IL297987B2 - Hot pressed, binder-including gas diffusion electrodes - Google Patents

Description

297987/ HOT PRESSED, BINDER-INCLUDING GAS DIFFUSION ELECTRODES BACKGROUND OF THE INVENTION 1. TECHNICAL FIELD [0001]The present invention relates to the field of electrochemical devices, and more particularly, to configurations of electrodes thereof. 2. DISCUSSION OF RELATED ART [0002]Electrolyzers and fuel cells are electrochemical devices that produce hydrogen and consume hydrogen to produce energy, respectively, which gain uses as alternative energy sources (fuel cells) and fuel sources (electrolyzers). Combined configurations provide independent sustainable energy sources that can regenerate their hydrogen supply. SUMMARY OF THE INVENTION [0003]The following is a simplified summary providing an initial understanding of the invention. The summary does not necessarily identify key elements nor limit the scope of the invention, but merely serves as an introduction to the following description. [0004]One aspect of the present invention provides a method of preparing a gas diffusion electrode (GDE) for an electrochemical device, the method comprising: applying a mixture on a gas diffusion layer (GDL), wherein the mixture comprises a catalyst dispersion and a binder (e.g., Teflon) dispersion, and hot pressing the GDL to form the GDE. [0005]One aspect of the present invention provides a gas diffusion electrode (GDE) for an electrochemical device, the GDE comprising: a gas diffusion layer (GDL), and a mixture comprising a catalyst dispersion and a binder dispersion, applied on the GDL, wherein the GDL with the applied mixture is hot pressed to form the GDE. [0006]One aspect of the present invention provides an electrolyzer comprising the GDE (with carbon-based GDL) as a hydrogen evolution reaction (HER) electrode, and optionally the GDE (with metal-based GDL) as an oxygen evolution reaction (OER) electrode. 297987/

[0007]One aspect of the present invention provides a fuel cell comprising the GDE (with carbon-based GDL) as either or both of an oxygen reduction reaction (ORR) electrode and a hydrogen oxidation reaction (HOR) electrode thereof. [0008]One aspect of the present invention provides a dual cell, operable alternately as an electrolyzer and as a fuel cell, comprising the GDE (with carbon-based GDL and ionomer) as a HER/HOR electrode and another GDE (with metal-based GDL and optionally ionomer) as a OER/ORR electrode thereof. [0009]These, additional, and/or other aspects and/or advantages of the present invention are set forth in the detailed description which follows; possibly inferable from the detailed description; and/or learnable by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS [0010]For a better understanding of embodiments of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout. In the accompanying drawings: [0011] Figure 1A is a high-level schematic block diagram of an electrolyzer, according to some embodiments of the invention [0012] Figure 1B is a high-level schematic block diagram of a fuel cell, according to some embodiments of the invention. [0013] Figure 1C is a high-level schematic block diagram of a dual cell, according to some embodiments of the invention. [0014] Figure 2 is a high-level flowchart illustrating a method, according to some embodiments of the invention [0015] Figures 3A and 3B provide initial experimental results related to the operation of an electrolyzer, according to some embodiments of the invention. [0016] Figures 4A and 4B provide initial experimental results related to the operation of a fuel cell, according to some embodiments of the invention. [0017] Figures 5A and 5B provide high resolution scanning electron microscope (HRSEM) images of fuel cell ORR GDE prepared with Teflon and hot pressing, before and after operation in a fuel cell (after durability test, 450h at 80°C under 0.5A/cm), 297987/ respectively, according to some embodiments of the invention, compared with Figures 5Cand 5D that provide HRSEM images of prior art electrodes prepared without Teflon and without hot pressing according to prior art procedures, before and after operation in a fuel cell (after durability test, 310h at 80°C under 0.5A/cm), respectively. [0018]It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. DETAILED DESCRIPTION OF THE INVENTION [0019]In the following description, various aspects of the present invention are described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the present invention. However, it will also be apparent to one skilled in the art that the present invention may be practiced without the specific details presented herein. Furthermore, well known features may have been omitted or simplified in order not to obscure the present invention. With specific reference to the drawings, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. [0020]Before at least one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments that may be practiced or carried out in various ways as well as to combinations of the disclosed embodiments. 297987/ Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. [0021]Embodiments of the present invention provide efficient and economical methods and mechanisms for preparing gas diffusion electrodes (GDEs) and thereby provide improvements to the technological field of electrochemical devices such as electrolyzers, fuel cells and combined bi-directional systems. Methods of preparing gas diffusion electrodes (GDEs) for electrochemical devices such as electrolyzers and fuel cells are provided. The GDEs comprise a gas diffusion layer (GDL), and a mixture comprising a catalyst dispersion and a binder (e.g., Teflon) dispersion, applied on the GDL, wherein the GDL with the applied mixture is hot pressed to form the GDE. GDLs may be carbon-based or metal-based, and ionomer may be added to improve performance if needed. Briefly hot pressing the layer at or near the glass temperature of the binder improves the adhesion of the layer and its cohesivity, which improves its long-term performance and durability in electrolyzer and/or fuel cell applications. For example, the catalyst dispersion may comprise a catalyst dispersion and the GDE may be a hydrogen evolution reaction (HER) electrode operable in an electrolyzer. In another example, the catalyst dispersion may comprise a catalyst dispersion, the mixture may further comprise an ionomer, and the GDE may be an oxygen reduction reaction (ORR) electrode operable in a fuel cell. Certain embodiments comprise electrodes that may be operable reversibly, e.g., be used as HER/HOR electrodes and/or OER/ORR electrodes, for example in reversible devices (e.g., dual cells) that can be operated alternately in fuel cell and electrolyzer modes. Typically fuel cell electrodes may be made with carbon-based GDLs and the fuel cells may be operated with ionomeric electrolyte, while electrolyzer OER electrode may be made with metal-based GDLs and the electrolyzer may be operated with liquid electrolyte. Dual cells may be configured with carbon-based GDLs for the HER/HOR electrodes and with metal-based GDLs for the OER/ORR electrodes. Either or both types of GDEs may be prepared with binder material and be hot-pressed to improve their performance and/or durability. [0022] Figure 1A is a high-level schematic block diagram of an electrolyzer 110 , according to some embodiments of the invention. Electrolyzer 110 comprises GDE 112 as HER made of the GDL with the applied mixture of catalyst dispersion and binder dispersion (e.g., comprising Teflon) - hot pressed thereupon, and further comprising a 297987/ catalyst-coated porous transfer layer as an oxygen evolution reaction (OER) electrode 114 and electrolyte 90 . In certain embodiments, OER electrode 114 with metal-based GDL may likewise include binder material and be hot-pressed. Electrolyte 90 may be alkaline and comprise e.g., KOH, K CO  and/or KHCO  solutions at concentrations up to 10M (e.g., 1M, 1-5M, 3-10M or intermediate values) or may possibly comprise water (with ionomer material combined in the catalytic layer providing ionic conductivity). [0023]The binder material may be selected to enhance the stability and the durability of the electrode, particularly when hot pressed. Binder materials may comprise one or more materials, which have (i) low glass transition temperatures (e.g., Tg <180°C), (ii) low swelling properties (e.g., less than 80% swelling in X-Y direction in wet conditions, at 80°C , OH- form) - to make the respective electrode mechanically stable, (iii) sufficient chemical stability at alkaline conditions (e.g., 1M KOH), (iv) prolonged thermal stability, e.g., being stable above 100°C for at least 1000h. Specific examples for alternative binders include chlorotrifluoroethylene, perfluoroalkoxy alkane (PFA), ethylene tetrafluoroethylene, polyvinylidene fluoride or poly (methyl-methacrylate) or any combination of these materials. In any of the disclosed embodiments, the binder material may comprise Teflon and/or any binder(s) which conform to these requirements. In any of the embodiments in which Teflon is used, Teflon may be partly or fully replaced by other types of appropriate binders. [0024]In any of the disclosed embodiments, hot pressing may be optimized with respect to the type of binder and with respect to other GDE components – to yield the most stable and most efficient electrode, depending on performance requirements. For example, hot pressing may be carried out within the temperature range of 80-180°C (depending on the Tg of the selected binder as well as on the type of ionomer and other electrode materials) and carried out for the ranges of few seconds to a few minutes (e.g., between ten seconds and ten minutes). [0025]In non-limiting examples, a mixture of catalyst (e.g., Pt) dispersion in a solvent (e.g., 2-propanol and DI (deionized) water) and binder (e.g., Teflon) dispersion in water may be applied (e.g., sonicated and sprayed) on the GDL, which may then be pressed between plates to form GDE 112 . OER electrode 114 may comprise catalyst (e.g., Ni) dispersion in the solvent (e.g., 2-propanol and DI water), applied (e.g., sonicated and 297987/ sprayed) on a Ni PTL (porous transport layer). In certain embodiments, OER electrode 114 may be produced as a PTL, using binder dispersion and hot pressing, e.g., with respective catalysts/binders coated on the metal-based PTL and hot pressing for OER electrode 114 . OER electrode 114 may further comprise ionomer material, or comprise catalyst and binder material (e.g., Teflon) without additional ionomer. [0026] Figure 1B is a high-level schematic block diagram of a fuel cell 120 , according to some embodiments of the invention. Fuel cell 120 comprises GDE 122 as ORR made of the GDL with the applied mixture of catalyst dispersion, ionomer and binder (e.g., Teflon) dispersion hot pressed thereupon, and further comprising a hydrogen oxidation reaction (HOR) electrode 124 . For example, HOR electrode 124 may comprises a catalyst (e.g., Pt) dispersion and ionomer, applied (e.g., sonicated and sprayed) on a HOR GDL. The ionomer material may provide ionic conductivity, without requiring electrolyte solution in fuel cell 120 . [0027]In non-limiting examples, a mixture of catalyst (e.g., Ag) dispersion in solvent (e.g., 2-propanol and DI water), ionomer and binder (e.g., Teflon) dispersion in water may be applied (e.g., sonicated and sprayed) on the GDL, which may then be pressed between plates, for example stainless steel plates or other types of plates, to form GDE 122 . HOR electrode 124 may comprise catalyst dispersion in solvent (e.g., 2-propanol and DI water) mixed with ionomer and applied (e.g., sonicated and sprayed) on a GDL. [0028]In various embodiments, the solvent(s) may comprise, e.g., any of water, 2-propanol, ethanol, methanol, N-methyl-2-pyrrolidone, toluene, tetra-hydro-furan and/or combinations thereof with different ratios. Any of the dispersions may be formulated as an ink for the corresponding form of application. [0029]In certain embodiments, GDEs (with carbon-based GDLs) may be used in fuel cells 120 both as ORR electrode 122 and as HOR electrode 124 , with corresponding adjustments. [0030] Figure 1C is a high-level schematic block diagram of a dual cell 115 , according to some embodiments of the invention. Dual cell 115 may be reversible, configured to operate alternately (and reversibly) as electrolyzer 110 and fuel cell 120 , depending on the operation conditions of dual cell 115 , namely whether electricity is provided to dual cell 115 to generate hydrogen and oxygen by electrolysis (and be operated as electrolyzer 110 , 297987/ with electrolyte 90 comprising water or an alkaline solution) or whether hydrogen and oxygen are delivered to dual cell 115 to generate electricity (and be operated as fuel cell 120 ). Correspondingly, both GDEs, namely HER/HOR electrode 112 / 124 and OER/ORR electrode 114/122 , may be produced as disclosed herein, by spraying catalyst dispersion, binder (e.g., Teflon) and ionomer material of respective GDLs and hot pressing them to form the respective GDEs. Clearly the exact details of the catalyst type, binder (e.g., Teflon) concentration and ionomer type and concentration may be optimized to provide the required ionic conductivity and electrode stability, e.g., from the options disclosed herein, with respect to specific assembly and operation parameters of dual cell 115 . In certain embodiments, OER/ORR electrode 114/122 may be produced as a PTL, using binder dispersion and hot pressing, e.g., with respective catalysts/binders coated on the metal-based PTL and hot pressing for OER/ORR electrode 114/122 . OER/ORR electrode 114/122may further comprise ionomer material, or comprise catalyst and binder material (e.g., Teflon) without additional ionomer. [0031] Figure 2 is a high-level flowchart illustrating a method 200 , according to some embodiments of the invention. The method stages may be carried out with respect to the disclosed GDE electrodes, electrolyzer 110 and/or fuel cells 120 described above, which may optionally be configured to implement method 200 . Method 200 may comprise the following stages, irrespective of their order. [0032]Method 200 may comprise preparing a gas diffusion electrode (GDE) for an electrochemical device (stage 205 ), the method comprising: sonicating and spraying a mixture on a gas diffusion layer (GDL), wherein the mixture comprises a catalyst dispersion and a binder dispersion (stage 210 ), and hot pressing the GDL to form the GDE (stage 220 ), for example at the glass transition temperature of the binder, and e.g., between plates. [0033]In certain embodiments, method 200 may comprise preparing the GDE using a catalyst dispersion (stage 212 ), e.g., Pt, and using the GDE as a hydrogen evolution reaction (HER) electrode operable in an electrolyzer (stage 222 ), e.g., with a catalyst-coated porous ransport layer (PTL) as an OER electrode and KOH electrolyte. [0034]In certain embodiments, method 200 may comprise preparing the GDE using a catalyst (e.g., Ag) dispersion and ionomer (stage 214 ) and using the GDE as an oxygen 297987/ reduction reaction (ORR) electrode operable in a fuel cell (stage 224 ), e.g., with a catalyst (e.g., Pt) dispersion and ionomer, sonicated and sprayed on a HOR GDL and KOH electrolyte. [0035]In certain embodiments, method 200 may comprise configuring the device as an electrolyzer, fuel cell and/or a dual device (stage 207 ), with respective GDEs as ORR electrodes for fuel cells, HER electrodes for electrolyzers and/or preparing and using GDEs as a HER/HOR electrode and as a OER/ORR electrode in a dual device (stage 226 ). Method 200 may thus comprise using the GDEs to form a dual cell, that is operable alternately as an electrolyzer and as a fuel cell (with both GDEs including ionomer). [0036]In various embodiments, disclosed uses of binder and hot pressing may be applied to one or both types of electrodes in each type of device. For example, in fuel cells, only ORR electrode or both ORR and HOR electrodes may be produced using binder dispersion and hot pressing, e.g., with respective catalysts/binders coated on respective carbon-based GDLs. In electrolyzers, only HER electrode or both HER and OER electrodes may be produced using binder dispersion and hot pressing, e.g., with respective catalysts/binders coated on carbon-based GDL for the HER electrode and on metal-based PTL for the OER electrode. In dual systems, the OER/ORR (on metal-based PTL) electrodes and the HER/HOR (on carbon-based GDL) electrodes may be produced using binder dispersion and hot pressing as disclsoed herein. Specifically, in certain embodiments, PTL electrodes may be prepared with added binder and hot pressing, and be used on the oxygen side of the electrolyzer or the dual device (stage 230 ). [0037]In various embodiments, catalyst dispersion for either electrode may include other types of catalysts, such as other members of the platinum group metals (PGMs), non-supported or supported on carbon. For example, the hydrogen-side catalyst layer may include ionomer(s) with embedded hydrogen oxidizing and/or hydrogen evolving (generating) catalyst particles such as nanoparticles made of any of Pt, Ir, Pd, Ru, Ni, Co, Fe, Pd-CeOX and their alloys, blends and/or combinations, optionally supported on carbon or other conducting substrates. Alternatively or complementarily, the hydrogen-side catalyst layer may comprise modified carbons with embedded catalytic groups such as nitrides or various transition metals. Alternatively or complementarily, the hydrogen-side catalyst layer may comprise transition metal oxides or hydroxides based on Ni, Co, Mn, 297987/ Mo, Fe, etc., nitrogen-doped and/or metal-doped carbon materials. The hydrogen-side catalyst layer may have an ionomer content of between 0% to 40%w/w (or within subranges such as 0% to 10%w/w, 5% to 20%w/w, 10% to 30%w/w, 20% to 40%w/w, or other intermediate ranges). The hydrogen-side catalyst layer may be configured to be stable over the full voltage range of electrode operation, e.g., from under about -0.2 V in electrolyzer mode to over about +0.4V in fuel cell mode, versus a reversing hydrogen electrode. In non-limiting examples, the oxygen-side catalyst layer may include ionomer(s) with embedded cathode catalyst particles such as nanoparticles made of oxygen reducing and/or oxygen evolving (generating) catalysts made of any of NiFe2O4, Perovskites, Fe, Zn, Ag, Ag alloyed with Pt, Pd, Cu, Zr, Ag, Ni, Fe, Mn, Co, Pt, Ir, Ru their alloys, blends and/or combinations, possibly combined with metal oxides such as, e.g., cerium oxide, zirconium oxide, their alloys, blends and/or combinations. Alternatively or complementarily, the oxygen-side catalyst layer may comprise the metal particles in oxide or hydroxide form and/or include surface oxide or hydroxide layers. Alternatively or complementarily, the oxygen-side catalyst layer may comprise transition metal(s), metal oxide(s) and/or metal hydroxide(s) that are based on Ni, Fe, Co, Mn, Mo and their alloys, mixed oxides or mixed hydroxides such as spinel, perovskite or layered double hydroxide (LDH) structures, potentially doped with or loaded with Pt, Ir, Ru, Ag or other elements to enhance oxygen generation and/or reduction performance. [0038]Gas diffusion layer(s) (GDLs) and/or may include any type of gas diffusion layers such as carbon paper, non-woven carbon felt, woven carbon cloth and the like, nickel, titanium or stainless steel meshes, felts, foams, sintered microspheres, or other porous and electrically conductive substrates. In some embodiments, the GDLs may be attached to a microporous layer (MPL), made, e.g., from sintered carbon and/or optionally polytetrafluoroethylene (PTFE) or other hydrophobic particles, or from various porous metallic or other porous conductive layers. [0039]In various embodiments, the PTL (porous transport layer) may be made of the following materials: Ni, various grades of stainless steel, titanium or any combination of all of them together. In addition, it can be either felt, mesh, or dual layers, with different porosity values and different thicknesses. The PTL may be used with or without a mesoporous layer (MPL). 297987/

[0040]In non-limiting examples of anion exchange membrane (AEM) device implementations, the ionomeric material matrix may comprise a continuous anion conducting ionomer comprising, e.g., polyolefin(s), polyphenylene(s) and/or polysulfones. The continuous anion conducting ionomer may further comprise polymers or copolymers of (vinylbenzyl)trimethylammonium chloride, wherein the chloride counterion may be exchanged to any desired anion, copolymers of diallyldimethylammonium chloride (DADMAC), wherein the counterion may be exchanged to any desired anion, styrene-based polymers having quaternary ammonium anion conducting group, quaternized poly(vinylalcohol) (QPVA), bi-phenyl or tri-phenyl backboned polymers with one or more functional groups that could include alkyl tether group(s) and/or alkyl halide group(s) and/or equivalent groups, poly(arylpiperidinium) and other polymers containing cyclic quaternary ammonium in the backbone or on tethered sidechains, poly(bis-arylimidazoliums), cation-functionalized poly(norbornenes), neutral polymers or polymer membranes with grafted anion-conductive sidechains, or any other anion-conducting polymer. In some embodiments, the anion conducting ionomer may be crosslinked, e.g., using crosslinking agent(s) selected according to the type of the ionomer to be crosslinked, such as divinylbenzne, N,N,N',N'-tetramethyl-1,6-hexanediamine (TMHDA), 1,4-diazabicyclo[2.2.2]octane (DABCO), glyoxal, glutaraldehyde, styrene based polymer(s) having quaternary ammonium anion conducting group(s), bi-phenyl or tri-phenyl backboned with one or more functional groups that could include alkene tether group(s) and/or alkyl halide group(s) and/or equivalent groups, hydrocarbon chains, sulfur groups, siloxy groups, N-hydroxybenzotriazole groups, azide groups and the like. In some embodiments, the anion conducting ionomer may be a blend of several polymers, some of which may not be anion conducting. [0041]In non-limiting examples of PEM implementations, the ionomeric material matrix may comprise a continuous cation conducting ionomer comprising, e.g., poly(aryl sulfones), perfluorinated polysulfonic acids such as Nafion®, polymers or copolymers of styrene sulfonic acid with various modifications, sulfonated polyimides, phosphoric acid-doped poly(benzimidazole), sulfonated poly(arylene ethers) such as sulfonated poly (ether ether ketone) (SPEEK) and/or other synthetic or natural cation exchange ionomers. 297987/

[0042]Non-limiting examples and experimental results are provided in the following. In these examples, the combination of using Teflon material and brief hot-pressing was used to enhance the performance of the respective electrodes with respect to their stability and durability. GDEs with 5cm active area were prepared and tested in respective sealed electrolyzer and fuel cell configurations. [0043]In the electrolyzer configurations, catalyst dispersion was applied to yield a loading of 0.17 mg/cm on the HER GDE. The Teflon dispersion had a 60% wt% and 1.5 gr/ml density (in water) with particle size between 0.05-0.5µm. Mixtures with Teflon content ranging between 3wt%, 6wt% and 10wt% were compared. The mixture was sonicated for minutes and sprayed by a spray gun on Freudenberg carbon paper GDLs, and then hot-pressed at 119°C to change the Teflon to amorphous structure near its Tg (glass transition temperature). The Ni PTL OER electrode was prepared in a similar manner of spraying, without using Teflon, ionomer or applying hot pressing. The electrolyzer cells were assembled using Ni200 flow fields, stainless steel end plates, 50 µm PTFE sub-gaskets and 250/160µm thick PTFE gaskets at the cathode/anode sides, respectively, sealed under a torque of 7Nm. [0044]In the fuel cell configurations, the catalyst dispersion was applied to yield a loading of 2.5 mg/cm on the ORR GDE, with a 4wt% commercial ionomer. The Teflon dispersion had a 60% wt% and 1.5 gr/ml density (in water) with particle size between 0.05-0.5µm and an overall Teflon content of 3wt%. The HOR electrode was prepared in a similar manner of spraying a mixture of catalyst dispersion applied to yield a loading of 1.4 mg/cm and including 12wt% commercial ionomer. Both mixtures were sonicated for 15 minutes and sprayed by a spray gun on Freudenberg nonwoven carbon GDLs with microporous layer. The ORR GDE was hot-pressed at 119°C for 3 minutes at a pressure of 106 kg/cm, to change the Teflon to amorphous structure at its Tg (glass transition temperature). The fuel cells were assembled and sealed using 200µm thick Kapton polyimide gaskets on both electrodes, under a torque of 7Nm. [0045] Figures 3A and 3B provide initial experimental results related to the operation of electrolyzer 110 , according to some embodiments of the invention. Figure 3A indicates the performance and Figure 3B illustrates the durability of electrolyzer 110 having HER GDE 112 prepared from mixtures having no Teflon (0% Teflon content as comparison) 297987/ and mixtures having Teflon content of 3wt%, 6wt% and 10wt%. The results were measured for electrolyzers 110 described above, operating HER GDE 112 at 60°C under 170 ml/h flow of 1M KOH electrolyte, while OER electrode 114 was operated at room temperature without flow of electrolyte. It is noted that in this experiment, in a non-limiting manner, KOH was pumped only to the anode side, from which the KOH penetrated through the membrane and reached the cathode side. In a performance experiment ( Figure 3A ) the resulting current density (corresponding to the hydrogen production level) was measured with respect to the applied voltage, while in a durability experiment ( Figure 3B ) a constant level of current density (1 A/cm) was applied, which corresponds to a constant level of hydrogen production. [0046]In Figure 3A , for a given voltage, the efficiency of electrolyzer 110 is higher (greater hydrogen production) with higher current density, indicating maximal performance at a Teflon content of 3 wt%, which is better than no Teflon. In Figure 3B , the increase in voltage level (for the same current density) indicates the degradation of HER GDE 112 , indicating maximal durability at a Teflon content of 3wt%. The inventors suggest that the intermediate level of Teflon content provides on the one hand a more stable GDE layer having better adhesion to the GDL (see also Figures 5A-5D for corresponding SEM images of fuel cell cathodes) while, on the other hand, not making the GDE too hydrophobic as to reduce the permeation of the liquid KOH electrolyte and of the gaseous oxygen release, which might have occurred at higher Teflon content. Clearly, the optimal Teflon content depends on the production and operation conditions of electrolyzers 110 and may change for different designs. [0047]For example, it is noted however, that while Teflon makes the layer more hydrophobic and therefor requires longer time to equilibrate with the KOH electrolyte (that may lead to initial lower performance), in the long term the Teflon increases the durability of the layer, so there is some trade-off between initial performance and durability, which may be optimized in different ways, with different Teflon content, depending on details of production and use. Additional considerations involve the different effects of adding Teflon at different current densities, the possibility to include both Teflon and ionomer in the catalyst layer, which make the considerations and optimization more complex - (enhancing conductivity, but increasing sensitivity to the hot press parameters), and 297987/ possibly requiring modification of catalyst loading. Accordingly, parameters of Teflon application and hot pressing may be modified and optimized with respect to the electrode composition and performance requirements. On the other hand, it is noted that replacing some or all of the ionomer in the electrode with Teflon may provide benefits such as less or no degradation in alkaline environment (as might occur to the functional groups of the ionomer) and reduction or prevention of swelling and of leaching out of the catalyst during operation, which are main causes for reduced lifetime and performance. [0048] Figures 4A and 4B provide initial experimental results related to the operation of fuel cell 120 , according to some embodiments of the invention. Figure 4A indicates the initial performance and Figure 4B illustrates the durability of fuel cell 120 having ORR GDE 122 prepared from mixtures having no Teflon (0% Teflon content as comparison) and mixtures having Teflon content of 3wt% and 7wt%. The results were measured for fuel cells 120 described above, operated at 80°C. The dew point for ORR GDE 122 was 76°C and the back pressure 1 barg (gauge pressure); the dew point for HOR electrode 124 was 67°C and the back pressure (BP) 3 barg (note, the non-limiting experimental application of back pressure to the H2 side increases the water transport through the layer). In an initial performance experiment ( Figure 4A ) air flow to ORR GDE 122 was 1 l/min and H2 flow to HOR electrode 124 was 0.15 l/min; in a durability experiment ( Figure 4B ) air flow to ORR GDE 122 was 0.2 ml/min and H2 flow to HOR electrode 124 was 0.07 ml/min, while the current density was 0.5A/cm (the lower flow in the durability tests are in order to keep the MEA – membrane electrode assembly, from dry-out). In Figure 4A , the efficiency of fuel cell 120 with Teflon content of 7wt% in ORR GDE 122 was the highest over the entire current density range. In Figure 4B , the lowest rate of degradation was in fuel cell 120 with Teflon content of 3wt% in ORR GDE 122 . The inventors suggest that the intermediate level of Teflon content provides on the one hand a more stable GDE layer having better internal adhesion and adhesion to the GDL (see also Figures 5A-5D for corresponding SEM images) while, on the other hand, maintaining sufficient ionic conductivity via the ionomer in the GDE, avoiding too much interference by the Teflon material. [0049]In cases the method of adding Teflon and hot pressing the electrode are carried out for electrodes that include ionomer material (e.g., in fuel cells), the inventors have noted that carrying the process out when the ionomer includes HCO3- as counter ions (rather than 297987/ OH- as it does during operation with the electrolyte) – significantly reduces damage to the functional groups. Therefore, brief hot pressing around the glass temperature of Teflon is sufficient to improve electrode structure and layer adhesion and stability, while minimizing the damage to the ionomer and to electrode performance. [0050] Figures 5A and 5B provide high resolution scanning electron microscope (HRSEM) images of fuel cell ORR GDE prepared with Teflon (3wt%) and hot pressing, before and after operation in fuel cell 120 (after durability test, 450h at 80°C under 0.5A/cm), respectively, according to some embodiments of the invention, compared with Figures 5Cand 5D that provide HRSEM images of prior art electrodes prepared without Teflon and without hot pressing according to prior art procedures, before and after operation in fuel cell 120 (after durability test, 310h at 80°C under 0.5A/cm), respectively. [0051]The inventors note that comparing Figures 5A , 5Bwith Figures 5C , 5D , it seems that the Teflon and hot pressing helped keep the uniformity and integrity of the catalyst layer, preventing cracks and voids from being created during the durability test, and practically maintaining the layer morphology throughout the fuel cell operation. It is suggested that the Teflon binder creates a fine net that contributes to the stability of the layer, fixes its morphology and in small quantity does not interfere too much with the hydrophilicity/hydrophobicity of the layer, thereby keeping its good ionic conductivity-that is essential for stable durability test. In contrast, in prior art electrodes without Teflon and hot pressing, the catalyst layer is less uniform and exhibits non-patterned channels, cracks and voids that seem to have been created during the durability test, probably decreasing the voltage are related to leaching of ionomer and catalyst material which, together with reduced conductivity and formation of inactive regions contribute to fuel cell degradation. It is noted that upon applying hot pressing without addition of Teflon only minor changes in electrode durability have been observed. It is further noted that the exact percentage of Teflon and parameters of the hot pressing may change and be optimized with respect to the size, constituents and purpose of the electrode. [0052]In the above description, an embodiment is an example or implementation of the invention. The various appearances of "one embodiment”, "an embodiment", "certain embodiments" or "some embodiments" do not necessarily all refer to the same embodiments. Although various features of the invention may be described in the context 297987/ of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment. Certain embodiments of the invention may include features from different embodiments disclosed above, and certain embodiments may incorporate elements from other embodiments disclosed above. The disclosure of elements of the invention in the context of a specific embodiment is not to be taken as limiting their use in the specific embodiment alone. Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in certain embodiments other than the ones outlined in the description above. [0053]The invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described. Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined. While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents.

Claims (32)

297987/ CLAIMS

1. A method of preparing a gas diffusion electrode (GDE) for an alkaline or anion exchange membrane (AEM) electrochemical device, the method comprising: mixing a catalyst dispersion and a binder dispersion to form a mixture, applying the mixture on a gas diffusion layer (GDL), and hot pressing the GDL with the applied mixture to form the GDE, at or near the glass transition temperature of the binder, to modify the binder into its amorphous structure.

2. The method of claim 1, wherein the hot pressing is carried out between hot plates.

3. The method of claim 1 or 2, wherein the application of the mixture on the GDL is carried out by sonicating and spraying the mixture on the GDL.

4. The method of any one of claims 1-3, wherein the GDL is carbon-based and the GDE is a hydrogen evolution reaction (HER) electrode operable in an electrolyzer.

5. The method of claim 4, wherein the catalyst dispersion comprises catalyst particles of one or more of: Pt, Ir, Pd, Ru, Ni, Co, Fe and their alloys, mixtures, oxides or mixed oxides, and the binder comprises polytetrafluoroethylene (PTFE), with the hot pressing carried out at or near 119°C.

6. The method of claim 4, wherein the binder comprises at least one of PTFE, chlorotrifluoroethylene, perfluoroalkoxy alkane (PFA), ethylene tetrafluoroethylene, polyvinylidene fluoride and poly (methyl-methacrylate). 297987/

7. The method of any one of claims 1-4, wherein the GDL is metal-based and the GDE is an oxygen evolution reaction (OER) electrode operable in an electrolyzer.

8. The method of any one of claims 1-4, wherein the mixture further comprises an ionomer, and the GDE is an oxygen reduction reaction (ORR) electrode and/or a hydrogen oxidation reaction (HOR) electrode operable in a fuel cell.

9. The method of claim 8, wherein the catalyst dispersion is a Ag-based catalyst dispersion and the binder comprises PTFE), with the hot pressing carried out at or near 119°C.

10. The method of claim 8, wherein the binder comprises at least one of PTFE, chlorotrifluoroethylene, perfluoroalkoxy alkane (PFA), ethylene tetrafluoroethylene, polyvinylidene fluoride and poly (methyl-methacrylate).

11. A gas diffusion electrode (GDE) for an alkaline or anion exchange membrane (AEM) electrochemical device, the GDE comprising: a gas diffusion layer (GDL), and a mixture comprising a catalyst dispersion and a binder dispersion, applied on the GDL, wherein the GDL with the applied mixture is hot pressed at or near the glass transition temperature of the binder to form the GDE, and wherein the binder in the GDE has an amorphous structure.

12. The GDE of claim 11, wherein the GDL is carbon-based and the GDE is configured as at least one of: a HER electrode operable in an electrolyzer, a HER/HOR electrode operable in a dual cell, an ORR electrode operable in a fuel cell and/or a HOR electrode operable in a fuel cell. 297987/

13. The GDE of claim 12, wherein the catalyst dispersion comprises catalyst particles of one or more of: Pt, Ir, Pd, Ru, Ni, Co, Fe and their alloys, mixtures, oxides or mixed oxides, and the binder comprises PTFE, with the hot pressing carried out at or near 119°C.

14. The GDE of claim 12, wherein the binder comprises at least one of PTFE, chlorotrifluoroethylene, perfluoroalkoxy alkane (PFA), ethylene tetrafluoroethylene, polyvinylidene fluoride and poly (methyl-methacrylate).

15. The GDE of claim 11, wherein the mixture further comprises an ionomer, and the GDE is a ORR electrode and/or a HOR electrode operable in a fuel cell.

16. The GDE of claim 15, wherein the catalyst dispersion is a Ag-based catalyst dispersion and the binder comprises PTFE, with the hot pressing carried out at or near 119°C.

17. The GDE of claim 15, wherein the binder comprises at least one of PTFE, chlorotrifluoroethylene, perfluoroalkoxy alkane (PFA), ethylene tetrafluoroethylene, polyvinylidene fluoride and poly (methyl-methacrylate).

18. The GDE of claim 11, wherein the GDL is metal-based and the GDE is configured as a OER electrode operable in an electrolyzer and/or a OER/ORR electrode operable in a dual cell.

19. The GDE of any one of claims 11-18, wherein the hot pressing is carried out between hot plates.

20. An alkaline or AEM electrolyzer comprising the GDE of any one of claims 11-17 and as a HER electrode and optionally the GDE of claim 18 as a OER electrode.

21. An alkaline or AEM fuel cell comprising the GDE of any one of claims 11-17 and as a ORR electrode and/or as a HOR electrode of the fuel cell. 297987/

22. An alkaline or AEM dual cell, operable alternately as the electrolyzer of claim 20 and as the fuel cell of claim 21, comprising the GDE of any one of claims 11-17 and 19, wherein the mixture further comprises ionomer and the GDE is configured as HER/HOR electrode, and the GDE of claim 18 as a OER/ORR electrode.

23. A gas diffusion electrode (GDE) for an alkaline or anion exchange membrane (AEM) electrochemical device, the GDE comprising: a gas diffusion layer (GDL), and a mixture comprising a catalyst dispersion and a PTFE dispersion, applied on the GDL, wherein the PTFE in the GDE has an amorphous structure formed by hot-pressing the GDL with the applied mixture at or near the glass transition temperature 119°C of the PTFE.

24. The GDE of claim 23, wherein the GDL is carbon-based and the GDE is configured as at least one of: a HER electrode operable in an electrolyzer, a HER/HOR electrode operable in a dual cell, an ORR electrode operable in a fuel cell and/or a HOR electrode operable in a fuel cell.

25. The GDE of claim 24, wherein the catalyst dispersion comprises catalyst particles of one or more of: Pt, Ir, Pd, Ru, Ni, Co, Fe and their alloys, mixtures, oxides or mixed oxides.

26. The GDE of claim 23, wherein the mixture further comprises an ionomer, and the GDE is a ORR electrode and/or a HOR electrode operable in a fuel cell.

27. The GDE of claim 26, wherein the catalyst dispersion is a Ag-based catalyst dispersion. 297987/

28. The GDE of claim 23, wherein the GDL is metal-based and the GDE is configured as a OER electrode operable in an electrolyzer and/or a OER/ORR electrode operable in a dual cell.

29. The GDE of any one of claims 23-28, wherein the hot pressing is carried out between hot plates.

30. An alkaline or AEM electrolyzer comprising the GDE of any one of claims 23-27 and as a HER electrode and optionally the GDE of claim 28 as a OER electrode.

31. An alkaline or AEM fuel cell comprising the GDE of any one of claims 23-27 and as a ORR electrode and/or as a HOR electrode of the fuel cell.

32. An alkaline or AEM dual cell, operable alternately as the electrolyzer of claim 30 and as the fuel cell of claim 31, comprising the GDE of any one of claims 23-27 and 29, wherein the mixture further comprises ionomer and the GDE is configured as HER/HOR electrode, and the GDE of claim 28 as a OER/ORR electrode.