Method for electrolyzing water, method for producing hydrogen, and method for producing cell of PEM water electrolysis device

Abstract

A method for electrolyzing water according to the present invention is a method for splitting water with the use of a PEM water electrolysis device which is provided with a cell in which a cathode, an electrolyte membrane, a porous transport layer, and an anode are stacked, wherein: the porous transport layer has a titanium porous body; in the electrolyte membrane-side surface of the titanium porous body, the average value of the areas of pores that open to the surface is 5 μm2 to 45 μm2 inclusive; the standard deviation value of the areas of the pores is 90 μm2 or less; the number of the pores that are present within a rectangular region that has an area of 22,000 μm2 and an aspect ratio of 4:3 is 120 or more; and the pressure applied in the stacking direction of the cathode, the electrolyte membrane, the porous transport layer, and the anode at the time of assembling the cell is set to 6 MPa or more.

Description

Method for electrolyzing water, method for producing hydrogen, and method for producing cells of PEM-type water electrolysis device

[0001] The present invention relates to a method for electrolyzing water using a PEM water electrolysis device having a cell including a stack of a cathode, an electrolyte membrane, a porous transport layer, and an anode, a method for producing hydrogen using the same, and a method for producing a cell for a PEM water electrolysis device.

[0002] Water electrolysis using a polymer electrolyte membrane (PEM)-type water electrolysis device can produce green hydrogen, particularly when using electricity derived from renewable energy, and is attracting great expectations in recent years as efforts to realize a decarbonized society accelerate.

[0003] A PEM-type water electrolysis device includes a cell composed of a cathode, an electrolyte membrane (proton exchange membrane), a porous transport layer, and an anode, which are stacked together. A plurality of cells are generally stacked together to form a stack. The porous transport layer (PTL) between the anode and the electrolyte membrane of the cell is required to have air or liquid permeability and electrical conductivity, as well as high corrosion resistance due to the environment in which corrosion may occur. To meet these requirements, the use of a titanium porous body for the porous transport layer has been considered.

[0004] Conventional techniques related to this include those described in Patent Documents 1 to 5 and Non-Patent Document 1.

[0005] International Publication No. 2023 / 068245 JP 2023-111139 A Japanese Patent No. 7467743 A Japanese Patent Application Laid-Open No. 2021-70849 A Special Publication No. 2024-516306

[0006] G. Bender et al., “Initial approaches in benchmarking and round robin testing for proton exchange membrane water electrolyzers”, INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 44, 2019, p. 9174-9187

[0007] The cells of a PEM water electrolysis device are assembled by applying a required pressure in the stacking direction of the cathode, electrolyte membrane, porous transport layer, and anode. This presses the porous transport layer against the electrolyte membrane. When a titanium porous body is used for the porous transport layer, if the pores on the surface of the titanium porous body are large, the electrolyte membrane pressed against the titanium porous body may partially penetrate into the pores, causing significant deformation in areas adjacent to the pores and potentially damaging the electrolyte membrane.

[0008] Therefore, from the viewpoint of preventing damage to the electrolyte membrane, it is desirable that the pores of the titanium porous body that open to the surface facing the electrolyte membrane are small and that the surface is smooth. Even if the pores on the surface are small, a certain degree of air or liquid permeability can be achieved as long as there are many of them.

[0009] However, it has been newly discovered that porous titanium with a smooth surface does not have very high electrical conductivity in a typical cell.

[0010] An object of the present invention is to provide a method for electrolyzing water, a method for producing hydrogen, and a method for producing a cell of a PEM water electrolysis device, in which a porous titanium body has a relatively smooth surface facing an electrolyte membrane in a cell, is air- and liquid-permeable, and exhibits relatively high electrical conductivity.

[0011] After extensive research, the inventors discovered that by increasing the pressure applied in the stacking direction of the cathode, electrolyte membrane, porous transport layer, and anode during cell assembly, a large current can be passed through a porous titanium body with a smooth surface and numerous pores even at low applied voltage. In cells assembled using porous titanium bodies with smooth surfaces, the electrolyte membrane is less likely to be damaged even when high pressure is applied in the stacking direction. This makes it possible to suppress damage to the electrolyte membrane, achieve the required air permeability, and perform good water electrolysis.

[0012] The water electrolysis method of the present invention is a method for electrolyzing water using a PEM water electrolysis device having a cell including a cathode, an electrolyte membrane, a porous transport layer, and an anode stacked together, wherein the porous transport layer has a titanium porous body, and the average area of ​​pores opening to the surface of the titanium porous body facing the electrolyte membrane is 5 μm 2 More than 45 μm 2 The standard deviation of the area of ​​the holes is 90 μm or less. 2 or less, and the area is 22000 μm 2 the number of holes present within a rectangular region having an aspect ratio of 4:3 is 120 or more, and a pressure of 6 MPa or more is applied in a stacking direction of the cathode, the electrolyte membrane, the porous transport layer, and the anode during assembly of the cell.

[0013] In the water electrolysis method, the thickness of the titanium porous body is preferably 500 μm or less. At least one of the electrolyte membrane-side surface and the anode-side surface of the titanium porous body is preferably coated with a platinum group metal. The titanium porous body preferably has a plurality of superposed porous layers.

[0014] The method for producing hydrogen of the present invention involves decomposing water using any of the electrolysis methods described above to produce hydrogen.

[0015] The cell manufacturing method of the present invention is a method for manufacturing a cell in a PEM water electrolysis device including a cell including a cathode, an electrolyte membrane, a porous transport layer, and an anode stacked together, wherein the porous transport layer has a titanium porous body, and the average area of ​​pores opening to the surface of the titanium porous body facing the electrolyte membrane is 5 μm 2 More than 45 μm 2 The standard deviation of the area of ​​the holes is 90 μm or less. 2 or less, and the area is 22000 μm 2 The number of holes present within a rectangular region having an aspect ratio of 4:3 is 120 or more, and a pressure of 6 MPa or more is applied in the stacking direction of the cathode, the electrolyte membrane, the porous transport layer, and the anode during assembly of the cell.

[0016] In the above-described cell manufacturing method, the thickness of the porous titanium body is preferably 500 μm or less. At least one of the electrolyte membrane-side surface and the anode-side surface of the porous titanium body is preferably coated with a platinum group metal. The porous titanium body preferably has a plurality of superposed porous layers.

[0017] According to this invention, the porous titanium body has a relatively smooth surface facing the electrolyte membrane in the cell, is air-permeable or liquid-permeable, and can exhibit relatively high electrical conductivity.

[0018] Embodiments of the water electrolysis method, hydrogen production method, and PEM water electrolysis cell manufacturing method of the present invention are described in detail below. In one embodiment of the water electrolysis method, water is decomposed by electrolysis using a PEM water electrolysis device. Hydrogen is produced together with oxygen by this water electrolysis. In another embodiment of the cell manufacturing method, a PEM water electrolysis cell is manufactured. The cell includes a cathode, an electrolyte membrane, a porous transport layer, and an anode stacked in this order. The porous transport layer has a titanium porous body.

[0019] In the above-mentioned method for electrolyzing water and the method for manufacturing a cell, the average area of ​​the pores opening on the surface of the titanium porous body on the electrolyte membrane side is 5 μm 2 More than 45 μm 2 The standard deviation of the area of ​​the hole is 90 μm or less. 2 or less, and the area is 22000 μm 2 The number of such pores present within a rectangular region with an aspect ratio of 4:3 is 120 or more. If a relatively large number of fine pores are formed on the surface of the titanium porous body in this way, damage to the electrolyte membrane pressed against the surface can be effectively suppressed. This is thought to be because it becomes difficult for the electrolyte membrane to partially penetrate into the fine pores, and because the load acting on the electrolyte membrane when the titanium porous body is pressed against it is dispersed among the large number of pores.

[0020] However, porous titanium bodies with numerous fine pores formed on their surfaces, as described above, tend not to have very high electrical conductivity in a typical cell. In contrast, in the water electrolysis method and cell manufacturing method according to this embodiment, a pressure of 6 MPa or more is applied in the stacking direction of the cathode, electrolyte membrane, porous transport layer, and anode during cell assembly. By increasing the pressure applied during cell assembly in this manner, it is believed that the contact area between the porous transport layer containing the porous titanium body and the electrolyte membrane is increased, reducing contact resistance while suppressing damage to the electrolyte membrane, allowing a large current to flow even when the applied voltage is relatively low. As a result, water electrolysis can be performed satisfactorily.

[0021] (Cell Manufacturing Method) A cell in a PEM water electrolysis device includes a cathode, an electrolyte membrane, a porous transport layer, and an anode, with the electrolyte membrane sandwiched between the cathode and the anode, and the porous transport layer sandwiched between the electrolyte membrane and the anode. A porous transport layer may also be provided between the cathode and the electrolyte membrane. In addition, in a stack, multiple cells may be formed by sandwiching a bipolar plate between the cathode and the anode.

[0022] The anode and cathode are often made of a highly conductive metal such as copper. The porous transport layer on the anode side includes a sheet-like porous titanium body. To enhance electrical conductivity, the porous titanium body may be coated on at least one surface with, for example, a noble metal, more specifically, a platinum group metal (platinum, ruthenium, rhodium, palladium, osmium, iridium, typically platinum). This coating may be applied to one or both of the electrolyte membrane-side and anode-side surfaces of the porous titanium body. The porous transport layer on the cathode side is made of, for example, carbon or graphite.

[0023] The electrolyte membrane, which may also be referred to as a proton exchange membrane, may typically include a sulfonic acid cation exchange membrane made of a fluororesin. An anode catalyst layer made of iridium oxide (IrO) or the like may be provided between the electrolyte membrane and the anode. Meanwhile, a cathode catalyst layer made of platinum (Pt) or the like may be provided between the electrolyte membrane and the cathode. Each catalyst layer may be bonded to the electrolyte membrane by coating or pasting, and an electrolyte membrane provided with such a catalyst layer may be referred to as a catalyst-coated membrane (CCM). However, the anode catalyst layer and / or cathode catalyst layer may be bonded to the porous transport layer on the anode or cathode side, or may simply be sandwiched between the porous transport layer on the anode or cathode side and the electrolyte membrane.

[0024] Separators may be provided between the anode and the anode-side porous transport layer, and between the cathode and the cathode-side porous transport layer. The cathode-side separator and the anode-side separator may each be made of titanium or stainless steel, and their surfaces may be coated with a precious metal such as a platinum group metal. To reduce the size of the device, the separator may be integrated with the anode or cathode, respectively, and in such cases, the separator material is used. As described above, separators may be arranged as appropriate depending on the requirements for the cell or stack.

[0025] In a PEM water electrolysis system, a plurality of cells as described above are stacked to form a stack, which is often arranged inside the system. When forming a stack, for cells other than the endmost cells, the cathode-side separator and cathode of the cell, as well as the anode and anode-side separator of the adjacent cell, are replaced with bipolar plates, allowing the cells to be connected and stacked together. In this case, the innermost cell includes an electrolyte membrane and a porous transport layer stacked between two bipolar plates. The bipolar plates may be made of the same material as the separators.

[0026] The porous titanium body of the porous transport layer on the anode side has pores with an average area of ​​5 μm2 that open to the surface facing the electrolyte membrane. 2 More than 45 μm 2 In this way, if the porous titanium body has pores on its surface with a relatively small area, the surface can be said to be generally smooth, and damage to the electrolyte membrane pressed against it can be effectively suppressed.

[0027] From this viewpoint, the average area of ​​the pores on the surface of the titanium porous body facing the electrolyte membrane is 5 μm 2 More than 38 μm 2 Below, 5μm 2 More than 34 μm 2 Further, 7 μm 2 More than 25 μm 2 It is preferable that the average area of ​​the holes is less than 1 / 2 mm. If the average area of ​​the holes is too small, the air permeability or liquid permeability may decrease. If the average area of ​​the holes is too large, the electrolyte membrane may partially penetrate into the holes, causing significant deformation in the areas close to the holes and potentially damaging the electrolyte membrane.

[0028] From the same viewpoint, the standard deviation of the pore area on the surface of the titanium porous body facing the electrolyte membrane is 90 μm 2 It is preferably 75 μm or less. 2 Further, 3 μm 2 ~50μm 2 , especially 5 μm 2 ~45μm 2A small standard deviation value means that most of the pores present on the surface facing the electrolyte membrane are of the required small size.

[0029] In addition, the surface of the titanium porous body located on the electrolyte membrane side has an area of ​​22,000 μm 2 The number of pores present in a rectangular region having an aspect ratio of length:width = 4:3 is 120 or more, preferably 130 or more, more preferably 200 or more, and even more preferably 250 or more. The presence of many fine pores on the surface makes it possible to achieve smoothness while ensuring the required breathability or liquid permeability. The number of pores within the rectangular region on the surface is not limited to this, but may be, for example, 700 or less, or 550 or less.

[0030] In order to improve the liquid permeability or air permeability of the surface of the titanium porous body opposite or behind the electrolyte membrane side (also referred to as the "opposite surface"), it is preferable that the average area of ​​the pores opening on the opposite surface is at least 1.5 times, and even at least 3 times, the average area of ​​the pores on the surface facing the electrolyte membrane. Other configurations can be appropriately determined taking into account the liquid permeability or air permeability required of the porous transport layer. In order to satisfy the above-mentioned pore configuration on the surface facing the electrolyte membrane and the surface facing the opposite surface, the titanium porous body may be constructed by stacking multiple porous layers.

[0031] The average value and standard deviation of the area of ​​the pores opening to the surface, as well as the number of pores within a predetermined rectangular area, are measured using a scanning electron microscope (Keyence Corporation, ultra-deep multi-angle lens VHX-D510). More specifically, for both the surface on the electrolyte membrane side and the surface on the opposite side, the average value and standard deviation of the area of ​​the pores opening to the surface are measured using a scanning electron microscope (Keyence Corporation, ultra-deep multi-angle lens VHX-D510). 2An SEM image was acquired at 2000x magnification for a rectangular region with an aspect ratio of 4:3. The SEM image was then analyzed using a scanning electron microscope. Half the maximum detected brightness value for the SEM image was used as a threshold, and closed regions with brightness ranging from 0 to the threshold were considered to be individual holes. If necessary, the SEM image may be binarized. After the binarization process, small grain removal (processing to convert black pixels after binarization) was performed on closed regions of 50 pixels or less, and then hole filling (processing to convert white pixels after binarization) was performed on closed regions of 50 pixels or less. This was used to calculate the number and area of ​​each hole, and the standard deviation, expressed as the square root of the variance, was determined. This SEM image analysis was performed on five rectangular regions on the surface, at least partially offset from each other. The average area and average number of holes in these rectangular regions were used as the average area and standard deviation of holes on the surface, and the number of holes in the rectangular regions, respectively. In the case of a surface that is a square or a rectangle in plan view, the five rectangular regions are the five rectangular regions at the center and four corners. If the surface of the titanium porous body facing the electrolyte membrane is coated with a coating (a coating of a noble metal, more specifically a platinum group metal) to enhance electrical conductivity, the pores on the coated surface are observed and the average value, standard deviation, and number of pores are measured.

[0032] In order to improve the air permeability or liquid permeability of the porous titanium body while maintaining high surface smoothness, the porous titanium body preferably has a porosity of 30% or more and 50% or less, or 35% or more and 50% or less. The porosity of the porous titanium body may be 30% or more and less than 60%. The porosity ε is calculated by multiplying the apparent density ρ' calculated from the volume and mass determined from the external dimensions of the porous titanium body, such as width, length, and thickness, by the true density ρ (4.51 g / cm) of titanium constituting the porous titanium body. 3 ) and calculate using the formula: ε = (1 - ρ' / ρ) x 100. Even if the surface of the porous titanium body is coated, the true density ρ is the true density of titanium.

[0033] The titanium porous body is made of titanium. If it is made of titanium, it can be said that the titanium porous body has high electrical conductivity at a certain relative density. The titanium content of the titanium porous body is preferably 97% by mass or more, and more preferably 98% by mass or more. The upper limit of the titanium content is not limited to, but may be, for example, 99.8% by mass or less, or 99% by mass or less. This titanium content refers to the purity of titanium taking into account not only metal components but also gas impurities such as oxygen. Therefore, the titanium content is determined by subtracting the total content of metal components and impurities, including gas components, from 100% by mass. The titanium porous body may have a purity equivalent to pure titanium grades 1 to 4, typically grades 1 to 2, specified in JIS H 4600 (2012), excluding the oxygen content. The oxygen content of the titanium porous body can be measured by inert gas fusion-infrared absorption spectroscopy. Metal components other than titanium in the titanium porous body can be measured by inductively coupled plasma (ICP) atomic emission spectroscopy. When the surface of the porous titanium body is coated with a precious metal or the like, the content of the precious metal or the like that constitutes the coating is not taken into consideration when determining the titanium content.

[0034] The thickness of the titanium porous body (when composed of multiple porous layers, the total thickness including all the porous layers) may be, for example, 40 μm or more and 3000 μm or less. The upper limit of the thickness range is preferably 1500 μm or less, 1000 μm or less, 500 μm or less, even 400 μm or less, and particularly preferably 300 μm or less. By making the titanium porous body somewhat thin, it is possible to increase the number of cells constituting the stack without significantly increasing the size of the PEM water electrolysis apparatus. The lower limit of the thickness range may be 50 μm or more, 80 μm or more. From the viewpoint of ensuring good mechanical strength of the entire sheet while maintaining the surface smoothness described above, it is preferable that the sheet-like titanium porous body be somewhat thick.

[0035] The surface area of ​​the sheet-like porous titanium body in a plan view is not particularly limited and can be determined appropriately depending on various conditions. For example, it is 70 mm 2 More than 2,000,000 mm 2Below, 10,000 mm 2 More than 150,000 mm 2 The term "sheet-like" as used herein means a plate-like or foil-like shape having a small thickness relative to the dimensions in a plan view, and the shape in a plan view is not particularly limited.

[0036] The above-described porous titanium body (at least the porous layer on the electrolyte membrane side when multiple porous layers are stacked) can be prepared, for example, as follows. Specifically, a paste prepared by mixing fine titanium powder with an organic solvent, an organic binder, or the like and defoaming the mixture is applied to a sheet, followed by drying, degreasing, and heating at predetermined temperatures to sinter the titanium powder. This results in a powder sintered body with a titanium sponge skeleton and a three-dimensional network structure. The titanium powder preferably has a volume-based average particle size D50 of 10 μm or more and 35 μm or less, preferably 10 μm or more and 28 μm or less, and particularly preferably 10 μm or more and 20 μm or less, as measured for 5,000 particles using a particle shape image analyzer (e.g., PITA-4). The use of such fine titanium powder facilitates the formation of a sintered titanium body with the above-described properties on the electrolyte membrane side. Titanium powder tends to have an average circularity of, for example, 0.6 or more, typically 0.7 or more, measured on 5,000 particles using a particle shape image analyzer (such as the PITA-4). A particle with an average circularity lower than this can be determined to be titanium fiber. When titanium fiber is used instead of titanium powder, the resulting sintered fiber compact has a nonwoven fabric-like skeleton. When pressed against the electrolyte membrane, the tips of the fibers protruding from the surface are likely to damage the membrane. The higher the pressure applied during cell assembly, the more likely this damage is to occur.

[0037] If a titanium porous body is formed by stacking multiple porous layers, the porous layer located on the electrolyte membrane side is preferably the powder sintered body described above. The porous layers other than the porous layer located on the electrolyte membrane side may be the powder sintered body described above, or may be sintered fiber, titanium mesh, titanium expanded metal, titanium punched metal, titanium lotus metal, or any of these with additional holes. The method for joining the multiple porous layers is not particularly limited, and various methods such as sintering, laser fusion joining, and spot welding can be used.

[0038] To manufacture a cell, a predetermined pressure (pressure during cell assembly) is applied in the stacking direction by, for example, fastening the stack of an anode, a porous transport layer containing the porous titanium body described above, an electrolyte membrane, and a cathode. The pressure applied here is 6 MPa or more. Applying such a high pressure increases the electrical conductivity of the porous titanium body within the cell, allowing a large current to flow through the porous titanium body even when a relatively low voltage is applied between the anode and cathode. Furthermore, because the porous titanium body has excellent surface smoothness on the electrolyte membrane side, the electrolyte membrane is less likely to be damaged even when the pressure during cell assembly is as high as described above.

[0039] The pressure during cell assembly is preferably 6 MPa or more, and more preferably 7 MPa or more. On the other hand, the pressure during cell assembly may be 50 MPa or less, and even 30 MPa or less. By not using too high a pressure during cell assembly, the electrolyte membrane is less likely to be damaged and the required size of pores in the titanium porous body can be ensured. Furthermore, the required mechanical strength of the cell can be reduced, thereby preventing cost increases. The pressure during cell assembly can be adjusted, for example, by changing the magnitude of the bolt tightening torque or the thickness and characteristics of sealing members such as gaskets that may be placed between the electrode plates.

[0040] To measure the pressure during cell assembly, a pressure-sensitive paper such as PRESCALE (manufactured by Fujifilm Corporation) is placed between the electrolyte membrane and the porous titanium body when assembling the cell by stacking the anode, porous transport layer containing the porous titanium body, electrolyte membrane (CCM), and cathode. If the pressure-sensitive paper is small relative to the area of ​​the porous titanium body, multiple sheets of pressure-sensitive paper can be used. After leaving the assembled cell for 2 minutes, the cell is disassembled, the pressure-sensitive paper is removed, and the pressure is measured. To measure the pressure, a photograph is taken so that the standard color sample and the pressure-sensitive paper appear in the same image, and the image is converted to 256 gradations. The image resolution is 0.125 mm / dot. The area that was in contact with the porous titanium body is designated. The higher the pressure, the darker the pressure-sensitive paper appears. A histogram of each pixel (arranged from dark to light) is created in 256 gradations, and the pressure at the point where the integrated value is 5% of the total number of pixels is taken as the pressure during cell assembly. Therefore, according to this method, the pressure closest to the maximum in the histogram is the pressure at the time of cell assembly. After the PEM water electrolysis device is used, the pressure acting on the cell may change, and the pressure after use may not match the pressure at the time of cell assembly. The cell may be included in a stack. The tightening torque of an already assembled cell can be estimated by the loosening torque method, the marking method, the retightening torque method, etc. When assembling the cell, the estimated tightening torque value can also be reproduced to estimate the pressure at the time of cell assembly.

[0041] Aside from the above, the cell can be manufactured according to known techniques. A plurality of cells may be stacked to manufacture a stack. Furthermore, a PEM-type water electrolysis device including such a cell or stack can be manufactured.

[0042] (Water Electrolysis Method) In an embodiment of a water electrolysis method, water is split using a PEM water electrolysis device including the above-described cell.

[0043] More specifically, when a voltage is applied between the anode and cathode of the cell, water (HO) supplied to the anode side is converted mainly into 2HO → 4H + + O2 + 4e - The oxidation reaction produces hydrogen ions (H +) and oxygen (O2). The hydrogen ions pass through the electrolyte membrane and - passes through an external circuit and moves to the cathode side, and 2H + +2e - →Hydrogen (H2) is produced by the reduction reaction of H2. Therefore, looking at the reaction as a whole, water is decomposed into oxygen and hydrogen. This allows hydrogen to be produced.

[0044] As mentioned above, the porous transport layer in the cell of the PEM water electrolysis device used here has a titanium porous body. This titanium porous body has pores with an average area of ​​5 μm2 on the surface facing the electrolyte membrane. 2 More than 45 μm 2 The standard deviation of the hole area is 90 μm or less. 2 or less, and the area is 22000 μm 2 The number of pores present within a rectangular region having an aspect ratio of 4:3 is 120 or more. As also mentioned above, the thickness of the porous titanium body is preferably 500 μm or less.

[0045] Next, tests were conducted to confirm the effectiveness of the water electrolysis method of the present invention, which will be described below. However, the description here is for illustrative purposes only and is not intended to be limiting.

[0046] The cathode, electrolyte membrane, porous transport layer, and anode were stacked and sandwiched between separators, and a predetermined pressure was applied to the cathode, electrolyte membrane, porous transport layer, and anode in the stacking direction. The electrolyte membrane consisted of a WEPEM (N-115) main body of the electrolyte membrane, with a cathode catalyst layer of TEC10E50E Pt 1.0 mg / cm on each side. 2 and IrO2 / TiO2 1.0 mg / cm2 of the anode catalyst layer 2A triple-layered CCM was used. The cathode-side porous transport layer contained carbon felt, and the anode-side porous transport layer contained a titanium porous powder sintered compact or fiber sintered compact. The separator was a Pt-coated Ti plate with a groove-shaped path. This was used to assemble and manufacture a cell (WER-1 (mini cell) manufactured by FC development Co., Ltd.). The pressure applied to the cathode, electrolyte membrane, porous transport layer, and anode in the stacking direction during cell assembly (pressure during cell assembly) was less than 0.1 MPa or 7.5 MPa.

[0047] In Example 1, a titanium powder (HDH powder) having an average particle size D50 of 26 μm and an average circularity of 0.75 was used as the titanium porous body for the anode-side porous transport layer. This was made into a paste, which was then dried, degreased, and heated for sintering to produce a powder sintered body. Example 2 was the same as Example 1, except that a powder sintered body made from titanium powder (HDH powder) having an average particle size D50 of 14 μm and an average circularity of 0.78 was used as the titanium porous body. Comparative Example 1 was the same as Example 1, except that a fiber sintered body made from titanium fiber was used as the titanium porous body.

[0048] In all of Examples 1 and 2 and Comparative Example 1, the thickness of the sheet-like porous titanium body was in the range of 100 μm or more and 250 μm or less. The porosity was 50% in Example 1, 40% in Example 2, and 52% in Comparative Example 1. The porous titanium body of Example 1 had an average pore area of ​​34 μm that opened to the surface facing the electrolyte membrane. 2 The standard deviation of the hole area is 71 μm 2 The number of pores present within a predetermined rectangular region was 133. The titanium porous body of Example 2 had an average pore area of ​​7 μm2 that opened to the surface on the electrolyte membrane side. 2 The standard deviation of the hole area is 10 μm 2 The number of pores present within a predetermined rectangular region was 214. The titanium porous body of Comparative Example 1 had an average pore area of ​​514 μm2 that opened to the surface on the electrolyte membrane side.2 The standard deviation of the hole area is 1148 μm 2 The number of holes present in the predetermined rectangular region was 43.

[0049] Using the above cell, a voltage was applied between the anode and cathode at a cell temperature of 60°C, and the other conditions were the same as those described in Non-Patent Document 1, and the relationship between voltage and current was confirmed. As a result, in Example 1, cells were assembled 10 times at a cell assembly pressure of 7.5 MPa, and it was possible to measure the voltage and current for each cell. In contrast, in Comparative Example 1, cells were assembled three times at a cell assembly pressure of 7.5 MPa, but it was not possible to measure the voltage and current for one cell. Note that for the cell in which the current could not be measured, it was not possible to measure the voltage and current at the cell assembly pressure of 1.75 MPa (i.e., before the cell assembly pressure reached 7.5 MPa). This is thought to be because the porous titanium body on the anode side damaged the electrolyte membrane, causing it to cease functioning. Therefore, it can be said that Example 1 suppresses electrolyte membrane damage compared to Comparative Example 1.

[0050] In addition, when the pressure during cell assembly in Example 1 was set to less than 0.1 MPa, and when the pressure during cell assembly in Comparative Example 1, in which cell assembly was possible, was set to less than 0.1 MPa, the relationship between current and voltage was compared. 2 In order to achieve a current density of 0.1 A / cm, Comparative Example 1 required a voltage of 1.9 V. In contrast, Example 1 required a current density of 0.1 A / cm even at a voltage exceeding 2.3 V. 2 It was.

[0051] In addition, when the pressure during cell assembly in Example 1 was set to 7.5 MPa, the relationship between current and voltage was compared to 1.0 A / cm 2The voltage required to achieve a current density of 1.8 V was 1.8 V. This result shows that by increasing the pressure during cell assembly, the porous titanium anode in the cell can exhibit high electrical conductivity. In Example 2, the relationship between current and voltage was not investigated, but since the porous titanium had a higher surface smoothness than that of Example 1, it is expected that it will exhibit even better electrical conductivity than Example 1. The cell used in Example 1 could be assembled without any problems even when the pressure during cell assembly was 24 MPa.

[0052] From the above, it has been suggested that, according to this invention, the porous titanium body has a relatively smooth surface on the electrolyte membrane side within the cell, is breathable or liquid permeable, and can exhibit relatively high electrical conductivity.

[0053] The thickness of the sheet-like porous titanium body used in Examples 1 and 2 of the above tests was within the range of 100 μm or more and 250 μm or less, but it is believed that similarly good results would be obtained if a porous titanium body with the same or greater surface smoothness but with an even greater thickness were used. This is because a porous titanium body with excellent surface smoothness, even if thick, is unlikely to damage the electrolyte membrane even when pressed against it with high pressure, and it is believed that the contact resistance with the electrolyte membrane is reduced, resulting in a high current density. It is also possible to form a porous titanium body with a large thickness by stacking multiple porous layers.

Claims

1. A method for splitting water using a PEM-type water electrolysis device having a cell including a cathode, an electrolyte membrane, a porous transport layer, and an anode stacked together, wherein the porous transport layer has a titanium porous body, and the average area of ​​the pores opening to the surface of the titanium porous body facing the electrolyte membrane is 5 μm 2 More than 45 μm 2 The standard deviation of the area of ​​the holes is 90 μm or less. 2 or less, and the area is 22000 μm 2 the number of holes present within a rectangular region having an aspect ratio of 4:3 is 120 or more, and a pressure of 6 MPa or more is applied in a stacking direction of the cathode, the electrolyte membrane, the porous transport layer, and the anode during assembly of the cell.

2. The electrolysis method according to claim 1, wherein the thickness of the porous titanium body is 500 μm or less.

3. The electrolysis method according to claim 1, wherein at least one of the surface of the porous titanium body on the electrolyte membrane side and the surface on the anode side is coated with a platinum group metal.

4. The electrolysis method according to claim 1, wherein the titanium porous body has a plurality of porous layers stacked one on top of the other.

5. A method for producing hydrogen, which comprises decomposing water using the electrolysis method according to any one of claims 1 to 4 to produce hydrogen.

6. A method for manufacturing a cell in a PEM-type water electrolysis device, the cell including a cathode, an electrolyte membrane, a porous transport layer, and an anode stacked together, wherein the porous transport layer has a titanium porous body, and the average area of ​​the pores opening to the surface of the titanium porous body facing the electrolyte membrane is 5 μm 2 More than 45 μm 2 The standard deviation of the area of ​​the holes is 90 μm or less. 2 or less, and the area is 22000 μm 2 the number of the holes present within a rectangular region having an aspect ratio of 4:3 is 120 or more, and a pressure of 6 MPa or more is applied in a stacking direction of the cathode, the electrolyte membrane, the porous transport layer, and the anode during assembly of the cell.

7. The manufacturing method according to claim 6, wherein the thickness of the porous titanium body is 500 μm or less.

8. The manufacturing method according to claim 6 or 7, wherein at least one of the surface of the porous titanium body facing the electrolyte membrane and the surface facing the anode is coated with a platinum group metal.

9. The manufacturing method according to claim 6 or 7, wherein the porous titanium body has a plurality of porous layers stacked one on top of the other.