JP2006066255A - Cathode for fuel cell and solid polymer fuel cell including the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cathode having an excellent electrode characteristic to an oxygen reduction reaction and comprising a carrier carrying a novel catalyst that reduces the amount of use of a platinum catalyst, and to provide a solid polymer fuel cell including the cathode and capable of providing a high battery output. <P>SOLUTION: The cathode for a fuel cell includes a catalyst layer comprising a catalyst-carrying conductive carrier and a polymer electrolyte. A composite oxide containing a noble metal element (element A) and an element constituting a nonstoichiometric oxide (element B) is carried as the catalyst by the conductive carrier. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a cathode for a fuel cell and a polymer electrolyte fuel cell including the same.

  Since a polymer electrolyte fuel cell having a polymer electrolyte membrane is easily reduced in size and weight, it is expected to be put to practical use as a mobile vehicle such as an electric vehicle or a power source for a small cogeneration system. However, since solid polymer fuel cells have a relatively low operating temperature and their exhaust heat is difficult to use effectively for auxiliary power, etc., the utilization rate of the anode reaction gas (pure hydrogen, etc.) and the cathode for practical use. There is a demand for performance capable of obtaining high power generation efficiency and high power density under operating conditions with a high utilization rate of reaction gas (such as air).

  The electrode reaction in each catalyst layer of the anode and cathode of the polymer electrolyte fuel cell is a three-phase interface (hereinafter referred to as reaction site) in which each reaction gas, catalyst, and fluorine-containing ion exchange resin (electrolyte) are present simultaneously. ). Therefore, in a polymer electrolyte fuel cell, a catalyst such as a metal catalyst or a metal-supported catalyst (for example, a metal-supported carbon in which a metal catalyst such as platinum is supported on a carbon black support having a large specific surface area) is used as a polymer. Using the same or different type of fluorine-containing ion exchange resin as the electrolyte membrane and using it as a constituent material of the catalyst layer, the reaction sites in the catalyst layer are three-dimensionalized to increase the reaction sites. Yes.

  The fluorine-containing ion exchange resin that coats the above catalyst has a high ionic conductivity as represented by “Nafion” manufactured by DuPont and the like, and has a sulfonic acid group that is chemically stable in an oxidizing and reducing atmosphere. A perfluorocarbon polymer (hereinafter referred to as sulfonic acid type perfluorocarbon polymer) is used.

  However, while the conventional fluorine-containing ion exchange resin contained in the catalyst layer of the cathode is excellent in ion conductivity and chemical stability, the oxygen gas permeability in the resin is insufficient. As a result, the oxygen permeability of the cathode becomes insufficient, the overvoltage of the oxygen reduction reaction at the cathode increases, and it is difficult to obtain high electrode characteristics.

  On the other hand, in Patent Document 1 below, the cathode overvoltage is reduced by increasing the oxygen permeability in the cathode catalyst layer by mixing a fluorine-containing ether compound with the fluorine-containing ion exchange resin coating the catalyst. A proposed polymer electrolyte fuel cell has been proposed.

  However, even in the polymer electrolyte fuel cell described in Patent Document 1, the oxygen permeability in the catalyst layer of the cathode is insufficient, and the cathode overvoltage cannot be sufficiently reduced. There is a problem that the durability of the catalyst layer is insufficient and the battery life is short. This is because the preferred fluorine-containing ether compound is an oily low molecular weight compound, so that it gradually dissolves in the reaction product water during power generation, or is accompanied by it and desorbed from the fluorine-containing ion exchange resin. It is thought that it is because it is discharged from the catalyst layer together with the produced water.

  Therefore, Patent Document 2 below improves the electrode characteristics particularly in the cathode by including a polymer compound having high oxygen permeability and substantially having no ion exchange group in the electrode catalyst layer of the fuel cell. It is disclosed.

  Patent Document 3 below discloses a solution of an oxygen transport carrier containing a metal complex that specifically mimics hemoglobin contained in blood and has a specific and reversible bond with oxygen. Diffusion electrode using oxygen as active material along the air intake hole in the battery container having the air intake hole communicating with the outside air by forming the dispersion liquid dispersed in the slow medium into a membrane to form an oxygen selective permeable membrane And a battery in which the oxygen selective permeable membrane is interposed between the gas diffusion electrode and the air intake hole.

JP 11-354129 A JP 2002-252001 A JP-A-8-173775

  If the oxygen-permeable resin is mixed with the catalyst-supporting carbon and the electrolyte when the catalyst is prepared as in Patent Document 2, the oxygen-permeable resin is dispersed in the electrode. Is difficult to be performed. That is, it is a problem that the drainage path of the generated water and the oxygen diffusion path are in the same place. Inside the electrode, the electrode reaction proceeds at a site called a three-phase interface where the reaction gas, catalyst, and electrolyte meet. Supplying oxygen to the three-phase interface is an important issue. In Patent Document 2, the efficiency of the reaction is increased by physically mixing a polymer having a high oxygen permeability coefficient, an electrolyte, and a catalyst. However, oxygen actually requires a three-phase interface, In physical mixing, a high oxygen-permeable polymer cannot be concentrated in the vicinity of the three-phase interface. Therefore, even if a high oxygen permeable material is used, the ability cannot be fully exhibited. After all, it can be said that the approach to the idea that the electrode reaction takes place at the three-phase interface is not sufficient.

  In addition, as in Patent Document 3, a battery in which an oxygen selective permeable membrane is simply interposed between the gas diffusion electrode and the air intake hole does not directly diffuse oxygen into the catalyst, and its catalytic activity is sufficient. I could not increase it. That is, when the output is increased, the reaction rate at the cathode is increased and the oxygen supply amount is insufficient.

  On the other hand, one of the problems in putting a polymer electrolyte fuel cell into practical use is material cost. One means for solving this is to reduce the amount of platinum. Of the oxygen supplied from the outside, there is a large amount of oxygen that is not reacted and is discharged to the outside without being adsorbed on the catalyst platinum (low oxygen utilization rate). It was necessary to increase the amount of reactive oxygen using platinum. This is the cause of the high cost due to the increased amount of platinum used.

  The present invention has been made in view of the above-described problems of the prior art, and has a cathode having excellent electrode characteristics with respect to an oxygen reduction reaction, and a solid polymer fuel cell capable of obtaining a high battery output equipped with the cathode. It is another object of the present invention to provide a cathode comprising a carrier carrying a novel catalyst that reduces the amount of platinum catalyst used.

  The present inventor has found that the above problems can be solved by securing a path through which oxygen molecules can diffuse into the catalytic element on carbon using a carrier carrying a novel catalyst, and has reached the present invention.

  That is, first, the present invention relates to a cathode for a fuel cell, which is a cathode for a fuel cell having a catalyst layer composed of a catalyst-supporting conductive carrier and a polymer electrolyte, and the catalyst-supporting conductive carrier. Is characterized in that a composite oxide containing a noble metal element and an element constituting a non-stoichiometric oxide is supported as a catalyst. Oxygen supplied to the cathode catalyst layer is taken in (adsorbed, absorbed, or occluded) by the elements constituting the non-stoichiometric oxide in the composite oxide, and supplied to adjacent noble metal elements, thereby improving oxygen utilization efficiency. With this function, the same battery performance can be exhibited with a smaller amount of noble metal than conventional catalysts (such as platinum-supported carbon).

The composite oxide used as a catalyst in the present invention has the following general formula A _a B _b C _c O _x
(A is a noble metal element, B is an element constituting a non-stoichiometric oxide, C is an arbitrary metal element which is an optional component, and 0 <a ≦ 1.0, 0 <b ≦ 1.0, 0 ≦ c ≦ 1.0 and 0 <z ≦ 6.)
It is represented by In the general formula, A is preferably exemplified by one or more noble metal elements selected from Pt, Pd, Rh, Ir, and Au, and B includes Ce, Mn, Ti, V, Fe, Co, Preferable examples include elements constituting one or more non-stoichiometric oxides selected from Cu, Nb, Tc, Re, Os, Sn, Pb, Sb, Bi, Pr, and W. That is, it is an oxidation number variable metal element that forms a non-stoichiometric oxide by changing the oxidation number. C is an optional component, and is an optional metal element other than the elements constituting the noble metal element and the nonstoichiometric oxide.

More specifically, as the composite oxide, Pt _0.1 Ce _0.6 Zr _0.3 BaO ₃ , Pt _0.1 Ce _0.6 Zr _0.2 Nd _0.1 BaO ₃ , Pt _0.2 Ce _0.8 MgO ₃ , Pt _0.1 Mn _0.6 Zr _0.3 BaO ₃ , Pt _0.1 Pr _0.6 Zr _0.3 BaO _{3 and the} like are exemplified, but not limited thereto. .

  In the cathode of the present invention, since the composite oxide catalyst composed of the noble metal element and the element constituting the non-stoichiometric oxide is supported on the conductive support, the diffusion path of oxygen molecules to the catalyst surface constitutes the non-stoichiometric oxide. Therefore, the concentration of the reaction gas in the vicinity of the reaction site in the catalyst layer can be made higher than before. As a result, the exchange current density in the electrode reaction can be increased, and the oxygen overvoltage can be reduced. That is, high electrode characteristics can be obtained. In particular, when used as a cathode of a polymer electrolyte fuel cell, the overvoltage of the oxygen reduction reaction at the cathode can be effectively reduced, so that the electrode characteristics of the cathode can be improved. Although the shortage of oxygen gas occurs particularly during operation of the fuel cell, the present invention makes it possible to maintain high electrode characteristics even during long-time operation. At the same time, the amount of expensive noble metal used can be reduced.

  In addition, in a fuel cell using air as an oxidant, the presence of nitrogen can cause an electrode reaction suppression. In the present invention, by supporting a composite oxide catalyst having an element constituting a non-stoichiometric oxide, diffusion of nitrogen gas is suppressed, a high oxygen concentration in the vicinity of the electrode is realized, and higher power generation performance is obtained. Can do.

  In the present invention, the conductive support on which the composite oxide catalyst containing the noble metal element and the element constituting the non-stoichiometric oxide is supported is preferably carbon powder or a fibrous carbon material.

  Second, the present invention is an invention of a method for producing a cathode for a fuel cell having a catalyst layer comprising a catalyst-supporting conductive carrier and a polymer electrolyte, the step of suspending the conductive carrier in a solvent, A step of dissolving a salt of one or more precious metal elements, a salt of an element constituting one or more nonstoichiometric oxides, and a salt of another metal element, if desired, in the suspension, and firing the suspension And a step of mixing an electrolyte solution with a conductive carrier on which the obtained noble metal element and the element composite oxide constituting the non-stoichiometric oxide are supported as a catalyst.

  Third, the present invention is a polymer electrolyte fuel cell having an anode, a cathode, and a polymer electrolyte membrane disposed between the anode and the cathode, comprising the above-described cathode as a cathode. This is a feature of a solid polymer fuel cell.

  Thus, by providing the cathode of the present invention having excellent electrode characteristics with respect to the oxygen reduction reaction described above, it becomes possible to configure a polymer electrolyte fuel cell having a high cell output. In addition, as described above, the cathode of the present invention can sufficiently prevent the occurrence of flooding and is excellent in durability. Therefore, the polymer electrolyte fuel cell of the present invention including the cathode has a high cell output. Can be obtained stably over a long period of time. At the same time, the amount of noble metal used is reduced, which helps to reduce the cost of the fuel cell.

  According to the present invention, the presence of the composite oxide containing the noble metal element and the element constituting the non-stoichiometric oxide in the catalyst layer of the cathode for the fuel cell is affected by the gas diffusion of the catalyst layer. In addition, a high oxygen concentration in the vicinity of the electrode was realized, and higher power generation performance could be obtained.

  Hereinafter, preferred embodiments of the cathode of the present invention and a polymer electrolyte fuel cell including the cathode will be described in detail.

FIG. 1 shows a conceptual diagram of the catalytic electrode reaction at the cathode of the prior art. Part of the oxygen supplied from the outside is exhausted without being used. Oxygen molecules that have diffused near the catalyst react with protons and electrons to generate water molecules.
O ₂ + 4H ⁺ + 4e ^{− −−} →→ 2H ₂ O

FIG. 2 shows a conceptual diagram of the catalytic electrode reaction at the cathode of the present invention. A composite oxide containing a noble metal element (element A) and an element constituting the nonstoichiometric oxide (element B) is supported on conductive carbon as a catalyst. The catalyst B takes in oxygen supplied from the outside (FIG. 2A) and supplies it to the adjacent catalyst A (FIG. 2B). Catalyst A is cathodic reaction O ₂ + 4H ⁺ + 4e ^{− −−} →→ 2H ₂ O
Wake up. Thereby, the utilization factor of oxygen improves.

  The electrode reaction proceeds at a site called a three-phase interface where reaction gas, catalyst, and electrolyte meet. The supply of oxygen to the three-phase interface is an important topic. When the output of the battery is increased, a large amount of oxygen is required for the reaction, and if there is no oxygen in the vicinity of the catalyst, the power generation characteristics deteriorate rapidly. In the conventional technique, a high concentration oxygen is supplied. However, as shown in FIG. 1, the actual reaction is performed at the three-phase interface (near the catalyst). The ability cannot be fully demonstrated. In particular, when the output is increased, the amount of oxygen consumed on the catalyst surface increases, but the diffusion rate of oxygen from the outside to the catalyst surface hardly changes. Therefore, when the oxygen consumption rate on the catalyst surface above a certain level exceeds the oxygen supply rate on the catalyst surface, the power generation characteristics deteriorate due to lack of oxygen in the vicinity of the catalyst. On the other hand, as shown in FIG. 2, in the present invention, by increasing the supply rate of oxygen to the catalyst A, a decrease in power generation characteristics due to lack of oxygen in the vicinity of the catalyst is prevented.

  The cathode of the polymer electrolyte fuel cell of the present invention comprises a catalyst layer, and preferably comprises a catalyst layer and a gas diffusion layer disposed adjacent to the catalyst layer. As a constituent material of the gas diffusion layer, for example, a porous body having electronic conductivity (for example, carbon cloth or carbon paper) is used.

In the present invention, a catalyst composed of a composite oxide containing a noble metal element (element A) and an element constituting a nonstoichiometric oxide (element B) contained in a catalyst-carrying conductor is supported on an electrically conductive carrier. It is preferable that Although this support | carrier is not specifically limited, The carbon material whose specific surface area is 200 m < ² > / g or more is preferable. For example, carbon black or activated carbon is preferably used.

  The polymer electrolyte contained in the catalyst layer of the present invention is preferably a fluorinated ion exchange resin, and particularly preferably a sulfonic acid type perfluorocarbon polymer. The sulfonic acid-type perfluorocarbon polymer enables proton conduction that is chemically stable and rapid in the cathode for a long period of time.

  The layer thickness of the catalyst layer of the cathode of the present invention may be the same as that of a normal gas diffusion electrode, preferably 1 to 100 μm, more preferably 3 to 50 μm.

  In a polymer electrolyte fuel cell, since the overvoltage of the cathode oxygen reduction reaction is usually very large compared to the overvoltage of the anode hydrogen oxidation reaction, the oxygen in the vicinity of the reaction site in the cathode catalyst layer as described above. Increasing the concentration to effectively use the reaction site and improving the electrode characteristics of the cathode is effective in improving the output characteristics of the battery.

  On the other hand, the configuration of the anode is not particularly limited. For example, the anode may have a configuration of a known gas diffusion electrode.

  In addition, the polymer electrolyte membrane used in the solid polymer fuel cell of the present invention is not particularly limited as long as it is an ion exchange membrane exhibiting good ion conductivity in a wet state. As the solid polymer material constituting the polymer electrolyte membrane, for example, a perfluorocarbon polymer having a sulfonic acid group, a polysulfone resin, a perfluorocarbon polymer having a phosphonic acid group or a carboxylic acid group can be used. Among these, a sulfonic acid type perfluorocarbon polymer is preferable. And this polymer electrolyte membrane may consist of the same resin as the fluorine-containing ion exchange resin contained in a catalyst layer, and may consist of different resin.

  The catalyst layer of the cathode of the present invention is prepared by previously dissolving a catalyst comprising a composite oxide containing a noble metal element and an element constituting a nonstoichiometric oxide on a conductive support and a polymer electrolyte dissolved in a solvent or dispersion medium. Or it can produce using the dispersed coating liquid. Alternatively, a coating liquid in which a catalyst-carrying conductive carrier, a polymer electrolyte, and a catalyst made of a composite oxide containing a noble metal element and an element constituting a nonstoichiometric oxide are dissolved or dispersed in a solvent or a dispersion medium is used. Can be produced. As the solvent or dispersion medium used here, for example, alcohol, fluorine-containing alcohol, fluorine-containing ether and the like can be used. And a catalyst layer is formed by apply | coating a coating liquid to the carbon cloth etc. which become an ion exchange membrane or a gas diffusion layer. Alternatively, the catalyst layer can be formed on the ion exchange membrane by coating the coating solution on a separately prepared substrate to form a coating layer and transferring the coating layer onto the ion exchange membrane.

  Here, when the catalyst layer is formed on the gas diffusion layer, it is preferable to join the catalyst layer and the ion exchange membrane by an adhesion method, a hot press method, or the like. Further, when the catalyst layer is formed on the ion exchange membrane, the cathode may be constituted only by the catalyst layer, but a gas diffusion layer may be further arranged adjacent to the catalyst layer to serve as the cathode.

  A separator having a normal gas flow path is disposed outside the cathode, and a gas containing hydrogen is supplied to the anode, and a gas containing oxygen is supplied to the cathode. Composed.

  Hereinafter, the cathode and the polymer electrolyte fuel cell of the present invention will be described in detail with reference to Examples and Comparative Examples.

[Sample preparation]
Usually, a Pt / C catalyst is used as an electrode catalyst for the cathode (comparative example). This time, Pt _0.1 Ce _0.6 Zr _0.3 BaO ₃ was supported and an experiment was performed (Example).

(Example)
A predetermined amount of carbon black powder (Ketjen EC) was suspended in ion-exchanged water. Platinum chloride (4) acid, diammonium cerium nitrate, zirconium oxynitrate, and barium nitrate were dissolved in predetermined amounts in the suspension. Here, platinum chloride (4) acid was dissolved so that the platinum amount was 30 wt% with respect to the total amount of carbon and composite oxide. While stirring, aqueous ammonia was added dropwise until pH = 9.5 ± 0.2. After stirring for 2 hours, water was evaporated to some extent at 120 ° C. After vacuum drying, it was baked at 350 ° C. for 5 hours in an atmospheric baking furnace. Thus _Pt 0.1 _Ce 0.6 _Zr 0.3 BaO ₃ was obtained.

Predetermined amounts of ion-exchanged water, an electrolyte (Nafion: trade name) solution, ethanol, and polyethylene glycol were mixed with the obtained Pt _0.1 Ce _0.6 Zr _0.3 BaO ₃ -supported carbon to prepare a catalyst ink. . The catalyst ink was cast on a Teflon (trade name) resin film to a film thickness of 6 mil, then dried and cut into 13 cm ² .

  The cast film was hot pressed onto an electrolyte film to prepare an MEA. MEA was assembled into the cell and evaluated as follows.

(Comparative example)
Commercially available Pt (60 wt%) supported carbon was used. Preparation of the catalyst ink, casting, production of MEA, assembly in the cell of MEA, and evaluation were the same as in the examples.

[Catalyst activity evaluation]
The following power generation evaluation test was conducted in a single cell having an electrode area of 12.96 cm ² .
“Gas flow rate” Anode: H ₂ 500 cc / min
Cathode: Air 1000cc / min
“Pressure” Anode: 0.2 MPa
Cathode: 0.2 MPa
“Cell temperature” 80 ℃

  FIG. 3 shows the evaluation results. From the results of FIG. 3, it can be seen that the fuel cell using the electrode carrying the catalyst of the present invention is inferior in power generation performance as compared with the comparative example using the electrode carrying the platinum catalyst. Moreover, the MEA using the catalyst of the example showed equivalent performance with a platinum amount ½ that of the MEA using the conventional catalyst of the comparative example.

The conceptual diagram of the catalyst electrode reaction in the conventional cathode is shown. The conceptual diagram of the catalyst electrode reaction in the cathode of this invention is shown. The voltage-current density curve of an Example and a comparative example is shown.

Claims (6)

  A cathode for a fuel cell having a catalyst layer composed of a catalyst-supporting conductive support and a polymer electrolyte, wherein the catalyst-supporting conductive support includes a composite oxide containing a noble metal element and an element constituting a non-stoichiometric oxide. A cathode for a fuel cell, which is supported as a catalyst. The composite oxide has the following general formula A _a B _b C _c O _x
(A is a noble metal element, B is an element constituting a non-stoichiometric oxide, C is an arbitrary metal element which is an optional component, and 0 <a ≦ 1.0, 0 <b ≦ 1.0, 0 ≦ c ≦ 1.0 and 0 <z ≦ 6.)
2. The fuel cell cathode according to claim 1, wherein   In the general formula, A is one or more noble metal elements selected from Pt, Pd, Rh, Ir, and Au, and B is Ce, Mn, Ti, V, Fe, Co, Cu, Nb, Tc, The cathode for a fuel cell according to claim 2, which is an element constituting one or more non-stoichiometric oxides selected from Re, Os, Sn, Pb, Sb, Bi, Pr, and W. The composite _{oxide, Pt 0.1 Ce 0.6 Zr 0.3 BaO} 3, Pt 0.1 Ce 0.6 Zr 0.2 Nd 0.1 BaO 3, Pt 0.2 Ce 0.8 MgO 3 The fuel cell cathode according to claim 2, wherein the cathode is one or more selected from the group consisting of:   A method for producing a cathode for a fuel cell having a catalyst layer comprising a catalyst-supporting conductive carrier and a polymer electrolyte, the step of suspending the conductive carrier in a solvent, and at least one noble metal in the suspension A step of dissolving a salt of an element, a salt of an element constituting one or more nonstoichiometric oxides, and a salt of another metal element if desired, a step of firing the suspension, and a noble metal element obtained And a step of mixing an electrolyte solution with a conductive support on which an elemental composite oxide constituting the non-stoichiometric oxide is supported as a catalyst.   5. A polymer electrolyte fuel cell comprising an anode, a cathode, and a polymer electrolyte membrane disposed between the anode and the cathode, wherein the fuel according to claim 1 is used as the cathode. A polymer electrolyte fuel cell comprising a battery cathode.

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