Titanium porous body and method for producing hydrogen

Abstract

This titanium porous body is in the form of a sheet. In the titanium porous body, the maximum height Rz of at least one surface is 5 μm or less, the irreversible deformation amount during pressure application at 100 MPa is 0.2% or less, and the thickness is 500 μm or less.

Description

Titanium porous body and method for producing hydrogen

[0001] The present invention relates to a sheet-shaped porous titanium body and a method for producing hydrogen.

[0002] Porous titanium bodies produced by sintering titanium powder or the like have air or liquid permeability due to their fine pores, electrical conductivity, and also high corrosion resistance due to the formation of a passivation film on the surface.

[0003] Porous titanium having such properties is being considered for use in porous transport layers (PTLs) in polymer electrolyte membrane (PEM)-type water electrolysis devices, which are in environments where corrosion may occur. In particular, hydrogen produced in PEM-type or other water electrolysis devices using electricity derived from renewable energy is called "green hydrogen," and great expectations are being placed on it in recent years as efforts to realize a decarbonized society accelerate.

[0004] In this regard, Patent Document 1 proposes "a sheet-like titanium-based porous body containing titanium, having a thickness of 0.8 mm or less, a porosity of 30% to 65%, a maximum height Rz1 of one sheet surface of 30 μm or less, a ratio (Rz2 / Rz1) of the maximum height Rz2 of the other sheet surface to the maximum height Rz1 of one sheet surface of 1.2 or more, and a compressive deformation rate of 19% or less," with the aim of "having one sheet surface that is relatively smooth, the other sheet surface that exhibits the required air or liquid permeability, and keeping compressive deformation when a predetermined pressure is applied to it relatively small."

[0005] Patent Document 2 aims to "provide a method for manufacturing a porous titanium body that can effectively suppress peeling defects and cracks even in the case of a low-density, thin sintered body made using spherical gas-atomized powder," and discloses "a method for manufacturing a porous titanium body, characterized in that, when manufacturing a porous powder sintered body made of titanium or a titanium alloy, a molybdenum material or molybdenum alloy material having a surface centerline average roughness Ra of 0.1 μm or less and a maximum height Rz of 1.0 μm or less is used as a setter on which the titanium or titanium alloy powder is placed."

[0006] Patent No. 7061735 Patent No. 4098169

[0007] In the above-mentioned water electrolysis apparatus, from the viewpoint of miniaturization of the apparatus, a thin sheet-like titanium porous body that has excellent compression resistance is sometimes required. This is because a titanium porous body with poor compression resistance will not be able to maintain the required thickness or shape against the compressive force that may act when incorporated into an apparatus such as a PEM-type water electrolysis apparatus, and there is a concern that the air permeability or liquid permeability will be reduced.

[0008] Furthermore, when a porous titanium body is used as a porous transport layer in a PEM-type water electrolysis device, the porous titanium body may be pressed against the electrolyte membrane when assembled. In this case, depending on the surface properties of the porous titanium body pressed against the electrolyte membrane, the electrolyte membrane may be largely deformed in parts, potentially resulting in damage to the electrolyte membrane.

[0009] Patent Document 1 describes a porous titanium body having a relatively small maximum surface height Rz and a small compressive deformation rate, but this porous titanium body leaves room for further improvement in terms of surface smoothness and compression resistance. Patent Document 2 does not consider the surface roughness or compression resistance of the porous titanium body at all.

[0010] An object of the present invention is to provide a porous titanium body having at least one surface that is relatively smooth and has excellent compression resistance, and a method for producing hydrogen.

[0011] The inventors have found that a porous titanium body having a maximum height Rz of at least one surface and an amount of irreversible deformation when pressurized at 100 MPa each below a predetermined value can be evaluated as having a smooth surface and excellent compression resistance. Furthermore, through extensive research by the inventors, it has been found that in order to produce such a porous titanium body, it is effective to appropriately adjust the drying and sintering conditions when drying, debinding, and sintering the paste in sequence.

[0012] The porous titanium body of the present invention is in the form of a sheet, and has a maximum height Rz of 5 μm or less on at least one surface, an irreversible deformation amount of 0.2% or less when pressurized at 100 MPa, and a thickness of 500 μm or less.

[0013] The porous titanium body preferably has a porosity of 30% or more and 50% or less.

[0014] The titanium porous body may be a powder sintered body. The titanium porous body preferably has a Ti content of 97 mass% or more. The titanium porous body preferably has an O content of 0.6 mass% or more and 2.0 mass% or less. The thickness is preferably 30 μm to 400 μm. The amount of irreversible deformation is preferably 0.1% or less. A method for producing hydrogen according to the present invention comprises decomposing water to generate hydrogen using a PEM water electrolysis device having any of the titanium porous bodies described above.

[0015] The porous titanium body of the present invention has at least one surface that is relatively smooth and has excellent compression resistance.

[0016] The porous titanium body of one embodiment of the present invention is sheet-shaped, has a maximum height Rz of 5 μm or less on at least one surface, an irreversible deformation amount of 0.2% or less when pressurized at 100 MPa, and a thickness of 500 μm or less.

[0017] (Composition) The titanium porous body is made of titanium. If it is made of titanium, a titanium porous body having high electrical conductivity at a certain relative density can be obtained. The Ti 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 Ti content is not limited to this, but may be, for example, 99.8% by mass or less, or 99% by mass or less. This Ti content refers to the purity of titanium taking into account not only metal components but also impurities such as gas components such as oxygen. Therefore, the Ti content can be calculated by subtracting the total content of metal components and impurity components, including gas components, from 100% by mass.

[0018] Porous titanium bodies may contain Fe as an impurity, and the Fe content may be, for example, 0.25% by mass or less. Porous titanium bodies may also contain Ni, Cr, Al, Cu, Zn, and Sn as unavoidable impurities resulting from the raw materials or manufacturing process. It is preferable that the content of each of Ni, Cr, Al, Cu, Zn, and Sn is less than 0.10% by mass, and that the total content of these elements is less than 0.30% by mass. To measure the content of unavoidable impurities in porous titanium bodies, the porous titanium body can be melted in an inert gas (such as argon gas) atmosphere to produce a cast piece, which can then be subjected to XRF analysis.

[0019] The O (oxygen) content of the titanium porous body is not particularly limited, but may be 0.6% by mass or more and 2.0% by mass or less, or 0.9% by mass or more and 1.6% by mass or less. The O content can be measured by inert gas fusion-infrared absorption spectroscopy.

[0020] The titanium porous body may have a purity equivalent to pure titanium grades 1 to 4, typically grades 1 to 2, of JIS H 4600 (2012), excluding the oxygen content.

[0021] (Sheet dimensions) The thickness of the sheet-shaped porous titanium body is 500 μm or less. For example, a porous transport layer of a PEM water electrolysis device may require a porous titanium body with a certain degree of thinness. If the thickness is too large, the PEM water electrolysis device may become large. The thickness of the porous titanium body may be, for example, 30 μm to 400 μm, typically 80 μm to 300 μm. Even with such a thin porous titanium body, in this embodiment, at least one surface is smooth and has excellent compression resistance, which allows for the miniaturization of the water electrolysis device and allows for long-term use within the water electrolysis device.

[0022] The thickness is measured at five points in total, four points on the periphery and one point in the center of the titanium porous body, using a digital thickness gauge with a flat probe having a diameter of 10 mm and a measurement accuracy of 0.001 to 0.01 mm, such as a digital thickness gauge (model number 547-321) manufactured by Mitutoyo Corporation, and the average of these measurements is used. When the sheet-like titanium porous body has a rectangular shape in plan view, the four peripheral points are the four corners.

[0023] The surface area of ​​the sheet-like porous titanium body in a plan view is not particularly limited and may be determined appropriately depending on the application, but for example, the upper limit is 0.75 m 2 Below, 0.42m 2 Below, 0.13m 2 Below, the lower limit is 70 mm 2 Above, 250mm 2 More than 10,000 mm 2 Above, 25000mm 2 There may be more than this.

[0024] The term "sheet-like" in reference to a porous titanium body refers to a plate or foil shape having a thickness small relative to its dimensions in a plan view. The shape of the porous titanium body in a plan view is not particularly limited, but may be a square, a square with an aspect ratio of 1:1 to 1:3 or a rectangular shape such as a rectangle, a diamond shape or other polygon, an ellipse, or a perfect circle. The corners of a polygonal porous titanium body may be chamfered.

[0025] (Surface roughness) The maximum height Rz of at least one surface of the titanium porous body is 5 μm or less. The maximum height Rz of both the front and back surfaces of the titanium porous body may be 5 μm or less. In titanium porous bodies produced by the method described below, the maximum height Rz of the surface that was in contact with the substrate during drying tends to be small.

[0026] The maximum height Rz means the sum of the height of the highest peak (maximum peak height) and the depth of the deepest valley (maximum valley depth) in the profile curve over the reference length. While the arithmetic mean roughness Ra represents the unevenness as an average value, the maximum height Rz can be used to check the presence or absence of large depressions or protrusions, and can therefore be used as an index for determining the degree of the possibility of damaging the electrolyte membrane.

[0027] The maximum height Rz of at least one surface of the titanium porous body is preferably 4 μm or less. The maximum height Rz of at least one surface is not limited to this, but may be, for example, 1 μm or more. The maximum height Rz is measured in accordance with ISO 4287-1997.

[0028] (Amount of irreversible deformation) When the porous titanium body is compressed in the thickness direction by applying a pressure of 100 MPa for 3 minutes and then unloaded twice, the amount of irreversible deformation, which is the rate of change in thickness before and after the operation, is 0.2% or less.

[0029] As a result, when the porous titanium body is incorporated into a device such as a PEM water electrolysis device, it will maintain a certain degree of thickness and shape when a compressive force is applied, and will be able to exhibit the required air and liquid permeability. The amount of irreversible deformation is preferably 0.1% or less. The smaller the amount of irreversible deformation, the better, so there is no particular restriction on the lower limit, and it can be set appropriately depending on the purpose.

[0030] The irreversible deformation amount Dc is calculated by measuring the thickness T1 of the porous titanium body before applying a pressure of 100 MPa and the thickness T2 of the porous titanium body after applying and unloading the pressure twice, using the formula: Dc = (1 - T2 / T1) x 100. More specifically, to measure the irreversible deformation amount Dc, the thickness T1 of a square sample of the porous titanium body (projected area 20 mm x 20 mm) in plan view is first measured. The sample is then sandwiched between the flat surfaces of two flat plates or the like in the thickness direction, and the flat surfaces are displaced toward each other, so that a pressure of 100 MPa is applied evenly to the surface of the sample in the thickness direction for three minutes. The displacement rate can be constant at 0.1 mm / min. After applying the pressure, the pressure is released. This pressure application and unloading operation is repeated twice, for a total of two times. The thickness T2 of the sample is then measured after removal from the flat surfaces. Here, various compression testing devices or other devices capable of applying pressure to the sample can be used. To measure the thicknesses T1 and T2 of the sample, the thickness is measured at one point at the center of gravity of the sample in a plan view. This measurement is performed on five different samples, and the average value is used as the amount of irreversible deformation.

[0031] (Porosity) The porous titanium body may be a sintered body having a skeleton formed by sintering and bonding titanium powder particles together, in which case the porous titanium body has a three-dimensional network structure in which pores are formed between the bonded titanium powder particles.

[0032] For example, when a porous titanium body is produced by sintering titanium powder as described below, the porous titanium body becomes a powder sintered body, and the three-dimensional network structure that defines the voids in the porous titanium body tends to resemble a titanium sponge. This sponge-like three-dimensional network structure is similar in shape to that of sponge titanium produced by the Kroll process. On the other hand, when titanium fibers are used, the porous titanium body becomes a fiber sintered body, and the three-dimensional network structure that defines the voids in the porous titanium body often resembles a nonwoven fabric. Furthermore, when a paste containing titanium powder and an organic binder is used and the paste is dried and then sintered, if a foaming agent is added to the paste, the porous titanium body produced thereby is prone to forming voids within the network due to the influence of the foaming agent. In this case, the voids in the porous titanium body tend to become large, which tends to increase the amount of irreversible deformation and the maximum surface height Rz, which is undesirable.

[0033] The porosity of the porous titanium body is preferably 30% or more and 50% or less, more preferably 35% or more and 47% or less. If the porosity is within this range, breakage during handling can be suppressed while ensuring the required breathability or liquid permeability depending on the application. When the porosity is 30% or more, good breathability or air permeability can be obtained. On the other hand, when the porosity is 50% or less, cracks are less likely to occur during handling.

[0034] The porosity ε of the porous titanium body is determined by the ratio of the apparent density ρ' calculated from the volume and mass determined from the width, length, and thickness of the porous titanium body, to 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.

[0035] (Applications) The porous titanium body can be particularly suitably used for the porous transport layer of a PEM-type water electrolysis device. A PEM-type water electrolysis device may include an anode, a cathode, an electrolyte membrane such as a perfluorocarbon sulfonic acid membrane disposed between the anode and the cathode and having electrode catalyst layers of a platinum group metal or the like on both sides, and a porous transport layer disposed between each electrode catalyst layer of the electrolyte membrane and the anode or cathode.

[0036] In the PEM water electrolysis device, when water is supplied to the anode and a voltage is applied, the water moves through the porous transport layer on the anode side and reaches the electrode catalyst layer, where it decomposes to produce oxygen and protons (H + The protons travel from the anode to the cathode through the electrolyte membrane, gaining electrons at the cathode catalyst layer, generating hydrogen at the cathode. Meanwhile, oxygen travels through the porous transport layer to the discharge channel and is discharged outside the device.

[0037] In such a PEM-type water electrolysis device, the space where the porous transport layer is placed, particularly on the anode side, is subject to strongly acidic and strongly oxidizing conditions, but a titanium porous body with high corrosion resistance can be used successfully as a porous transport layer in such an extremely harsh environment. Furthermore, as described above, the titanium porous body of the present invention has a small maximum height Rz of at least one surface, which makes it possible to suppress damage to the electrolyte membrane when the titanium porous body is placed in a PEM-type water electrolysis device with its surface side pressed against the electrolyte membrane.

[0038] In addition to the PEM-type water electrolysis devices described above, the use of porous titanium materials in organic electrolysis synthesis using PEM-type reactors is also being considered. In such devices, electrolysis is carried out by passing protons through a proton exchange membrane. The porous titanium materials described here may also be well suited for organic electrolysis synthesis using PEM-type reactors, and are considered to be usable as a porous transport layer (PTL) on the anode side of electrolysis devices that use proton exchange membranes.

[0039] (Manufacturing Method) To manufacture the porous titanium body described above, for example, a paste containing titanium powder can be dried, and the resulting molded body can be subjected to binder removal and sintering in this order.

[0040] The paste is prepared by mixing titanium powder, an organic binder, an organic solvent, and a dispersant, and does not contain water as a solvent or a foaming agent. However, water that inevitably gets mixed in during the paste preparation process due to moisture absorption or other reasons is acceptable. The addition of a dispersant makes it easier for the titanium powder to disperse in the paste, ultimately resulting in a porous titanium body with the desired maximum surface height Rz and porosity. The absence of water as a solvent and a foaming agent can suppress the aggregation of titanium powder due to differences in drying behavior between organic solvents and water, as well as the generation of localized voids due to foaming.

[0041] The titanium powder can be a crushed powder, a spherical powder such as atomized powder, a titanium hydride powder obtained by hydrogenating and crushing a titanium raw material such as sponge titanium, a hydrogenated and dehydrogenated titanium powder (HDH powder) obtained by dehydrogenating titanium hydride powder, or any other powder. The mass ratio (Ms / Mb) of the mass Ms of the organic solvent to the mass Mb of the organic binder in the paste can be in the range of 2.5 to 4.5, or in the range of 2.5 to 3.5.

[0042] The paste is applied thinly to a substrate, such as a resin substrate, with a release agent on its surface, and then dried. During drying, it is important to heat the paste at a relatively low temperature for a long period of time. This allows the titanium powder particles in the paste to be properly oriented and in contact with each other to form a molded body. If the heating temperature during drying is high, the organic solvent will volatilize immediately, disrupting the arrangement of the titanium powder particles, which will make the surface of the final titanium porous body more likely to become rough. Heating the paste for a long period of time will allow the organic solvent to volatilize sufficiently, but an appropriate time can be set taking into account the lead time. For example, the heating temperature during drying may be 100°C and the heating time may be 3 hours.

[0043] The molded body obtained by drying the paste is debindered by heating at 360°C for approximately 6 hours to volatilize the organic binder, and then the titanium powder contained therein is sintered. Typically, the molded body is debindered after being peeled from the substrate. During sintering, it is desirable to heat the molded body at a low temperature (approximately 775°C) sufficient to sinter the titanium powder for a relatively long time (approximately 5 hours). Low-temperature heating allows the titanium powder in the molded body to be properly aligned and in contact, and the sintering proceeds slowly. As a result, even a relatively thin sheet-like porous titanium body after sintering has a smooth surface and excellent compression resistance.

[0044] Next, a prototype of the porous titanium body of the present invention was produced and its effects were confirmed, which will be described below. However, the description here is for illustrative purposes only and is not intended to be limiting.

[0045] A paste was prepared by mixing titanium powder from HDH powder with an average particle size of 13 μm with polyvinyl butyral as an organic binder, isopropyl alcohol as an organic solvent, and SN Sperse 2190 (manufactured by San Nopco Ltd.) as a dispersant. The dispersant was added in an amount that resulted in a content of 0.1% by mass in the paste. The mass ratio Ms / Mb of the organic solvent (Ms) to the organic binder (Mb) was set to a range of 2.5 to 3.5. No water or foaming agent was added to the paste.

[0046] The paste was applied in the form of a sheet onto a PET sheet of resin substrate having a release agent on the surface, and the sheet was heated and dried in an air atmosphere at the temperature and for the time shown in Table 1 to remove the organic solvent, thereby obtaining a molded body. Next, the molded body was peeled off from the PET sheet, and then heated in an air atmosphere at 360°C for 6 hours to remove the binder. Thereafter, the molded body was heated in a vacuum atmosphere (1.0 x 10 -3 The titanium powder in the compact was sintered by heating under a pressure of 100 Pa or less at the temperature and for the time shown in Table 1, to obtain a sheet-like porous titanium body as a sintered body. The surface size of the sheet-like porous titanium body in a plan view was 700 mm x 600 mm.

[0047] In Comparative Example 3, the drying temperature was relatively low and the drying time was short, resulting in insufficient drying and a molded body not being formed. As a result, the subsequent binder removal and sintering could not be carried out, and a titanium porous body could not be produced.

[0048] In the examples and comparative examples 1, 2, 4, and 6, the molded body was placed on a graphite setter and heated during sintering. Of these, in comparative example 4, the sintering temperature was too high, causing the porous titanium body to adhere to the setter. When an attempt was made to peel it off from the setter, cracks occurred, making it impossible to produce a porous titanium body. Based on this, in comparative example 5, the setter was coated with BN (boron nitride) as a release layer, and the molded body was placed on top of that and heated. As a result, it became possible to peel it off from the setter, and a porous titanium body could be produced.

[0049] The titanium porous bodies obtained in each of the Examples and Comparative Examples 1, 2, 5, and 6 were measured for thickness, maximum height Rz of one surface (the surface that had been in contact with the resin substrate during production), amount of irreversible deformation, and porosity using the methods described above. The results are shown in Table 1. The oxygen content of these titanium porous bodies was within the range of 0.9% by mass or more and 1.6% by mass or less.

[0050]

[0051] In the Examples, the drying and sintering temperatures were relatively low, and the drying and sintering times were relatively long, which is thought to have resulted in porous titanium bodies with small maximum surface heights Rz and small amounts of irreversible deformation. In Comparative Examples 1 and 2, the drying temperature was high, which is thought to have caused the large maximum surface heights Rz of the porous titanium bodies. In Comparative Example 1, the sintering time was shortened compared to Comparative Example 2 in an attempt to improve the surface properties, but the desired maximum height Rz was not achieved. These results suggest that if the arrangement of titanium powder in the compact is disrupted during drying, it will be difficult to improve the surface properties of the final porous titanium body, even if the subsequent sintering conditions are changed. In Comparative Example 5, the sintering temperature was too high, which is thought to have caused excessive sintering, resulting in a large maximum surface height Rz of the porous titanium body. In Comparative Example 6, the sintering temperature was too low, resulting in a porous titanium body with large irreversible deformation and poor compression resistance.

[0052] From the above, it has been found that according to the present invention, a porous titanium body having at least one surface that is relatively smooth and has excellent compression resistance can be obtained.

Claims

[CLAIMS]

1. A sheet-shaped titanium porous body,wherein a maximum height Rz of at least one surface is 5 gm or less, an irreversible deformation under a pressure of 100 MPa is 0.2% or less, and a thickness is 500 gm or less.

2. The titanium porous body according to claim 1, wherein the titanium porous body has a porosity of 30% or more and 50% or less.

3. The titanium porous body according to claim 1 or 2, which is a powder sintered body.

4. The titanium porous body according to any one of claims 1 to 3, wherein the titanium porous body has a Ti content of 97 % by mass or more.

5. The titanium porous body according to any one of claims 1 to 4, wherein the titanium porous body has an O content of 0.6 % by mass or more and 2.0 % by mass or less.

6. The titanium porous body according to any one of claims 1 to 5, wherein the thickness is 30 gm to 400 gm.

7. The titanium porous body according to any one of claims 1 to 6, wherein the irreversible deformation is 0.1% or less.

8. A method for producing hydrogen, the method comprising decomposing water and generating hydrogen using a PEM-type water electrolysis device comprising the titanium porous body according to any one of claims 1 to 7.