JP2007149417A - Electrode for fuel cell, and method of manufacturing same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a fuel cell wherein a power generation voltage is not reduced even if a long operation, starting, and stopping are repeated, by preventing a catalyst layer from being deteriorated without lowering the initial power generation voltage of the fuel cell. <P>SOLUTION: The electrode for the fuel cell carries catalyst particles of platinum or the like on a part of a conductive carrier surface made of carbon. A corrosion resistant coating film of silicon oxide or the like is formed on the surface of the carrier except for portions on which the catalyst particles are carried, and thereby, the carrier is prevented from being deteriorated by oxidation. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell electrode used in a polymer electrolyte fuel cell and a phosphoric acid fuel cell.

  A polymer electrolyte fuel cell is a system that generates electricity by causing an electrochemical reaction between a fuel gas containing hydrogen and an oxidant gas (for example, air) containing oxygen via a catalyst disposed through an electrolyte membrane. Such a power generation system is expected as clean energy because the product of power generation is mainly water. Recently, development has been promoted as a power source for electric vehicles and a stationary power source for home use, and ensuring long-term reliability has become an important issue for application to such applications.

  In the polymer electrolyte fuel cell, an electrode having a catalyst layer formed on the surface of a conductive electrode support such as carbon paper having high gas diffusibility is used. The catalyst layer is composed of a catalyst in which noble metal catalyst fine particles are supported on a carbon conductive carrier such as carbon black. The pair of electrodes are arranged and bonded so that the catalyst layer is in contact with the polymer electrolyte membrane. Supplying fuel gas and oxidant gas to each electrode, causing an electrochemical reaction in the catalyst layer and moving hydrogen ions through the polymer electrolyte membrane to generate electricity by passing current between the electrodes Yes. The electrode to which the fuel gas is supplied is called an anode, and the electrode to which the oxidant gas is supplied is a cathode.

  In the polymer electrolyte fuel cell configured as described above, the generated voltage may be reduced by repeating the long-time operation, starting and stopping. Specifically, the volume of the catalyst is reduced due to the oxidation of the conductive carrier that constitutes the catalyst, and the efficiency of the electrochemical reaction in the catalyst layer decreases due to the dropping of the catalyst particles from the carrier and the accompanying aggregation of the catalyst catalyst particles. As a result, the generated voltage decreases. In particular, in the catalyst layer of the cathode to which the oxidant gas is supplied, there is a case where a carbon carrier reacts with oxygen and decomposes into carbon dioxide and is consumed due to the reaction shown in the following formula.

C + O ₂ → CO ₂
On the other hand, in the anode catalyst layer, water electrolysis and carrier oxidation may occur together with the oxidation reaction of the fuel gas. For example, the carrier may be decomposed into carbon dioxide and consumed (causes oxidative deterioration) by the reaction shown in the following formula using water as an oxidizing agent.

C + H ₂ O → CO ₂ + 4H ⁺ + 4e ⁻
For such oxidative deterioration (hereinafter referred to as corrosion) of the carbon carrier, carbon black or the like is heat treated at high temperature to be graphitized to enhance corrosion resistance, and the graphitization degree and specific surface area are optimized. A material having improved drainage is used as a carrier.

  However, in the method of increasing the corrosion resistance by graphitization, the specific surface area of the carrier is reduced, so that the dispersibility of the catalyst using this carrier is reduced, and the catalyst fine particles are likely to aggregate and the initial power generation voltage is lowered. It was. Further, the method for improving drainage by optimizing the degree of graphitization and the specific surface area can improve short-term corrosion, but there is a problem that long-term corrosion is hardly improved.

  Therefore, as another method, there is a method in which a metal carbide coating film is formed on the surface of carbon to prevent an oxidation reaction and used as a carrier. Furthermore, when a catalyst layer is formed by supporting catalyst fine particles on the carbon surface, metal carbide fine particles having properties that are more susceptible to corrosion than the metal carbide used in the coating film are mixed to be used for the coating film. A method for further suppressing the corrosion of the metal carbide was disclosed (for example, see Patent Document 1).

JP 2004-172107 A (page 4-5, FIG. 2)

  However, in the conventional method in which a metal carbide coating film is formed on the surface of carbon to prevent corrosion, the catalyst fine particles are supported on the metal carbide coating film, and the catalyst fine particles and the carbon are directly There is no contact. As a result, the catalyst fine particles and the carbon are electrically connected via the metal carbide having a relatively high electric resistance, so that the resistance of the catalyst layer is large, and the movement of electrons and ions between the catalyst fine particles and the carbon is large. This hinders the efficiency of the electrochemical reaction, that is, the initial power generation voltage is reduced.

  In addition, when the catalyst layer is formed, the method of mixing fine particles of metal carbide having the property of being more easily corroded than the metal carbide used in the coating film, the metal carbide that is easily corroded reacts with water and metal ions are formed. Elute. As a result, poisoning of catalyst fine particles by the eluted metal ions, deterioration of the polymer electrolyte membrane, and the like occurred, and there was a problem that the generated voltage decreased.

  The present invention has been made to solve the above-mentioned problems, and prevents deterioration of the catalyst layer without lowering the initial power generation voltage, and can also generate power by repeating long-time operation, starting and stopping. An object of the present invention is to provide a fuel cell electrode in which the voltage does not decrease.

  In the fuel cell electrode according to the present invention, catalyst fine particles are supported on a part of the surface of the carrier having conductivity, and a corrosion-resistant coating film is formed on the surface of the carrier excluding the part where the catalyst fine particles are supported. is there.

  In the present invention, since the corrosion-resistant coating film is formed on the surface of the carrier excluding the part where the catalyst fine particles are supported, the conductivity between the catalyst fine particles and the carrier is not lowered, and the initial stage associated with the formation of the coating film The battery characteristics can be prevented from deteriorating. Furthermore, since corrosion of the carrier is prevented by the corrosion-resistant coating film, it is possible to prevent the catalyst fine particles from falling off the carrier and the accompanying aggregation of the catalyst catalyst fine particles, and by repeating the operation, starting and stopping for a long time. Even high power generation voltage can be maintained.

Embodiment 1 FIG.
FIG. 1 is a schematic diagram of a fuel cell electrode according to Embodiment 1 for carrying out the present invention. In FIG. 1, platinum fine particles 2 as catalyst fine particles are supported on a part of the surface of a carbon black carrier 1. The platinum fine particles 2 are partly in contact with the carrier 1, and the carrier 1 and the platinum fine particles 2 are electrically connected at this contact portion. A corrosion-resistant coating film 3 mainly composed of silicon oxide is formed on the surface of the carrier 1 excluding the portion where the platinum fine particles 2 are supported. The catalyst configured as described above is applied to the surface of the conductive substrate 4 which becomes the cathode or anode of the fuel cell to form the fuel cell electrode 5.

  In the fuel cell electrode thus configured, the platinum fine particles 2 and the carrier 1 are in direct contact with each other, so that the conductivity between the platinum fine particles 2 and the carrier 1 does not decrease. Further, since the platinum fine particles 2 and the carrier 1 are in direct contact and are electrically connected, the corrosion-resistant coating film 3 does not necessarily have conductivity, and the selection range of materials used for the coating film 3 Becomes wider. Furthermore, since the corrosion of the carrier 1 can be prevented by the corrosion-resistant coating film 3, the carrier 1 is corroded even if it contacts with water, and the volume of the carrier 1 does not decrease. Can be prevented from falling off and aggregation of the platinum fine particles 2 associated therewith. As a result, in the fuel cell using the fuel cell electrode according to the present embodiment, the initial power generation voltage does not decrease, and a high power generation voltage is maintained even by repeated operation, start and stop for a long time. be able to.

  In the present embodiment, carbon black is used as the carrier, but the carrier is not particularly limited as long as it is a conductive material. However, in order to widen the reaction area of the electrochemical reaction, it is desirable that the specific surface area be large, and therefore it is generally desirable that the main component is carbon. As the carrier, in addition to carbon black, graphitized carbon, activated carbon, or the like can be used.

  In addition, an example of a film mainly composed of silicon dioxide has been shown as a corrosion-resistant coating film. However, in addition to silicon oxide, a metal oxide having high corrosion resistance, such as titanium, zirconium, aluminum, tantalum, or niobium, should be used. You can also.

  Furthermore, in the present embodiment, platinum fine particles are used as the catalyst fine particles, but other noble metal fine particles, for example, fine particles of platinum, iridium, gold, silver, palladium and the like can also be used.

Embodiment 2. FIG.
The second embodiment describes a method for manufacturing the fuel cell electrode described in the first embodiment. FIG. 2 is a schematic diagram showing the manufacturing process of the fuel cell electrode in the present embodiment. First, the carbon carrier 1 is applied to the surface of the conductive substrate 4 that becomes the cathode or anode of the fuel cell (FIG. 2 (a)). As a coating method of the carrier 1, a screen printing method or the like can be used. Next, nickel fine particles 6 that are more easily electrochemically dissolved than the noble metal used as a catalyst are formed on a part of the surface of the carrier 1 (FIG. 2B). As a method for forming the nickel fine particles 6, a spray coating method or the like can be used. Further, a coating film 3 mainly composed of silicon oxide is formed so as to cover the entire surface of the carrier 1 and the nickel fine particles 6 (FIG. 2C). As a method of forming the coating film 3, a molecular beam epitaxy method or the like can be used.

  Next, the surface is etched by dry etching until the surface of the nickel fine particles 6 is exposed (FIG. 2D). When the coating film 3 is formed by the molecular beam epitaxy method in the previous step, the coating film 3 at the boundary portion between the nickel fine particles 6 and the carrier 1 becomes a shadow of the nickel fine particles 6, and thus is thinner and rougher than the other portions. become. Therefore, in the dry etching, the coating film 3 at the boundary portion between the nickel fine particles 6 and the carrier 1 is etched faster than the other portions. The surface of the nickel fine particle 6 is exposed before the surface is exposed.

  Next, the platinum fine particles 2 that are catalyst fine particles are substituted at positions where the nickel fine particles 6 are formed by displacement plating in a chloroplatinic acid aqueous solution. In this substitution reaction, since nickel has a higher ionization tendency than platinum, the nickel fine particles 6 are ionized and dissolved in the solution, and the platinum in the solution is combined with the electrons to form the nickel fine particles 6. Is plated (replaced) as platinum fine particles 2 (FIG. 2E).

  In the fuel cell electrode obtained by the manufacturing method configured as described above, the platinum fine particles 2 are directly brought into contact with the carrier 1 because the platinum fine particles 2 as the catalyst particles are replaced with the positions where the nickel fine particles 6 were formed. Therefore, the conductivity between the platinum fine particles 2 and the carrier 1 does not decrease. Further, the portion of the carrier 1 where the platinum fine particles 2 are not formed is covered with the silicon oxide coating film 3, so that the carrier 1 is corroded even when it comes into contact with water, and the volume of the carrier 1 is reduced. As a result, it is possible to prevent the platinum fine particles 2 from dropping from the carrier 1 and the accompanying aggregation of the platinum fine particles 2. Furthermore, since the platinum fine particles 2 are formed in the gaps between the coating films 3, the platinum fine particles 2 are firmly held by the anchor effect, so that the platinum fine particles 2 can be further prevented from falling off.

  In the present embodiment, the screen printing method is used as the method for applying the carrier 1, but other methods such as a table coater or a spray coating method can be used. Further, although nickel fine particles are used as a material that is more easily dissolved electrochemically than a noble metal used as a catalyst, fine particles such as other metals such as tin can also be used. In addition, the molecular beam epitaxy method is used as a method for forming the coating film. However, other methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), and sputtering are used. A vacuum arc deposition method, a melt deposition method, a sol-gel method, a molten salt electrolysis method, a plating method, or the like can be used. The thickness of the coating film is desirably in the range of 100 to 300 nm. If the film thickness is within this range, the film thickness of the coating film necessary for preventing corrosion remains on the surface of the carrier even after the etching in the next step. Moreover, although dry etching was used as an etching method for the coating film, other etching methods such as a wet etching method and a chemical etching method can be used.

Embodiment 3 FIG.
In the third embodiment, a method of manufacturing a fuel cell electrode different from that of the second embodiment will be described. In the present embodiment, the nickel fine particles described in the second embodiment are formed by a wet method. A fuel cell electrode having a carbon black carrier coated on the surface of a conductive substrate is immersed in a plating solution containing nickel hydrochloride and boric acid as main components. Nickel fine particles are formed on the surface of the carbon black by flowing a pulse current between the fuel cell electrode and a counter electrode immersed in a position facing the fuel cell electrode. Further, the fuel cell electrode is washed with water and then dried.

  Next, a titanium oxide coating film is coated on the fuel cell electrode by CVD. Further, by holding the fuel cell electrode in dilute sulfuric acid and irradiating with ultraviolet rays, the titanium oxide coating film is etched to expose the surface of the nickel fine particles.

  Next, the fuel cell electrode is immersed in a chloroplatinic acid aqueous solution. At this time, since nickel has a higher ionization tendency than platinum, the nickel fine particles exposed on the surface are ionized and dissolved into the solution, and the electrons released from the nickel react with the platinum ions in the solution, so that the nickel fine particles are formed. Platinum fine particles are formed in the existing region.

  In the fuel cell electrode obtained in the manufacturing method configured as described above, the platinum fine particles, which are catalyst particles, are replaced with the positions where the nickel fine particles are formed, so the platinum fine particles are in direct contact with the carrier. The conductivity between the platinum fine particles and the carrier does not decrease. In addition, since the portion where the platinum fine particles are not formed on the surface of the carrier is covered with a titanium oxide coating film, the carrier does not corrode even if it comes into contact with water, and the volume of the carrier does not decrease. Dropping of the fine particles from the carrier and accompanying aggregation of the platinum fine particles can be prevented. In particular, since the platinum fine particles are formed in the gaps in the coating film, the platinum fine particles are firmly held by the anchor effect, so that the platinum fine particles can be further prevented from falling off.

Embodiment 4 FIG.
In the fourth embodiment, the life characteristics of a polymer electrolyte fuel cell manufactured using the fuel cell electrode obtained by the same manufacturing method as in the second embodiment are measured. For comparison, the life characteristics of a fuel cell manufactured using a fuel cell electrode in which a coating film was not formed on the carrier were also measured. The thickness of the catalyst layer of the fuel cell electrode is in the range of 10 to 15 μm regardless of the presence or absence of the coating film.

A pair of fuel cell electrodes with or without a coating film was prepared and immersed in a polymer electrolyte solution. Thereafter, the catalyst layer is placed in contact with the polymer electrolyte membrane with each pair of fuel cell electrodes, the polymer electrolyte membrane is sandwiched, and hot-pressed at 120 ° C. and a pressure of 0.2 MPa for 15 minutes. A membrane electrode assembly was prepared by joining the fuel cell electrode and the polymer electrolyte membrane. The used polymer electrolyte membrane has a thickness of about 50 μm. Each of the membrane electrode assemblies that differ in the presence or absence of the coating film thus obtained has each electrode corresponding to an anode and a cathode, and the platinum fine particles of both electrodes are about 0.5 mg per 1 cm ^{2 of the} apparent electrode area. Yes, the electrode area is 100 cm ² .

Next, each of the membrane electrode assemblies, which differ depending on the presence or absence of a coating film, is sandwiched between a separator and a current collector to form a fuel cell, and hydrogen gas is supplied to the anode and air is supplied to the cathode and the output of the fuel cell is Electric power was generated by connecting to an electrical load, and the generated voltage was measured for a long time. The power generation conditions are as follows. The temperature of the fuel cell is 70 ° C., hydrogen and air are 65 ° C., saturated humidification, hydrogen utilization efficiency is 70%, air utilization efficiency is 40%, and current density is 0.25 A / cm ² . The operating conditions of the fuel cell are as follows. Once the output of the fuel cell is disconnected from the electrical load once every 24 hours, the supply of hydrogen gas and air to the anode and cathode is stopped, and then the anode and cathode are purged with nitrogen gas as an inert gas. Stop power generation for a minute. Thereafter, the supply of hydrogen gas and air to the anode and cathode was resumed, and after 30 minutes had elapsed, an electric load was connected to the output of the fuel cell, and power generation was resumed.

  FIG. 3 is a characteristic diagram showing the life characteristics of the polymer electrolyte fuel cell in the present embodiment. FIG. 3 shows the change with time of the power generation voltage of the fuel cells produced by the respective fuel cell electrodes that differ depending on the presence or absence of the coating film. As can be seen from this figure, the fuel cell using the fuel cell electrode with the coating film formed had a power generation voltage of about 0.71 V even after 1200 hours, and the fuel cell electrode without the coating film was used. The voltage about 0.1 V higher than the power generation voltage of the fuel cell is maintained. From this, in the fuel cell electrode having the catalyst layer on which the coating film is formed, corrosion of the catalyst layer can be effectively prevented, and a fuel cell having a small decrease in generated voltage can be obtained.

It is a schematic diagram of the electrode for fuel cells by Embodiment 1 of this invention. It is a schematic diagram which shows the manufacturing process of the electrode for fuel cells by Embodiment 2 of this invention. It is a characteristic view of the fuel battery cell by Embodiment 4 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Carrier 2 Platinum particle 3 Coating film 4 Conductive substrate 5 Electrode for fuel cell 6 Nickel particle

Claims (6)

A conductive substrate and a conductive carrier attached to the surface of the conductive substrate;
Catalyst fine particles supported on a part of the surface of the carrier;
An electrode for a fuel cell, comprising: a corrosion-resistant coating film formed on a surface of the carrier excluding a portion on which the catalyst fine particles are supported. 2. The fuel cell electrode according to claim 1, wherein the coating film contains any one of silicon oxide, zirconium oxide, aluminum oxide, tantalum oxide and niobium oxide. Attaching a conductive carrier to the surface of the conductive substrate;
A step of supporting metal fine particles on a part of the surface of the carrier;
Forming a corrosion-resistant coating film on the surface of the carrier excluding the surface of the metal fine particles and the portion on which the metal fine particles are supported;
Removing the coating film formed on the surface of the metal fine particles;
Removing the fine metal particles;
And a process for forming catalyst fine particles on the surface of the carrier from which the metal fine particles have been removed. 4. The method for producing a fuel cell electrode according to claim 3, wherein the material of the metal fine particles is a material having a higher ionization tendency than the material of the catalyst fine particles. 4. The method for producing an electrode for a fuel cell according to claim 3, wherein the material of the metal fine particles is one of nickel and tin. 4. The method for producing an electrode for a fuel cell according to claim 3, wherein the step of forming the coating film is a liquid phase forming method or a gas phase forming method.

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