one or more openings designed for water drainage through the gas diffusion layer, wherein the one or more pores are comparatively smaller in size and the one or more openings are comparatively larger in size; and (b) a catalyst layer, the catalyst layer being disposed on one of the pair of opposing sides.

In a more detailed feature of the invention, the catalyst layer may not cover the one or more openings of the gas diffusion layer.

In a more detailed feature of the invention, the gas diffusion layer may be electron conductive.

In a more detailed feature of the invention, the gas diffusion layer may comprise a material selected from the group consisting of a metal or carbon mesh, woven or felt material, paper, or disc, a porous graphite film, a metal foam, a metal sinter, and combinations thereof.

In a more detailed feature of the invention, the gas diffusion layer may further comprise a hydrophobic coating.

In a more detailed feature of the invention, the gas diffusion layer may be electron non-conductive, and the gas diffusion electrode may further comprise an electron conductive layer.

In a more detailed feature of the invention, the gas diffusion layer may comprise one of a hydrophobic porous polymer membrane, a hydrophobic polymer-woven material or paper, and combinations thereof. In a more detailed feature of the invention, the gas diffusion layer may comprise one of a porous ePTFE (expanded polytetrafluoroethylene) membrane and a porous PVDF (polyvinylidene difluoride) membrane.

In a more detailed feature of the invention, the electron conductive layer may be a porous structure made of at least one of carbon and a metal.

In a more detailed feature of the invention, each opening of the set of one or more openings may extend entirely through the gas diffusion layer in a direct fashion from one of the opposing sides to the other of the opposing sides.

In a more detailed feature of the invention, the set of one or more openings may comprise a plurality of openings that are uniformly arranged.

In a more detailed feature of the invention, the set of one or more openings may comprise a plurality of openings that are randomly arranged.

In a more detailed feature of the invention, the set of one or more openings may comprise a plurality of regularly-shaped openings.

In a more detailed feature of the invention, the regularly-shaped openings may be circular openings.

In a more detailed feature of the invention, the circular openings may have a diameter that is larger than about 30 pm.

In a more detailed feature of the invention, the regularly-shaped openings may be square openings.

In a more detailed feature of the invention, the regularly-shaped openings may be rectangular slits.

In a more detailed feature of the invention, the rectangular slits may be intersecting lines arranged in a rectangular grid pattern.

In a more detailed feature of the invention, the rectangular slits may be nonintersecting lines arranged in a rectangular grid pattern.

In a more detailed feature of the invention, the set of one or more openings may comprise a plurality of irregularly- shaped openings.

In a more detailed feature of the invention, the irregularly- shaped openings may be slits.

In a more detailed feature of the invention, the set of one or more openings may comprise a plurality of slits, and the slits may have a length that exceeds about 1 millimeter.

In a more detailed feature of the invention, the gas diffusion layer may consist of a single piece of material. In a more detailed feature of the invention, the gas diffusion layer may comprise a plurality of distinct pieces separated from one another by spaces, and the spaces may form the one or more openings of the gas diffusion layer.

In a more detailed feature of the invention, the plurality of distinct pieces may be arranged uniformly in a single plane in a rectangular grid pattern.

In a more detailed feature of the invention, the plurality of distinct pieces may be arranged randomly in a single plane.

According to another aspect of the present invention, there is provided a novel membrane electrode assembly suitable for use in a carbon dioxide electrolyzer, the membrane electrode assembly comprising (a) an ion exchange membrane, the ion exchange membrane having an anode side and a cathode side; (b) an anode coupled to the anode side of the ion exchange membrane; and (c) the above-described gas diffusion electrode coupled to the cathode side of the ion exchange membrane.

In a more detailed feature of the invention, the gas diffusion layer may be electron conductive, and the membrane electrode assembly may further comprise a water barrier layer positioned between the catalyst layer and the ion exchange membrane.

In a more detailed feature of the invention, the water barrier layer may be a porous structure comprising one or more materials selected from the group of hydrophobic polymer particles, porous hydrophobic polymer particles, hydrophobic polymer fibers, and hydrophobic polymer pellets.

In a more detailed feature of the invention, the water barrier layer may have a thickness in a range of about 200 nm to about 1 mm.

In a more detailed feature of the invention, the gas diffusion layer may be electron conductive, and the membrane electrode assembly may further comprise a water capillary membrane positioned between the catalyst layer and the ion exchange membrane.

In a more detailed feature of the invention, the water capillary membrane may be a porous and hydrophilic structure having a thickness in a range of about 30 micrometers to about 1 millimeter.

In a more detailed feature of the invention, the membrane electrode assembly may further comprise a water barrier layer positioned between the water capillary membrane and the catalyst layer.

In a more detailed feature of the invention, the gas diffusion layer may be electron non-conductive, and the membrane electrode assembly may further comprise an electron conductive layer positioned between the catalyst layer and the ion exchange membrane. In a more detailed feature of the invention, the electron conductive layer may be a porous structure comprising one or more materials selected from the group of carbon, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, brass, and a metal alloy.

In a more detailed feature of the invention, the electron conductive layer may have a thickness in a range from about 30 micrometers to about 2 millimeters.

In a more detailed feature of the invention, the membrane electrode assembly may further comprise a water capillary membrane positioned between the ion exchange membrane and the electron conductive layer.

According to yet another aspect of the invention, there is provided a carbon dioxide electrolyzer, the carbon dioxide electrolyzer comprising (a) the membrane electrode assembly as described above; and (b) a voltage source, the voltage source operatively coupled to the membrane electrode assembly.

The present invention is also directed at novel methods for fabricating a gas diffusion electrode, a membrane electrode assembly, and a carbon dioxide electrolyzer.

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 one embodiment of an electrochemical cell constructed according to the present invention;

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

FIGS. 3A and 3B are top and bottom views, respectively, of the gas diffusion layer shown in FIG. 2;

FIG. 4 is a schematic diagram illustrating the mass transport mechanism in the membrane electrode assembly of FIG. 2, the anode of the membrane electrode assembly not being shown for simplicity;

FIGS. 5A through 5G are top views of alternative gas diffusion layers to the gas diffusion layer of FIG. 3A;

FIGS. 6 A and 6B are top views of two additional alternative gas diffusion layers to the gas diffusion layer of FIG. 3A;

FIGS. 7 A and 7B are top views of yet another two alternative gas diffusion layers to the gas diffusion layer of FIG. 3A;

FIG. 8 is a top perspective view of an alternative gas diffusion electrode to the gas diffusion electrode of FIG. 2, the alternative gas diffusion electrode comprising the gas diffusion layer of FIG. 7A;

FIG. 9 is a simplified fragmentary section view of a first alternative membrane electrode assembly to that shown in FIG. 2;

FIG. 10 is a schematic diagram illustrating the mass transport mechanism in the membrane electrode assembly of FIG. 9, the anode of the membrane electrode assembly and the intrinsic pores of the gas diffusion layer not being shown for simplicity;

FIG. 11 is a simplified fragmentary section view of a second alternative membrane electrode assembly to that shown in FIG. 2; FIG. 12 is a schematic diagram illustrating the mass transport mechanism in the membrane electrode assembly of FIG. 11, the anode of the membrane electrode assembly and the intrinsic pores of the gas diffusion layer not being shown for simplicity;

FIG. 13 is a simplified fragmentary section view of a third alternative membrane electrode assembly to that shown in FIG. 2;

FIG. 14 is a schematic diagram illustrating the mass transport mechanism in the membrane electrode assembly of FIG. 13, the anode of the membrane electrode assembly and the intrinsic pores of the gas diffusion layer not being shown for simplicity;

FIG. 15 is a graph depicting the stability test performance for the gas diffusion electrode of Example 1 ; and

FIG. 16 is a graph depicting the stability test performance for the gas diffusion electrode of 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, in at least some embodiments, the gas diffusion electrode of the present invention may be designed as a pressure relief matrix electrode. More specifically, in at least one embodiment, the gas diffusion electrode may comprise a gas diffusion layer and a catalyst layer, wherein the gas diffusion layer may be a porous structure that additionally includes one or more openings or channels that extend perpendicularly through the entire thickness of the gas diffusion layer in a straight-line fashion, thereby creating specific areas to drain water from an adjacent portion of the catalyst layer and, thus, providing water pressure relief in the adjacent areas of the catalyst layer. In at least one embodiment, the gas diffusion layer may comprise an electron-conductive material, whereas, in at least one embodiment, the gas diffusion layer may comprise an electron non-conductive material. 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 carbon dioxide electrolyzer, that includes such a membrane electrode assembly.

Referring now to FIG. 1, there is shown a simplified side view of one 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 is preferably an electrolytic cell and more preferably is a carbon dioxide electrolyzer, which may be 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 13 may comprise an ion exchange membrane 15, an anode 17 positioned on one side of ion exchange membrane 15, and a cathode 19 positioned on the opposite side of 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. 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.

Cathode 19 may comprise a gas diffusion electrode and, thus, may comprise a gas diffusion layer 21 and a catalyst layer 23. (Catalyst layer 23, which may be conventional, may be deposited or otherwise positioned directly on top of gas diffusion layer 21 in the conventional manner. For example, catalyst layer 23 may be a layer of catalyst particles coated on top of gas diffusion layer 21 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 gas diffusion layer 21.) Gas diffusion layer 21, which is also shown separately in FIGS. 3A and 3B, may comprise an electron- conductive gas diffusion layer and, as will be discussed further below, may be similar in certain respects to any type of electron-conductive gas diffusion layer, whether conventional or otherwise. Examples of materials suitable for use in making gas diffusion layer 21 may comprise, for example, a porous electron-conductive material, such as a metal or carbon mesh, woven or felt material, paper, or disc, a porous graphite film, a metal foam, a metal sinter, or combinations thereof. As such, gas diffusion layer 21 may include a plurality of intrinsic pores or channels 26 for gas transport therethrough, such as transport of carbon dioxide upwardly through gas diffusion layer 21 to catalyst layer 23. (For simplicity, the intrinsic pores or channels 26 of gas diffusion layer 21 are not shown in FIG. 2.) In the present embodiment, gas diffusion layer 21 may be a unitary member consisting of a single piece of material; however, it is to be understood that, alternatively, gas diffusion layer 21 could be made by joining together a plurality of separate pieces of material to form a coherent structure.

Where gas diffusion layer 21 is made of carbon and/or metal, gas diffusion layer 21 may be of limited hydrophobicity. Such limited hydrophobicity is undesirable as it may cause and/or exacerbate one or more of the problems discussed above. To improve the hydrophobicity of gas diffusion layer 21, gas diffusion layer 21 may be coated or treated in the conventional manner with one or more hydrophobic materials.

Gas diffusion layer 21 may differ from conventional electron-conductive gas diffusion layers in that gas diffusion layer 21 may additionally be provided with one or more openings (or channels) 27 that extend through the entire thickness of gas diffusion layer 21, preferably perpendicularly through gas diffusion layer 21 in a direct (i.e., straight-line) fashion from a top surface 28-1 of gas diffusion layer 21 to a bottom surface 28-29 of gas diffusion layer 21. In the present embodiment, openings 27 may be in the shape of circular holes and may be uniformly arranged throughout gas diffusion layer 21 in a linear pattern; however, as will be discussed further below, openings 27 need not be in the form of circular holes and/or need not be arranged in a linear or uniform pattern. In any event, in contrast with conventional gas diffusion electrodes, gas diffusion layer 21 may be constructed so that the reaction zone of catalyst layer 23 may effectively be divided by openings 27 into a plurality of sub-reaction zones, with an opening 27 being provided adjacent to one or more of said sub-reaction zones. In this manner, by providing a plurality of openings 27 in gas diffusion layer 21, water may be drained from the adjacent sub-reaction zones. As such, openings 27 may serve as water drainage or pressure release channels. More specifically, the water breakthrough pressure at openings 27 is low; consequently, due to openings 27, water pressure does not build up in the nearby sub-reaction zones of catalyst layer 23.

It is to be understood that the number, size, and distribution of openings 27 shown in FIGS. 3A and 3B is merely illustrative and that a greater or lesser number of openings 27 of greater or lesser size may be provided in any sort of arrangement.

Openings 27 may be formed in gas diffusion layer 21 by a variety of cutting techniques, such as, but not limited to, shape die cutting and laser cutting. Openings 27 may vary in size from the nanometer scale to the millimeter scale; preferably, openings 27 are larger in diameter than the intrinsic pores 26 of gas diffusion layer 21. Where, as in the present embodiment, openings 27 are circular or similar in shape, openings 27 preferably have a diameter that is larger than about 30 pm. By contrast, where openings 27 have a slit shape, the width of openings 27 can be down to the nanometer scale, but the length should be longer than about 1 millimeter.

In addition, openings 27 may be fewer in number than the intrinsic pores 26 of gas diffusion layer. In fact, in theory, there may be as few as one opening 27. The size of intrinsic pores 26 may be no more than 10 pm in diameter. Additionally, the porosity of gas diffusion layer 21 attributable to intrinsic pores 26 may be more than 50%.

Referring back now to FIG. 2, membrane electrode assembly 13 may further comprise a water barrier layer 29, wherein water barrier layer 29 may be positioned between ion exchange membrane 15 and cathode 19 and, more specifically, may be applied directly to the top surface of catalyst layer 23 (i.e., the surface of catalyst layer 23 that is opposite gas diffusion layer 21). Water barrier layer 29 may consist of or comprise one or more porous materials possessing a high degree of hydrophobicity. Water barrier layer 29 may have a thickness that varies from the nanometer scale to hundreds of micrometers (e.g., about 200 nm to about 1 mm), depending on its porosity and hydrophobicity. The hydrophobicity of water barrier layer 29 may come from the properties of the material used to make water barrier layer 29 or from a hydrophobic coating applied to the material used to make water barrier layer 29. Water barrier layer 29 may comprise one or more of hydrophobic polymer particles, porous hydrophobic polymer particles, hydrophobic polymer fibers, and hydrophobic polymer pellets. For example, water barrier layer 29 may be made by coating a layer of 100 nm PTFE particles on catalyst layer 23. The function of water barrier layer 29 is to withstand part of the water pressure in the ion exchange membrane 15 and to reduce the quantity of water going into catalyst layer 23. Notwithstanding the above, the inclusion of water barrier layer 29 in membrane electrode assembly 13 is optional. While providing some resistance to water diffusion, water barrier layer 29 may still allow some water to pass therethrough.

Catalyst layer 23 and water barrier layer 29 are preferably absent in the areas above openings 27 of gas diffusion layer 21. By contrast, catalyst layer 23 and water barrier layer 29 preferably are present in the remaining areas above gas diffusion layer 21.

It is to be understood that, although each of ion exchange membrane 15, anode 17, gas diffusion layer 21, catalyst layer 23, and water barrier layer 29 is shown in FIG. 2 as a monolithic layer of material, one or more of said layers may comprise a composite layer or laminate material comprising a plurality of distinct sublayers.

Referring now to FIG. 4, there is schematically shown the mass transport mechanism of membrane electrode assembly 13. As can be seen, carbon dioxide may flow upwardly through gas diffusion layer 21 via its intrinsic pores, which pores are not shown in FIG. 4 for simplicity, to catalyst layer 23. In addition, electrons may be conducted upwardly via the conductive structural framework of gas diffusion layer 21 to catalyst layer 23. Much of the water in ion exchange membrane 15 may be kept from passing from ion exchange membrane 15 to catalyst 23 by water barrier layer 29; instead, such water may be diverted to areas aligned with openings 27, and such water may drain through openings 27. In addition, at least some of the water that passes through water barrier layer 29 into catalyst layer 23 may drain into openings 27.

Referring now to FIGS. 5 A through 5G, there are shown top views of alternative gas diffusion layers that may be used in place of gas diffusion layer 21 to form cathode 19 of membrane electrode assembly 13. For simplicity, details of such gas diffusion layers that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from one or more of FIGS. 5A through 5G and/or from the accompanying description herein or may be shown in one or more of FIGS. 5A through 5G and/or described herein in a simplified manner. More specifically, in FIG. 5A, there is shown a gas diffusion layer 51. Gas diffusion layer 51 may differ from gas diffusion layer 21 in that, whereas gas diffusion layer 21 may include openings (or channels) 27 in the shape of circular holes that are uniformly arranged in a linear pattern, gas diffusion layer 51 may, instead, include openings (or channels) 53 in the shape of square holes that are uniformly arranged in a linear pattern. In FIG. 5B, there is shown a gas diffusion layer 57. Gas diffusion layer 57 may differ from gas diffusion layer 21 in that, whereas gas diffusion layer 21 may include openings (or channels) 27 in the shape of circular holes that are uniformly arranged in a linear pattern, gas diffusion layer 57 may, instead, include openings (or channels) 59 in the shape of irregularly-shaped holes that are uniformly arranged in a linear pattern. In FIG. 5C, there is shown a gas diffusion layer 61. Gas diffusion layer 61 may differ from gas diffusion layer 21 in that, whereas gas diffusion layer 21 may include openings (or channels) 27 in the shape of circular holes that are uniformly arranged in a linear pattern, gas diffusion layer 61 may, instead, include openings (or channels) 63 in the shape of irregularly-shaped slits that are generally uniformly arranged in a linear pattern. In FIG. 5D, there is shown a gas diffusion layer 67. Gas diffusion layer 67 may differ from gas diffusion layer 21 in that, whereas gas diffusion layer 21 may include openings (or channels) 27 in the shape of circular holes that are uniformly arranged in a linear pattern, gas diffusion layer 67 may, instead, include openings (or channels) 69 in the shape of circular holes that are randomly arranged. In FIG. 5E, there is shown a gas diffusion layer 71. Gas diffusion layer 71 may differ from gas diffusion layer 21 in that, whereas gas diffusion layer 21 may include openings (or channels) 27 in the shape of circular holes that are uniformly arranged in a linear pattern, gas diffusion layer 71 may, instead, include openings (or channels) 73 in the shape of square holes that are randomly arranged. In FIG. 5F, there is shown a gas diffusion layer 77. Gas diffusion layer 77 may differ from gas diffusion layer 21 in that, whereas gas diffusion layer 21 may include openings (or channels) 27 in the shape of circular holes that are uniformly arranged in a linear pattern, gas diffusion layer 77 may, instead, include openings (or channels) 79 in the shape of irregularly-shaped holes that are randomly arranged. In FIG. 5G, there is shown a gas diffusion layer 81. Gas diffusion layer 81 may differ from gas diffusion layer 21 in that, whereas gas diffusion layer 21 may include openings (or channels) 27 in the shape of circular holes that are uniformly arranged in a linear pattern, gas diffusion layer 81 may, instead, include openings (or channels) 83 in the shape of irregularly-shaped slits that are randomly arranged.

It is to be understood that, although the water drainage or pressure-relief openings or channels of each of gas diffusion layers 51, 57, 61, 67, 71, 77 and 81 are similar or identical in size and shape to the other water drainage or pressure-relief openings or channels of the same gas diffusion layer, this need not be the case as a single gas diffusion layer may include a plurality of water drainage or pressure-relief openings or channels of different sizes and/or shapes, and such water drainage or pressure-relief openings or channels may be arranged in an ordered pattern or in a random pattern.

Referring now to FIGS. 6 A and 6B, there are shown top views of two additional alternative gas diffusion layers that may be used in place of gas diffusion layer 21 in membrane electrode assembly 13. For simplicity, details of such gas diffusion layers that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from one or more of FIGS. 6 A and 6B and/or from the accompanying description herein or may be shown in one or more of FIGS. 6 A and 6B and/or described herein in a simplified manner. More specifically, in FIG. 6A, there is shown a gas diffusion layer 91. Gas diffusion layer 91 may differ from gas diffusion layer 21 in that, whereas gas diffusion layer 21 may include openings (or channels) 27 in the shape of circular holes that are uniformly arranged in a linear pattern, gas diffusion layer 91 may, instead, include openings (or channels) 93 and 94 in the shape of non-intersecting horizontal and vertical lines, respectively, openings 93 and 94 being arranged in a rectangular grid pattern. In FIG. 6B, there is shown a gas diffusion layer 97. Gas diffusion layer 97 may differ from gas diffusion layer 21 in that, whereas gas diffusion layer 21 may include openings (or channels) 27 in the shape of circular holes that are uniformly arranged in a linear pattern, gas diffusion layer 97 may, instead, include openings (or channels) 98 and 99 in the shape of intersecting horizontal and vertical lines, respectively, openings 98 and 99 being arranged in a rectangular grid pattern.

Referring now to FIGS. 7 A and 7B, there are shown top views of two additional alternative gas diffusion layers that may be used in place of gas diffusion layer 21 in membrane electrode assembly 13. For simplicity, details of such gas diffusion layers that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from one or more of FIGS. 7A and 7B and/or from the accompanying description herein or may be shown in one or more of FIGS. 7 A and 7B and/or described herein in a simplified manner. More specifically, in FIG. 7A, there is shown a gas diffusion layer 101. Gas diffusion layer 101 may comprise a plurality of gas diffusion layer pieces 103. Each of gas diffusion layer pieces 103 may be similar or identical in composition and structure to a gas diffusion layer of the electron-conductive type but may be reduced in footprint thereto. In addition, each of gas diffusion layer pieces 103 may have a generally rectangular footprint, each of gas diffusion layer pieces 103 may be similar or identical to one another in size, shape and composition, and each of gas diffusion layer pieces 103 may be arranged in a single plane in a rectangular grid pattern such that each gas diffusion layer piece 103 is spaced apart from each of its neighboring gas diffusion layer pieces 103 by a space (or channel) 105. In this manner, spaces 105 may function like openings 98 and 99 of gas diffusion layer 97 to permit water to pass through gas diffusion layer 101. Although not shown, gas diffusion layer pieces 103 may be mounted on a suitable substrate or may otherwise be fixed in space relative to one another to maintain the desired spacing between gas diffusion layer pieces 103. Also, although, in the present embodiment, gas diffusion layer pieces 103 of gas diffusion layer 101 are said to be identical to one another in size, shape, and composition, some of gas diffusion layer pieces 103 could be made to be different than other of gas diffusion layer pieces 103 in one or more of size, shape, and composition.

In FIG. 7B, there is shown a gas diffusion layer 111. Gas diffusion layer 111 may comprise a plurality of gas diffusion layer pieces 113, wherein each of gas diffusion layer pieces 113 may be similar or identical to gas diffusion layer pieces 103 of gas diffusion layer 101. Gas diffusion layer 111 may differ from gas diffusion layer 101 in that, whereas gas diffusion layer pieces 103 may be arranged in a single plane in a rectangular grid pattern, gas diffusion layer pieces 113 may be arranged in a single plane in a non-uniform or random pattern. As a result, depending on the arrangement of gas diffusion layer pieces 113, the spacing between any two given adjacent gas diffusion layer pieces 113 may be uniform or may vary, and the spacing across the entirety of gas diffusion layer 111 may be non-uniform. Although not shown, gas diffusion layer pieces 113 may be mounted on a suitable substrate or may otherwise be fixed in space relative to one another to maintain the desired spacing between gas diffusion layer pieces 113. Also, although, in the present embodiment, gas diffusion layer pieces 113 of gas diffusion layer 111 are said to be identical to one another in size, shape, and composition, some of gas diffusion layer pieces 113 could be made to be different than other of gas diffusion layer pieces 113 in one or more of size, shape, and composition.

Referring now to FIG. 8, there is shown a top perspective view of an alternative gas diffusion electrode that may be used in place of cathode 19 in membrane electrode assembly 13, the alternative gas diffusion electrode being represented generally by reference numeral 131. For simplicity, details of gas diffusion electrode 131 that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from FIG. 8 and/or from the accompanying description herein or may be shown in FIG. 8 and/or described herein in a simplified manner. In the present embodiment, gas diffusion electrode 131 may comprise a gas diffusion layer 133 and a catalyst layer 135. Gas diffusion layer 133 may be similar or identical to gas diffusion layer 101 and may comprise a plurality of identical gas diffusion layer pieces 137. Catalyst layer 135, which may be applied to the top surfaces of gas diffusion layer pieces 137, may be similar or identical in thickness and/or composition to catalyst layer 23 and may comprise a plurality of catalyst layer pieces 139. As can be appreciated, each combination of a gas diffusion layer piece 137 and its associated catalyst layer piece 139 may be regarded as a gas diffusion electrode piece 141, with adjacent gas diffusion electrode pieces 141 being separated from one another by a space (or channel) 143.

As can be appreciated, one could modify gas diffusion electrode 131 by replacing gas diffusion layer 133 with gas diffusion layer 111.

Referring now to FIG. 9, there is shown a simplified section view of a first alternative membrane electrode assembly to membrane electrode assembly 13, said first alternative membrane electrode assembly being represented generally by reference numeral 171. Details of membrane electrode assembly 171 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. For example, the intrinsic pores of gas diffusion layer 21 are not shown in FIG. 9.

Membrane electrode assembly 171 may be similar in most respects to membrane electrode assembly 13, wherein the principal difference between the two membrane electrode assemblies may be that membrane electrode assembly 171 may additionally include a water capillary membrane 173 that may be positioned between ion exchange membrane 15 and water barrier layer 29. Water capillary membrane 173, which acts as a buffer between catalyst layer 23 and ion exchange membrane 15, is preferably a porous and hydrophilic structure that facilitates water flow and removal in a horizontal direction, enabling the excess water to then circumvent the reaction zone and to avoid flooding catalyst layer 23. Water capillary membrane 173 may be, for example, a piece of cellulose fiber sheet or non-woven fiber paper. Water capillary membrane 173 also allows gas to flow in the horizontal direction. Water capillary membrane 173 may have a thickness ranging from about 30 micrometers to about 1 millimeter, depending on its porosity and hydrophilicity.

The mass transport mechanism of membrane electrode assembly 171 is schematically shown in FIG. 10.

As will be discussed further below, although the various gas diffusion layers described above are said to be electron-conductive, it is to be understood that corresponding gas diffusion layers could alternatively be made of electron non-conductive materials. However, where gas diffusion layers are made of electron non-conductive materials, a gas diffusion electrode comprising the non-conductive gas diffusion layer may further comprise an electron conductive layer deposited over the catalyst layer.

Referring now to FIG. 11, there is shown a simplified section view of a second alternative membrane electrode assembly to membrane electrode assembly 13, said second alternative membrane electrode assembly being represented generally by reference numeral 201. Details of membrane electrode assembly 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.

Membrane electrode assembly 201 may be similar in many respects to membrane electrode 13. One difference between the two membrane electrode assemblies may be that, whereas membrane electrode assembly 13 may comprise a gas diffusion layer 21 that is electron-conductive, membrane electrode assembly 201 may, instead, comprise a gas diffusion layer 203 that is not electron-conductive (i.e., electron non-conductive). Examples of materials that may be used to make gas diffusion layer 203 may include hydrophobic porous electron non-conductive materials, such as, but not limited to, hydrophobic porous polymer membranes, such as porous ePTFE membranes or porous PVDF (polyvinylidene difluoride) membranes, hydrophobic polymer-woven materials or papers, and the like. Gas diffusion layer 203 may comprise a plurality of openings 205 that may be similar to any of the various types of water drainage or pressure-relief openings or channels that are described in the present patent application. Although, in the present embodiment, gas diffusion layer 203 is a one-piece structure, it is to be understood that gas diffusion layer 203 may be made from a plurality of pieces, as in the cases of gas diffusion layers 101 and 111. For simplicity, the intrinsic pores of gas diffusion layer 203 are not shown in FIG. 11.

Another difference between membrane electrode assembly 201 and membrane electrode assembly 13 may be that membrane electrode assembly 201 need not include a water barrier layer like water barrier layer 29. This is particularly the case where gas diffusion layer 203 is made of a material, such as ePTFE, that possesses good hydrophobicity.

Still another difference between membrane electrode assembly 201 and membrane electrode assembly 13 may be that membrane electrode assembly 201 may further comprise an electron conductive layer 207. Electron conductive layer 207 may be necessary since, as noted above, gas diffusion layer is electron non-conductive. Electron conductive layer 207 may be a porous structure and may comprise, for example, one or more materials like carbon, copper, iron, stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc, titanium, brass, or a metal alloy. Materials with low catalytic activity, such as carbon, may be preferred for electron conductive layer 207. Examples of carbon materials may include carbon paper, carbon disk, and carbon foam. The thickness of electron conductive layer 207 may range from about 30 micrometers to about 2 millimeters. Conductive carbon particles or carbon blacks can be added to electron conductive layer 207 to reduce its porosity and enhance in-plane electron conductivity. The conductive carbon particles may be treated with hydrophilic polymers to make them more hydrophilic. In addition or alternatively, electron conductive layer 207 may further comprise certain amounts of hydrophilic materials or ionomers to increase its ion conductivity. On the other hand, electron conductive layer 207 may contain certain amounts of hydrophobic materials to increase the water breakthrough pressure so that electron conductive layer 207 can withstand part of the water pressure and reduce the quantity of water going into catalyst layer 23. When electrons are conducted through electron conductive layer 207, water and gas also may flow horizontally therethrough.

The combination of electron conductive layer 207, catalyst layer 23, and gas diffusion layer 203 may collectively form a cathode 209.

The mass transport mechanism of membrane electrode assembly 201 is schematically shown in FIG. 12.

Referring now to FIG. 13, there is shown a simplified section view of a third alternative membrane electrode assembly to membrane electrode assembly 13, said third alternative membrane electrode assembly being represented generally by reference numeral 301. Details of membrane electrode assembly 301 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.

Membrane electrode assembly 301 may be similar in many respects to membrane electrode 201, wherein the principal difference between the two membrane electrode assemblies may be that membrane electrode assembly 301 may further comprise a water capillary membrane 303 positioned between ion exchange membrane 15 and electron conductive layer 207. Water capillary membrane 303 may be porous and hydrophilic and may be identical to water capillary membrane 173 of membrane electrode assembly 171. Water capillary membrane 303 may facilitate water flow in a horizontal direction, thereby enabling excess water to circumvent the reaction zone and avoid flooding catalyst layer 23. Water capillary membrane 303 also may allow gas to flow in the horizontal direction. Water capillary membrane 303 may be omitted if electron conductive layer 207 is thick enough.

The mass transport mechanism of membrane electrode assembly 201 is schematically shown in FIG. 14.

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

• According to one aspect of the invention, a Pressure Relief Matrix Electrode (PRME) is provided that includes a Pressure Relief Matrix Gas Diffusion Layer (PRMGDL) serving as the gas diffusion media and a Catalyst Layer (CL) functioning as the reaction zone.

• The Pressure Relief Matrix Gas Diffusion Layer is preferably a porous and hydrophobic membrane, which can be either conductive or non-conductive. It preferably contains numerous water pressure relief channels that extend throughout its thickness.

• The diameters and lengths for the water pressure relief channels can vary from the nanometer scale to the millimeter scale. If a water pressure relief channel has a round shape, its diameter should be larger than about 30 pm. If the water pressure relief channel has a slit shape, its width can be down to nanometer scale, but its length should be longer than about 1 millimeter.

• The water pressure relief channels can have various shapes and patterns, such as linear or non-linear. A reason for incorporating multiple holes and slits in the gas diffusion layer is to create specific areas for draining water originating from the anode via membrane crossover. The water breakthrough pressure at these holes and slits is low, preventing water buildup in the catalyst layer.

• The water pressure relief channels can be periodically patterned or randomly distributed, with a preference for uniform distribution.

• Various cutting methods like shape die cutting or laser cutting can be employed to create the water pressure relief channels.

• Alternatively, the water pressure relief channels can be formed by assembling multiple small sub-gas diffusion layers together, leaving gaps between each pair of sub-gas diffusion layers.

• In the case of a non-conductive gas diffusion layer, it can be made using hydrophobic porous polymer membranes, such as porous PTFE or porous PVDF. Hydrophobic polymer-woven materials or papers can also be utilized.

• The electron conductive layer can be a piece of regular carbon paper, metal foam, metal mesh, metal woven, or metal sinter. • The gas diffusion electrode may incorporate numerous through-holes and slits that penetrate its entire thickness due to the structure of the gas diffusion layer.

• A membrane electrode assembly (MEA) design may be based on an electron- conductive pressure relief matrix (PRM) gas diffusion layer (GDL) that includes four layers and one optional layer (not including the anode). These five layers may be as follows: 1) PRM GDL, 2) cathode catalyst layer, 3) optional water barrier layer, 4) hydrophilic porous media, and 5) ion exchange membrane.

• The electron-conductive PRM GDL and the cathode catalyst layer collectively form a PRME (pressure relief matrix electrode).

• The aforementioned optional water barrier layer may be a hydrophobic polymer coating on the surface of the catalyst layer. While providing some resistance to water diffusion, this layer may still allow water to pass through.

• The thickness of the water barrier layer may vary from about 200 nanometers to about 1 millimeter, depending on its porosity and hydrophobicity.

• The water barrier layer may comprise one or more of hydrophobic polymer particles, porous hydrophobic polymer particles, hydrophobic polymer fibers, or hydrophobic polymer pellets.

• The aforementioned hydrophilic porous media may facilitate water diffusion or flow within the plane.

• The hydrophilic porous media may act as a buffer between the catalyst layer and the membrane. Its thickness may range from about 30 micrometers to about 1 millimeter, depending on the porosity and hydrophilicity.

• Any hydrophilic polymer material may be utilized to create the hydrophilic porous media.

• The ion exchange membrane may be a cation exchange membrane, an anion exchange membrane, or a bipolar membrane.

• In operation of the aforementioned membrane electrode assembly, electrons may be conducted through the electron-conductive PRM GDL, while gas reactants and products may be transported via the intrinsic PRM GDL hydrophobic channels. Water may originate from the anode and may cross the ion exchange membrane to the cathode side. The hydrophilic porous media may enable water transport within the plane. The water barrier layer may restrict water diffusion, allowing only a portion of the water to enter the catalyst layer. A significant portion of the water may escape through the water pressure relief channel in the PRM GDL. • An alternative MEA design may be based on a non-conductive PRM GDL that comprises four layers and one optional layer (not including the anode). These five layers may be as follows: 1) non-conductive PRM GDL, 2) cathode catalyst layer, 3) carbon conductive layer, 4) optional hydrophilic porous media, and 5) ion exchange membrane.

• The non-conductive PRM GDL and the cathode catalyst layer together may form the PRME.

• The aforementioned carbon conductive layer may facilitate electron conduction.

• The carbon conductive layer may be made of materials, such as carbon paper, carbon disk, or carbon foam.

• The thickness of the carbon conductive layer may range from about 30 micrometers to about 2 millimeters.

• Conductive carbon particles or carbon blacks may be added to the carbon conductive layer to reduce its porosity and enhance in-plane electron conductivity.

• The conductive carbon particles may be treated with hydrophilic polymers to make them hydrophilic, allowing water to flow or diffuse within the plane.

• The carbon conductive layer may serve the dual functions of electron conduction and water buffering, making the hydrophilic porous media optional in this design.

• The ion exchange membrane may be a cation exchange membrane, an anion exchange membrane, or a bipolar membrane.

• The working mechanism may involve the gas diffusion layer (GDL) primarily serving as the pathway for gas diffusion while electrons conduct from the electrode's edges, passing through the carbon conductive layer to reach the catalyst layer. Water transport within the carbon conductive layer may follow a similar process as described above in the hydrophilic porous media and may eventually escape through the water pressure relief channels in the PRME.

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:

A stability test was performed on a membrane electrode assembly, like that shown in Fig. 13, that included a 5 cm ² cathode including a non-conductive gas diffusion layer having water drainage or pressure relief channels in accordance with the present invention. The test involved feeding a dry CO2 stream at a flow rate of 100 standard cubic centimeters per minute (seem) to the cathode side, while supplying 1 M KOH at a flow rate of 0.3 mL/min to the anode. A current density of 300 milliamperes per square centimeter (mA cm’ ) was applied, and the cell voltage was maintained at approximately 3.25 volts for a duration of 75 hours. Throughout the test, the C2H4 selectivity remained stable at around 35%. The results of such testing are found in FIG. 15.

Example 2: A stability test was performed on a membrane electrode assembly, like that shown in

Fig. 2, that included a 5 cm cathode comprising a conductive gas diffusion layer having water drainage or pressure relief channels in accordance with the present invention. The C2H4 Faradaic efficiency improved to around 40% and remained stable over 100 hours. The test was under a current density of 300 mA cm". The results of such testing are found in FIG. 16.

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.