Porous electrolytic cell gas diffusion layer and method for producing the same

Abstract

A porous titanium sheet configured to function as an anode-side gas diffusion layer in a proton exchange membrane (PEM) electrolyzer is formed by a powder technique such as tape casting or powder metallurgy. A first major side of the porous titanium sheet has a higher porosity than an opposite second major side of the porous titanium sheet. The first major side of the porous titanium sheet is configured to face the anode-side flow plate, and the second major side of the porous titanium sheet is configured to face the anode electrode.

Description

Technical Field

[0001] (Cross - reference to Related Applications) This application claims the benefit of U.S. Provisional Patent Application No. 63 / 056,820, filed Jul. 27, 2020, the entire contents of which are incorporated herein by reference.

[0002] (Field) The present disclosure generally relates to electrolytic cells, and more specifically to gas diffusion layers for electrolytic cells and methods of making the same.

Background Art

[0003] Proton exchange membrane (PEM) electrolytic cells can be used to convert water into separate hydrogen and oxygen streams. Such PEM electrolytic cells include a polymeric electrolyte positioned between an anode electrode and a cathode electrode. Porous gas diffusion layers on the anode side and the cathode side are positioned adjacent to the respective anode and cathode electrodes.

Summary of the Invention

Means for Solving the Problems

[0004] (Abstract) In one embodiment, a porous titanium sheet configured to function as the anode - side gas diffusion layer of a proton exchange membrane (PEM) electrolytic cell is formed by a powder technique.

[0005] In one embodiment, a method includes fabricating, by a powder technique, a porous titanium sheet configured to function as the anode - side gas diffusion layer of a proton exchange membrane (PEM) electrolytic cell. The present invention provides, for example, the following: (Item 1) A porous titanium sheet configured to function as an anode-side gas diffusion layer in a proton exchange membrane (PEM) electrolytic cell, wherein the porous titanium sheet is formed by powder technology. (Item 2) The porous titanium sheet according to item 1, wherein the first main surface of the porous titanium sheet has higher porosity than the second main surface opposite the porous titanium sheet. (Item 3) The porous titanium sheet according to item 2, wherein the first main surface of the porous titanium sheet is configured to face the anode-side flow plate, and the second main surface of the porous titanium sheet is configured to face the anode electrode. (Item 4) The porous titanium sheet according to item 2, wherein the first main surface of the porous titanium sheet has porosity that is at least 10 percent higher than the opposite second main surface of the porous titanium sheet. (Item 5) The porous titanium sheet according to item 1, wherein the first main surface of the porous titanium sheet includes grooves, and the second main surface opposite the porous titanium sheet has a substantially flat surface without grooves. (Item 6) The porous titanium sheet contains a titanium nitride coating on at least one of its surfaces. The porous titanium sheet according to item 1, wherein the porous titanium sheet comprises pure titanium, or a titanium alloy containing more than 50 atomic percent of titanium and less than 50 atomic percent of at least one of molybdenum, vanadium, niobium, tantalum, or zirconium. (Item 7) The porous titanium sheet according to item 1, comprising a bimodal pore size distribution with micropores having an average pore size in the range of 1 to 5 microns and macropores having an average pore size in the range of 30 to 40 microns. (Item 8) The porous titanium sheet according to item 1, further comprising conductive vias extending through the porous titanium sheet in the thickness direction of the porous titanium sheet. (Item 9) PEM electrolytic cell, Anode-side fluid plate, Cathode-side fluid plate and A PEM polymer electrolyte is located between the anode-side fluid plate and the cathode-side fluid plate, An anode-side gas diffusion layer comprising a porous titanium sheet as described in item 1, located between the electrolyte and the anode-side fluid plate, An anode electrode is located between the anode-side gas diffusion layer and the electrolyte, A cathode-side gas diffusion layer is located between the electrolyte and the cathode-side fluid plate, A cathode electrode and A PEM electrolytic cell equipped with the following features. (Item 10) The first main surface of the porous titanium sheet has higher porosity than the second main surface of the porous titanium sheet opposite to it, or The first main surface of the porous titanium sheet faces the anode-side flow plate, and the second main surface of the porous titanium sheet faces the anode electrode, or The first main surface of the porous titanium sheet has porosity that is at least 10 percent higher than the opposite second main surface of the porous titanium sheet, or The PEM electrolytic cell according to item 9, wherein the first main surface of the anode-side fluid plate facing the anode-side gas diffusion layer contains water flow channel grooves, the first main side surface of the porous titanium sheet facing the first main surface of the anode-side fluid plate contains grooves that are substantially mirror images of the water flow channel grooves, and the opposite second main side surface of the porous titanium sheet has a substantially planar surface without grooves. (Item 11) A method comprising preparing a porous titanium sheet by powder technology, configured to function as an anode gas diffusion layer in a proton exchange membrane (PEM) electrolytic cell. (Item 12) The method according to item 11, comprising a powder metallurgy technique, the powder technique comprising: providing a mixture of titanium powder and a lubricant in a die cavity; compressing the mixture of titanium powder and lubricant in the die cavity to form a green sheet; debinding the green sheet; and sintering the green sheet to form the porous titanium sheet. (Item 13) The aforementioned powder technique is the method described in item 11, including tape molding. (Item 14) The aforementioned tape molding is The process involves mixing titanium-containing powder with a binder, solvent, and plasticizer to form a sliding material. Dispense the aforementioned sliding material onto the tape carrier web, Using a doctor blade, the sliding material moving on the tape carrier web is flattened into a green titanium-containing tape, The process involves drying the aforementioned green titanium-containing tape, The aforementioned green titanium-containing tape is cut into a first titanium green sheet having a first porous structure. The first green sheet is sintered to form the porous titanium sheet. The method described in item 13, including the method described in item 13. (Item 15) The titanium-containing powder comprises a mixture of elemental titanium and titanium hydride powder. The method according to item 14, wherein the titanium hydride is thermally converted to elemental titanium in an exothermic reaction. (Item 16) The method according to item 14, further comprising adding a pore-forming agent material to the sliding material, removing the pore-forming agent material during the sintering process, and forming pores within the porous titanium sheet. (Item 17) The method according to item 14, wherein the planarization, drying, cutting, and sintering steps occur continuously on the same movable tape carrier web. (Item 18) The method of item 14, further comprising, prior to sintering, placing a second titanium green sheet having a second porosity different from the first porosity on the first titanium green sheet. (Item 19) The first and second titanium green sheets are sintered in contact with each other such that the first main surface of the porous titanium sheet has higher porosity than the opposite second main surface of the porous titanium sheet. The method according to item 18, wherein the first main surface of the porous titanium sheet is configured to face the anode-side flow plate, and the second main surface of the porous titanium sheet is configured to face the anode electrode. (Item 20) The titanium-containing powder has an average diameter of less than 50% of the thickness of the porous titanium sheet. The titanium-containing powder is further mixed with at least one of the following: additional titanium-containing particles having an average diameter of no more than 20% of the thickness of the porous titanium sheet, or titanium-containing wires having an average length of no more than 20% of the thickness of the porous titanium sheet. The method according to item 14, wherein at least one of the additional titanium-containing particles or the titanium-containing wires extends throughout the entire thickness of the porous titanium sheet and functions as the conductive via. (Item 21) The method according to item 11, further comprising reaction-sintering the porous titanium sheet in a nitrogen-containing atmosphere to form a titanium nitride coating on at least one surface of the porous titanium sheet. (Item 22) The method according to item 11, further comprising placing the porous titanium sheet inside the PEM electrolytic cell. [Brief explanation of the drawing]

[0006] [Figure 1] Figure 1 is an oblique view of the internal structure of a PEM electrolytic cell.

[0007] [Figure 2] Figures 2A-2F are side cross-sectional views of the steps of a powder metallurgy method for fabricating a gas diffusion layer for a PEM electrolytic cell.

[0008] [Figure 3] Figure 3 is a perspective view of a gas diffusion layer according to one embodiment of the present disclosure.

[0009] [Figure 4] Figures 4 and 5 are other perspective views of the gas diffusion layer according to embodiments of the present disclosure. [Figure 5] Figures 4 and 5 are other perspective views of the gas diffusion layer according to embodiments of the present disclosure.

[0010] [Figure 6] Figure 6 is a side cross-sectional view of a tape forming apparatus that may be used to form a gas diffusion layer according to an embodiment of the present disclosure. [Modes for carrying out the invention]

[0011] (Detailed explanation) Figure 1 illustrates an oblique cut internal view of a PEM electrolytic cell, as described in the article “3D printed flow plates for The electrolysis of water: An economic and adaptable approach to device manufacture” by Greig Chisholm et al., Energy Environ. Sci., 2014, 7, 3026-3032. The PEM electrolytic cell 1 includes an anode-side flow plate 2 and a cathode-side flow plate 4, each with a fluid flow channel 6 and separate openings 8, 9, and 10; a PEM polymer electrolyte 12 located between the flow plates 2 and 4; an anode-side gas diffusion layer 14 located between the electrolyte 12 and the anode-side flow plate 2; an anode electrode 16 located between the anode-side gas diffusion layer 14 and the electrolyte 12; a cathode-side gas diffusion layer 18 located between the electrolyte 12 and the cathode-side flow plate 4; and a cathode electrode 20 located between the cathode-side gas diffusion layer 18 and the electrolyte 12.

[0012] The anode-side fluid plate 2 may include a water inlet opening 8, an oxygen outlet opening 9, and a water flow channel (e.g., a meandering channel groove) 6 connecting the water inlet opening 8 and the oxygen outlet opening 9 on the side of the fluid plate 2 facing the anode-side gas diffusion layer 14. The anode-side gas diffusion layer 14 may include a porous titanium layer. The cathode-side gas diffusion layer 18 may include a porous carbon layer. The anode electrode 16 may comprise any suitable anode catalyst, such as an iridium layer. The cathode electrode 20 may comprise any suitable cathode catalyst, such as a platinum layer. Other noble metal catalyst layers may also be used for the anode and / or cathode electrodes. The electrolyte 12 has the chemical formula C7HF 13 It may consist of any suitable proton exchange (e.g., hydrogen ion transport) polymer membrane, such as a Nafion® membrane, which is composed of a sulfonated tetrafluoroethylene-based fluoropolymer copolymer having O5S·C2F4.

[0013] During operation, water is supplied into the flow channel 6 through the water inlet opening 8. The water flows through the water flow channel 6 and through the anode-side gas diffusion layer 14 to the anode electrode 16. In response to the application of an external current or voltage between the anode electrode 16 and the cathode electrode 20, the water is electrochemically separated into oxygen and hydrogen at the anode electrode 16. The oxygen diffuses back to the anode-side flow plate 2 through the anode-side gas diffusion layer 14 and exits the electrolytic cell 1 through the oxygen outlet opening 9. Hydrogen ions diffuse through the electrolyte 12 to the cathode electrode 20 and then exit the electrolytic cell 1 through the cathode-side gas diffusion layer 18 and the hydrogen outlet opening 10 in the cathode-side flow plate 4.

[0014] A porous titanium layer (e.g., a sheet) may be used as the anode gas diffusion layer (i.e., transport layer) 14. In one embodiment, the porous titanium layer (e.g., a sheet) used as the anode gas diffusion layer 14 is formed by a powder process.

[0015] In one embodiment, the powder process includes tape formation. After the porous titanium sheet is sintered, it may be coated on both sides (e.g., the anode electrode side and the fluid plate side) with a conductivity-enhancing and / or corrosion-resistant coating, such as platinum and / or gold coating, to provide good conductivity and corrosion resistance. The coating may be formed by physical vapor deposition, such as evaporation.

[0016] In another embodiment, a porous titanium layer (e.g., a sheet) used as the anode gas diffusion layer 14 is formed by powder metallurgy techniques, in which titanium powder is press-formed into a porous titanium sheet using a compaction process. The compacted sheet is then sintered to yield a gas diffusion layer (e.g., a sheet) with established metallurgical bonds. The porous titanium sheet may have 40–60 percent porosity.

[0017] Figures 2A-2F illustrate exemplary steps of a powder metallurgy technique (e.g., the technique illustrated on azom.com (https: / / www.azom.com / article.aspx?ArticleID=155)) that can be used to form a porous anode-side titanium gas diffusion layer. The powder metallurgy press apparatus 100 includes a chamber 101 containing an upper puncher 102 and a lower puncher 104 that move axially relative to a die cavity 106 located within a die 108.

[0018] The steps of the powder metallurgy consolidation cycle include lowering the lower puncher 104 from a high position to expose the die cavity 106, as shown in Figures 2A and 2B. Titanium powder 112 is mixed with a lubricant and then supplied into the powder shoe 110. The powder shoe 110, containing the titanium powder 112 mixed with the lubricant, moves across the die cavity, filling the die cavity with the mixture of titanium powder 112 and lubricant. After the shoe 110 is withdrawn, the upper and / or bottom punchers 102, 104 move against the die 108, as shown in Figures 2C and 2D, to compress the titanium powder 112. After consolidation, as shown in Figure 2E, the upper puncher 102 is retracted upward and the bottom puncher 104 moves against the die 108, injecting the consolidated green titanium sheet 114 from the die cavity 106. The shoe 110 then moves again across the upper surface of the die 108, as shown in Figure 2F, which fills the die cavity with an additional mixture of titanium powder 112 and lubricant, pushing the green titanium sheet 114 out of the die 108.

[0019] The green titanium sheet 114 may be provided on a movable belt through one or more belt furnaces to move the green titanium sheet. The sheet may first be annealed at a lower temperature in a debindering process to sinter all organic lubricants (i.e., binders), and then annealed at a higher temperature to sinter the delubricated titanium sheet. The sintered porous titanium sheet 14 is then provided in an electrolytic cell (e.g., electrolytic cell 1 in Figure 1) between the anode-side fluid plate 2 and the membrane / electrode assembly (i.e., the electrolyte 12 with the anode electrode 16 and cathode electrode 20 on either side thereof) to function as the anode-side gas diffusion layer 14.

[0020] In one embodiment shown in Figure 3, the first main surface 14A of the porous titanium sheet 14 has higher porosity than the opposite second main surface 14B of the porous titanium sheet 14. The first main surface 14A of the porous titanium sheet 14 faces the anode-side fluid plate 2, and the opposite second main surface 14B of the porous titanium sheet 14 faces the anode electrode 16. For example, the first main surface 14A of the porous titanium sheet 14 may have at least 10 percent higher porosity than the opposite second main surface 14B of the porous titanium sheet 14. The first main surface 14A of the porous titanium sheet may have 40-50% porosity, while the opposite second main surface 14B of the porous titanium sheet may have 50-60% porosity. In one embodiment, the higher porosity of the porous titanium sheet 14 on the fluid plate 2 side 14A allows more water to enter the pores, while the lower porosity of the titanium sheet 14 on the anode electrode 16 side 14B provides improved electrical contact with the anode electrode 16.

[0021] The difference in porosity may be a continuous porosity gradient, where porosity increases continuously from the first main surface 14A to the second main surface 14B (i.e., between opposing main surfaces). Alternatively, porosity may vary in a stepwise manner, such that the porous titanium sheet 14 has at least first and second portions (14A, 14B) having distinct porosities different from each other. To form different porous regions, different amounts of lubricant are added to the upper and bottom portions of the titanium powder 112 in the die cavity 106, either in a continuous gradient or in a stepwise manner. The porous titanium sheet 14 has higher porosity in portion 14A, which is made from a powder portion containing a higher lubricant concentration than portion 14B, which is made from a powder portion containing a lower lubricant concentration. Different lubricant concentrations within the die cavity 106 may be formed by two or more different passes of the shoe 110, where the titanium powder 112 to lubricant ratio differs in each pass (e.g., higher or lower). The die cavity 106 is partially filled with a first portion of a mixture of titanium powder 112 and lubricant having a first titanium powder to lubricant ratio during the pass of the first shoe 110, and the die cavity 106 is further filled with a second portion of a mixture of titanium powder 112 and lubricant having a second titanium powder to lubricant ratio different from the first ratio during the pass of the second shoe 110. A lower titanium powder 112 to lubricant ratio results in higher porosity during the debinder annealing step.

[0022] In another embodiment, as shown in Figure 1, the first main surface of the anode-side fluid plate facing the anode-side gas diffusion layer 14 contains water flow channel grooves 6. The first main side surface 14A of the porous titanium sheet 14 facing the anode-side fluid plate 2 contains grooves 26, which are substantially mirror images of the water flow channel grooves 6, as shown in Figure 3. As used herein, substantially mirror images of water flow channel grooves 26 mean a shape that is a general mirror image of water flow channel grooves 6, but may have dimensions that differ by less than 20%. In contrast, the opposite second main side surface 14B of the porous titanium sheet 14 facing the anode electrode 16 has a substantially planar surface without grooves 26. Such a design maintains good electrical contact between the gas diffusion layer 14 and the anode electrode 16 while promoting water flow on the fluid plate 2 side 14A of the gas diffusion layer 14. The grooves 26 within the porous titanium sheet 14 may be formed by using the upper punching machine 102 or the lower punching machine 104 in the apparatus shown in Figure 2A, which include protrusions that are mirror images of the grooves 26.

[0023] In another embodiment shown in Figure 4, the porous titanium sheet 14 has a bimodal distribution of pore size as part of a porous network. The two modes of pore size are broadly classified as micropores 15B and macropores 15A, which are larger than micropores 15B. Micropores 15B act as capillaries, helping to transport water from the groove 6 to the anode electrode (e.g., the anode catalyst layer) 16, while macropores 15A provide a low-flow, resistant pathway for generated gas to exit from the electrocatalyst to the groove 6 and be transported by flowing water. The pore size may be calculated using the Young-Laplace equation. In one embodiment, the average pore size of micropores 15B is in the range of 1 to 5 microns, and the average pore size of macropores 15A is in the range of 30 to 40 microns.

[0024] In another embodiment, the formation of the porous titanium sheet 14 in the powder press apparatus 100 shown in Figures 2A-2F is carried out in an inert, low-oxygen partial pressure atmosphere in a chamber 101 so that the titanium is not oxidized to titanium dioxide, reducing resistance loss and avoiding sintering caused by titanium oxidation. The inert atmosphere may contain any suitable inert gas, such as a noble gas, such as argon. The atmosphere may contain an oxygen partial pressure of less than 0.1 atmospheres, such as 0.0001 to 0.01 atmospheres. The inert gas may be supplied into the chamber 101 containing the die 108 and the punching machines 102 and 104. Alternatively, the inert gas may be supplied as a gas blanket into the die cavity from an inert gas conduit after the die cavity has been filled with titanium powder.

[0025] In another embodiment, the formation of a porous titanium sheet 14 in the powder press apparatus 100 shown in Figure 2A-2F is carried out using titanium alloy powder. Titanium may be alloyed with one or more of molybdenum, vanadium, niobium, tantalum, and / or zirconium. Therefore, titanium alloy powder containing more than 50 atomic percent of titanium (e.g., 60-99 atomic percent of Ti and less than 50 atomic percent of molybdenum, vanadium, niobium, tantalum, and / or zirconium, such as 1-40 atomic percent) may be used in the powder press apparatus. The powder particles include titanium alloy particles. Alternatively, a combination of two powders (e.g., a mixture), i.e., pure titanium powder and a titanium-containing powder such as an alloying element powder containing molybdenum, vanadium, niobium, tantalum, and / or zirconium, may be used in the powder press apparatus. The powder combination contains titanium powder particles with more than 50 atomic percent (e.g., 60-99 atomic percent Ti) and alloying element powder particles with less than 50 atomic percent, such as 1-40 atomic percent. The porous titanium alloy sheet is formed during compaction in a powder press. A combination of chemical stability requirements and processing capacity requirements may be used to select the grade of the titanium alloy. The alloy grades can be grades 1, 2, 3, 6, 7, 9, 11, 16, 17, 18, 21, 24, 26, 27, 29, 32, 36, and / or 37.

[0026] In another embodiment, a precious metal coating (e.g., a gold or platinum-based metal coating) may be formed on a porous titanium sheet 14 during the powder metallurgy process, which increases the coating process speed and reduces the coating process cost. In this embodiment, the precious metal powder is supplied into the die cavities 106 below and above the titanium powder 112, as shown in Figures 2A-2F, followed by the compression of all the powder together using punching machines 102, 104. Different powder layers may be formed by three or more different passes of the shoe 110, in which the powder composition varies in different passes. The die cavity 106 is partially filled with a mixture of precious metal powder and lubricant during the first shoe pass, then the die cavity is further filled with a mixture of titanium powder 112 and lubricant during the second shoe pass, and finally the die cavity is filled with a mixture of precious metal powder and lubricant during the third shoe pass. In one embodiment, two different shoes 110 filled with different powders may be used. A first shoe, filled with a mixture of precious metal powder and lubricant, is used between the passage of the first and third shoes. A different second shoe, filled with a mixture of titanium powder and lubricant, is used between the passage of the second shoe. This forms a triple layer of precious metal / titanium / precious metal powder within the die cavity. The triple layer is then compressed and coated on both main surfaces with precious metal, forming a porous titanium plate.

[0027] In another embodiment shown in Figure 5, the porous titanium sheet has intrinsic vias filled with one or more precious metal powders, which produce highly conductive vias 22. In other words, the porous titanium or titanium alloy sheet 14 contains precious metal vias 22 extending through the thickness direction of the sheet 14. The precious metal vias 22 may contain any suitable precious metal such as ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and / or gold. The vias provide good conductivity and thus separate the conductive function from the water / gas transport function provided by the chemically stable porous sheet 14.

[0028] In another embodiment, a binder removal sintering protocol is selected to obtain a continuous noble metal coating on the opposite main surface of the porous titanium layer 14 while improving the porosity of the porous titanium sheet. This can be achieved by using a higher binder removal temperature increment rate and / or a lower sintering temperature. In one embodiment, a relatively high binder removal temperature increment rate may be 1°C / min to 5°C / min, for example, 2°C / min to 4°C / min. In one embodiment, a relatively low sintering temperature may be 1,100°C to 1,300°C, such as 1,150°C to 1,250°C.

[0029] In another embodiment, the method for forming the anode-side gas diffusion layer 14 includes controlling the formation of silicon compounds. For example, some metal alloys form a silicon compound phase during sintering (see JD Bolton, M. Youseffi & BS Becker (1998), Silicide Phase Formation and Its Influence on Liquid Phase Sintering in 316L Stainless Steel with Elemental Silicon Additions, Powder Metallurgy 41:2, (1998) 93-101 (which is incorporated herein in whole by reference), available at https: / / www.tandfonline.com / doi / abs / 10.1179 / pom.1998.41.2.93). Specifically, silicon added to 316L stainless steel causes the formation of a liquid silicon compound phase during sintering, which leaves large pores within the stainless steel portion after sintering is complete.

[0030] In one aspect of this embodiment, controlling silicon compound formation involves using titanium powder 112 with less than 0.1 weight percent silicon (e.g., 0 to 0.01 weight percent silicon) in the powder metallurgy process used to form the anode gas diffusion layer. The complete or substantial absence of silicon in the titanium powder avoids or reduces the formation of a titanium-silicon compound phase or multiple phases on the surface of the anode gas diffusion layer, which may provide undesirable surface properties for the anode gas diffusion layer.

[0031] In another aspect, the control of silicon compound formation involves using titanium powder 112 with at least 1 weight percent silicon (e.g., 1 to 10 weight percent silicon) dispersed throughout the die cavity in the powder metallurgy process used to form the anode gas diffusion layer 14. In this embodiment, the titanium-silicon compound phase forms throughout the entire thickness of the anode gas diffusion layer during sintering (e.g., liquid-phase sintering) and acts as a pore-forming agent. Thus, the porosity of the anode gas diffusion layer 14 is increased by generating a silicon compound phase pore-forming agent that intentionally leaves pores within the anode gas diffusion layer during sintering. Furthermore, because the silicon compound phase is dispersed throughout the anode gas diffusion layer 14, the silicon compound phase does not aggregate on the surface of the anode gas diffusion layer, and undesirable surface effects are avoided or reduced.

[0032] Therefore, a powder metallurgy method for producing a porous titanium sheet configured to function as an anode-side gas diffusion layer in a proton exchange membrane (PEM) electrolytic cell includes providing a mixture of titanium powder and lubricant in a die cavity, compressing the mixture of titanium powder and lubricant in the die cavity to form a green sheet, debinding the green sheet, and sintering the green sheet to form a porous titanium sheet.

[0033] In one embodiment, the method also includes providing a first mixture of noble metal powder and lubricant into the die cavity prior to providing a mixture of titanium powder and lubricant into the die cavity, and providing a second mixture of noble metal powder and lubricant into the die cavity after providing a mixture of titanium powder and lubricant into the die cavity. Compressing the mixture of titanium powder and lubricant in the die cavity occurs together with compressing the first and second mixtures of noble metal and lubricant to form a green sheet, and the step of sintering the green sheet forms a porous titanium sheet having a noble metal coating on both main surfaces of the porous titanium sheet.

[0034] In another embodiment, the porous titanium layer (e.g., a sheet) used as the anode gas diffusion layer 14 is formed by a tape forming process. Figure 6 illustrates an exemplary tape forming apparatus 200 that may be used to form the anode gas diffusion layer 14. A low-cost and scalable tape forming process can sustainably produce large-format titanium-containing tapes used to form the anode gas diffusion layer 14.

[0035] In one embodiment, the tape forming process shown in Figure 6 involves supplying a lubricant 202, such as a mixture of titanium powder, a solvent, a binder, and optionally one or more additional raw materials such as a plasticizer and / or surfactant (e.g., slurry), from a storage container such as a fluid-holding tank into a dispensing chamber 204. The titanium powder may consist of elemental (i.e., metallic) titanium powder and / or titanium hydride (e.g., TiH2) powder. Titanium hydride powder may be used to reduce production costs by lower raw material costs and less energy consumption during sintering, due to its exothermic reaction when it undergoes Ti conversion at approximately 600-800 degrees Celsius. Thus, in one embodiment, the titanium-containing powder consists of a mixture of elemental titanium and titanium hydride powder, the titanium hydride is subsequently thermally converted to elemental titanium in an exothermic reaction, such as during sintering and / or during a separate annealing step.

[0036] In one embodiment, titanium and titanium hydride powders with a desired particle size and distribution are mixed together with a polymer binder, an organic solvent, and an organic plasticizer, which can be removed during a low-oxygen or oxygen-free sintering process to form pores within the tape-formed anode-side gas diffusion layer 14. The binder, solvent, and plasticizer may consist of polypropylene carbonate ("PPC"), methyl ethyl ketone ("MEK"), and polycarbonate ("PC"), respectively. Other suitable materials may also be used.

[0037] The sliding material 202 is dispensed from the dispensing chamber 204 onto the movable tape carrier web 206. The tape carrier web 206 may include a belt made of metal (e.g., steel), glass, polymer, etc., which moves over the doctor blade 208. Moving on the tape carrier web 206 below the doctor blade 208, the sliding material 202 is flattened by the doctor blade 208 into a green titanium-containing tape 210. Green titanium-containing tapes 210 with various formulations of powder diameter, specific content, and solid ratio can be produced with slight variations in the sliding material (i.e., slurry) 202 formulation.

[0038] The tape carrier web 206 may then move the green titanium-containing tape 210 through a drying chamber 212. The drying chamber 212 may include a heated air inlet 214 and a saturated air outlet 216. Heated air (or another heat source) dries the green titanium-containing tape 210, and the solvent evaporated from the tape 210 is removed with the air through the saturated air outlet 216. The dried green titanium-containing tape 210 may then be cut in a cutting station 218 into titanium green sheets 210S having an anode-side gas diffusion layer.

[0039] The dried green titanium-containing tape 210, cut into titanium green sheets 210S, is subsequently sintered in a sintering chamber 220 at a desired temperature to form an anode-side gas diffusion layer 14. Preferably, the titanium green sheets 210S are sintered at 1,000 to 1,100 degrees Celsius in an oxygen-free or low-oxygen atmosphere. The atmosphere may consist of an inert atmosphere of any suitable inert gas such as argon or a noble gas. The atmosphere may contain an oxygen partial pressure of less than 0.1 atmospheres, such as 0.0001 to 0.01 atmospheres.

[0040] In one embodiment, the dried green titanium-containing tape 210 may be supplied from the drying chamber 212 into the cutting station 218, and the cut tape (i.e., titanium green sheet 210S) is then supplied from the cutting station 218 into the sintering chamber 220 for sintering, using the same tape carrier web 206. Optionally, if a debinding step is desired, performed at a temperature between the drying temperature and the sintering temperature, an additional chamber, such as a debinding chamber, may be located between the drying chamber 212 and the sintering chamber 220.

[0041] In one embodiment, the sintering chamber 220 may comprise a resistance-heated or gas-heated continuous furnace (e.g., a belt furnace). In this embodiment, the dried green titanium-containing tape 210 (i.e., titanium green sheet 210S) moves through the drying chamber 212, cutting station 218, and continuous furnace on the same tape carrier web 206. In another embodiment, the sintering chamber 220 may comprise a rapid heat annealing ("RTA") apparatus (also referred to as a rapid heat treatment ("RTP") apparatus) in which the cut and dried green titanium-containing tape 210 (i.e., titanium green sheet 210S) is heated by a flash lamp or laser beam. In this embodiment, the green titanium-containing tape 210 moves through the drying chamber 212, cutting station 218, and RTA apparatus on the same tape carrier web 206. Thus, the steps of planarizing the sliding material 202, drying the tape, cutting the tape, and sintering the cut tape can occur continuously on the same movable tape carrier web 206.

[0042] In one embodiment, the sintering chamber 220 may comprise an upstream portion 220A and a downstream portion 220B located downstream of the upstream portion 220A with respect to the direction of movement of the tape carrier web 206. An optional partition 220P may be provided between the upstream and downstream portions of the sintering chamber 220. The upstream portion 220A may be maintained in an oxygen-free or low-oxygen atmosphere, such as a rare gas (e.g., argon) atmosphere. The downstream portion 220B may be maintained in a nitrogen-containing atmosphere, such as a nitrogen gas or ammonia-containing atmosphere (e.g., a low-pressure or vacuum atmosphere with a partial pressure of nitrogen-containing gas). The titanium green sheet 210S may be reaction-sintered in the downstream portion 220B to form a titanium nitride layer on its surface. Thus, the titanium gas diffusion layer 14 may have a titanium nitride coating on one or both of the main surfaces 14A, 14B. The titanium nitride forms a hard, corrosion-resistant, and conductive coating on the titanium gas diffusion layer 14. This coating improves the performance of the titanium gas diffusion layer 14. Furthermore, a precious metal (e.g., Au or Pt) corrosion-resistant coating may be omitted if a titanium nitride corrosion-resistant coating is formed. It should be noted that a titanium gas diffusion layer 14 formed by methods other than tape molding, such as powder metallurgy, can also be reaction-sintered and have a titanium nitride coating formed on it.

[0043] In alternative embodiments, the sliding material 202 may be supplied onto the tape carrier web 206 from the side and / or bottom instead of from the top, as shown in Figure 6. When the sliding material 202 is supplied from the side, the apparatus 200 may be referred to as a slot die coater apparatus. When the sliding material 202 is supplied from the bottom, the apparatus 200 may be referred to as a margin coater or microgravure coater.

[0044] In summary, the tape forming method may include: mixing titanium-containing powder with a binder, a solvent, and a plasticizer to form a sliding material 202; dispensing the sliding material 202 onto a tape carrier web 206; planarizing the sliding material 202 moving on the tape carrier web 206 into a green titanium-containing tape 210 using a doctor blade 208; drying the green titanium-containing tape 210 in a drying chamber 212; cutting the green titanium-containing tape 210 into a first titanium green sheet 210S having a first porous structure in a cutting station 218; and sintering the first green sheet 210S to form a porous titanium sheet 14.

[0045] In one embodiment, a sacrificial pore-forming agent material powder may be added to the sliding material (i.e., slurry) 202 to enable a higher sintering temperature, and thus lead to an anode-side gas diffusion layer 14 with higher flexural strength and toughness. Such a pore-forming agent material may consist of engineered carbon powder, i.e., poly(methyl methacrylate) ("PMMA") powder sized to spherical microns and / or less than microns. The pore-forming agent material is removed during the sintering step to form pores within the gas diffusion layer 14.

[0046] In one embodiment, a porous titanium sheet gas diffusion layer 14, as shown in Figure 3, having higher porosity in one portion 14A than in another portion 14B, may be formed by tape forming. Such a porous titanium sheet can be formed by forming two separate green titanium sheets 210S having different porosity. Different porosity can be obtained by using tape material with different titanium powder diameters, different solid ratios, different content, and / or different volumes, compositions, and / or sizes within a sliding material 202. The two green titanium sheets 210S having different porosity are then placed in contact with each other and then sintered. Sintering forms the porous titanium sheet gas diffusion layer 14, as shown in Figure 3, with functional grading of pore size and microstructure. Thus, in this embodiment, the tape forming method also includes, prior to sintering, placing a second titanium green sheet 210S having a second porosity different from that of the first titanium green sheet 210S on top of the first titanium green sheet 210S. The first and second titanium green sheets 210S are sintered in contact with each other such that the first main surface 14A of the resulting sintered porous titanium sheet 14 has higher porosity than the opposite second main surface 14B of the porous titanium sheet 14B.

[0047] In another embodiment, the conductive vias 22 shown in Figure 5 may be formed within the tape-formed gas diffusion layer 14. The conductive vias 22 may be formed by forming precious metal or conductive titanium compound vias within the green and / or sintered titanium-containing tape 210. Alternatively, or in addition, larger titanium or titanium hydride particles and / or wires may be added to the sliding material 202. The larger particles may have an average diameter of no more than 20%, such as no more than 10% of the thickness of the porous titanium sheet 14. The wires (e.g., fibers) may have an average length of no more than 20%, such as no more than 10% of the thickness of the porous titanium sheet 14. The larger particles and / or wires are mixed with titanium or titanium hydride powder particles having an average diameter of less than 50% of the thickness of the porous titanium sheet 14. The larger particles and / or wires extend throughout the entire thickness of the porous titanium sheet 14 and function as conductive vias 22. As used herein, the thickness of the porous titanium sheet 14 is the dimension between the first main surface 14A and the second main surface 14A of the porous titanium sheet 14.

[0048] In another embodiment, the green titanium-containing tape 210 may have its surface embossed (e.g., roughened or patterned) to create raised and recessed surface areas. The pattern may include dimples recessed into the tape surface and / or waffle-shaped grids protruding from or recessed into the tape surface. Embossing the tape can increase its surface area and improve water flow. Embossing (e.g., roughening or patterning the surface) can be carried out by pressing a textured or patterned roller onto the surface of the green titanium-containing tape 210 prior to sintering, such as before or after drying the tape.

[0049] In another embodiment, a precious metal coating (e.g., a gold or platinum-based metal coating) or another corrosion inhibitor material coating may be formed on the porous titanium sheet 14 during tape formation. In this embodiment, the corrosion inhibitor material powder may be formed on the surface of the dried green titanium-containing tape 210 prior to sintering, and the corrosion inhibitor coating may be sintered on at least one surface of the porous titanium sheet 14.

[0050] In another embodiment, ions of materials other than titanium may be embedded in the porous titanium sheet 14 or the dried green titanium-containing tape 210. For example, silicon atoms may be ion-embedded to form the titanium-silicon compound pore-forming agent regions described above.

[0051] While the foregoing refers to certain preferred embodiments, it should be understood that the invention is not limited thereto. Those skilled in the art will recall that various modifications can be made to the disclosed embodiments, and that such modifications are intended to be within the scope of the invention. All published documents, patent applications, and patents cited herein are incorporated herein by reference as a whole.

Claims

1. A porous titanium sheet configured to function as an anode-side gas diffusion layer in a proton exchange membrane (PEM) electrolytic cell, The first main surface of the porous titanium sheet includes grooves, and the second main surface of the porous titanium sheet opposite to it has a substantially flat surface without grooves. The first main surface of the porous titanium sheet has a higher degree of porosity than the opposite second main surface of the porous titanium sheet. A porous titanium sheet in which a continuous porous gradient is formed between the first main surface of the porous titanium sheet and the opposite second main surface of the porous titanium sheet.

2. The porous titanium sheet according to claim 1, wherein the first main surface of the porous titanium sheet is configured to face the anode-side flow plate, and the second main surface of the porous titanium sheet is configured to face the anode electrode.

3. The porous titanium sheet according to claim 1, wherein the first main surface of the porous titanium sheet has a porosity at least 10 percent higher than the porosity of the opposite second main surface of the porous titanium sheet.

4. The porous titanium sheet contains a titanium nitride coating on at least one of its surfaces. The porous titanium sheet according to claim 1, wherein the porous titanium sheet comprises pure titanium, or a titanium alloy containing more than 50 atomic percent of titanium and less than 50 atomic percent of at least one of molybdenum, vanadium, niobium, tantalum, or zirconium.

5. PEM electrolytic cell, Anode-side fluid plate, Cathode-side fluid plate and A PEM polymer electrolyte located between the anode-side fluid plate and the cathode-side fluid plate, An anode-side gas diffusion layer comprising a porous titanium sheet according to claim 1, located between the electrolyte and the anode-side fluid plate, The anode electrode is located between the anode-side gas diffusion layer and the electrolyte, A cathode-side gas diffusion layer is located between the electrolyte and the cathode-side fluid plate, The cathode electrode located between the cathode-side gas diffusion layer and the electrolyte, Equipped with, The first main surface of the porous titanium sheet has a higher degree of porosity than the second main surface of the porous titanium sheet opposite to it. A PEM electrolytic cell in which a continuous porous gradient is formed between the first main surface of the porous titanium sheet and the opposite second main surface of the porous titanium sheet.

6. The PEM electrolytic cell according to claim 5, wherein the first main surface of the porous titanium sheet faces the anode-side flow plate, and the second main surface of the porous titanium sheet faces the anode electrode.

7. The PEM electrolytic cell according to claim 5, wherein the first main surface of the porous titanium sheet has at least 10 percent higher porosity than the opposite second main surface of the porous titanium sheet.

8. The PEM electrolytic cell according to claim 5, wherein the first main surface of the anode-side fluid plate facing the anode-side gas diffusion layer contains water flow channel grooves, the first main side surface of the porous titanium sheet facing the first main surface of the anode-side fluid plate contains grooves that are substantially mirror images of the water flow channel grooves, and the opposite second main side surface of the porous titanium sheet has a substantially planar surface without grooves.

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