JP7563156B2 - Catalyst ink for polymer electrolyte fuel cells - Google Patents

Description

The present invention relates to a catalyst ink for a polymer electrolyte fuel cell.

In recent years, in order to solve environmental problems such as global warming, there is a demand for the development of new power sources aimed at reducing _{CO2 emissions} . As a power source, fuel cells that do not emit _CO2 have attracted attention. A fuel cell uses a fuel (e.g., hydrogen) and an oxidant (e.g., oxygen) to oxidize and generate harmless water. The chemical energy obtained by generating water is converted into electrical energy, which is used as a power source or power source.

Fuel cells are classified according to the type of electrolyte, so they are highly dependent on the operating temperature, and the appropriate fuel cell is used depending on the temperature range in which the load operates. Polymer electrolyte fuel cells (PEFCs) operate at low temperatures, have high output density, and can be made small and lightweight, so they are being developed as home power sources and vehicle power sources.

A polymer electrolyte fuel cell (PEFC) has a structure (membrane electrode assembly) in which a polymer electrolyte membrane is sandwiched between a fuel electrode (anode) and an air electrode (cathode). Hydrogen is supplied as fuel gas to the fuel electrode side, and air gas containing oxygen is supplied to the air electrode side, generating electricity through the following electrochemical reaction:
Anode: _H2 → 2H ⁺⁺ 2e ^-... (Reaction 1)
Cathode: 1/2O2 + _2H ^{++ 2e-} → _H2O ... (Reaction 2)
The anode and cathode each have a laminated structure of a catalyst layer and a gas diffusion layer. Protons and electrons are generated from hydrogen supplied to the anode catalyst layer by the electrode catalyst (Reaction 1). The protons pass through the polymer electrolyte and polymer electrolyte membrane in the anode catalyst layer and move to the cathode. The electrons pass through an external circuit and move to the cathode. In the cathode catalyst layer, the protons, electrons, and oxygen contained in the air supplied from the outside react to generate water (Reaction 2).

In order to reduce the cost of fuel cells, efforts are being made to develop fuel cells that exhibit high output characteristics. However, there is a problem that the proton conductivity of the polymer electrolyte decreases due to a lack of water caused by low humidification operation, which reduces the power generation performance. The catalyst for fuel cells is a conductive carrier, for example, a carbon particle surface on which a metal with catalytic activity such as platinum is supported. The surface of the carbon particle is originally hydrophobic, and the surface state of the surface of the catalytic metal is also uncontrollable due to the influence of impurities. When such catalyst particles are mixed with a hydrophilic polymer solution, the particles become loose immediately after mixing due to the surface tension of the catalyst particles, making it difficult to uniformly disperse the catalyst particles. Therefore, a technology is known in which the particle size distribution peak of the catalyst-supported particles is 1 μm (Patent Document 1). In general, when the surface of the catalyst-supported particles is activated, the surface area increases, so that they tend to adhere to each other and form agglomerates. Even if catalyst-supported particles with a particle size of 1 μm or less are used, when the particle size of the catalyst-supported particles after activation is measured, a peak may appear in the region of particle sizes of 1 μm or more. This peak is thought to be due to agglomerations. If an electrolyte membrane is manufactured using catalyst-supported particles containing such agglomerates, the agglomerates may be pushed into the electrolyte membrane, causing it to break and otherwise adversely affecting the electrolyte membrane. To solve the above problem, the polymer electrolyte that performs the proton conduction function and the catalyst are uniformly intertwined in the catalyst ink to increase proton conductivity and improve the output characteristics of solid polymer electrolyte fuel cells.

JP 2005-285511 A

The present invention has been made in view of the above circumstances, and has an object to provide a catalyst ink for a polymer electrolyte fuel cell that can improve proton conductivity and enable high output.

One aspect for solving the above problem is a catalyst ink for a polymer electrolyte fuel cell, which is composed of catalyst particles, a conductive support, a polymer electrolyte, a fibrous material and a solvent, and is characterized in that the ratio (P2/P1) of a frequency peak (P1) existing in a particle diameter range of 1.0 μm or more and 12 μm or less in the dispersoid of the catalyst ink measured by a particle size distribution measuring device to a frequency peak (P2) existing in a particle diameter range of 0.3 μm or more and less than 1.0 μm is 1.2 or more and less than 2.0.

2.0 > P2 / P1 ≧ 1.2 ... Formula (1)
In addition , the catalyst ink for a polymer electrolyte fuel cell according to one embodiment is characterized in that the catalyst particles are made of a metal selected from the group consisting of platinum, palladium, ruthenium, iridium, rhodium, and osmium .
A catalyst layer for a polymer electrolyte fuel cell according to one embodiment is characterized in that it is produced from the catalyst ink for a polymer electrolyte fuel cell.

A membrane-electrode assembly for a polymer electrolyte fuel cell according to one embodiment is characterized in that the above-mentioned catalyst layer for a polymer electrolyte fuel cell is provided on at least one of an anode and a cathode.

According to one aspect of the present invention, it is possible to provide a catalyst ink for a polymer electrolyte fuel cell which has improved proton conductivity and exhibits high power generation performance.

FIG . 1 is a diagram showing the configuration of a catalyst ink for a polymer electrolyte fuel cell according to an embodiment. FIG. 2 is a diagram showing the configuration of a catalyst layer for a polymer electrolyte fuel cell according to one embodiment. FIG. 1 is a diagram illustrating the configuration of a membrane electrode assembly according to an embodiment.

The following describes an embodiment of the present invention with reference to the drawings. Note that the present invention is not limited to the embodiment described below, and modifications such as design changes can be made based on the knowledge of those skilled in the art. Such modified embodiments are also included in the scope of the present invention.

Fig. 1 shows a catalyst ink for a polymer electrolyte fuel cell according to one embodiment, comprising catalyst particles 1, a conductive support 2, a polymer electrolyte 3, and a fibrous material 4. Fig. 2 shows catalyst layers (cathode catalyst layer 5, anode catalyst layer 6) for a polymer electrolyte fuel cell, comprising catalyst particles 1, a conductive support 2, a polymer electrolyte 3, and a fibrous material 4. Fig. 3 shows a membrane electrode assembly comprising a polymer electrolyte membrane 7, a cathode catalyst layer 5, an anode catalyst layer 6, a gasket material 8, and a gas diffusion layer 9.

Next, the configuration of the catalyst layers (cathode catalyst layer 5, anode catalyst layer 6) will be described with reference to FIG. 2. As the catalyst particles 1 used in this embodiment, in addition to platinum group elements such as platinum, palladium, ruthenium, iridium, rhodium, and osmium, metals such as iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, or alloys thereof can be used. Oxides, double oxides, etc. can also be used. Furthermore, the particle size of these catalysts is, for example, 0.1 nm to 1 μm, preferably 0.5 nm to 100 nm, and more preferably 1 nm to 10 nm.

Carbon particles are generally used as the conductive support 2 that supports these catalyst particles 1. Any type of carbon particles can be used as long as they are fine particles, conductive, and not corroded by the catalyst particles 1, but examples of usable carbon particles include carbon black, graphite, activated carbon, and fullerene.

The polymer electrolyte 3 used in this embodiment may be any material having proton conductivity, and may be made of the same material as the polymer electrolyte membrane 7 (see FIG. 3), such as a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. As the fluorine-based polymer electrolyte, for example, a Nafion (registered trademark)-based material manufactured by DuPont may be used.

The ratio I/(C+CF) of the mass C of the conductive support 2 and the mass CF of the fibrous material 4 supported on the catalyst particle 1 to the mass I of the polymer electrolyte 3 is preferably 0.2 or more and 1.5 or less.
In particular, when the IEC (ion exchange capacity of the polymer electrolyte) is 1.1 or more and less than 1.3, I/(C+CF) is more preferably 0.2 or more and less than 0.8. Also, when the IEC is 1.3 or more and less than 1.5, I/(C+CF) is more preferably 0.8 or more and 1.5 or less. This is because the lower the IEC, the more easily the polymer electrolyte 3 and the fibrous material 4 are entangled, and the effective range of I/(C+CF) for power generation performance changes depending on the IEC.
When I/(C+CF) is less than 0.2, the proton path is poor, and the power generation performance is significantly impaired.When I/(C+CF) is more than 1.5, the drainage of the catalyst layer is reduced, and the power generation performance of the fuel cell is significantly impaired.

The fibrous material 4 used in this embodiment can be carbon fibers such as carbon fibers, carbon nanofibers, and carbon nanotubes, and polymer electrolyte fibers with proton conductivity. Carbon nanofibers and carbon nanotubes are preferable. For example, materials such as VGCF-H (registered trademark) manufactured by Showa Denko KK can be used. The fiber diameter of the carbon fiber is preferably 30 to 500 nm, more preferably 60 to 300 nm. By setting it in the above range, the voids in the catalyst layer can be increased, and high power generation performance is shown. The fiber length of the carbon fiber is preferably 0.5 to 20 μm, more preferably 1 to 10 μm. By setting it in the above range, the strength of the catalyst layer can be increased and the occurrence of cracks during formation can be suppressed. In addition, the voids in the catalyst layer can be increased, and high power generation performance is shown. In addition, the solvent used as the dispersion medium of the catalyst ink is not particularly limited as long as it does not erode the conductive carrier 2 supporting the catalyst particles 1, the polymer electrolyte 3, and the fibrous material 4, and can dissolve or disperse the polymer electrolyte 3 as a fine gel in a highly fluid state.

In addition, it is preferable that the solvent contains a volatile organic solvent or water. The organic solvent is not particularly limited, but examples of the organic solvent include alcohol solvents such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, and pentanol; ketone solvents such as acetone, methyl ethyl ketone, pentanone, methyl isobutyl ketone, heptanone, cyclohexanone, methylcyclohexanone, acetonylacetone, and diisobutyl ketone; ether solvents such as tetrahydrofuran, dioxane, diethylene glycol dimethyl ether, anisole, methoxytoluene, and dibutyl ether; and polar solvents such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, diacetone alcohol, and 1-methoxy-2-propanol. A mixture of three or more solvents is preferable, and a mixture containing two or more alcohols is particularly preferable. Furthermore, by mixing multiple solvents with different polarities and dielectric constants in the catalyst ink, the polymer electrolyte 3 penetrates every corner of the catalyst, improving power generation performance.

Next, the preparation and configuration of the membrane electrode assembly will be described with reference to FIG.
The membrane electrode assembly includes catalyst layers (cathode catalyst layer 5 on the front side and anode catalyst layer 6 on the back side), a gasket material 8, and a gas diffusion layer 9 on the front and back sides of a polymer electrolyte membrane 7. The gasket material 8 and the gas diffusion layer 9 are laminated in this order from the polymer electrolyte membrane 7 side. The gasket material 8 is disposed so as to surround the periphery of the catalyst layers (cathode catalyst layer 5, anode catalyst layer 6).

The polymer electrolyte membrane 7 used in the membrane electrode assembly may be any one having proton conductivity, and may be a fluorine-based polymer electrolyte, a hydrocarbon-based polymer electrolyte, or the like. As the fluorine-based polymer electrolyte, for example, Nafion (registered trademark) manufactured by DuPont may be used. As the hydrocarbon-based polymer electrolyte membrane, for example, an electrolyte membrane such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, or sulfonated polyphenylene may be used. In particular, a material containing perfluorosulfonic acid as a fluorine-based polymer electrolyte may be preferably used as the polymer electrolyte membrane 7.

The gasket material 8 and the plastic film having the adhesive layer may be heat-resistant to such an extent that they do not melt when heated and pressed. For example, polymer films such as polyethylene naphthalate, polyethylene terephthalate, polyimide, polypalvanic acid aramid, polyamide (nylon), polysulfone, polyethersulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone, polyetherimide, and polyacrylate can be used. Heat-resistant fluororesins such as ethylene tetrafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroperfluoroalkylvinylether copolymer, and polytetrafluoroethylene can also be used. When gas barrier properties and heat resistance are taken into consideration, polyethylene naphthalate is particularly preferable as the base material for the gasket material 8.

The gasket material 8 and the adhesive layer of the plastic film having an adhesive layer may be an acrylic, urethane, silicone, rubber, or other adhesive, and is preferably an acrylic adhesive in consideration of adhesion between the gasket material 8 and the polymer electrolyte membrane 5 and heat resistance during heating and pressurization. The adhesion between the gasket material 8 and the adhesive layer of the plastic film having an adhesive layer is preferably such that the adhesive strength between the polymer electrolyte membrane 5 and the gasket material 8 is greater than the adhesive strength between the gasket material and the plastic film having an adhesive layer, since this makes it easier to apply the gasket material 8 to the membrane electrode assembly.

Next, the manufacture of the catalyst ink, catalyst layer, and membrane electrode assembly will be described. Various devices can be used for the dispersion process when preparing the catalyst ink. For example, the dispersion process can be a process using a ball mill or roll mill, a process using a shear mill, a process using a wet mill, or an ultrasonic dispersion process. A homogenizer that stirs by centrifugal force may also be used.

The dispersibility and particle size of the dispersed catalyst ink can be measured, for example, using the following method. First, for the catalyst ink that has completed the dispersion process, the particle size of the dispersoid contained in the catalyst ink is measured using a particle size distribution measuring device that incorporates the laser diffraction scattering method.

The equipment and measurement conditions used are as follows:
Device name: Laser diffraction/scattering type particle size distribution measuring device Model number: MT3000II SERIES
Manufacturer: MICROTRACK MRB
Transmission setting: Absorption In the catalyst ink measured using the particle size distribution measuring device, two or more and three or less peaks are confirmed. If the frequency peak existing in the particle diameter range of 1.0 μm or more and 12 μm or less is the primary peak (P ₁ ), and the frequency peak existing in the particle diameter range of 0.3 μm or more and less than 1.0 μm is the secondary peak (P ₂ ), it is preferable that the secondary peak/primary peak ratio (P ₂ /P ₁ ) is 1.2 or more and less than 2.0. Furthermore, if the peak ratio is 1.0 or less, the entanglement between the catalyst and the electrolyte is weakened, the proton conductivity is reduced, and the output characteristics are impaired. Furthermore, if the peak ratio is 2.0 or more, the entanglement between the catalyst and the electrolyte is excessively strong, the pores in the formed catalyst layer are narrowed, water clogging occurs, and the output characteristics are impaired.

Coating methods for forming the catalyst ink on a coating substrate include die coating, gravure coating, and spray coating, but are not limited to these methods in the present invention. The coating substrate on which the catalyst layer, which is a component of the membrane electrode assembly, is formed is the polymer electrolyte membrane 7 and the transfer substrate, but are not limited to these methods in the present invention.

When fabricating by the transfer method, the material constituting the transfer substrate may be one on whose surface a catalyst layer can be formed and from which the catalyst layer can be transferred to the polymer electrolyte membrane 7. For example, polymer films such as polyimide, polyethylene terephthalate, polypalvanic acid aramid, polyamide (nylon), polysulfone, polyethersulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone, polyetherimide, polyacrylate, and polyethylene naphthalate can be used. Heat-resistant fluororesins such as ethylene tetrafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroperfluoroalkylvinyl ether copolymer, and polytetrafluoroethylene can also be used.

The ion exchange capacity of the cathode catalyst layer 5 and the anode catalyst layer 6 is desirably IEC 1.1 or more and 1.5 or less. In this range, the gas diffusion and drainage of the catalyst layer are improved, and good power generation performance is obtained. On the other hand, when the IEC is less than 1.1, the entanglement of the polymer electrolyte 3 and the fibrous material 4 is suppressed, and the polymer electrolyte 3 aggregates, which is not preferable. Also, when the IEC exceeds 1.5, the bond between the polymer electrolyte 3 and the conductive support 2 or the carbon fiber 4 becomes strong, and the drainage is reduced, which is not preferable.
From the above, it is understood that a membrane electrode assembly having a catalyst layer containing carbon fibers with an IEC of 1.1 or more and 1.5 or less exhibits high power generation performance.

In one embodiment of the membrane electrode assembly, the above-mentioned catalyst layer is provided on at least one of the anode and the cathode, and by adopting such a configuration, a membrane electrode assembly with excellent power generation performance can be obtained. According to the manufacturing method of the membrane electrode assembly described above, it is possible to manufacture a membrane electrode assembly in which catalyst layers are joined in a good shape to both sides of the polymer electrolyte membrane 7.

The present invention will be specifically described below with reference to examples. However, the present invention is not limited to these examples.

First, the common manufacturing process will be described.
<Preparation of catalyst ink>
The catalyst ink for forming the catalyst layer was composed of a dispersion solution of a fluorine-based polymer electrolyte (DE2020: manufactured by Chemours), platinum-supported carbon using platinum as a catalyst (TEC10E50E: manufactured by Tanaka Kikinzoku Kogyo K.K.), a fibrous material, two types of alcohol, and water. The catalyst ink for the catalyst layer was prepared by mixing these in a ball mill.
<Measurement of particle size distribution of catalyst ink>
The particle sizes of the dispersoids contained in the catalyst inks prepared in Examples 1 to 3 and Comparative Examples 1 to 3 described below were evaluated using a laser diffraction/scattering type particle size distribution measuring device.
<Formation of catalyst layer and production of membrane-electrode assembly>
An anode catalyst layer and a cathode catalyst layer were formed on the polymer electrolyte membrane by an applicator to produce a membrane-electrode assembly.
<Power generation evaluation>
The membrane-electrode assemblies produced in Examples 1 to 3 and Comparative Examples 1 to 3 described below were sandwiched between carbon papers to be used as gas diffusion layers, and placed in a power generation evaluation cell.

Next, a fuel cell measuring device was used to measure current and voltage at a cell temperature of 80° C. Hydrogen was used as the fuel gas and air was used as the oxidizing gas, and the flow rates were controlled to keep the utilization rate constant.
[Example 1]
A cathode catalyst ink having a frequency peak ratio ( _P2 / _P1 ) of 1.23 obtained by a particle size distribution measurement device was applied to the cathode catalyst layer so that the platinum loading amount was 0.3 mg/ ^cm2 , and a membrane-electrode assembly equipped with this was produced.
[Example 2]
A cathode catalyst ink having a frequency peak ratio ( _P2 / _P1 ) of 1.47 obtained by a particle size distribution measurement device was applied to the cathode catalyst layer so that the platinum loading amount was 0.3 mg/ ^cm2 , and a membrane-electrode assembly equipped with this was produced.
[Example 3]
A cathode catalyst ink having a frequency peak ratio ( _P2 / _P1 ) of 1.93 obtained by a particle size distribution measurement device was applied to the cathode catalyst layer so that the platinum loading amount was 0.3 mg/ ^cm2 , and a membrane-electrode assembly was produced.
[Comparative Example 1]
A cathode catalyst ink having a frequency peak ratio ( _P2 / _P1 ) of 0.52 obtained by a particle size distribution measurement device was applied to the cathode catalyst layer so that the platinum loading amount was 0.3 mg/ ^cm2 , and a membrane-electrode assembly equipped with this was produced.
[Comparative Example 2]
A cathode catalyst ink having a frequency peak ratio ( _P2 / _P1 ) of 0.95 obtained by a particle size distribution measurement device was applied to the cathode catalyst layer so that the platinum loading amount was 0.3 mg/ ^cm2 , and a membrane-electrode assembly equipped with this was produced.
[Comparative Example 3]
A cathode catalyst ink having a frequency peak ratio ( _P2 / _P1 ) of 2.12 obtained by a particle size distribution measurement device was applied to the cathode catalyst layer so that the platinum loading amount was 0.3 mg/ ^cm2 , and a membrane-electrode assembly equipped with this was produced.
<Evaluation Results>
The values of P ₂ /P ₁ and output characteristics of the examples and comparative examples are summarized in the table.

From Table 1, it can be seen that in Examples 1 to 3, when the frequency peak ratio (P ₂ /P ₁ ) was 1.2 or more and less than 2.0, the output was 906 to 970 mW/cm ² , and in Comparative Examples 1 and 2, when the frequency peak ratio (P ₂ /P ₁ ) was less than 1.0, the output was 825 to 870 mW/cm ^2. In Comparative Example 3, when the frequency peak ratio (P ₂ /P ₁ ) was 2.0 or more, the output was 892 mW/cm ^2. These results demonstrate that high output characteristics can be obtained by the catalyst ink according to this embodiment and the membrane-electrode assembly using the same.

By employing the catalyst layer and membrane-electrode assembly according to this embodiment, it becomes possible to have sufficient proton conductivity and gas diffusibility, and to exhibit high output characteristics over the long term. That is, according to this embodiment, by making the frequency peak ratio (P ₂ /P ₁ ) in the particle size distribution of the dispersoid contained in the catalyst ink 1.2 or more and less than 2.0, it is possible to provide an electrode catalyst layer, a membrane-electrode assembly, and a solid polymer fuel cell that have sufficient gas diffusibility and proton conductivity in the operation of the solid polymer fuel cell and can exhibit high power generation performance over the long term. Therefore, the present invention can be suitably used in stationary cogeneration systems, fuel cell vehicles, and the like that utilize solid polymer fuel cells, and has great industrial utility value.

Reference Signs List 1 catalyst particle 2 conductive carrier 3 polymer electrolyte 4 fibrous material 5 cathode catalyst layer 6 anode catalyst layer 7 polymer electrolyte membrane 8 gasket material 9 gas diffusion layer

Claims (2)

A catalyst ink for a polymer electrolyte fuel cell, comprising catalyst particles, a conductive carrier, a polymer electrolyte, a fibrous material, and a solvent,
the fibrous material is a carbon fiber or a polymer electrolyte fiber having proton conductivity;
The fiber length of the fibrous material is 1 μm or more and 10 μm or less,
a ratio of the mass of the polymer electrolyte to the total mass of the conductive support and the fibrous material is 0.2 or more and 1.5 or less;
A catalyst ink for a polymer electrolyte fuel cell, characterized in that the ratio (P2/P1) of a frequency peak (P1) existing in the particle diameter range of 1.0 μm or more and 12 μm or less in the dispersoid of the catalyst ink, measured with a particle size distribution measuring device, to a frequency peak (P2) existing in the particle diameter range of 0.3 μm or more and less than 1.0 μm, is 1.2 or more and less than 2.0, satisfying formula (1).
2.0 > P2 / P1 ≧ 1.2 ... Formula (1) The catalyst ink for polymer electrolyte fuel cells according to claim 1, characterized in that the catalyst particles are a metal selected from platinum, palladium, ruthenium, iridium, rhodium, and osmium.