JP2002289208A - Electrode catalyst for fuel cell and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode catalyst for a fuel cell, in particular, a cathode-side catalyst capable of exerting high durability. SOLUTION: This electrode catalyst is formed by supporting particles of metal other than platinum and harder to oxidize than platinum in an acidic condition by a conductive carbon material in order to restrain particle growth of platinum particles and by covering the outside surfaces of the particles with platinum.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel cell electrode catalyst used in a polymer electrolyte fuel cell and the like, and a method for producing the same.

[0002]

2. Description of the Related Art As a cathode catalyst for an electrode catalyst of a polymer electrolyte fuel cell and a phosphoric acid fuel cell, a catalyst in which a noble metal containing platinum is supported on carbon black has been used. Platinum-supported carbon black is obtained by adding sodium hydrogen sulfite to aqueous chloroplatinic acid solution, then reacting with hydrogen peroxide solution, supporting the resulting platinum colloid on carbon black, washing, and heat-treating as necessary. It is generally prepared. Electrodes of a polymer electrolyte fuel cell are prepared by dispersing platinum-supported carbon black in a polymer electrolyte solution to prepare an ink, applying the ink to a gas diffusion substrate such as carbon paper, and drying. . The polymer electrolyte membrane is sandwiched between the two electrodes, and hot pressing is performed to assemble an electrolyte membrane-electrode assembly (MEA). When a reformed gas such as a hydrocarbon or methanol is used as a fuel, about tens of ppm of carbon monoxide is mixed in addition to hydrogen and carbon dioxide. Since this carbon monoxide poisons the platinum catalyst of the anode, a catalyst obtained by alloying ruthenium and platinum is used as a catalyst for the anode. Hydroxyl groups formed on ruthenium oxidize carbon monoxide adsorbed on platinum and maintain good catalytic activity.

[0003] Platinum is expensive, and it is desired to exhibit sufficient performance with a small amount of platinum. For this reason, platinum is made into fine particles to increase the exposed surface area. However, the oxygen reduction activity per exposed platinum greatly decreases when the platinum particle size becomes 2.5 nm or less. This is thought to be because in such small platinum particles, the proportion of the coordinatively unsaturated platinum atoms such as edges and steps is relatively large, and the activity of these platinum atoms is low. For this reason, usually, about 3 nm of platinum is often supported. However, in practice, when used under the operating conditions of a polymer electrolyte fuel cell, some platinum particle growth is observed during use. The cause of this grain growth is still unknown. A similar phenomenon has been observed with the cathode catalyst of the phosphoric acid fuel cell. In this case, it is presumed that this is due to particle growth due to the mechanism of dissolution and precipitation of platinum, fusion of platinum particles caused by peeling of the joint between platinum and the carbon material, and the like. The operating temperature of the polymer electrolyte fuel cell is lower than the operating temperature of the phosphoric acid fuel cell (190 ° C.) and is about 70 ° C. to 80 ° C., which is a condition under which particle growth hardly occurs. However, a similar mechanism is assumed, given that the potentials are equally applied under acidic conditions. The particle growth of the platinum catalyst does not seem to have a large change in characteristics with an electrode having a sufficient amount of supported platinum, but when the amount of platinum is small, the characteristics are deteriorated. As described above, it is necessary to suppress the growth of platinum particles during operation from the viewpoint of reducing the amount of platinum.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide an electrode catalyst for a fuel cell which has high durability while suppressing the growth of platinum particles during operation.

[0005]

An electrode catalyst for a fuel cell according to the present invention comprises a conductive carbon material, metal particles supported on the conductive carbon material and less oxidizable than platinum under acidic conditions;
And platinum that covers the outer surfaces of the metal particles. The metal particles are preferably gold. As the metal particles, an alloy composed of platinum and at least one metal selected from the group consisting of chromium, iron, nickel, cobalt, titanium, vanadium, copper, and manganese is preferably used.

The present invention comprises the steps of supporting metal particles which are less oxidized than platinum under acidic conditions on a conductive carbon material, dispersing the carbon material in an aqueous solution of a platinum salt, and reducing platinum with a reducing agent. Provided is a method for manufacturing an electrode catalyst for a fuel cell. Here, hydrogen is preferably used as the reducing agent. It is preferable to include a step of heat-treating the conductive carbon material in an inert gas atmosphere or a reducing gas atmosphere before or after the catalyst is supported.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The electrode catalyst for a fuel cell of the present invention comprises:
In order to suppress the particle growth of platinum particles, particles of a metal other than platinum and less oxidizable than platinum under acidic conditions are supported on a conductive carbon material, and the outer surface of the particles is covered with platinum. It has a modified configuration. A metal that is less oxidizable than platinum is defined as a metal whose oxidation potential is more noble than platinum in a potential-pH diagram as described below.

[0008] It is said that one of the factors for the platinum particle growth is the oxidation-reduction of Pt and PtO (or Pt (OH) ₂ ) occurring at around 0.8 V or less. That is, at a higher potential, the surface of the platinum is oxidized, but at a lower potential, the surface is reduced. However, this oxidation-reduction can also occur at the interface between the carbon black as a carrier and the platinum particles, and as a result, oxidation of the carbon black by platinum occurs, which triggers the movement of the platinum particles. The movement of the platinum particles also depends on the interaction (degree of bonding) between platinum and carbon black.
It is considered that it may be changed by the pretreatment of carbon black. On the other hand, on the surface of the platinum particles, the platinum surface is reconstructed by the movement of the platinum atoms, so that the particles in contact with each other due to the movement of the platinum particles eventually fuse. In order to prevent this, particles that undergo oxidation-reduction at a higher potential under acidic conditions are supported on carbon black, and the particles are covered with platinum. This makes it difficult for the particles to move away from the carbon black and keeps the catalytic action of platinum. Reconstitution of platinum on the particle surface occurs,
Since the movement of the particles hardly occurs, the particle growth due to the fusion of the particles is suppressed.

The particles covered with platinum include gold, and in addition, an alloy comprising platinum and at least one metal selected from the group consisting of chromium, iron, nickel, cobalt, titanium, vanadium and copper. Also, redox occurs at a higher potential than that of platinum alone. For example, in the case of gold, surface oxidation-reduction occurs at around 1V. As the conductive carbon material of the carrier, carbon black heat-treated in an inert gas atmosphere or a reducing gas atmosphere is desirable. Carbon black has a functional group such as a carboxyl group on the surface and is oxidized to some extent at room temperature and in the air. For this reason, the surface is oxidized during the battery characteristic test, which causes desorption of platinum particles. Carbon black in an inert gas atmosphere of 700 ° C. or more or 500
By performing the heat treatment in a reducing gas atmosphere at a temperature of not less than ℃, the surface functional groups of carbon black are removed, and the interface between platinum and carbon black becomes chemically stable. This process
It may be carried out after loading platinum, but in that case, it is desirable to carry out at a low temperature in a reducing gas atmosphere in order to suppress the aggregation of platinum.

In the electrode catalyst for a fuel cell of the present invention, particles of an acid which are less oxidized than platinum are supported on a conductive carbon material, the carbon material is dispersed in an aqueous solution of a platinum salt, and the platinum is oxidized by a reducing agent. It can be produced by reducing and precipitating on the surface of particles that are difficult to be formed. As a reducing agent,
Although formaldehyde and sodium borohydride can be used, bubbling with hydrogen is preferable from the viewpoint of suppressing contamination of impurities.

[0011]

EXAMPLES Specific examples will be described below. Example 1 Gold was supported on carbon black (Ketjen EC) which had been subjected to a hydrogen reduction treatment at 800 ° C. at a weight ratio of 80:20 by a precipitation method. Subsequently, the gold-supported carbon black was dispersed in water using an ultrasonic homogenizer. Meanwhile, an aqueous solution of potassium chloroplatinate was dissolved in water and allowed to stand for one day. The aqueous solution of potassium chloroplatinate was added to the aqueous dispersion of gold-supported carbon black, and hydrogen gas was bubbled for 5 minutes. Thereafter, sealing was performed overnight, and platinum was reduced and precipitated on the gold particles with dissolved hydrogen. This is filtered and dried at 100 ° C.
Heat treated at 00 ° C. and washed with water. Thus, Pt-A having a weight ratio of carbon black, gold, and platinum of 50:20:30.
u / carbon black (catalyst A) was prepared. When the catalyst thus prepared was observed with a high-resolution transmission electron microscope at a magnification of 2,000,000, the average particle size of the catalyst particles supported on carbon black was 4.5 nm, and it was assumed that the catalyst particles were composed only of platinum. Particles having a particle size of 2 nm or less
It was about 0%. The XPS spectrum of the catalyst confirmed that about 90% of the gold was covered with platinum.

Subsequently, water and an ethanol solution of a polymer electrolyte perfluorosulfonic acid ionomer (Flemion manufactured by Asahi Glass Co., Ltd .: 9 wt%) were added to the catalyst A to prepare a catalyst ink a. Here, the weight ratio between the polymer electrolyte and the carbon black was set to 1: 1. Catalyst A
This catalyst layer was formed on a platinum electrode plate using ink a and dried at 130 ° C. in order to measure the cyclic voltammetry of the above by a rotating electrode method. The electrode was immersed in a 0.5 M sulfuric acid aqueous solution, oxygen was bubbled, and scanning was performed between 0 V and 1.2 V. The peak position of the reduction wave at which the oxidized metal releases oxygen was 0.95V. The higher the potential, the less the metal is oxidized.

On the other hand, sodium bisulfite is added to an aqueous solution of chloroplatinic acid, and then reacted with an aqueous solution of hydrogen peroxide. The resulting platinum colloid is supported on carbon black (Ketjen EC) which has been hydrogen-reduced at 800 ° C. Electrode catalyst B having a composition of Pt / Ketjen EC with a weight ratio of black to platinum of 50:50 was prepared. For this catalyst B, cyclic voltammetry was performed in the same manner as for catalyst A, and the potential at which oxidized platinum was reduced was examined.
0.76 V, which was easily oxidized compared to gold.

Next, the catalyst ink a was added to Pt 0.3 mg /
The solution was applied to carbon paper by a doctor blade method so as to have a volume of cm ² and dried at 60 ° C. to produce a cathode. On the other hand, the anode was made of Pt-Ru / Ketjen EC (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) having a weight ratio of carbon black, platinum and ruthenium of 50:30:24.
It was produced by the same method so as to obtain 0.3 mg / cm ² . An MEA was assembled by sandwiching a polymer electrolyte membrane (Nafion 112 membrane manufactured by DuPont) between the thus prepared cathode and anode and hot pressing at 130 ° C.

With respect to this MEA, at a cell temperature of 75 ° C., air heated and humidified so as to have a dew point of 65 ° C. to the cathode and hydrogen heated and humidified so as to have a dew point of 70 ° C. to the anode are supplied, respectively. After operating at a rate of 40%, a hydrogen utilization rate of 70%, and a current density of 0.2 A / cm ² for 100 hours, it was operated at 0.7 A / cm ² for 100 hours. After that, the operation was stopped, the MEA was decomposed, and the catalyst layer on the cathode side was wiped with ethanol to dissolve the perfluorosulfonic acid ionomer, and the electrode catalyst after the life test was observed with a high-resolution transmission electron microscope. The average particle size was 5.1 nm. On the other hand, an MEA was assembled using Catalyst B as a cathode catalyst, and a battery life test was performed under the same conditions as described above. When the anode catalyst before and after the life test was observed with a high-resolution transmission electron microscope, particles having an average particle size of 3.5 nm before the life test grew to an average particle size of 6.7 nm.

Example 2 In the same manner as in Catalyst B, the weight ratio of carbon black (Ketjen EC) to platinum was 90:
Ten Pt / Ketjen ECs were prepared. In order to perform platinum alloying, chromium was supported on the above-mentioned carbon black by an impregnation method using an aqueous chromium nitrate solution, and hydrogen reduction was performed at 900 ° C. The atomic ratio of platinum to chromium was set to 1: 1. Subsequently, platinum is supported in the same manner as in the case of the catalyst A,
The content of platinum supported later was adjusted to 30 wt% (weight ratio of carbon black, platinum initially supported, chromium and platinum subsequently supported was 90: 10: 2.7: 44). Using this catalyst, in the same manner as Catalyst A,
A battery characteristic test was performed, and the average particle diameter was measured before and after that. Further, with respect to the catalyst before supporting platinum, the potential at which the surface oxide was reduced was examined by cyclic voltammetry. Table 1 shows the results. As with chrome, iron,
Alloys with platinum were prepared from nitrates of nickel, cobalt, copper, and manganese, and the reduction potential of oxides was examined, and the change in particle size due to battery characteristics was examined. The results are shown in Table 1.

Further, an alloy of vanadium and platinum from vanadium (III) chloride and an alloy of titanium and platinum from titanium tetrachloride were prepared in the same manner as in the case of chromium, respectively, and the reduction potential of the oxide was examined. Further, platinum was coated, and a change in particle diameter due to battery characteristics was examined. The results are shown in Table 1. As is clear from Table 1, the above alloy catalyst is more easily reduced than platinum alone, and the change in particle diameter after the life test is small.

[0018]

[Table 1]

Example 3 In the same manner as in the case of the catalyst A, a catalyst C in which gold was supported on untreated carbon black (Ketjen EC), and then platinum was supported was prepared. this,
Using catalyst C as a cathode catalyst, ME
A was assembled and a battery life test was performed. When the cathode catalyst before and after the life test was observed with a high-resolution transmission electron microscope, particles having an average particle size of 4.5 nm before the life test grew to an average particle size of 6.2 nm. On the other hand, carbon black (Ketjen E) heat-treated in the same manner as Catalyst A in an argon gas atmosphere at 1200 ° C.
Catalyst D was prepared by supporting gold on C) and subsequently supporting platinum. Using this catalyst D as a cathode catalyst, an MEA was assembled and a battery life test was performed. When the electrode catalyst before and after the life test was observed with a high-resolution transmission electron microscope, particles having an average particle diameter of 4.6 nm before the life test had an average particle diameter of 5.2 nm after the life test.

As is clear from the comparison between the catalysts C and D as described above, when carbon black is heat-treated in an inert gas atmosphere, particles after a life test of battery characteristics can be kept small. In the above example, the carbon black was heat-treated in an inert gas atmosphere, but heat treatment in a reducing atmosphere has the same effect.

[0021]

As described above, according to the present invention, an electrode catalyst for a fuel cell having high durability can be provided.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) H01M 4/86 H01M 4/86 B 4/88 4/88 K C 4/90 4/90 M 8/10 8/10 (72) Inventor Teruhisa Kamihara 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture F-term in Matsushita Electric Industrial Co., Ltd. (reference) BC75A BC75B BC75C CC32 DA06 EA01X EA01Y EB19 FA02 FB08 FB29 FB44 5H018 AA06 AS02 AS03 BB01 BB08 BB17 EE02 EE03 EE05 5H026 AA06 BB00 BB01 BB04 BB10 EE02 EE05

Claims (6)

[Claims] 1. A fuel comprising: a conductive carbon material; metal particles supported on the conductive carbon material, which are less oxidizable than platinum under acidic conditions; and platinum covering an outer surface of the metal particles. Electrode catalyst for batteries. 2. The fuel cell electrode catalyst according to claim 1, wherein the metal particles are gold. 3. The method according to claim 1, wherein the metal particles are an alloy composed of platinum and at least one metal selected from the group consisting of chromium, iron, nickel, cobalt, titanium, vanadium, copper, and manganese. Electrode catalyst for fuel cells. 4. A fuel cell comprising: a step of supporting metal particles less oxidizable than platinum under acidic conditions on a conductive carbon material; and a step of dispersing the carbon material in an aqueous solution of a platinum salt and reducing platinum with a reducing agent. For producing an electrode catalyst for use. 5. The method for producing an electrode catalyst for a fuel cell according to claim 4, wherein the reducing agent is hydrogen. 6. The method for producing an electrode catalyst for a fuel cell according to claim 4, further comprising a step of subjecting the conductive carbon material to a heat treatment in an inert gas atmosphere or a reducing gas atmosphere before or after supporting the catalyst.