JP2004179095A - Substrate for fuel cells, electrode for fuel cells, and method of manufacturing substrate for fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent drop in water repellency of a gas diffusion electrode and drop in performance of a fuel cell. <P>SOLUTION: Carbon paper is immersed in a PTFE dispersion (S10), heated and dried to remove a solvent (S12). The carbon paper from which the solvent is removed is heated to the melting point or more of the PTFE to melt the PTFE contained in the carbon paper (S14). Ionized radiation is radiated to the carbon paper in which the PTFE is melted (S16). The PTFE is crosslinked, and the carbon paper covered with the crosslinked PTFE is manufactured. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a fuel cell base and a fuel cell using the same as an electrode.
[0002]
[Prior art]
2. Description of the Related Art In recent years, fuel cells that have high energy conversion efficiency and do not generate harmful substances due to power generation reactions have attracted attention. As one of such fuel cells, a polymer electrolyte fuel cell that operates at a low temperature of 100 ° C. or less is known.
[0003]
A polymer electrolyte fuel cell has a basic structure in which a solid polymer membrane, which is an electrolyte membrane, is disposed between a fuel electrode and an air electrode. Hydrogen is supplied to the fuel electrode and oxygen is supplied to the air electrode. Electric power is generated by a chemical reaction.
Fuel _{^{electrode: H 2 → 2H + + 2e}} - (1)
Air electrode: 1 / 2O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O (2)
The fuel electrode and the air electrode have a structure in which a catalyst layer and a gas diffusion layer are stacked. The catalyst layers of the respective electrodes are opposed to each other with the solid polymer film interposed therebetween, and constitute a fuel cell. The catalyst layer is a layer formed by binding carbon particles carrying a catalyst with an ion exchange resin or the like. The gas diffusion layer serves as a passage for oxygen and hydrogen. The power generation reaction proceeds at a so-called three-phase interface between the catalyst, the ion exchange resin, and hydrogen in the catalyst layer.
[0004]
At the fuel electrode, hydrogen contained in the supplied fuel is decomposed into hydrogen ions and electrons as shown in the above formula (1). Among them, hydrogen ions move inside the solid polymer electrolyte membrane toward the air electrode, and electrons move to the air electrode through an external circuit. On the other hand, in the air electrode, oxygen contained in the supplied oxidizing agent reacts with hydrogen ions and electrons moved from the fuel electrode, and water is generated as shown in the above equation (2). As described above, in the external circuit, the electrons move from the fuel electrode toward the air electrode, so that electric power is extracted.
[0005]
In such a polymer electrolyte fuel cell, the gas diffusion layer covers the gas diffusion layer surface with a water-repellent resin such as polytetrafluoroethylene (PTFE) in order to suppress a decrease in gas permeability due to water wetting. (See Patent Document 1).
[0006]
[Patent Document 1]
JP 2001-15123 A
[Problems to be solved by the invention]
However, the inventor has come to recognize that when the fuel cell is operated for a long time, the water repellency of the gas diffusion layer decreases with the passage of time, which causes a decrease in the performance of the fuel cell.
[0008]
In view of such circumstances, an object of the present invention is to provide a technique for suppressing a decrease in the performance of a fuel cell. Still another object is to provide a technique for suppressing a decrease in water repellency of a gas diffusion layer of a fuel cell electrode.
[0009]
[Means for Solving the Problems]
One embodiment of the present invention relates to a fuel cell substrate. The fuel cell substrate includes a conductive porous substrate, and the surface of the porous substrate is coated with a crosslinkable polymer. The fuel cell base functions as a gas diffusion layer in the fuel cell electrode. Further, the conductive porous substrate has many pores having a diameter of several μm to several tens μm, and an example thereof is carbon paper. Here, the “cross-linkable polymer” refers to a polymer having a cross-linking structure, and the “cross-linking structure” means that any two points of a chain molecule are directly or bridged by one or more atoms. It refers to such a structure. By coating the surface of the porous base material with the crosslinkable polymer, the performance deterioration that is observed with the elapse of the operation time of the fuel cell is suppressed. In addition, since the fuel cell base includes the porous base, good gas permeability can be obtained.
[0010]
The crosslinkable polymer may include a fluororesin. By coating the surface of the porous substrate with a fluororesin, the gas diffusion layer is suppressed from decreasing in gas permeability due to wetting with water. Further, for example, when a gas diffusion electrode containing a fluororesin having a cross-linking structure is used in a fuel cell, deterioration of performance over time of the fuel cell is suppressed. Further, the fluororesin may be PTFE. PTFE has good water repellency.
[0011]
Another embodiment of the present invention relates to a method for manufacturing a fuel cell substrate. This method for producing a fuel cell substrate includes a step of impregnating a porous substrate having conductivity with a dispersion liquid of a water-repellent polymer, and a step of drying the porous substrate impregnated with the dispersion liquid. Heat treating the dried porous substrate, and crosslinking the water-repellent polymer during the heat treatment.
[0012]
The water-repellent polymer may be brought into a molten state by heating during the heat treatment step. The upper limit of the heat treatment temperature is preferably not lower than the melting point and not exceeding the melting point, for example, about (melting point + 20) ° C. With such a heat treatment temperature, the water-repellent polymer is in a molten state. Specifically, in the case of PTFE having a melting point of 327 ° C., the temperature is preferably set to 327 ° C. or higher. The molten water repellent polymer may be irradiated with ionizing radiation. Here, ionizing radiation includes γ-rays and electron beams. Decomposition of a water-repellent polymer is generally promoted by radiation, but the ionizing radiation is emitted to the water-repellent polymer in a molten state, so that the water-repellent polymer has a good cross-linked structure. Formed.
[0013]
Further, the water-repellent polymer may be a fluororesin. Fluororesins exhibit good water repellency. Further, the fluororesin may be PTFE. However, when a crosslinked structure is formed in PTFE, it is preferably performed in a low oxygen atmosphere or an inert gas atmosphere in order to prevent an oxidation reaction of PTFE.
[0014]
Still another embodiment of the present invention relates to an electrode for a fuel cell. The fuel cell electrode includes the fuel cell base and a catalyst layer. An electrode for a fuel cell using this electrode for a fuel cell has good gas permeability and little deterioration of water repellency with time.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
In the embodiment, it is possible to prevent the water repellency of the gas diffusion layer from decreasing with the elapse of the operation time of the fuel cell. For this purpose, a fluororesin used for the gas diffusion layer is melted in the step of forming the gas diffusion layer, and the molten fluororesin is irradiated with ionizing radiation to form a crosslinked structure.
[0016]
FIG. 1 schematically shows a cross-sectional structure of a fuel cell 10 according to an embodiment of the present invention. The fuel cell 10 includes a flat cell 50, and a separator 34 and a separator 36 are provided on both sides of the cell 50. In this example, only one cell 50 is shown, but the fuel cell 10 may be configured by stacking a plurality of cells 50 via the separator 34 or the separator 36. The cell 50 has the solid polymer electrolyte membrane 20, the fuel electrode 22, and the air electrode 24. The fuel electrode 22 and the air electrode 24 may be referred to as “catalyst electrodes”. The fuel electrode 22 has a stacked catalyst layer 26 and a gas diffusion layer 28, and the air electrode 24 similarly has a stacked catalyst layer 30 and a gas diffusion layer 32. The catalyst layer 26 of the fuel electrode 22 and the catalyst layer 30 of the air electrode 24 are provided so as to face each other with the solid polymer electrolyte membrane 20 interposed therebetween.
[0017]
The separator 34 provided on the fuel electrode 22 side is provided with a gas flow path 38, and fuel gas is supplied to the cell 50 through the gas flow path 38. Similarly, a gas passage 40 is also provided in the separator 36 provided on the air electrode 24 side, and oxygen is supplied to the cell 50 through the gas passage 40. Specifically, during operation of the fuel cell 10, a fuel gas, for example, hydrogen gas is supplied from the gas flow channel 38 to the fuel electrode 22, and an oxidant gas, for example, air is supplied from the gas flow channel 40 to the air electrode 24. . Thereby, a power generation reaction occurs in the cell 50. When hydrogen gas is supplied to the catalyst layer 26 via the gas diffusion layer 28, hydrogen in the gas becomes protons, and the protons move through the solid polymer electrolyte membrane 20 toward the air electrode 24. The electrons emitted at this time move to the external circuit and flow into the air electrode 24 from the external circuit. On the other hand, when air is supplied to the catalyst layer 30 via the gas diffusion layer 32, oxygen is combined with protons to form water. As a result, in the external circuit, electrons flow from the fuel electrode 22 to the air electrode 24, and power can be taken out.
[0018]
The solid polymer electrolyte membrane 20 preferably exhibits good ion conductivity in a wet state, and functions as an ion exchange membrane that moves protons between the fuel electrode 22 and the air electrode 24. The solid polymer electrolyte membrane 20 is formed of a solid polymer material such as a fluoropolymer, for example, a sulfonic acid type perfluorocarbon polymer, a polysulfone resin, a perfluorocarbon polymer having a phosphonic acid group or a carboxylic acid group, or the like. Can be used. Examples of the sulfonic acid-type perfluorocarbon polymer include Nafion (registered trademark) 112 and the like. Examples of the non-fluorinated polymer include aromatic polyetheretherketone and polysulfone.
[0019]
The gas diffusion layer 28 in the fuel electrode 22 and the gas diffusion layer 32 in the air electrode 24 have a function of supplying the supplied hydrogen gas or air to the catalyst layers 26 and 30. It also has a function of transferring electric charges generated by the power generation reaction to an external circuit and a function of discharging water, unreacted gas, and the like to the outside. The gas diffusion layer 28 and the gas diffusion layer 32 are preferably made of a porous material having electron conductivity, and are made of, for example, carbon paper or carbon cloth. Here, the porous body is coated with a fluoropolymer to have water repellency. Preferred examples of the solid polymer material such as a fluorine-containing polymer include PTFE, ethylene tetrafluoride / perfluoroalkyl vinyl ether copolymer resin (PFA), and ethylene tetrafluoride / propylene hexafluoride copolymer resin (FEP). And ethylene tetrafluoride / ethylene copolymer resin (ETFE).
[0020]
The catalyst layer 26 in the fuel electrode 22 and the catalyst layer 30 in the air electrode 24 are porous membranes, and are preferably composed of an ion exchange resin and carbon particles supporting a catalyst. Examples of the supported catalyst include one or a mixture of two or more of platinum, rhodium, ruthenium, and the like. The carbon particles supporting the catalyst include acetylene black, Ketjen black, and carbon nanotubes.
[0021]
The ion exchange resin has a function of electrochemically connecting the carbon particles supporting the catalyst and the solid polymer electrolyte membrane 20. The fuel electrode 22 is required to have proton permeability, and the air electrode 24 is required to have oxygen permeability. The ion exchange resin may be formed from the same polymer material as the solid polymer electrolyte membrane 20.
[0022]
Hereinafter, an example of a method for manufacturing the cell 50 is shown in FIG. Here, PTFE is used as the water-repellent resin. In order to produce the fuel electrode 22 and the air electrode 24, first, carbon paper is immersed in a PTFE dispersion in which PTFE fine particles are dispersed in an alcohol solvent such as isopropyl alcohol (S10). And heat drying for about 1 hour to remove the solvent (S12). Further, the carbon paper from which the solvent has been removed is heated to a temperature equal to or higher than the melting point of PTFE, for example, to 330 ° C., and the PTFE contained in the carbon paper is melted (S14). Next, an ionizing radiation typified by an electron beam is irradiated at an intensity of 100 kGy to the carbon paper in which the PTFE is melted (S16). As a result, the PTFE is crosslinked, and carbon paper coated with the crosslinked PTFE is produced.
[0023]
Next, the catalyst particles are mixed with the solid polymer electrolyte solution (Nafion solution) to produce a catalyst ink (S18). As a method for supporting the catalyst on the carbon particles, a general impregnation method or a colloid method is used. Subsequently, the generated catalyst ink is applied to the carbon paper coated with the crosslinked PTFE by using, for example, a brush coating or a spray coating technique (S20). Thereafter, the carbon paper coated with the catalyst ink is dried by heating (S22). Thus, the fuel electrode 22 and the air electrode 24 are manufactured.
[0024]
Thereafter, the solid polymer electrolyte membrane 20 is sandwiched between the catalyst layer 26 of the fuel electrode 22 and the catalyst layer 30 of the air electrode 24, and joined by hot pressing (S24). Thereby, the cell 50 is manufactured. When the ion exchange resin in the solid polymer electrolyte membrane 20, the catalyst layer 26, and the catalyst layer 30 is made of a polymer material having a softening point or a glass transition, hot pressing should be performed at a temperature exceeding the softening temperature or the glass transition temperature. Is preferred.
[0025]
FIG. 3 schematically shows a cross-sectional structure of the cell 50. In the fuel electrode 22, it is shown that the catalyst layer 26 has entered inside the surface of the gas diffusion layer 28 made of carbon paper or the like. Also in the air electrode 24, the catalyst layer 30 enters inside the gas diffusion layer 32.
[0026]
Irradiating ionizing radiation in a molten state produced a crosslinked PTFE sheet (treated sheet). Compared to a sheet of PTFE not irradiated with ionizing radiation (untreated sheet), the light transmittance was greatly improved, for example, the transmittance of light having a wavelength of 600 nm was improved about 10 times. Further, the radiation resistance was improved about 100 times. As a further appearance feature, the untreated sheet was opaque while the treated sheet became transparent.
[0027]
In addition, when an electrode sheet containing carbon paper coated with crosslinked PTFE was produced using the same method as the above method, an electrode sheet with water repellency and little deterioration over time was obtained. Further, in the fuel cell using the electrode manufactured by the above method, the deterioration with time of the performance was improved.
[0028]
The present invention has been described based on the embodiments. This embodiment is an exemplification, and it is understood by those skilled in the art that various modifications can be made to the combination of each component and each processing process, and that such modifications are also within the scope of the present invention. is there.
[0029]
As described above, according to the embodiment, the PTFE coated on the surface of the gas diffusion layer of the electrode of the fuel cell is crosslinked, whereby the deterioration over time of the water repellency is improved. Further, by using such an electrode in a fuel cell, the performance of the fuel cell over time is prevented from deteriorating.
[0030]
【The invention's effect】
According to the present invention, it is possible to suppress a decrease in the performance of the fuel cell. From another viewpoint, a decrease in water repellency of the gas diffusion layer can be suppressed.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing a cross-sectional structure of a fuel cell according to an embodiment of the present invention.
FIG. 2 is a flowchart showing a procedure for manufacturing a cell.
FIG. 3 is a diagram schematically showing a cross-sectional structure of a cell.
[Explanation of symbols]
Reference Signs List 10 fuel cell, 20 solid polymer electrolyte membrane, 22 fuel electrode, 24 air electrode, 26 catalyst layer, 28 gas diffusion layer, 30 catalyst layer, 32 gas diffusion layer, 34 separator, 36 separator, 36 air electrode, 38 gas flow Roads, 40 gas channels, 50 cells.

Claims (10)

A fuel cell substrate including a conductive porous substrate, wherein the surface of the porous substrate is coated with a crosslinkable polymer. The fuel cell substrate according to claim 1, wherein the crosslinkable polymer contains a fluororesin. The fuel cell substrate according to claim 2, wherein the fluororesin is polytetrafluoroethylene. An electrode for a fuel cell, comprising the fuel cell substrate according to claim 1 and a catalyst layer. A fuel cell comprising the fuel cell electrode according to claim 4. A step of impregnating a conductive porous substrate with a water-repellent polymer dispersion liquid,
Drying the porous substrate impregnated with the dispersion liquid,
Heat treating the dried porous substrate,
Has,
A method for producing a fuel cell substrate, comprising crosslinking the water-repellent polymer during the heat treatment step. The method for producing a fuel cell substrate according to claim 6, wherein the water-repellent polymer is brought into a molten state by heating during the heat treatment step. The method according to claim 7, wherein the molten water-repellent polymer is irradiated with ionizing radiation. The method according to any one of claims 6 to 8, wherein the water-repellent polymer contains a fluororesin. The method according to claim 9, wherein the fluororesin is polytetrafluoroethylene.

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