EP4680784A1

EP4680784A1 - Selective separators for water electrolysis applications and methods for making the same

Info

Publication number: EP4680784A1
Application number: EP24718947.5A
Authority: EP
Inventors: Murat Unlu
Original assignee: Chemours Co FC LLC
Current assignee: Chemours Co FC LLC
Priority date: 2023-03-13
Filing date: 2024-03-12
Publication date: 2026-01-21
Also published as: JP2026509522A; CN120981607A; KR20250160976A; WO2024191979A1; AU2024234845A1

Abstract

A selective separator is described that comprises a porous polymeric separator and selective material on at least one outer surface. Selective material comprising a composite of ion exchange polymer and zirconium oxide particles (ZrO2) distributed throughout the ion exchange polymer may be applied as a liquid by a spray coating method. Selective separators made by methods described herein are suitable for use in alkaline water electrolysis applications.

Description

TITLE OF THE INVENTION

SELECTIVE SEPARATORS FOR WATER ELECTROLYSIS APPLICATIONS AND METHODS FOR MAKING THE SAME

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 63/451 ,655, filed on March 13, 2023, the disclosure of which is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] Alkaline water electrolysis (AWE) plays a major role in hydrogen economy with its well understood chemistry. Improving efficiency of alkaline water electrolyzer is critical for meeting low-cost targets for green hydrogen production. High current density operation is a primary strategy to reduce the operational cost. High current density is possible by reduction in ohmic resistance that is typically achieved by use of a thinner separator, larger pore size, and/or a shorter distance between separator and electrode. However, these approaches may have unfavorable consequences in mechanical strength, gas crossover and gas blinding.

[0003] In addition to mechanical strength losses, a separator that is thinner or has larger pore sizes can result in significant gas crossover reducing coulombic efficiency and safety. Decreasing the gap between the electrode and separator to reduce resistance has been shown to cause a substantial voltage loss. Hydrogen and oxygen gases generated on the electrodes are trapped between electrode and separator interface, reducing the active membrane area for ion conduction. The reduced active area results in an increase in ohmic resistance. There have been several approaches to mitigate gas blinding effects in electrolyzers. These approaches include increasing flow by new electrode architectures, materials and hydrophobic diffusion layers. Additionally, various operational conditions, such as pressure swings, ultrasound, and magnetic fields, were examined to diminish gas stagnation on the separator surface in lab experiments. [0004] The electrodes, separators and electrolyzer design dictates the overall efficiency of electrolyzer. High pressure operation is desired in water electrolyzers to improve efficiency. The porous separator limits high pressure operation.

[0005] Conventional non-porous ion exchange membranes having relatively small narrow ion channels have been shown to be inferior to porous separators in AWE applications due to high resistance. For example, Nation™ membrane having channel sizes significantly smaller than porous separators may cause high ionic resistance. Additionally, where porous separators enable the transport of both cation and anions, cation exchange membranes allowing only the transport of cations, limit the mobility of hydroxide ions (OH-).

SUMMARY OF THE INVENTION

[0006] A selective separator for alkaline water electrolysis (AWE) as described herein, comprises a porous separator layer or structure on which a selective material is applied as a coating to form an outermost surface of the selective separator. The selective material coating comprises ion exchange polymer and inorganic particles that provide good ionic conduction, low gas permeability and improved gas release suitable for use in an AWE process.

[0007] An AWE porous selective separator may comprise a porous polyolefin separator layer and a selective material coating comprising an ion exchange polymer and inorganic particles, wherein the coating is on one or both opposing surfaces of the porous polyolefin separator. In one embodiment, a multilayer selective separator comprises a layer of porous polyethylene (PE) and a coating of a selective material comprising perfluorosulfonic acid ion exchange polymer and zirconium oxide particles forming the outermost layer of the multilayer selective separator wherein the particles are adhered to one or both opposing surfaces of the porous polyethylene. A method for making the selective separator comprises spray coating a liquid blend of the selective material to form a thin selective layer on one or both surfaces of a porous PE support. The ratio of ion exchange polymer to inorganic particles in the selective material, and the dried solids loading of selective material onto a porous polymeric separator, are optimized for properties such as gas adhesion and/or gas permeability. [0008] Novel AWE porous selective separators described herein have a significant reduction in gas permeability and gas release properties without a significant increase in ionic resistance. A significant decrease in gas release angle correlates to enhanced gas release in use. Without wishing to be bound by theory it is believed that application of selective material to the porous separator advantageously provides higher operational pressure of gases, which among other things, may result in reduced operational costs in AWE applications. A challenge in AWEs compared to PEM water electrolyzers is limited pressure range due to high gas crossover through large pores of conventional separators. Thus, pore size optimized by application of novel selective material and coating techniques, advantageously results in reduced gas crossover through the separator. Where conventional approaches to reduce gas crossover have resulted in an increase in ohmic resistance, novel porous selective separators described herein realize minimal impact on the overall ohmic resistance of the separator. Pore sizes can be selectively altered by changing the particle size, ratio of ion exchange polymer to inorganic particle, and coating technique.

BRIEF DESCRIPTION OF THE FIGURES

[0009] Figure 1 is a schematic representation of one embodiment of a highly selective multilayer separator.

[0010] Figure 2 is a top-down view of an image obtained by Scanning Electron Microscopy (SEM) of one embodiment of a selective layer on a porous separator.

[0011] Figure 3 is an SEM image of a cross-sectional view of one embodiment of a porous selective separator.

[0012] Figure 4 is an SEM image of a cross-sectional view of one embodiment of a porous selective separator.

[0013] Figure 5 is a schematic representation of one embodiment of an alkaline water electrolyzer.

DETAILED DESCRIPTION OF THE INVENTION

[0014] Novel porous selective separators, and method of making the same, are described. The selective separators are suitable for use in AWE applications providing improved gas release and gas permeation properties without significant change in ionic resistance. As illustrated in Figure 1, the selective separator (100) comprises a porous polymeric separator (101) having an outer surface (105a and/or 105b) coated with a selective material (102a and/or 102b) which becomes the outermost layer(s) (104a and 104b) of the selective separator, that provides hydrophilicity, surface roughness, controlled pore size, pore diameter, and/or percent porosity through the selective separator to increase efficiency of a porous separator in water electrolysis processes.

[0015] In an embodiment, a selective separator membrane is described that consists essentially of 1) a porous polyolefin separator as an unsupported film (101); and, 2) a coating of selective material (102a, 102b) comprising an ion exchange polymer (106) and inorganic particles (103) applied to at least one surface (105a, 105b) of the porous polyolefin separator, wherein the selective material becomes the outermost layer of the selective separator. The selective material may be applied to both first and second opposing surfaces (105a, 105b) of the porous polymeric separator. In one embodiment, the polyolefin is polyethylene (PE), and the selective material comprises an ion exchange polymer and zirconium oxide.

Porous Polymeric Separator

[0016] A porous separator suitable for use herein may comprise a polymer having no chemically reactive functional groups on the polymer structure. A porous polymer separator may comprise a polyolefin such as polyethylene (PE), including but not limited to, ultrahigh molecular weight polyethylene (LIHMWPE), low density polyethylene (LDPE) and high-density polyethylene (HDPE), or polypropylene (PP). In other embodiments, the porous polymeric separator may comprise a fluoropolymer such as polytetrafluoroethylene (PTFE) or expanded polytetrafluoroethylene (ePTFE) or polyvinylidene fluoride (PVDF), polystyrene, polysulfone, polyethersulfone or polyarylethersulfone, polyphenylenesulfide (PPS), or a combination thereof, that are suitable for use in water electrolysis applications, and may include materials under the trade name Zirfon® separator membrane (trademark of AgfaGevaert N.V.), and materials under the trade name Celgard® 5550, Celgard® 3419S and Celgard® 3420 (trademark of Celgard, LLC). [0017] Porous separators may be hydrophobic or hydrophilic as measured, for example, by contact angle method described herein. Hydrophilicity of a porous separator polymer may be increased by the addition of an additive or inorganic material, such as an ionic surfactant, or metal oxide microparticles such as silica powder, to the polymer, forming a polymer composite. Metal oxides suitable for use in optimizing hydrophobicity or hydrophilicity include zirconium oxide, titanium oxide, bismuth oxide, cerium oxide, and magnesium oxide, and combinations thereof. Other hydrophilic particles that may be used include nitrides and carbides of Group IV elements of the periodic table. Inorganic particles may have a mean particle size in the range of approximately 0.5 pm to 5 pm, or 0.5 pm to 2 pm, or 0.15 pm to 1 pm, or within the range of 0.15 pm to 0.75 pm, as determined by laser diffraction particle size analysis. A composite of a hydrophobic polymer, such as polyethylene, and metal oxide particles, such as titanium dioxide or zirconium oxide, integrated into and/or distributed throughout the polymer matrix, may be used to form a hydrophilic porous polyethylene separator.

[0018] The average pore size of a porous separator prior to application of the selective material may be approximately greater than 0.1 pm, or less than approximately 4 pm, or 0.1 pm to 4 pm, 0.1 pm to 2 pm, 0.1 pm to 1 pm, 0.5 pm to 4 pm, 0.5 pm to 3 pm, 0.5 pm to 2 pm, and pm 0.5 pm to 1 pm, measured by a capillary flow porometer. In some embodiments, a porous separator comprises a microporous layer having an average pore size in the range of, for example, approximately 0.1 pm to 5 pm, or approximately 0.3 pm to 2 pm, such as approximately 0.1 pm, or approximately 0.5 pm, or approximately 1 pm, prior to the application of the selective material coating. Microporous layer may include a microporous polymer, such as a fluoropolymer, such as microporous ePTFE.

[0019] Percent porosity of the porous polymeric separator material prior to coating with the selective material may be approximately from 40% to 80%, or approximately from 50% to 80%, or from 60% to 80%, or from 50% to 70%, based on volume. Pore size may be symmetrical or non-symmetrical through the thickness of the porous polymeric separator of the selective separator construct. Further, percent porosity may be uniform or non-uniform through the thickness of the porous polymeric separator. In some embodiments, wherein the selective separator comprises a multilayer porous polymeric separator, the pore size and the percent porosity, independently, may be the same or may be different in each polymeric material layer. The pore size distribution of each layer of a multilayer porous polymeric separate may be independently symmetrical or non-symmetrical throughout the thickness of each layer, and the percent porosity of each layer is independently uniform or non-uniform, throughout the layer.

[0020] By way of example and not limitation, the porous polymeric separator may be fabricated by casting, extrusion, or phase inversion, or may be woven, or nonwoven, including but not limited to a woven or non-woven fabric, spun woven mat, mesh, web, or cast or extruded layer or film. The porous polymer separator may comprise one or more than one layer, or other structure or form.

[0021] A porous separator may be reinforced or self-supporting. One or more reinforcing components may be integrated or embedded in the separator polymer, for example, to increase mechanical strength, chemical durability and/or dimensional stability of the final selective separator. A reinforcing component of a multilayer porous selective separator may comprise a porous continuous reinforcing layer or a discrete structure optionally comprised of a polymer, such as PTFE, for example, ePTFE, or PPS, polypropylene, polyphenylene sulfide, polyether ether ketone (PEEK) and the like. Alternatively, a discontinuous reinforcing component may be in the form of reinforcing fibers or threads. Fabrics, including, woven or non-woven support, may be embedded into the polymeric structure. Reinforcing components may comprise the same or different polymer composition as the porous polymeric separator.

[0022] In some embodiments, a porous polymeric separator comprises a microporous ePTFE membrane bonded to nonwoven fabric wherein a surface hydrophilic treatment may be applied to one or both opposing non-bonded surfaces of the ePTFE membrane and nonwoven fabric. Non-woven fabrics may include, but are not limited to, melt-spun PP or PE. In some embodiments, an ePTFE layer of a porous separator may have a thickness in the range of approximately 0.5 pm to approximately 2 pm, such as approximately 1 pm.

[0023] In an embodiment, prior to the application of the selective material, a porous polymeric separator is non-functional and does not contain a functional polymer such as an ion exchange polymer having sulfonic acid functional groups or carboxylic acid functional groups, or a group convertible to a sulfonic acid, or carboxylic acid, functional group. In one embodiment where the porous separator contains a reinforcing material, the reinforcing material does not contain an ion exchange polymer having sulfonic acid functional groups or carboxylic acid functional groups. In a further embodiment, neither the porous polymeric separator material nor the reinforcing material contains an ion exchange polymer having sulfonic acid or carboxylic acid functional groups prior to coating with a selective material. In other embodiments, a surface treatment may be applied to render surface functionality, such as hydrophilicity, to the underlying polymer structure; in some embodiments, an ionic surface treatment is present though neither the porous polymeric separator nor a reinforcing material, if present, contains ion exchange functionality. Surface hydrophilic treatments suitable for use herein include surface coating, plasma treatment, chemical grafting with sulfonate or phosphate functionalities, UV irradiation, alkali treatment and the like.

[0024] Thickness of the porous polymeric separator may be less than 300 pm, or less than 250 pm, or less than 200 pm, or less than 150 pm, or less than 100 pm, or the porous polymeric separator layer may have a thickness from 50 pm to 250 pm, or from 50 pm to 225 pm, or from 50 pm to 200 pm, or from 75 pm to 225 pm, or from 75 pm to 200 pm, or from 75 pm to 150 pm. Thickness may be measured by SEM cross sectional analysis by obtaining the average of at least 3 thickness measurements through the cross-section.

Selective Material

[0025] A selective material mixture is applied to one or more outer surfaces (105a, 105b) of the porous polymeric separator. The selective material (102a, 102b) comprises an ion exchange polymer (106) and inorganic particles (103), such as inorganic particles that impart good ion conduction, low gas permeability and improved gas release features in the final selective separator, compared to the same porous polymeric separator material without a selective material coating.

[0026] An ion exchange polymer may be a cation exchange polymer or anion exchange polymer. Cation exchange polymers suitable for use herein may include one or more fluorinated polymers, such as perfluorinated or partially fluorinated alkyl compounds including fluorinated hydrocarbon or aromatic polymers having ionic functional sites. The polymer composition may include, for example, a perfluorosulfonic acid (PFSAs) sold under the trade name NAFION, including materials commonly known for use as a solid polymer electrolyte membrane (PEM) in electrochemical devices. A selective polymeric material may further include a multivalent PFSA composition. The selective polymer may be crosslinked, for example, by treatment with or exposure to physical or chemical crosslinking methods, including but not limited to irradiation and/or free radical. Alternatively, the selective polymer may be non-crosslinked, such that non-crosslinked polymer has not been exposed to a crosslinking method.

[0027] The ion exchange polymer backbone, the main chain of a polymer, may include units represented by the following formulae:

- CF₂-CF[-O-CF₂CF(CF₃)-O-(CF₂)_m-SO₃M) -

(1) wherein m is 1-6, and M is an alkali metal;

4-CF₂-CF(-O-(CF₂)m-SO₃M) -]-

(2) wherein m is 1-6, and M is an alkali metal; wherein Q¹ is a perfluoroalkylene group optionally having an etheric oxygen atom, Q² is a single bond or a perfluoroalkylene group optionally having an etheric oxygen atom, R⁷ is a perfluoroalkyl group optionally having an etheric oxygen atom, X¹ is an oxygen atom, a nitrogen atom or a carbon atom, when X¹ is an oxygen atom, a is 0, when X¹ is a nitrogen atom, a is 1 , and when X¹ is a carbon atom, a is 2, Y is a fluorine atom or a monovalent perfluoro organic group, r is 0 or 1 , and M is an alkali metal; or wherein R² is a single bond or a C1-6 linear perfluoroalkylene group which may have an etheric oxygen atom, and R³ is a C1-6 linear perfluoroalkylene group, m is 0 or 1 ; n is 1 , and M is an alkali metal. In some embodiments, the ion exchange polymer backbone, the main chain of a polymer, may include units represented by the following formulae: wherein m is 0,1 or 2; n is 0, 1 or 2; Rfi is a C1-6 linear perfluoroalkylene; Rf2 is a C1-6 linear perfluoroalkylene group; and M is a cation, which may be a proton, alkali metal or quaternary ammonium; or wherein m is 0,1 or 2; n is 0, 1 or 2; Rfi is a C1-6 linear perfluoroalkylene group; Rf2 is a Rfi is a C1-6 linear perfluoroalkylene group; and M is a cation, which may be a proton, alkali metal or quaternary ammonium. Alkali metals and quaternary ammonium suitable for use herein include, but are not limited to for example, K⁺, Na⁺ and Li⁺, and tetramethyl ammonium, respectively. [0028] In other embodiments, the ion exchange polymer backbone comprises polyphenylene oxide, polysulfone, polyethylene, or poly(aryl ether sulfone). Functional groups of a cation exchange polymer may include but are not limited to sulfonate, carboxylate or phosphate functional groups. In an embodiment, the ion exchange polymer suitable for use herein comprises a copolymer derived from the polymerization of tetrafluoroethylene (TFE) and a derivative of a perfluoro (alkyl vinyl ether) with sulfonyl acid fluoride, such as a Nation™ brand polymer.

[0029] An anion exchange polymer used herein may have ionic sites selected from ammonium, quaternary ammonium, piperidinium, imidazolium, guanidinium, benzimidazolium, pyrrolidinium, spirocyclic or phosphonium, functional groups and their derivatives to improve ionic conductivity, chemical stability and mechanical properties. In one embodiment, an AEM separator comprises a selective material that comprises poly(aryl piperidinium) (Versogen, Inc., Newark, DE). In an embodiment, anion exchange capacity of a cationic or anionic exchange polymer, or polyelectrolyte, suitable for use in a selective material composition, herein, is approximately 0.8 meq/g to 2.5 meq/g dry polymer.

[0030] Selective material comprises a liquid mixture of inorganic particles dispersed within the polymer and applied as a coating. Inorganic particles, such as metal oxides suitable for use in selective materials include zirconium oxide, titanium oxide, bismuth oxide, cerium oxide, and magnesium oxide, and combinations thereof. Other hydrophilic particles that may be used include nitrides and carbides of Group IV elements of the periodic table. Inorganic particles may have a mean particle size in the range of approximately 0.5 pm to 8 pm 0.5 pm to 5 pm, or 0.5 pm to 2 pm, or 0.15 pm to 1 pm, or within the range of 0.15 pm to 0.75 pm, as determined by laser diffraction particle size analysis. In an embodiment the metal oxide particles may comprise a bimodal particle size distribution, such as 0.5 pm and 5 pm.

[0031] A liquid mixture comprising, for example, zirconium oxide particles and ion exchange polymer may be diluted to a viscosity optimized for the selected application technique. The diluent may comprise a volatile such as ethanol, n- propanol or iso-propanol. Methods of applying selective material to the porous polymeric separator layer include, but are not limited to, a spray coating technique, application by doctor blade, Meyer rod, gravure coating, and the like. The coating coverage of the selective material on the outer surface of the porous polymeric separator provides a dried solids loading of approximately from 0.1 mg/cm² to 2 mg/cm² porous polymeric separator, or a loading may be from 0.5 mg/cm² to 1.5 mg/cm², and in one specific embodiment coverage is approximately 0.9 mg/cm², of the total combination of ionomer and inorganic solids.

[0032] In some embodiments, a selective material coating may be applied to one or both opposing outer surfaces of the porous polymeric separator at a loading of approximately 0.9 mg/cm² per side of the porous polymeric support (i.e., an ePTFE surface and a non-woven fabric surface) based on the total dried solids weight of the ionomer and inorganic particles. In an embodiment, the dried solids weight ratio of ionomer to inorganic particles maybe in the range from 0.1 to 0.8, or from 0.1 to 0.6, or from 0.3 to 0.4. In other embodiments, the weight ratio of dried ion exchange polymer solids to ZrC>2 solids of the selective material is in the range of approximately 0.1 to 0.8, such as, approximately 0.1 to 0.5.

[0033] In one embodiment, selective material covers up to approximately 80%, or 90%, or 100% of the outer surface area of the porous separator, and the coated outer surface of the porous selective separator maintains sufficient porosity or permeability to achieve optimized ohmic resistance and gas crossover levels. While a conventional liquid ion exchange polymer may form a continuous non-porous coating that occludes the porosity of a porous polymeric separator, in some embodiments disclosed herein the addition of dispersed metal oxide particles, such as zirconia, may prevent or break up the formation of a compact continuous polymer matrix on the outer surface of the porous separator so that the porous separator described herein retains at least some continuous open porosity that extends through and between opposing outer surfaces of the selective separator. Thus, selective material comprising an ion exchange polymer and inorganic particles may be applied, forming a porous, thin film on the surface of the porous polymeric separator. In some embodiments, a porous polymeric selective separator has a surface roughness in the range of approximately 0.1 pm to 4 pm, when measured on the coated surface. [0034] In some embodiments, selective separators described herein have an air permeability of less than 0.008 ft³/min*ft² when measured according to methods provided herein. In other embodiments, selective separators have an air permeability of less than 0.007 ft³/min*ft², or less than 0.005 ft³/min*ft², or less than 0.004 ft³/min*ft², or between 0.001 ft³/min*ft² and 0.008 ft³/min*ft², or between 0.001 ft³/min*ft² and 0.007 ft³/min*ft², or between 0.002 ft³/min*ft² and 0.004 ft³/min*ft², when tested according to methods described herein. In other embodiments, selective separators having an air permeability of less than 0.008 ft³/min*ft², or between 0.001 ft³/min*ft² and 0.008 ft³/min*ft², have an ionic resistance of less than 200 mfrcm² or less than 150 mOcm² or less than 100 mfrcm².

[0035] A porous selective material layer having a thickness of approximately 1 pm to 10 pm, or 4 pm to 10 pm, or less than 20 pm, may be formed on at least one outer surface of the porous polymeric separator wherein the inorganic particles of the selective material are greater than or equal to 0.1 pm. Resulting selective separators have acceptable ohmic resistance that is, for example, within + 20%, or within + 10%, of the ohmic resistance of the porous polymer separator material from which the selective separator is constructed when tested without the selective material layer(s). In other embodiments, a selective separator is formed having an ohmic resistance that is increased no more than about 50% of the ohmic resistance of the porous polymer separator material used to form the selective separator.

[0036] In some embodiments, application of a selective material to a porous selective separator results in an increase in ohmic resistance of less than 200 mOhm-cm² when compared to substantially the same porous separator material without a selective material coating by testing according to the test conditions described herein. In some embodiments, selective separator membranes having a selective material coating beneficially have ionic resistance less than 200 mOhm-cm² or less than 180 mOhm-cm² or less than 150 mOhm-cm², while having a gas release angle less than 20 degrees, or less than 10 degrees, when measured according to the methods described herein. In still further embodiments, selective separators have a gas release angle less than 20 degrees, an air permeability less than 0.008 ft³/min*ft² and an ionic resistance of less than 200 mOhm-cm², when tested according to methods described herein. [0037] In one embodiment, a selective separator membrane comprises a multilayer porous polymer separator having a uniform distribution of inorganic particles throughout the thickness of the selective material layer applied to at least one surface of the porous polymer separator. In a further embodiment, the porosity of a first layer of the multilayer porous polymer separator is substantially free of the selective material, for example, where only an insubstantial amount of the inorganic particles penetrates the porous structure, for example, as visualized by SEM, and at least a second layer of the multilayer porous polymer separator is substantially penetrated by the selective material coating. In a particular embodiment comprising a multilayer ePTFE microporous film/PP layer separator, inorganic particles, such as zirconia particles, form a uniform layer on the outer surface of an ePTFE film layer without any substantial penetration of the zirconia particles into the ePTFE microstructure, and the selective coating mixture applied to the opposing surface substantially penetrates the porosity and coats fibers of, the non-woven PP layer; and optionally, the zirconia particles penetrate through the porosity of non-woven PP layer to which it is applied coating portions of the ePTFE surface that is bonded to the PP layer, opposite the outer ePTFE surface.

[0038] With reference to the orientation of a porous selective separator within an electrolysis cell, the selective material may be applied to an anode-facing outer surface, cathode-facing outer surface, or both anode and cathode-facing outer surfaces of the porous polymeric separator. The interaction of a separator surface varies with gas type, and therefore, a selective material composition applied to anode-facing surface may be different than a selective material composition applied to the opposing cathode-facing surface of the porous polymeric separator. In an embodiment, selective material does not significantly impact the ionic resistance of the final porous separator.

[0039] An electrochemical cell for use in AWE applications is also provided comprising an anode compartment, an anode situated within the anode compartment, a cathode compartment, a cathode situated within the cathode compartment, and a porous selective separator located between, and separating, the anode and the cathode, the electrochemical cell configured to hold a liquid electrolyte solution and porous selective separator providing ionic contact between the electrodes. In one embodiment the porous selective separator is comprised of 1) a hydrophilic porous polymeric separator and 2) a selective material provided as a coating on at least one outer surface of the hydrophilic porous polymeric separator. In one embodiment, the polymer material from which the porous polymeric separator structure is formed consists essentially of non-functional polymeric material; nonetheless, in some embodiments, a treatment material may be applied to the porous polymer separator, and the treatment material may comprise a functional or non-functional composition that enhances or provides wettability to the porous polymer structure made from non-functional polymer. In some embodiments, the selective material layer forms the outermost surface of the selective separator surface, comprising a composite matrix of discontinuous ion exchange polymer and inorganic particles, such as metal oxide particles, distributed throughout the ion exchange polymer. The mean particle size of the inorganic particles of the selective material may be in the range of approximately from 0.1 pm to 4 pm, or from 0.1 pm to 5 pm, or from 0.1 pm to 6 pm, or from 0.1 pm to 8 pm.

[0040] In the schematic representation of Figure 5, one embodiment of an alkaline water electrolyzer (500) is provided for hydrogen production comprising a porous selective separator (501) separating an anode and cathode operating in an alkaline electrolyte solution (502), such as an aqueous 25 wt% to 40 wt% KOH. A first selective material coating (503) is provided on an outer surface (505) of a porous polymeric separator (504) adjacent the anode and a second selective material (503) coating is applied to an opposing outer surface of the porous polymeric separator adjacent the cathode to form the porous selective separator that prevents gas stagnation on the surface of and/or within the porosity of the selective separator, that mitigates crossover of oxygen and hydrogen gases, and potassium and hydroxide ions are transported across the separator between cathode and anode.

[0041] The selective material composition and solids loading of the selective material coating may be altered to adjust surface energy and/or gas adhesion while maintaining low ionic resistance and low gas crossover. The selective material composition may be homogenous or form a gradient of the selective throughout the thickness of the selective layer.

[0042] A method of making a selective separator is provided comprising the steps of obtaining a porous polymeric separator, obtaining a selective material comprising a liquid composition of an ion exchange polymer and inorganic particles having a mean particle size between 0.1 pm and 4 pm, and applying the liquid composition to one or both outer surfaces of the porous polymeric separator forming a selective layer, wherein the inorganic particles are distributed uniformly throughout the thickness of the selective layer without substantially penetrating the thickness of the porous polymeric separator layer.

Test Methods

[0043] Ionic Resistance in KOH: Ionic Resistance was measured to determine the resistance of spray-coated AWE separators in 30% KOH solution.

[0044] Spray coated separators were dried in the oven at about 90°C. Measurements were performed on conditioned separators. Conditioned separators were heated in 60°C water for 6 hours. They were stored in 30% KOH overnight before testing.

[0045] Ionic resistance was characterized by four-probe impedance spectroscopy in caustic electrolyte. A separator sample was placed between two chambers filled with 30% KOH. The impedance -resistance was measured at ambient conditions (~22°C) using impedance analyzer on BioLogic Potentiostat SP-240 (Lambda System). The impedance scan was frequency-sweep from 50 kHz to 1 MHz. The resistance value was manually determined from impedance at high frequency intercept in Nyquist plot.

[0046] Maximum Tilt Angle for Bubble Release: The Maximum Tilt Angle for Bubble Release was used to measure gas release on the surface of the coated selective separator. The maximum angle that bubbles will stay on the surface of the AWE separator by tilting was measured as follows.

[0047] Separators were heated in 60°C water for six hour and stored in water after treatment. The separator was fixed in the captive bubble cell and the cell was filled with water. An air bubble was introduced at a tilt angle of 0 degrees (flat). The average bubble size was 2 mm diameter. The stage (the cell holder) was tilted manually until the bubble moved from the surface of the separator. The maximum angle reached when the bubble moved was measured in triplicate and the average was recorded. Air Permeability Measurement by Gurley Densometer

[0048] A Gurley densometer was used to measure porosity of separator. Separators were dried in the oven for one hour at about 90°C, then soaked in water for six hours at 60°C prior to testing. The separators were equilibrated overnight at 50% RH condition. The test was conducted by measuring the time required for a certain volume of air to pass through the sample. The separators were tested in triplicate, and the average value was reported in ft³/min*ft². The standard deviation for all samples was +0.0002 or less.

EXAMPLES

Coating Preparation

[0049] A selective material mixture for coating on the porous polymeric was prepared as follows. In preparing the selective material mixture, components were added in the order as follows: zirconium oxide, ion exchange polymer dispersion and reagent grade ethanol. The weight ratio of ion exchange polymer to ZrC>2 solids was about 0.1 to 0.6 by dry weight. Nation™ dispersion had about a 10 wt% perfluorosulfonic acid polymer (1000 EW) solids content and about 90 wt% water (dispersion Nation™ D1021 from The Chemours Company FC, LLC). Ethanol was added in an amount to render the material suitable for the selected spray application.

[0050] Selective material mixture ingredients were added to 20 ml glass scintillation vials for mixing. The vials (not shaken prior to sonication) were sonicated for 5 minutes, then shaken well, and sonicated again for another 5 minutes. Prior to loading into the application device vials containing the selective material mixture were briefly shaken using vortex mixer.

Spray Coating Application Technique

[0051] Prior to spray coating, a sample porous polymeric separator was placed into a frame template and secured around the porous substrate perimeter with clips.

[0052] An airbrush set, Paasche® model VL was used to apply the selective material mixture. The selective material mixture was gravity-fed into the airbrush by a cut-off syringe used to contain and feed the selective material mixture in the air brush gun. Air pressure of 15 psi was maintained during coating. For each substrate sample, an area of about 12 cm x 12 cm was spray coated.

[0053] The transfer rate of the selective material mixture, determined with gravimetric method by spraying on Kapton™ films, was approximately 30 wt% to 50 wt%, providing a nominal loading of approximately 0.9 mg/cm².

Example 1

[0054] An AWE selective separator membrane was prepared by spray coating the selective material mixture on only one outer surface of a porous PE substrate.

[0055] The PE substrate was a composite structure of silica powder and polyethylene having an average thickness of approximately 160 microns (obtained from Entek). The selective coating mixture comprising ZrC>2, Nation™ perfluoro sulfonic acid polymer (1000 EW dispersion) in a ratio of approximately 0.1 (ZrC>2/ dry ionomer), and ethanol as a solvent, was prepared as described above. ZrC>2 mean particle size was approximately 0.8 pm (MEL Chemical, MS2 grade), as determined by laser diffraction particle size analysis. The coating mixture was applied by the spray coating method described herein. The hydrophilic particles of the selective material adhered to the PE porous separator by the ion exchange polymer. The average thickness of the final selective separator was approximately 167 pm.

[0056] The loading of the selective coating solids on the PE substrate was approximately 0.9 mg/cm², measured by gravimetric analysis. The porous selective separator was tested for gas release angle on the coated surface, gas permeability and ionic resistance by methods provided herein, and compared to conventional materials. Results are provided in Table 1. An SEM image of the AWE selective separator membrane (200) in shown in Figure 2 which depicts a top-down view of the selective separator showing a uniform application of the selective coating mixture (202) having small and large zirconia particles (201a, 201b).

Example 2

[0057] A selective separator was prepared substantially similarly to Example 1 ; however, the selective material mixture was applied by spray coating to both anode- facing and cathode-facing outer surfaces of the porous PE. The nominal loading of coating solids on each surface was approximately 0.9mg/cm².

[0058] The porous selective separator was tested for gas release angle, gas permeability and ionic resistance by methods provided herein, and compared to conventional materials. Results are provided in Table 1. An SEM image of the AWE selective separator membrane (300) in shown in Figure 3 which depicts a cross- sectional view of the selective separator showing a uniform distribution of zirconia particles (304) seen as white particles uniformly distributed throughout the thickness of the selective coating mixture (303), forming the outermost surfaces (301a, 301 b) of the selective separator membrane. Minimal penetration of the selective coating mixture penetrates into and through the porous PE separator structure (302) which remains substantially free of the zirconia particles throughout the porous PE structure (302).

Example 3

[0059] A selective separator was prepared substantially similarly to Example 2, however, the porous separator was multilayer having an ePTFE film laminated to non-woven polypropylene film, wherein the ePTFE/polypropylene substrate was obtained from Membrane Solutions, LLC (FPL100A12 hydrophilic PTFE membrane having 1.0 pm pore size, bubble point 0.12 to 0.15 (MPa) at 23C, with PP support layer, and reporting bubble point test using purified water as wetting fluid.) The selective material layer was applied to the porous separator substrate by spray coating both anode-facing and cathode-facing outer surfaces of the ePTFE/polypropylene substrate to form the selective separator. The nominal loading of coating solids on each surface was approximately 0.9 mg/cm².

[0060] An SEM image of the selective separator membrane (400) in shown in Figure 4 which depicts a cross-sectional view showing a uniform coating of the selective coating mixture (401) where zirconia particles are seen on the outer surface of the ePTFE film (402a) of the porous separator layer (403) with minimal zirconia penetrating the ePTFE microstructure, and the selective coating mixture (401) applied to the opposing surface, substantially penetrating the porosity of, and coating fibers (405) of, the non-woven PP layer (404), where zirconia particles are shown to reach the ePTFE surface (402b) that is opposite the outer ePTFE surface (402a).

[0061] The weight ratio of ion exchange polymer to ZrC>2 solids was approximately 0.4 by dry weight. The porous selective separator was tested for gas release angle, gas permeability and ionic resistance by methods provided herein, and compared to conventional materials. Results are provided in Table 1. The thickness measurements of final porous selective separator were from approximately 170 pm to 220 pm when measured at 10 points.

Comparative Example 4

[0062] The polyethylene porous substrate used in Examples 1 and 2, was tested without any surface modification and without the addition of a selective material. The substrate was tested for gas release angle, gas permeability and ionic resistance by methods provided herein, and compared to conventional materials. Results are provided in Table 1. As reported in Table 1, the coated separators of Examples 1 and 2 showed significant improvement in gas release angle compared to uncoated plain PE, without significant impact on ionic resistance values which were 71 and 78 mQ cm², respectively, similar to the plain PE separator (PE) of Comparative Example 4.

Comparative Example 5

[0063] A comparative selective separator was prepared substantially similarly to Example 2, however, the inorganic particles in the selective material mixture were ZrC>2 nanoparticles, having a mean particle size of 100 nm. The nominal loading of coating solids on each surface of the porous PE substrate was approximately 0.9 mg/cm², and the weight ratio of ion exchange polymer to ZrC>2 solids was approximately 0.11 by dry weight, substantially similar to Example 2. The coated substrate of Comparative Example 5 was tested for gas release angle, gas permeability and ionic resistance by methods provided herein, and compared to conventional materials. Results provided in Table 1, showed a significantly higher gas release angle (>45 degrees) compared to Ex. 2 (<2 degrees), without a significant change in ionic resistance (approximately 81 mfrcm² for Comp. Ex. 5 compared to 78 mQ-cm²for Ex. 2). Comparative Examples 6 and 7

[0064] The ePTFE/PP porous separator substrate according to Example 3 was prepared both without any surface modification or selective material (Comparative Example 6) and with an ionomer of a selective material (Nation ™ ionomer), without inorganic particles, applied to the ePTFE side of the porous separator. The materials were tested for gas release angle, gas permeability and ionic resistance by methods provided herein, and compared to conventional materials. Results are provided in Table 1 , which show significantly higher gas release angles compared to Ex. 3, therefore lower gas release from the surface; and Comp. Ex. 7 also showed higher ionic resistance (217 mQ-cm²) compared to Example 3 (Approx. 136 mQ-cm²), while Comp. Ex, 6 showed high air permeability equating to higher gas cross-over.

Comparative Example 8

[0065] A perfluoro sulfonic acid based Nation ™ membrane was also tested for comparison to Examples 1 and 2. A Nation™ NR220 membrane having 10 pm thick Nation™ dispersion cast on a 160 pm thick PE separator, was tested for gas release angle, gas permeability and ionic resistance by methods provided herein, and compared to conventional materials. Results are provided in Table 1 and show a significantly higher ionic resistance than any of the selective separators made according to Examples 1 to 3. Visual observations indicated the surface of Nation™ polymer was not fully wetted in 30% KOH. While not wishing to be bound by theory, high resistance may be attributed to low water content in Nation™ polymer layer in 30% KOH.

Comparative Example 9

[0066] Zirfon® membrane bleu (LITP500; Agfa-Gevaert N.V. Belgium), a porous separator and benchmark membrane in AWE technology, was obtained. Resistance value reported in Table 1 was obtained in the literature (AGFA Technical Data Sheet Zirfon® Perl UTP 500).

Comparative Example 10

[0067] A Nation ™ 211 dispersion-cast, 25 pm thick separator membrane was obtained (The Chemours Company FC, LLC, Wilmington, DE) and tested as reported in Table 1. The dispersion cast membrane showed significantly higher ionic resistance than any of the selective separators made according to Examples 1 to 3.

Table 1. Gas Release Angle and Ionic Resistance of Separators.

*Based on literature value (AGFA Technical Data Sheet Zirfon® Perl UTP 500)

[0068] The porous selective separators of Example 1 having a first outer surface coated with a selective material, and Example 2 having both outer surfaces coated with a selective material, resulted in significantly lower gas release angles of approximately 3.9 degrees, and approximately 1.4 degrees, respectively, than the sample of Comparative Example 4, having the same porous selective separator material on which no selective material layer has been applied and which has a gas release angle of approximately 33 degrees. The material of Comparative Example 7, a separator comprising an ion exchange polymer with sulfonic acid functional groups, had a gas release angle greater than 40 degrees. The lower gas release angles of the Examples 1 through 3 described here are advantageous in mitigating gas blinding on the membrane/electrode interface.

[0069] The porous samples of Comparative Example 5 having both outer surfaces coated with zirconium oxide nanoparticles (particle size of <100 nm), showed higher gas release angle (equating to lower gas release) and higher air permeability (equating to higher gas crossover), when compared with Examples 1 and 2 both having a selective material layer comprised of ZrC>2 particles having a larger particle size (approximately 106 pm) , while Examples 1 and 2 maintained similar ionic resistance values. The gas bubbles were still attached to the surface of Comparative Example 5 sample material at the maximum angle on the test instrument.

[0070] The porous selective separators of Example 3, ePTFE laminated to nonwoven polypropylene with both outer surfaces coated with a selective material, resulted in low gas release angles of approximately 8.1 degrees on its ePTFE outermost surface. The plain porous polymer separator of Comparative Example 6 that lacked a selective layer, did not release the gas at the maximum angle limit (45 degrees) on the instrument during testing. The lack of the selective material on the ePTFE/PP structure resulted in a substantial drop in air permeability, from approximately 2.48 ft³/min*ft² (Comparative Example 6) without the selective layer to approximately 0.0029 ft³/min*ft² with the selective layer (Example 3).

Claims

CLAIMS What is claimed is:

1 . A selective separator for water electrolysis applications comprising: a porous polymeric separator layer having first and second opposing outer surfaces and a selective material layer comprising a composite of an ion exchange polymer and inorganic particles distributed throughout the ion exchange polymer having a mean particle size in the range of 0.1 pm to 8 pm on at least one of the porous polymeric separator layer surfaces, wherein the selective material layer forms an outermost surface of the selective separator and has gas release angle of less than 20 degrees.

2. A selective separator for water electrolysis application comprising: a porous polymer separator layer comprising a polymeric microporous film laminated to a non-woven porous material and a selective material layer on at least one surface of the porous polymer separator layer, the selective material comprising a composite of an ion exchange polymer comprising a perfluorinated sulfonic acid polymer and ZrC>2 particles uniformly distributed throughout the selective material layer, wherein the mean ZrC>2 particle size is in the range of 0.1 pm to 4 pm, and the selective material is the outermost surface of the selective separator surface on which it is provided.

3. A selective separator for water electrolysis applications consisting essentially of: a porous polymeric separator layer having a composite that comprises of polymer and inorganic particles in an amount sufficient to increase the hydrophilicity of the porous polymeric separator polymer, a selective material layer on at least one outer surface of the porous polymeric separator layer, the selective material comprising a composite of an ion exchange polymer comprising perfluorosulfonic acid functional groups and zirconium oxide particles having a mean particle size in the range of 0.1 pm to 5 pm distributed within the selective material, wherein the selective material layer is the outermost surface of the selective separator surface on which it is applied.

4. The selective separator of any of claims 1 to 3, wherein the porous polymeric separator layer comprises polyethylene, polypropylene, fluoropolymer, PTFE, expanded PTFE, PVDF, polysulfone, polyphenylenesulfide, or a combination thereof.

5. The selective separator of claim 1 , wherein the porous polymeric separator layer comprises a polymer composite comprising metal oxide microparticles selected from zirconium oxide, titanium oxide, bismuth oxide, cerium oxide, magnesium oxide, and combinations thereof, that increase the hydrophilicity of the polymer.

6. The selective separator of any of claims 1 to 3, wherein the porous polymeric separator layer has an average pore size in the range of 0.1 pm to 5 pm.

7. The selective separator of any of claims 1 to 3, wherein the porous polymer separator layer is self-supporting.

8. The selective separator of any of claims 1 to 3, wherein the selective separator has a gas release angle less than 20 degrees, an ionic resistance of less than 200 m£>cm² and an air permeability less than 0.008 ft³/min*ft².

9. The selective separator of any of claims 1 to 3, wherein the selective material layer is applied to both outer surfaces of the porous polymeric separator.

10. The selective separator of claim 1 , wherein the selective material layer has a dried solids loading comprising ion exchange polymer and inorganic particles that is from 0.1 mg/cm² to 2 mg/cm².

11 . The selective separator of claims 1 or 2, wherein the selective material layer has a dried solids loading comprising ion exchange polymer and ZrC>2 particles that is from 0.1 mg/cm² to 2 mg/cm².

12. The selective separator of claim 1 , wherein the solids weight ratio of ion exchange polymer to inorganic particles of the selective material layer is in the range of 0.1 to 0.6.

13. The selective separator of claims 2 or 3, wherein the solids weight ratio of ion exchange polymer to ZrC>2 solids is 0.1 to 0.8.

14. The selective separator of claim 1 , wherein the inorganic particles of the selective material layer comprise at least one material selected from the group consisting of an oxide, nitride and carbide of a Group 4 or Group 14 element or mixture thereof.

15. The selective separator of claim 1 , wherein less than 20 wt% of the inorganic particles penetrates the pores of the porous polymeric separator.

16. The selective separator of any of claims 1 to 3, wherein the thickness of the selective material layer on one of the porous polymeric separator surfaces is less than 20 pm.

17. The selective separator of any of claims 1 to 3, wherein the average thickness of the selective material layer is in the range of 1 pm to 10 pm.

18. The selective separator of claim 1, wherein the ion exchange polymer is a cation exchange polymer.

19. The selective separator of claim 1, wherein the ion exchange polymer is an anion exchange polymer.

20. The selective separator of claim 1 , wherein the ion exchange polymer functional groups include but are not limited to sulfonate, carboxylate or phosphate functional groups.

21. The selective separator of claim 1 , wherein the ion exchange polymer is a perfluorinated sulfonic acid polymer.

22. The selective separator of any of claims 1 to 3, wherein the ion exchange polymer has an ion exchange capacity of 0.8 meq/g to 2.5 meq/g dry polymer.

23. The selective separator of any of claims 1 to 3, further comprising zirconium oxide particles adhered to the selective separator by the ionomer exchange polymer.

24. The selective separator of any of claims 1 to 3, wherein the selective separator surface roughness is 0.1 pm to 4 pm.

25. The selective separator of any of claims 1 to 3, wherein the thickness of the selective material layer on one or both outermost surfaces is less than or equal to 300 pm.

26. The selective separator of any of claims 1 to 3, wherein the thickness of the selective material layer is less than 250 pm.

27. The selective separator of claims 2 or 3, wherein the gas release angle when measured on the selective separator outermost surface is less than 20 degrees.

28. The selective separator of any of claims 1 to 3, wherein the selective separator outermost surface has a gas release angle of less than 10 degrees.

29. The selective separator of claim 1 , having a first selective material layer on an anode-facing surface and a second selective material layer on a cathode-facing surface and the amount of inorganic particles that penetrates the first selective material layer is different from the second selective material layer.

30. The selective separator of claim 1 or 3, wherein the porous polymeric separator comprises at least two layers.

31 . The selective separator of claim 30, wherein a first porous polymeric separator layer has an average pore size that is less than the average pore size of a second porous polymeric separator layer.

32. The selective separator of claim 31 , where in the porous polymeric separator layer comprises a microporous film and a non-woven fabric.

33. The selective separator of any of claims 30 to 32, where in the porous polymeric separator layer comprises an ePTFE layer and a non-woven fabric layer.

34. The selective separator of any of claims 30 to 33, wherein the porous polymeric separator layer comprises a non-woven fabric between two outer layers of ePTFE.

35. The selective separator of any of claims 32 to 34, wherein the non-woven fabric comprises polypropylene or polyethylene.

36. The selective separator of claim 1 , wherein the ohmic resistance is less than 100 m£>cm2 when measured at ambient temperature.

37. Use of a selective separator of any of claims 1 to 36, in a water electrolyzer.

38. An electrochemical cell for use in alkaline water electrolysis (AWE) applications, comprising an anode compartment, an anode situated within the anode compartment, a cathode compartment, a cathode situated within the cathode compartment, the electrochemical cell configured to hold a liquid electrolyte and a selective separator between the anode and the cathode, comprising: a hydrophilic porous polymeric separator and a selective material layer on at least one outer surface of the porous polymeric separator that comprises a composite of an ion exchange polymer and inorganic particles distributed throughout the ion exchange polymer, wherein the mean inorganic particle size is in a range from 0.1 pm to 4 pm, and wherein the selective material layer forms the outermost surface of the selective separator surface to which it is applied.

39. The electrochemical cell of claim 38, wherein a selective separator is in contact with at least one of the anode and cathode.

40. The electrochemical cell of claim 38, wherein the selective separator is configured to not contact with the anode or cathode.

41 . A method of making a selective separator comprising the steps of: obtaining a porous polymeric separator, obtaining a selective material comprising a liquid composition of an ion exchange polymer and inorganic particles having a mean particle size between 0.1 pm and 4 pm, and applying the liquid composition to one or both outer surfaces of the porous polymeric separator forming a selective layer without substantially penetrating the thickness of at least a portion of the porous polymeric separator layer, wherein the inorganic particles are distributed uniformly throughout the thickness of the selective layer.

42. The method of making a selective separator of claim 41 , wherein the selective separator has a gas release angle of less than 20 degrees.

43. The method of claim 41 , wherein the porous polymeric separator comprises polyethylene, and the selective material comprises an ion exchange polymer and zirconium oxide particles.

44. The method of claim 41 , comprising the step of forming a selective material layer having a thickness of less than 10 pm on an outer surface of the porous polymeric separator.

45. The method of claim 41 , comprising the step of forming a selective material layer having a thickness of between 4pm or 10pm on at least one outer surface of the porous polymeric separator.

46. The method of claim 41 , wherein the porous polymeric separator is formed from a polymer mixture comprising polyethylene and silica, and wherein the ion exchange polymer comprises perfluorosulfonic acid.

47. The method of claim 41, wherein the porous polymeric separator is formed by applying the selective material comprising a perfluorosulfonic acid polymer to a multilayer porous polymeric separator that comprises a microporous ePTFE film laminated to a non-woven polypropylene.

48. The method of claim 41, comprising applying the selective material on the outer surface of the porous polymeric separator forming a coating having a dried solids loading of approximately from 0.1 mg/cm² to 2 mg/cm² forming the outermost layer of the selective separator.

9. The method of claim 47, wherein the selective material comprising the inorganic particles forms a layer on the microporous ePTFE film without the inorganic particles substantially penetrating the microporous ePTFE and the selective material penetrates the thickness of the non-woven layer with the inorganic particles coating fibers of the non-woven layers.