Porous carbon sheet and water electrolysis device

Abstract

The purpose of the present invention is to provide a low-cost porous carbon sheet that has a high compression deformation rate when constituting a water electrolysis cell, does not have the problems of penetration and short-circuiting, and has excellent electrical conductivity. The porous carbon sheet is a sheet-shaped structure having a porous structure in which carbon fibers are bound by a binder. The porous carbon sheet has a thickness d0 under a pressure of 0.15 MPa of 1.8-3.0 mm, a thickness d1 under a pressure of 1.0 MPa of 85% or more of the thickness d0 under the pressure of 0.15 MPa, and a thickness d2 under a pressure of 4.5 MPa of 75% or less of the thickness d0 under the pressure of 0.15 MPa.

Description

Porous carbon sheet and water electrolysis device

[0001] The present invention relates to a porous carbon sheet that is suitable for use as a diffusion layer in a water electrolysis cell or a fuel cell, particularly as a diffusion layer in a cathode electrode member of a water electrolysis device.

[0002] Polymer electrolyte fuel cells generate electricity through an electrochemical reaction between the anode and cathode by supplying a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas to the anode. The reaction between hydrogen and oxygen generates water and an electric current. Water electrolysis devices generate hydrogen and oxygen by passing an electric current through water. Such fuel cells and water electrolysis devices generally have cells that are composed of a separator, a diffusion layer, a catalyst layer, an electrolyte membrane, another catalyst layer, a diffusion layer, and a separator, stacked in this order. A cell stack, consisting of multiple stacked cells, is installed in the water electrolysis device.

[0003] Of these, the diffusion layers of the cells of the water electrolysis device must have high water permeability to supply water to the anode catalyst layer and remove it from the cathode catalyst layer, high gas diffusivity to discharge the generated hydrogen gas and oxygen gas, and high electrical conductivity to achieve high water electrolysis efficiency. Conventionally, porous sheets such as sintered titanium particles and titanium fiber aggregates have been widely used as diffusion layers to achieve high strength, thinness, and improved porosity.

[0004] However, such porous sheets have the problem of being difficult to widely use in consumer applications because titanium itself is very expensive. For this reason, porous carbon sheets such as porous carbon paper, which have traditionally been commonly used as diffusion layers in fuel cells, are sometimes applied, as described in Patent Document 1, for example. In water electrolysis cells, the cells are tightened in the stacking direction of the cell stack to increase the adhesion between the components, thereby improving gas sealing and reducing contact resistance. For this reason, porous carbon plates having a large number of carbon fibers dispersed in their surfaces, as described in Patent Document 2, for example, are generally prone to elastic deformation in the thickness direction (high cushioning properties) and therefore have the characteristic of being difficult to break.

[0005] JP 2023-61911 A JP 1-77625 A

[0006] However, carbon fiber porous bodies for fuel cells, such as those described in Patent Document 1, are generally manufactured by a wet process in which short carbon fibers are dispersed in water and then paper-made. This paper-making process results in carbon fibers oriented in the thickness direction of the carbon fiber porous body. Furthermore, the thickness of the carbon fiber porous body is thin, on the order of several hundred micrometers, and the catalyst layer and electrolyte membrane are even thinner, on the order of several micrometers. Therefore, the carbon fibers oriented in the thickness direction can penetrate the catalyst layer and electrolyte membrane when they are tightened after stacking in a cell stack. Furthermore, the penetration can cause a short circuit, resulting in the cell not operating properly.

[0007] Furthermore, since the porous carbon plate described in Patent Document 2 is thick, the above-mentioned catalyst layer and electrolyte membrane are less likely to short-circuit; however, the increased thickness increases electrical resistance in the thickness direction and deteriorates fluid permeability.

[0008] As described above, it is difficult to eliminate penetration by carbon fibers while maintaining porosity or fluid permeability in a porous carbon sheet, and it has been difficult to obtain a porous carbon sheet suitable for a diffusion layer in a water electrolysis cell.

[0009] As a result of extensive research aimed at solving the above-mentioned problems, the present invention has found that by making the thickness of a porous carbon sheet sufficiently thicker than that of conventional porous carbon sheets for general fuel cells and adjusting the internal structure and carbonization temperature of the porous carbon sheet to provide a porous carbon sheet with the compression characteristics described below, it is possible to provide a low-cost porous carbon sheet that has a high compression deformation rate and no penetration (short-circuit) problem when forming a water electrolysis cell, which has been difficult with conventional porous carbon sheets for fuel cells. Specifically, the present invention employs the following means: (1) A porous carbon sheet having a sheet-like structure with a porous structure in which carbon fibers are bound with a binder, the porous carbon sheet having a thickness d0 of 1.8 to 3.0 mm under a pressure of 0.15 MPa, a thickness d1 under a pressure of 1.0 MPa that is 85% or more of the thickness d0 under a pressure of 0.15 MPa, and a thickness d2 under a pressure of 4.5 MPa that is 75% or less of the thickness d0 under a pressure of 0.15 MPa. (2) The porous carbon sheet according to (1), having an electrical resistivity in the thickness direction of 120 mΩ cm or less under a pressure of 1.0 MPa. (3) The porous carbon sheet according to (1), having an electrical resistivity of 80 mΩ cm or less under a pressure of 1.0 MPa. (4) The porous carbon sheet according to any one of (1) to (3), having an electrical resistivity in the thickness direction of 90 mΩ cm or less under a pressure of 4.5 MPa. (5) The porous carbon sheet according to any one of (1) to (3), having an electrical resistivity of 60 mΩ cm or less under a pressure of 4.5 MPa. (6) Any one of the porous carbon sheets according to (1) to (5), containing conductive non-metallic particles having an equivalent sphere diameter of 10 to 30 μm. (7) The porous carbon sheet according to (6), containing the conductive non-metallic particles in a ratio of 10 to 50 mass % relative to 100 mass % of the porous carbon sheet. (8) The porous carbon sheet according to any one of (1) to (7), which has a thickness recovery rate R1 of 80% or more when pressurized to 4.5 MPa and then depressurized to 1.0 MPa.(9) The porous carbon sheet according to any one of (1) to (8), wherein the difference between the thickness recovery rate R1 when pressurized to 4.5 MPa and then reduced to 1.0 MPa and the thickness recovery rate R2 when pressurized again to 4.5 MPa and then reduced to 1.0 MPa is 5% or less. (10) A water electrolysis device comprising the porous carbon sheet according to any one of (1) to (9) as a cathode-side electrode member.

[0010] By using the porous carbon sheet of the present invention as a diffusion layer of a water electrolysis cell, hydrogen can be generated stably for a long period of time.

[0011] Fig. 1 is an example of a schematic cross-sectional view of a water electrolysis cell in a water electrolysis device of the present invention. Fig. 2 is a schematic view showing an embodiment of a compression step in an example of a method for producing a porous carbon sheet of the present invention. Fig. 3 is a schematic view showing an embodiment of a method for measuring the thickness of a porous carbon sheet of the present invention. Fig. 4 is a schematic view showing an embodiment of a method for measuring the electrical resistance in the thickness direction of a porous carbon sheet of the present invention. Fig. 5 is a schematic view showing an embodiment of a method for checking whether short carbon fibers protrude from the porous carbon sheet of the present invention.

[0012] Hereinafter, an example of an embodiment of the porous carbon sheet of the present invention will be described with reference to the drawings. The present invention is not limited to these embodiments, and the descriptions of preferred devices, their configurations, and numerical ranges in the descriptions of individual embodiments can also be interpreted as descriptions of the porous carbon sheet of the present invention as a general concept.

[0013] 1 is a schematic cross-sectional view of an example of a water electrolysis cell in the water electrolysis apparatus of the present invention. The basic structure of the water electrolysis cell includes a stack in which an electrolyte membrane 1 is sandwiched between a pair of catalyst layers 2 a, 2 c, and further sandwiched between a pair of diffusion layers 3 a, 3 c, and separators 4 a, 4 c having flow channels formed to surround the periphery of the stack, and main electrodes 5 a, 5 c are further disposed outside the separators 4 a, 4 c.

[0014] The electrolyte membrane 1 is typically a proton-conductive ion-exchange membrane approximately 8 μm thick formed from a polymer electrolyte material, such as a perfluorosulfonic acid polymer having sulfo groups at the side chain terminals. The catalyst layers 2 a and 2 c are formed in layers approximately 5 μm thick, each containing a material including carbon particles carrying a catalytic metal that promotes an electrochemical reaction such as water electrolysis, and a proton-conductive polymer electrolyte. Examples of the catalytic metal include platinum and platinum alloys containing platinum and other metals such as ruthenium. In the water electrolysis device of the present invention, the cathode-side diffusion layer 3 c is a layer made of the porous carbon sheet of the present invention. The anode-side diffusion layer 3 a may also be a layer made of the porous carbon sheet of the present invention.

[0015] In a water electrolysis cell, a porous material made of a highly corrosion-resistant metal such as titanium is often used for the anode-side diffusion layer 3a where oxygen gas is generated by the water electrolysis reaction to prevent oxidation. The thickness of the porous material is typically about 175 μm. The cathode-side diffusion layer 3c of the water electrolysis cell is formed from the porous carbon sheet of the present invention.

[0016] 1 , with positive and negative voltages applied to the main electrodes 5a and 5c, respectively, water is supplied through the flow paths in the anode separator 4a, passes through the anode diffusion layer 3a, and generates oxygen gas and protons at the anode catalyst layer 2a, while electrons are released and transferred to the main electrode 5a. The oxygen gas is recovered through the flow paths in the anode separator 4a. The protons pass through the electrolyte membrane 1 to reach the cathode catalyst layer 2c, where they accept electrons from the cathode main electrode 5c to become hydrogen gas. The hydrogen gas passes through the cathode diffusion layer 3c and is recovered into the flow paths in the cathode separator 4c.

[0017] The porous carbon sheet of the present invention is a sheet-like structure having a porous structure in which carbon fibers are bound with a binder, and examples thereof include carbon paper in which short carbon fibers are bound with a resin carbide or the like, and carbon fiber woven fabric or carbon fiber nonwoven fabric in which carbon fibers are bound with a resin carbide or the like. When using carbon paper, it is preferable that the short carbon fibers do not have a significant orientation within the sheet plane and are present in a generally random direction. Furthermore, the binder in the present invention refers to a substance that functions to fix the carbon fibers and improve the mechanical strength of the porous carbon sheet.

[0018] To prevent the high clamping pressure of the water electrolysis cell stack from damaging the diffusion layer due to shear or compression caused by the unevenness of the gas flow paths provided in the separator, and further from damaging the electrolyte layer due to the pressure or deformation being transmitted to the electrolyte layer, the porous carbon sheet of the present invention has a thickness d0 of 1.8 to 3.0 mm under a pressure of 0.15 MPa. A thickness of 1.9 to 2.4 mm is preferred. If the thickness d0 is less than 1.8 mm, damage to the diffusion layer or membrane due to high clamping pressure may not be fully prevented when the porous carbon sheet is used as a diffusion layer of a water electrolysis cell. Furthermore, if the thickness d0 exceeds 3.0 mm, the supply and discharge of water and generated gas may be hindered, and the cost of the diffusion layer may increase, leading to increased facility costs due to the increased cost of the diffusion layer and the larger size of the water electrolysis cell stack.

[0019] In the present invention, when the thickness of a porous carbon sheet is measured under a constant pressure, the thickness d0 at 0.15 MPa is measured by placing the porous carbon sheet on a smooth surface plate and applying 0.15 MPa. Measurements are taken at five different locations, and the arithmetic average of the measured values ​​is taken as the thickness d0 at 0.15 MPa. Furthermore, the thickness under pressures of 1.0 MPa and 4.5 MPa is measured by applying a predetermined pressure to a small sample of the porous carbon sheet using a universal testing machine and a pressure jig, and using multiple micrometers attached to the pressure jig. The arithmetic average of the values ​​measured by the multiple micrometers is taken as the thickness under the predetermined pressure.

[0020] When measuring the thickness of a porous carbon sheet pressurized at the above-mentioned predetermined pressure, the porous carbon sheet should be one that has never been subjected to a pressure exceeding the predetermined pressure. The thickness should be measured within 5 to 30 seconds after the application of the predetermined pressure.

[0021] In conventional water electrolysis cells, a thin porous carbon sheet and a titanium mesh are laminated together to form a diffusion layer, with the porous carbon sheet disposed on the catalyst layer side. As described above, titanium mesh is widely used because titanium has excellent permeability to gases (hydrogen and oxygen) produced by water electrolysis, as well as excellent strength, electrical conductivity, and corrosion resistance. However, titanium itself is expensive, which inevitably makes the titanium mesh and water electrolysis device expensive. In contrast, the porous carbon sheet of the present application has excellent gas permeability, strength, and electrical conductivity. Therefore, by using a porous carbon sheet having a thickness within the above range under a pressure of 0.15 MPa as at least the cathode-side diffusion layer, the need for an expensive titanium mesh is eliminated, and significant cost reductions are expected.

[0022] The porous carbon sheet of the present invention exhibits various compression characteristics depending on the applied pressure. Specifically, the thickness d1 under a pressure of 1.0 MPa, which corresponds to the stopped state of the water electrolysis device, is at least 85%, preferably at least 90%, of the thickness d0 under a pressure of 0.15 MPa. Furthermore, the thickness d2 under a pressure of 4.5 MPa, which corresponds to the operating state of the water electrolysis device, is at most 75%, preferably at most 70%, of the thickness d0 under a pressure of 0.15 MPa. If the thickness d1 of the porous carbon sheet under a pressure of 1.0 MPa is less than 85% of the thickness d0 under a pressure of 0.15 MPa, the amount of compressive deformation required until further pressure is increased to 4.5 MPa, which corresponds to the operating state, is reduced. The high clamping pressure of the cell stack creates a pressure difference between the groove and rib portions of the separator, which can cause damage due to shear or compression of the porous carbon sheet used as a diffusion layer. Furthermore, the pressure difference and deformation of the porous carbon sheet can easily damage the electrolyte layer. Furthermore, short carbon fibers can penetrate the catalyst layer and electrolyte membrane, making the membrane more susceptible to tearing. If the thickness d2 under a pressure of 4.5 MPa is greater than 75% of the thickness d0 under a pressure of 0.15 MPa, the electrical resistance in the thickness direction of the porous carbon sheet during operation of the water electrolysis apparatus will increase, undesirably reducing the efficiency of water electrolysis. By using a porous carbon sheet having the above-described compressibility as a diffusion layer, the adhesion between the diffusion layer and other members during operation of the water electrolysis apparatus will be improved, maximizing the amount of current contributing to water electrolysis and maintaining high hydrogen production efficiency.

[0023] The compression properties of the porous carbon sheet of the present invention can be achieved by controlling the shape of the binder so that it does not cause steric hindrance when the porous carbon sheet is compressed. Specifically, in an example of the manufacturing method described below, increasing the organic compound ratio in the solution containing the organic compound to be impregnated into the precursor fiber sheet can be used to achieve a solution viscosity at 25°C in the range of 2.0 to 5.0 mPa·s. Alternatively, decreasing the solvent ratio can be used to suppress the migration of the organic compound in the precursor fiber sheet associated with solvent evaporation during drying after impregnation. By maintaining the solution viscosity within the above range or decreasing the solvent ratio, the binder does not become too closely connected in the thickness direction of the porous carbon sheet inside the porous carbon sheet. This prevents the carbide from becoming a steric hindrance and making the porous carbon sheet difficult to compress when pressure is applied in the thickness direction of the porous carbon sheet when mounted in a water electrolysis device. Additionally, the shape of the carbide of the organic compound can be easily controlled by adding conductive nonmetallic particles to the solution. Furthermore, increasing the carbonization temperature in the carbonization step of the example of the manufacturing method described below can also control the compression properties to fall within the range of the porous carbon sheet of the present invention.

[0024] The porous carbon sheet of the present invention preferably has a through-thickness electrical resistivity of 120 mΩ·cm or less under a pressure of 1.0 MPa, more preferably 100 mΩ·cm or less, and even more preferably 80 mΩ·cm or less. Furthermore, the electrical resistivity under a pressure of 4.5 MPa is preferably 90 mΩ·cm or less, more preferably 70 mΩ·cm or less, and even more preferably 60 mΩ·cm or less. When the electrical resistivity of the porous carbon sheet of the present invention is within the above-mentioned ranges under each pressure, a decrease in current density can be suppressed when the porous carbon sheet of the present invention is used in a water electrolysis device, thereby achieving good hydrogen generation efficiency. One method for achieving these electrical resistivity ranges is to carbonize the precursor fiber sheet at a temperature of 2,500°C or higher in the carbonization step of an example of a porous carbon sheet manufacturing method described below, thereby graphitizing the carbon fibers and binder in the precursor fiber sheet. Furthermore, the electrical resistance of the porous carbon sheet can be reduced by incorporating conductive non-metallic particles into the precursor fiber sheet. Furthermore, further reduction in electrical resistance can be achieved by controlling the particle size of the conductive non-metallic particles within the range described below.

[0025] Since a water electrolysis cell equipped with a porous carbon sheet is used repeatedly, the thickness recovery rate R1 of the porous carbon sheet when it is pressurized once under the operating conditions of the water electrolysis apparatus, i.e., to 4.5 MPa, and then depressurized under the stopped conditions, i.e., to 1.0 MPa, is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. A thickness recovery rate R1 of 80% or more allows the cell components to maintain an intimate contact state even when the water electrolysis apparatus is repeatedly operated and stopped, enabling stable, highly efficient water electrolysis over a long period of time.

[0026] In the present invention, the thickness recovery rate R1 is calculated as R1=d1' / d1×100(%), where d1 is the thickness under a pressure of 1.0 MPa and d1' is the thickness when the pressure is once increased to 4.5 MPa and then reduced to 1.0 MPa.

[0027] Furthermore, when the thickness recovery rate when the pressure is again increased to 4.5 MPa after measuring the thickness recovery rate and then reduced to 1.0 MPa is defined as R2, the difference between the thickness recovery rate R1 and the thickness recovery rate R2 is preferably 5% or less, more preferably 3% or less, and even more preferably 2% or less. When the thickness recovery rate R2 is measured, the thickness when the pressure is again increased to 4.5 MPa after measuring d1' and then reduced to 1.0 MPa is defined as d1'', and the thickness recovery rate R2 is calculated as R2 = d1'' / d1 × 100 (%). Note that d1, d1', and d1'' should be measured within 5 to 30 seconds after the application of a pressure of 1.0 MPa. When measuring d1, the porous carbon sheet should be measured on a sheet that has never been subjected to a pressure of 1.0 MPa or more.

[0028] Next, an example of a specific configuration of the porous carbon sheet having the above characteristics will be described.

[0029] The porous carbon sheet of the present invention is preferably carbon paper, which preferably has a structure containing short carbon fibers not significantly oriented in the plane of the sheet and a binder, and at least a portion of the short carbon fibers are bound by the binder.

[0030] The binder is preferably a carbonized organic compound, and as the organic compound, it is preferable to use a phenolic resin, which has a small change in mass before and after carbonization and is easy to handle, and among these, it is more preferable to use a novolac-type phenolic resin and / or a resol-type phenolic resin.

[0031] The average diameter of the carbon fibers constituting the porous carbon sheet is preferably 6 to 20 μm, more preferably 6 to 13 μm, and even more preferably 6 to 10 μm, in order to obtain a suitable compression deformation rate and mechanical strength. If the average diameter of the carbon fibers exceeds 20 μm, the number of carbon fibers contained per unit volume of the porous carbon sheet decreases, making it difficult to form conductive paths in the thickness direction and resulting in a deterioration in electrical resistance. Here, the average diameter of the carbon fibers can be determined by measuring the fiber diameters of any 20 carbon fibers from a 1,000x magnification image of the porous carbon sheet obtained by electron microscopy and calculating the arithmetic mean value.

[0032] As the carbon fiber, polyacrylonitrile (PAN)-based, rayon-based, phenol-based, pitch-based, and other carbon fibers can be used alone or in combination. Among these, it is preferable to use PAN-based or pitch-based carbon fibers, and it is particularly preferable to use PAN-based carbon fibers, because they provide a porous carbon sheet that is excellent in mechanical strength and handleability.

[0033] The porous carbon sheet preferably contains conductive non-metallic particles. The conductive non-metallic particles preferably contain carbon powder and / or carbon black. The carbon powder is preferably graphite powder. The inclusion of conductive non-metallic particles improves the conductivity of the porous fiber sheet. Considering only the purpose of improving conductivity, it may seem possible to include metal particles, but when the porous carbon sheet is installed in a water electrolysis cell, this may result in uneven current density and reduced hydrogen production efficiency. In addition, radical generation reactions may be promoted in the metal particle-containing portions, and the generated radicals may decompose the electrolyte membrane.

[0034] In the case of a porous carbon sheet containing conductive non-metallic particles, the conductive non-metallic particles preferably have a spherical equivalent diameter of 0.01 to 30 μm, more preferably 3 to 30 μm, and even more preferably 10 to 30 μm. When the spherical equivalent diameter of the conductive non-metallic particles is within this range, the conductive non-metallic particles form a conductive path in the thickness direction of the sheet inside the porous carbon sheet, improving conductivity. When the spherical equivalent diameter is greater than 30 μm, the conductive non-metallic particles may hinder the flow of gas generated by water electrolysis, resulting in increased gas permeation resistance inside the sheet. Furthermore, the conductive non-metallic particles may not be uniformly distributed within the porous carbon sheet, which may cause steric hindrance when the porous carbon sheet is compressed. Herein, when referring to the spherical equivalent diameter of the conductive non-metallic particles, we are assuming a sphere with the same volume as the volume of the particle determined by laser diffraction scattering, and refer to the median (volume-based) of the spherical diameter distribution.

[0035] Next, an example of a specific method for producing a porous carbon sheet having the above characteristics will be described.

[0036] The method for producing the porous carbon sheet of the present invention may include an impregnation step of impregnating a precursor fiber sheet containing carbon fibers or carbon fiber precursors with an organic substance, a compression step of stacking one or more precursor sheets after the impregnation step and subjecting them to a heat and pressure treatment, and a carbonization step of carbonizing the precursor fiber sheet after the compression step.

[0037] As the precursor fiber sheet, a carbon fiber paper sheet, a carbon fiber nonwoven fabric, a carbon fiber woven fabric, etc. A method may be adopted in which a sheet using precursor fibers of carbon fiber, such as a flame-resistant fiber nonwoven fabric using acrylic flame-resistant fiber or the like, or a flame-resistant yarn woven fabric, is used as the precursor fiber sheet, and a sheet made of carbon fiber is produced in a carbonization step described below.

[0038] By using a carbon fiber paper sheet as the precursor fiber sheet, it is possible to obtain carbon paper as the porous carbon sheet. Carbon paper is particularly preferred because the carbon fibers are uniformly oriented in the in-plane direction of the sheet and there is little carbon fiber oriented in the direction penetrating the sheet surface, which makes it easy to handle, improves the mechanical strength and thickness recovery force of the porous carbon sheet, and further reduces the occurrence of short-circuits through the electrolyte membrane.

[0039] When a carbon fiber paper sheet is used as the precursor fiber sheet, the length of the carbon fibers is preferably within a range of 3 to 12 mm. When the length of the carbon fibers is within this range, good dispersibility can be obtained during papermaking of the carbon fibers, and a carbon fiber sheet with high tensile strength and resistance to tearing can be obtained, which is preferable.

[0040] When a precursor fiber sheet containing carbon fiber is used, the carbon fiber can be any of polyacrylonitrile (PAN)-based carbon fiber, pitch-based carbon fiber, rayon-based carbon fiber, and phenol-based carbon fiber, but it is preferable to use PAN-based carbon fiber or pitch-based carbon fiber, which can increase the bending strength and tensile strength of the obtained carbon fiber sheet, and it is more preferable to use PAN-based carbon fiber. Starch, PVA, etc. can also be added to the carbon fiber paper sheet for the purpose of improving the processability of obtaining the precursor fiber sheet.

[0041] In the impregnation process, the precursor fiber sheet is impregnated with a solution in which a binder or a binder precursor is dissolved in an organic solvent. The binder precursor refers to a substance that acts as an adhesive to form the laminated precursor fiber sheets into a thin, integrated body when heated and pressurized in the compression process and becomes a binder after the carbonization process described below. An organic compound is preferably used. The content of the organic compound is preferably 30% by mass or more and 70% by mass or less, based on 100% by mass of the precursor fiber sheet. Having an organic compound content of 30% by mass or more can prevent peeling during the carbonization process when multiple precursor fiber sheets are laminated in the compression process. Furthermore, the restoring force of the organic compound can suppress the restoring force of the short carbon fibers curved inside the precursor fiber sheet, thereby suppressing destruction of the internal structure of the porous carbon sheet caused by the restoration of the curved short carbon fibers. Furthermore, having an organic compound content of 70% by mass or less can improve the fluid permeability of the porous carbon sheet. Examples of organic compounds that can be used include thermoplastic resins such as polyvinyl alcohol (PVA) and phenolic resins. When conductive non-metallic particles are to be contained in the porous carbon sheet, the conductive non-metallic particles may be dispersed in the above solution.

[0042] The content of the conductive non-metallic particles is preferably 10 to 50 mass% relative to 100 mass% of the porous carbon sheet. A content of 10 mass% or more is preferable because it improves the conductivity. A content of 50 mass% or less can prevent the adhesion between precursor fiber sheets from deteriorating during the compression step, making it difficult to integrate them.

[0043] In the compression step, the precursor fiber sheets are stacked and subjected to a heating and pressure treatment, preferably using a pair of hot plates positioned parallel to each other.

[0044] 2 is a schematic diagram showing one embodiment of the compression step in the porous carbon sheet manufacturing method of the present invention. An arbitrary number of precursor fiber sheets 6 are stacked as shown in the figure to achieve a desired thickness, and then compressed and integrated by a heated and pressurized press 7 using an upper heating plate 8 and a lower heating plate 9. It is also preferable to use a spacer 10 to regulate the thickness.

[0045] By adjusting the number of precursor fiber sheets stacked in the compression step and performing carbonization at a temperature of 2,500°C or higher, it is possible to obtain a porous carbon sheet of the present invention that combines high electrical conductivity with compression properties under various thickness and conditions.

[0046] A batch-type heating furnace can be used in the carbonization process. In the carbonization process, it is preferable to perform primary carbonization at a maximum temperature of 500 to 1,000°C and secondary carbonization at a maximum temperature of 2,500°C or higher. When primary and secondary carbonization are performed, it is preferable to place the precursor fiber sheet after the compression process into a heating furnace, heat it to 500 to 1,000°C, hold it for a certain period of time, and then lower the temperature. In this case, multiple precursor fiber sheets may be carbonized simultaneously. However, for the purpose of uniform heat treatment efficiency and efficient lamination, a certain number of precursor fiber sheets and carbon plates may be alternately stacked. In the secondary carbonization, it is preferable to heat treat the precursor fiber sheet after primary carbonization at a maximum temperature of 2,500°C or higher. Heat treatment at a maximum temperature of 2,500°C or higher sufficiently advances the graphitization of the carbon fiber and organic compound. In particular, phenolic resin carbide, which is often used as a binder, is also called glassy carbon and is brittle. However, graphitization changes the brittle material into an elastic material and increases its strength. This is also preferable because the increased mechanical strength of the short carbon fibers allows for favorable compression characteristics and good electrical conductivity. Furthermore, a higher maximum temperature is more preferable from the perspective of the degree of graphitization progression, but a temperature of 3,000°C or less is preferred from the perspective of furnace durability and energy consumption. The heat treatment time for the secondary carbonization is preferably 10 minutes or more, more preferably 20 minutes or more, to ensure sufficient graphitization of the carbon fiber and binder. As with the primary carbonization, multiple precursor fiber sheets may be carbonized simultaneously. However, to achieve uniform heat treatment efficiency, a certain number of precursor fiber sheets and carbon plates may be alternately stacked.

[0047] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to the descriptions of these examples.

[0048] <Measurement of the thickness of porous carbon sheet> Thickness (d0) under a pressure of 0.15 MPa A sample of 0.1 m square was cut out from a porous carbon sheet. Using a micrometer with a circular probe having a diameter of 5 mm, a surface pressure of 0.15 MPa was applied to the sample placed on a smooth surface plate for measurement. The thickness was measured 10 seconds after the surface pressure was applied. Measurements were taken at five points in total, the four corners and the center of the sample, and the arithmetic average value was taken as the thickness d0 under a pressure of 0.15 MPa.

[0049] Thickness (d1) under a pressure of 1.0 MPa and thickness (d2) under a pressure of 4.5 MPa. The measurement mode is shown in FIG. 3. From the porous carbon sheet on which the d0 was measured, a 2.23 cm square sample 11 (5.0 cm ２ ) was cut out. First, the pressure jig 20 alone was set in a precision universal testing machine, AUTOGRAPH AG-Xplus, manufactured by Shimadzu Corporation, and a sufficient pressure (2,500 N in this testing machine) was applied. The values ​​of the micrometers 13 fixed at four points around the pressure jig 20 were set to 0. Note that only one micrometer 13 is shown in Figure 3 for simplification. Next, the sample 11 was sandwiched between the pressure jig 20 from above and below, and when a pressure of 1.0 MPa was applied to the sample 11 from above, the values ​​of the four micrometers 13 were read and the average value was taken as the thickness d1 of 1.0 MPa. Thereafter, a pressure of 4.5 MPa was applied, and the thickness d2 at that time was measured in the same manner. Note that d1 and d2 were measured for the thickness 10 seconds after the above pressure was applied.

[0050] <Measurement of thickness recovery rate of porous carbon sheet> After measuring the thickness under a pressure of 4.5 MPa according to the procedure of <Measurement of thickness of porous carbon sheet>, the thickness d1' of the porous carbon sheet when reduced to 1.0 MPa was measured in the same manner. Note that d1' was measured 10 seconds after the pressure was reduced to 1.0 MPa. After measuring d1', the thickness d1'' of the porous carbon sheet when pressurized again to 4.5 MPa and then reduced to 1.0 MPa was measured in the same manner as d1'. The thickness recovery rate R1 was calculated using the following formula (I), and the thickness recovery rate R2 was calculated using the following formula (II).

[0051] Thickness recovery rate R1 (%)=d1' / d1×100 (I) Thickness recovery rate R2 (%)=d1'' / d1×100 (II).

[0052] <Measurement of Electrical Resistivity in the Thickness Direction of Porous Carbon Sheet> Fig. 4 shows the measurement mode. ２ ) was cut out. The sample was sandwiched between two gold-plated stainless steel blocks 12 equipped with terminals for current and voltage, and a load was applied from above the gold-plated stainless steel blocks 12 to apply a predetermined pressure (1.0 MPa or 4.5 MPa) to the sample 11 using a precision universal testing machine, AUTOGRAPH AG-Xplus (not shown in Figure 4), manufactured by Shimadzu Corporation. A current of 1 A was passed between the current terminals connected to the two gold-plated stainless steel blocks 12, and the voltage V between the voltage terminals also connected to the gold-plated stainless steel blocks 12 was measured, and the electrical resistivity in the thickness direction was calculated using the following formula (III). Note that the voltage V was measured 10 seconds after the predetermined pressure was applied. Here, d indicates the thickness under the corresponding pressure measured in <Measurement of the Thickness of the Porous Carbon Sheet>. That is, when calculating the electrical resistivity under a pressure of 1.0 MPa, d indicates the thickness d1 of the porous carbon sheet under a pressure of 1.0 MPa, and when calculating the electrical resistivity under a pressure of 4.5 MPa, d indicates the thickness d2 of the porous carbon sheet under a pressure of 4.5 MPa. Electrical resistivity (mΩ cm) = (V (mV) / 1 (A)) × (5.0 (cm ２ ) / d(cm))...(III).

[0053] <Measurement of Basis Weight of Porous Carbon Sheet> Five samples of 0.1 m square were cut out from the porous carbon sheet. Each sample was placed on a precision electronic balance and weighed. ２ ) and the arithmetic mean of the values ​​was calculated to calculate the basis weight of the porous carbon sheet.

[0054] <Confirmation of the Presence or Absence of Short Carbon Fibers Protruding from the Surface of the Porous Carbon Sheet> The presence or absence of short carbon fibers protruding from the surface of the porous carbon sheet as defined in the present invention was confirmed by the following procedures (1) to (3). FIG. 5 shows the measurement procedure. (1) Five 4 cm square samples (hereinafter referred to as Sample 11) were cut out from the porous carbon sheet, and a 5.5 cm square polymer electrolyte membrane 14 "Nafion (registered trademark)" NR211 (manufactured by DuPont, membrane thickness 25 μm) was placed on one surface of the sample. Here, the polymer electrolyte membrane 14 was placed so that each side of the sample 11 was parallel to each side of the polymer electrolyte membrane 14 and so that the center of the polymer electrolyte membrane 14 coincided with the center of the sample 11. (2) The stacked polymer electrolyte membrane 14 and sample 11 were sandwiched between two gold-plated stainless steel blocks 12 from above and below, and a pressure of 1.0 MPa was applied to the sample 11 using a precision universal testing machine, AUTOGRAPH AG-Xplus (not shown in Figure 5), manufactured by Shimadzu Corporation. The two gold-plated stainless steel blocks 12 were sandwiched so that their centers coincided with the center of the sample 11, avoiding direct contact between them. (3) A DC voltage of 1.0 V was applied between the gold-plated stainless steel blocks 12 using a digital multimeter, and the current between the blocks was measured to confirm whether a current flowed. The presence of a current indicated that short carbon fibers had protruded from the sample 11 and penetrated the polymer electrolyte membrane 14. After performing steps (1) to (3) above, the sample 11 was turned upside down, so that the other side of the sample 11 was in contact with the polymer electrolyte membrane 12, and steps (1) to (3) above were repeated. There were five samples 11, and measurements were taken twice on the front and back sides. Of the total 10 measurements, if the current did not exceed 10 mA in 9 or more measurements, it was judged as "absent" that short carbon fibers had protruded, and if a current exceeding 10 mA flowed two or more times, it was judged as "present."

[0055] (Example 1) Toray Industries, Inc.'s polyacrylonitrile carbon fiber "TORAYCA (registered trademark)" T300 (average single fiber diameter: 7 μm, number of single fibers: 6,000) was cut to a length of 12 mm, and a long sheet-like carbon fiber paper body with a width of 1,000 mm and a length of 1,000 m was continuously produced using water as a papermaking medium. A binder was applied to the long carbon fiber paper body and dried to produce a carbon fiber paper body with a basis weight of 50 g / m. ２ A long carbon fiber paper sheet was produced.

[0056] The long carbon fiber paper body was continuously impregnated with a 25 mass % methanol solution (viscosity at 25°C: 2.5 mPa s) of a resin component (a mixture of novolac-type phenolic resin and resol-type phenolic resin in a solid content mass ratio of 1:1), dried, and then cut into 1.0 m sections, resulting in a paper body with a basis weight of 100 g / m ２ A precursor fiber sheet of 1000 nm was obtained.

[0057] Next, 15 of the precursor fiber sheets were stacked and placed between the hot plates of a hot-press press, with a pair of hot plates set parallel to each other, and then heated and pressed at 155°C and 0.6 MPa for 30 minutes to cure the resin component. The precursor fiber sheet with the cured resin component was placed in a batch-type heating furnace and subjected to primary carbonization at a maximum temperature of 1,000°C in an inert atmosphere, followed by secondary carbonization at a maximum temperature of 2,500°C in an inert atmosphere, to obtain a porous carbon sheet. The obtained porous carbon sheet was subjected to various measurements according to the above-mentioned <Measurement of the thickness of the porous carbon sheet>, <Measurement of the electrical resistance of the porous carbon sheet>, <Measurement of the basis weight of the porous carbon sheet>, and <Confirmation of the presence or absence of short carbon fibers protruding from the surface of the porous carbon sheet>. The evaluation results are shown in Table 1.

[0058] Example 2 Instead of a 25 mass % methanol solution of the resin component, graphite powder (equivalent volume sphere diameter 5 μm), resin component (a mixture of novolac-type phenolic resin and resol-type phenolic resin in a solid content mass ratio of 1:1), and methanol were mixed in a mass ratio of 8:17:75 to prepare a uniformly dispersed resin composition (viscosity at 25° C. 3.2 mPa s), and the resin composition was impregnated into a long carbon fiber paper sheet. A porous carbon sheet was obtained in the same manner as in Example 1, except that the evaluation results of the obtained porous carbon sheet were as shown in Table 1.

[0059] Example 3 A porous carbon sheet was obtained in the same manner as in Example 2, except that graphite powder having an equivalent volume sphere diameter of 15 μm was used. The viscosity of the resin composition at 25°C was 3.2 mPa·s. The evaluation results of the obtained porous carbon sheet are shown in Table 1. Example 4 A porous carbon sheet was obtained in the same manner as in Example 3, except that 14 precursor fiber sheets were stacked and arranged, and hot-pressed under conditions of 155°C and 0.4 MPa. The evaluation results of the obtained porous carbon sheet are shown in Table 1. Example 5 A porous carbon sheet was obtained in the same manner as in Example 4, except that graphite powder having an equivalent volume sphere diameter of 10 μm was used. The evaluation results of the obtained porous carbon sheet are shown in Table 1.

[0060] Comparative Example 1 A porous carbon sheet was obtained in the same manner as in Example 1, except that the resin component concentration in the methanol solution containing the resin component was 15 mass %. The evaluation results of the obtained porous carbon sheet are shown in Table 1.

[0061] (Comparative Example 2) A porous carbon sheet was obtained in the same manner as in Example 1, except that the maximum temperature during the secondary carbonization of the precursor fiber sheet was set to 2,000° C. The evaluation results of the obtained porous carbon sheet are shown in Table 1.

[0062] (Comparative Example 3) A porous carbon sheet was obtained in the same manner as in Example 2, except that the maximum temperature during the secondary carbonization of the precursor fiber sheet was set to 2,000° C. The evaluation results of the obtained porous carbon sheet are shown in Table 1.

[0063]

[0064] As shown in Table 1, Examples 1, 2, and 3 of the present invention exhibited compression properties suitable for use in water electrolysis cells, with no protrusion of short carbon fibers from the sheet surface and good electrical conductivity. Example 2, which added graphite particles with a volume-equivalent sphere diameter of 5 μm as conductive nonmetallic particles, had better compression properties and electrical conductivity, while Example 3, which added graphite particles with a volume-equivalent sphere diameter of 15 μm, had the best electrical conductivity. In Examples 4 and 5, even when the number of precursor fiber sheets integrated was reduced compared to Example 3 and the pressure during hot and pressurized pressing was reduced, good compression properties and electrical conductivity were achieved by adjusting the viscosity of the resin solution and the volume-equivalent sphere diameter of the contained conductive nonmetallic particles. In Comparative Example 1, the viscosity of the methanol solution of the resin component was low, so the phenolic resin carbide in the porous carbon sheet formed a structure that presented steric hindrance when the porous carbon sheet was compressed. This made it difficult to crush the porous carbon sheet even when a pressure of 4.5 MPa was applied, making it difficult to form a conductive path in the thickness direction, and increasing electrical resistance. In Comparative Examples 2 and 3, the carbonization temperature was low, and the phenolic resin carbonized binder was brittle, so it was significantly crushed when a pressure of 1.0 MPa was applied, and short carbon fibers were observed to protrude. Furthermore, in Comparative Examples 2 and 3, graphitization of the carbon fibers and phenolic resin did not proceed sufficiently, and the electrical conductivity and thickness recovery rate of the porous carbon sheet also deteriorated.

[0065] REFERENCE SIGNS LIST 1 Electrolyte membrane 2a Anode side catalyst layer 2c Cathode side catalyst layer 3a Anode side diffusion layer 3c Cathode side diffusion layer 4a Anode side separator 4c Cathode side separator 5a Cathode side main electrode 5c Cathode side main electrode 6 Precursor fiber sheet 7 Heating and pressure press 8 Upper heating plate 9 Lower heating plate 10 Spacer 11 Sample 12 Gold-plated stainless steel block 13 Micrometer 14 Polymer electrolyte membrane 20 Pressure jig

[0066] The present invention relates to a porous carbon sheet suitable for use in a water electrolysis cell. By using the porous carbon sheet of the present invention as a diffusion layer of a water electrolysis cell, it is possible to provide a water electrolysis cell that can stably produce hydrogen for a long period of time.

Claims

1. A porous carbon sheet having a sheet-like structure in which carbon fibers are bound with a binder, the porous carbon sheet having a thickness d0 of 1.8 to 3.0 mm under a pressure of 0.15 MPa, a thickness d1 under a pressure of 1.0 MPa that is 85% or more of the thickness d0 under a pressure of 0.15 MPa, and a thickness d2 under a pressure of 4.5 MPa that is 75% or less of the thickness d0 under a pressure of 0.15 MPa.

2. The porous carbon sheet according to claim 1, which has an electrical resistivity in the thickness direction under a pressure of 1.0 MPa of 120 mΩ·cm or less.

3. The porous carbon sheet according to claim 1, which has an electrical resistivity of 80 mΩ·cm or less under a pressure of 1.0 MPa.

4. The porous carbon sheet according to claim 1, which has an electrical resistivity in the thickness direction of 90 mΩ·cm or less under a pressure of 4.5 MPa.

5. The porous carbon sheet according to claim 1, which has an electrical resistivity of 60 mΩ·cm or less under a pressure of 4.5 MPa.

6. The porous carbon sheet according to claim 1, which contains conductive nonmetallic particles having an equivalent sphere diameter of 10 to 30 μm.

7. The porous carbon sheet according to claim 6, wherein the conductive nonmetallic particles are contained in a ratio of 10 to 50 mass % based on 100 mass % of the porous carbon sheet.

8. A porous carbon sheet according to claim 1, which has a thickness recovery rate R1 of 80% or more when pressurized to 4.5 MPa and then depressurized to 1.0 MPa.

9. A porous carbon sheet according to claim 1, wherein the difference between the thickness recovery rate R1 when the pressure is pressurized to 4.5 MPa and then reduced to 1.0 MPa and the thickness recovery rate R2 when the pressure is again pressurized to 4.5 MPa and then reduced to 1.0 MPa is 5% or less.

10. A water electrolysis device comprising the porous carbon sheet according to claim 1 as a cathode electrode member.

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