JP2007335259A - Electrode for polymer electrolyte fuel cell, and polymer electrolyte fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a solid polymer fuel cell capable of improving power generation performance by improving reaction activity in a gas diffusion electrode using an ion liquid as an electrolyte, and the solid polymer fuel cell using such an electrode. <P>SOLUTION: In the electrode for the solid polymer fuel cell provided with a gas diffusion layer and a catalyst layer, the thickness of the catalyst layer formed from a carbon to carry platinum, the ion liquid provided with proton conductivity, and an organic compound to fix this ion liquid is made to have a range of 1 to 10 μm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a polymer electrolyte fuel cell (PEFC) using an ionic liquid as an electrolyte, and in particular, an electrode (GDE: gas diffusion electrode) used in such a polymer electrolyte fuel cell and the electrode are applied. The present invention relates to a polymer electrolyte fuel cell.

An ionic liquid is a low-melting point “salt” having an ionic conductivity, also called an ionic liquid or a room temperature molten salt, most of which combine an organic onium ion as a cation and an organic or inorganic anion as an anion. The characteristic of the comparatively low melting point obtained by this is shown.
As a fuel cell using such an ionic liquid as an electrolyte, for example, a report has been made on the power generation performance of an MEA (membrane electrode assembly) manufactured using EMIm (FH) nF as an ionic liquid (non-patent). Reference 1).
46th Battery Symposium Proceedings, The Institute of Electrical Engineers of Japan, November 2005, p. 808

However, the power generation performance by the MEA is about 30 mW / cm ² at the maximum output density, which is only about 1/10 of the power generation capacity of a normal polymer electrolyte fuel cell, and sufficient power generation performance is obtained. There is no actual situation.

  The present invention has been made in view of the above problems in a polymer electrolyte fuel cell using an ionic liquid as an electrolyte. The object of the present invention is to improve the reaction activity in the catalyst electrode, and to generate power. An object of the present invention is to provide a polymer electrolyte fuel cell electrode capable of improving the performance, and a polymer electrolyte fuel cell excellent in power generation performance, comprising such an electrode.

  As a result of repeating earnest studies to achieve the above object, the present inventors have reduced the thickness of a catalyst layer composed of a carbon support carrying platinum and an ionic liquid having proton conductivity. As a result, the present invention has been completed.

  The present invention is based on the above knowledge, and the electrode for a polymer electrolyte fuel cell of the present invention comprises a gas diffusion layer and a catalyst layer, and the catalyst layer has a carbon carrier carrying platinum, and proton conductivity. The solid polymer fuel cell of the present invention is characterized in that it is formed from an ionic liquid provided and an organic compound that immobilizes the ionic liquid, and the thickness of the catalyst layer is 1 to 10 μm. It is characterized by having an electrode.

  According to the present invention, the catalyst layer in the polymer electrolyte fuel cell electrode is composed of a carbon carrier carrying platinum, an ionic liquid having proton conductivity, and an organic compound for immobilizing the ionic liquid. On the other hand, since the thickness is set to 1 to 10 μm, the proton conduction resistance in the catalyst layer, which is one factor for increasing the catalytic reaction resistance, and the deficient state of diffusion protons (reactants) in the vicinity of platinum due to this resistance. Thus, the catalytic activity of platinum is improved and the power generation performance of the battery can be improved.

  Hereinafter, the polymer electrolyte fuel cell electrode of the present invention and the polymer electrolyte fuel cell using such an electrode will be described in more detail and specifically. In the present specification, “%” means mass percentage unless otherwise specified.

  The solid polymer fuel cell electrode of the present invention is a gas diffusion electrode comprising a gas diffusion layer and a catalyst layer as described above, wherein the catalyst layer in the gas diffusion electrode is composed of a platinum-supported carbon carrier, a proton transmission An ionic liquid and an organic compound that immobilizes the ionic liquid in a predetermined shape and has a thickness of 1 to 10 μm. Therefore, proton conduction resistance in the catalyst layer and diffusion protons in the vicinity of platinum The deficient state is improved and the catalytic activity of platinum is improved. As a result, the amount of noble metal catalyst having insufficient catalytic activity can be reduced, and a highly efficient gas diffusion catalyst electrode for fuel cells can be obtained.

  In the electrocatalyst phase in which the fuel cell reaction proceeds, it is necessary that all of electrons, protons, hydrogen, oxygen, and water diffuse quickly, and these conduction or diffusion is caused by the carbon support, the electrolyte, and the carbon support, respectively. It proceeds through voids formed in the aggregate of particles. Therefore, the types, shapes, and characteristics of these materials greatly affect the supply of reactants to the reaction field (platinum surface) and the removal of products from the reaction field.

  In the past, it was estimated that among these diffusion phenomena, gas diffusion and proton conduction, in particular, largely controlled the cell reaction, and there was a large amount of platinum with low reaction activity due to insufficient supply of reactants. As an improvement measure, an electrode with a thin electrode catalyst layer was studied by making the platinum content constant and increasing the platinum support concentration on the carbon support, but the support carbon was changed. Therefore, it is considered that the state of the electrolyte between carbons and the shape of the gap do not change greatly, and the proton conduction resistance and gas diffusion resistance per unit volume do not change.

In the present invention, regardless of the amount of platinum supported, the effect of increasing the current density can be obtained by defining the thickness of the catalyst layer in the range of 1 to 10 μm.
That is, in the region where the thickness of the catalyst layer is 10 μm or less, the proton and the reaction gas are sufficiently supplied to the platinum. Therefore, the length of the ion conduction path formed of the ionic liquid in the catalyst layer is shortened, While the effect of lowering the resistance of gas diffusion can be sufficiently obtained, in the region where the thickness of the catalyst layer exceeds 10 μm, the supply of protons and reaction gas to platinum becomes insufficient, so that the effect can be sufficiently obtained. It is not possible. When the thickness of the catalyst layer is less than 1 μm, a portion where the catalyst layer made of Pt-supported carbon is not formed occurs, and as a result, only the surface of the electrolyte membrane is formed, and the power generation performance is deteriorated.

As described above, the catalyst layer in the electrode for the polymer electrolyte fuel cell of the present invention is formed of a carbon carrier supporting platinum, an ionic liquid having proton conductivity, and an organic compound that immobilizes the ionic liquid. However, the carbon support for supporting the platinum catalyst preferably has a specific surface area of 200 to 1100 m ² / g, so that the catalyst layer is maintained while maintaining platinum in a highly dispersed state. The electrolyte can be held therein, and a gap through which the gas can pass can be obtained.

Here, the specific surface area means a measured value by the BET (Brunauer-Emmette-Teller) method, and when the specific surface area by the BET method is less than 200 m ² / g, the dispersion state of platinum deteriorates, Power generation performance deteriorates. On the other hand, if it exceeds 1100 m ² / g, with the increase in the bulk density of carbon, in order to ensure the amount of noble metal contained in the constant catalyst layer thickness, it is necessary to carry a high concentration of noble metal concentration. As the noble metal particle size increases, the performance tends to deteriorate.
In addition, as such a carbon support | carrier, carbon black, graphitized carbon black, etc. can be used, for example.

The ionic liquid constituting the catalyst layer of the polymer electrolyte fuel cell electrode of the present invention is preferably an electrolyte composed of a molecular cation and a molecular anion.
As a result, the properties of the room temperature molten salt of the ionic liquid, specifically, the vapor pressure is very low, it is difficult to evaporate, it is flame retardant, and it has a high thermal decomposition temperature (> 250-400 ° C.). Characteristics such as having a low freezing point (<−20 ° C.) are favorable as an electrolyte used in a polymer electrolyte fuel cell.

  As the cation component constituting such an ionic liquid, for example, as shown in FIG. 1, an imidazolium derivative (Imidazolium Derivatives), a pyridinium derivative (Pyrridinium Derivatives), a pyrrolidinium derivative (A), an ammonium derivative (V) Phosphonium derivatives (Phosphonium Derivatives), guanidinium derivatives (Guanidinium Derivatives), isouronium derivatives (Isouronium Derivatives), thiourea derivatives (Thiourea Derivatives), and the like.

  On the other hand, examples of the anion component include sulfates (Sulfates), sulfonic acids (Sulfonates), amides (Imides), imides (Imides), methanes (Methanes), and borates (Borates) as shown in FIG. ), Phosphates (Phosphates), antimonies (Antimonates), other salts, and the like.

  In addition, the above-mentioned cation component and anion component can be used not only individually but also in combination of two or more.

Since the ionic liquid is literally liquid and has fluidity, the position as an electrolyte cannot be determined in the gas diffusion electrode as it is, and the gas can move freely through the catalyst layer and pass through the gas. Since the gap is also closed, it is necessary to fix the organic compound in a predetermined shape with an organic compound as described below, for example.
In the present invention, “immobilization” means that a solid material containing an ionic liquid component is formed by physical gel, chemical gel, polymerization of the ionic liquid component, or the like.

As the organic compound used for immobilizing the ionic liquid, an organic compound having a polymer monomer as a precursor can be used, whereby the organic compound can be uniformly dispersed in the ionic liquid.
That is, by dispersing in an ionic liquid in a monomer state and then polymerizing, the organic compound can be uniformly dispersed in the ionic liquid, and a good fixed state can be obtained.

Specific examples of organic compounds used to immobilize the ionic liquid include, for example, polymers such as polyethylene oxide, polypropylene oxide, polyacrylonitrile, polymethyl methacrylate, and polyvinylidene fluoride, or acrylic acid monomers and methacrylic acid monomers. Polymers of monomers such as acrylamide monomers, allyl monomers, styrene monomers, and epoxy monomers can be used.
In addition, these organic compounds can be used alone, and can be used as a mixture of two or more kinds or as a copolymer.

  The organic compound used for immobilizing the ionic liquid is preferably added at a rate of about 1 to 500 g per liter of the ionic liquid, although the preferred range differs depending on the type.

  In addition, as an organic compound used to immobilize the ionic liquid, a structure having a three-dimensional network structure formed from nano-sized fibrous aggregates due to the effect of intermolecular force such as hydrogen bonding may be used. In other words, the electrolyte composed of the ionic liquid is included and held by the three-dimensional network cross-linked structure in which the low molecular weight compounds are associated. At this time, by selecting a material that does not cause a chemical reaction between the fibrous aggregate and the ionic liquid, it is possible to suppress a decrease in ionic conductivity accompanying the immobilization of the ionic liquid. That is, it is possible to suppress a decrease in ionic conductivity as compared with the case where a gel-like ionic liquid formed from another polymer, a polymer, or a crosslinked body accompanied by a chemical reaction is used as an electrolyte.

As the above-mentioned three-dimensional network crosslinked structure, for example, in carrageenan represented by polysaccharides, as shown in FIGS. 3 and 4, the unit component forms an aggregate after forming a double helix (c). And (f) which forms an aggregate after the unit component forms a single helix.
Furthermore, molecular compounds such as 1,2-hydroxystearic acid, trans-1,2-diaminocyclohexane diamide derivatives, L-isoleucine derivatives and L-valine derivatives, which are put into practical use as solidifying agents for waste tempura oil, are used in a wide range of solvents. It is known to gel oil, and this gelation is a fibrous aggregate in which molecules self-assemble through non-covalent bonds such as hydrogen bonds acting between molecules and van der Waals forces of long-chain alkyl groups. Form.

These exemplified aggregates can form a gel by finally forming a three-dimensional network structure and incorporating solvent molecules between the network structures.
As described above, since the unit component acts as a gelling material, an ion conductive gel having a desired viscosity (hardness) can be produced regardless of the viscosity of the electrolyte by adjusting the blending condition. Even if it is electrolyte of this, it can be gelatinized by simple operation which only melt | dissolves and mixes the said unit component.

  Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

(Ionic liquid)
As the ionic liquid, 2EtIm / Tf, that is, loumidazolium as a cation and triflate as an anion was used.

(Production of GDE)
As a conductive carbon material, 4.0 g of carbon black (Ketjen Black TMEC manufactured by Ketjen Black International Co., Ltd., BET surface area = 800 m ² / g) was prepared, and 400 g of dinitrodiammine platinum aqueous solution (Pt concentration: 1.0%) was prepared. After stirring for 1 hour, 50 g of methanol was further mixed as a reducing agent and stirred for 1 hour.
Then, it heated up to 80 degreeC in 30 minutes, and stirred at 80 degreeC for 6 hours, Then, it cooled to room temperature over 1 hour. After filtering the precipitate, the obtained solid was dried under reduced pressure at 85 ° C. for 12 hours and pulverized in a mortar to obtain a catalyst (1) having a platinum loading concentration of 46%.

To the catalyst (1) obtained above, twice the amount of ethanol was added with respect to its mass, and a vacuum degassing operation was added for 5 minutes. To this, 0.5 times the amount of n-propyl alcohol was added, and further PTFE dispersion was added.
About content of PTFE in a solution, solid content mass ratio with respect to the carbon mass of the said catalyst (1) was adjusted so that it might become Carbon / PTFE = 1.0 / 0.5.

  The obtained mixed slurry was well dispersed with an ultrasonic homogenizer, and a catalyst slurry was prepared by applying a vacuum degassing operation. An amount of catalyst slurry corresponding to the desired thickness was printed on one side of the PTFE sheet by screen printing, and dried at 60 ° C. for 24 hours. The size of the formed catalyst layer was 5 cm × 5 cm.

Then, the electrode catalyst layer formed on the PTFE sheet and the carbon paper (gas diffusion layer) were superposed, and then hot pressed at 110 ° C. and 1.0 MPa for 10 minutes.
At this time, in order to change the thickness of the electrode catalyst layer, 20 types of GDEs (gas diffusion electrodes) having different platinum concentrations and platinum loadings on carbon as shown in Table 1 were prepared. The thickness of the electrode catalyst layer was confirmed by SEM (scanning electron microscope).

(Production of MEA)
FIG. 5 shows a production procedure of MEA (membrane electrode assembly). First, a composite polymer is obtained by radical polymerization of 2 hydroxyethyl methacrylate (HEMA) in the ionic liquid 2EtIm / Tf. An electrolyte was synthesized.
That is, the ionic liquid 2EtIm / Tf was mixed at a HEMA molar ratio of 5: 5 to prepare a mixed liquid, and then a peroxide as a polymerization initiator was added to the mixed liquid of the ionic liquid and the acrylic monomer. Benzoyl (Benzoyl peroxide) was mixed at 0.5% and impregnated in a porous membrane mainly composed of PTFE, while the mixed solution was also applied to the GDE obtained above.

As described above, after porous polymer impregnated or coated with a mixed solution of an ionic liquid, an acrylic monomer, and a polymerization initiator and GDE are heated at 80 ° C. for 1 hour, the above-mentioned monomer is polymerized, An MEA was produced by sandwiching between two GDEs, heating while applying pressure at a temperature of 80 ° C. for 24 hours, polymerizing and joining.
In this example, as described above, HEMA was used as the polymer monomer. However, other than this, for example, polyethylene oxide, polypropylene oxide, polyacrylonitrile, polymethyl methacrylate, polyvinylidene fluoride, etc. Various acrylic acid monomers, methacrylic acid monomers, acrylamide monomers, allyl monomers, styrene monomers, and epoxy monomers may be used.

Next, the power generation performance of the polymer electrolyte fuel cell using the MEA obtained above was investigated. The following experiments were all conducted at room temperature.
FIG. 6 shows a schematic diagram of the fuel cell used in this example. The MEA produced in the previous procedure was sandwiched between carbon separators, and hydrogen and oxygen were introduced into the gas flow path carved in the carbon separator. It is the structure which each supplies and generates electric power.

The result is shown in FIG. As evaluation conditions for power generation performance, anode H ₂ (SR: 1.3), cathode O ₂ (SR: 1.3), cell temperature of 120 ° C., no humidification, and current density at 0.9 V were measured. .

  As is clear from the results of FIG. 7, the contribution ratio of the catalyst layer thickness changes at 10 μm as a boundary regardless of the amount of platinum supported, and in the region of 10 μm or less, the effect of increasing the current density is large. can get.

  That is, in the region where the catalyst layer thickness is 10 μm or less, the supply of protons and reaction gas to the platinum catalyst is sufficient, and thus the effect of lowering the resistance of proton diffusion is sufficiently exhibited. In the region where the catalyst layer exceeds 10 μm, the supply of protons to platinum is insufficient, so the performance deteriorates.

In the above example, platinum was supported at a high concentration of 98%. Normally, when the noble metal loading concentration increases, the surface area of the platinum catalyst decreases due to the aggregation of the noble metal. In this example, however, no performance decrease that seems to be due to a decrease in the platinum surface area was observed.
Although the cause of this is not necessarily clear, it is considered that the platinum surface area and the effective active surface area that becomes a substantially active point by forming a three-phase interface in the catalyst phase do not always coincide.

It is a figure which shows the example of the cation component applicable to the ionic liquid applied to this invention. It is a figure which shows the example of the anion component applicable to the ionic liquid applied to this invention. It is the schematic which shows an example of the three-dimensional network structure used in order to fix an ionic liquid in this invention. It is the schematic which shows the other example of the said three-dimensional network structure. It is process drawing which shows the manufacture procedure of the membrane electrode assembly using the electrode for polymer electrolyte fuel cells of this invention. It is a perspective view which shows schematic structure of the polymer electrolyte fuel cell used in the Example of this invention. 3 is a graph showing the relationship between the thickness of a catalyst layer and power generation performance in each polymer electrolyte fuel cell according to an embodiment of the present invention.

Claims (7)

In a polymer electrolyte fuel cell electrode having a gas diffusion layer and a catalyst layer,
The catalyst layer is formed of a carbon support carrying platinum, an ionic liquid having proton conductivity, and an organic compound that immobilizes the ionic liquid, and the catalyst layer has a thickness of 1 to 10 μm. An electrode for a polymer electrolyte fuel cell.   2. The polymer electrolyte fuel cell electrode according to claim 1, wherein the ionic liquid is an electrolyte composed of a molecular cation and a molecular anion. 3. The polymer electrolyte fuel cell electrode according to claim 1, wherein the carbon support has a specific surface area of 200 to 1100 m ² / g.   The electrode for a solid polymer fuel cell according to any one of claims 1 to 3, wherein the organic compound has a polymer monomer as a precursor.   The organic compound is at least one polymer selected from the group consisting of polyethylene oxide, polypropylene oxide, polyacrylonitrile, polymethyl methacrylate, and polyvinylidene fluoride, and / or acrylic acid monomers, methacrylic monomers, and acrylamide monomers. 5. A polymer of at least one monomer selected from the group consisting of allylic monomers, styrenic monomers, and epoxy monomers, according to claim 1. Electrode for molecular fuel cell.   5. The polymer electrolyte fuel cell according to any one of claims 1 to 4, wherein the organic compound has a three-dimensional network structure formed from nano-sized fibrous aggregates. electrode.   A polymer electrolyte fuel cell using the electrode according to any one of claims 1 to 6.

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