Electrode catalyst and electrochemical devices using the same

Details Electrode catalyst and electrochemical devices using the same Electrode catalyst and electrochemical devices using the same

2001-05-18

2003-11-18

Bell, Bruce F. (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C429S047000, C429S047000, C429S006000, C429S006000, C502S101000, C502S182000, C502S185000, C204S283000, C204S284000, C204S282000

Reexamination Certificate

active

06649300

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an electrode catalyst for electrochemical devices such as fuel cells, and its utilization.
2. Description of the Prior Art
Solid polymer electrolyte fuel cells can provide high current density at low temperature, and hence they are on development as power sources of automobiles and also as power sources of cogeneration for public use.
In the solid polymer electrolyte fuel cells, catalysts are used to accelerate electrode reaction at the anode and the cathode, where it is important to improve their activity per unit mass of catalytic metal in order to improve the efficiency and output power or cells and to manufacture cells at a low cost.
In the beginning, it was considered that in solid polymer electrolyte fuel cells only a catalyst present at the joint interface with an electrolyte ion-exchange membrane participated in the electrode reaction. Accordingly, platinum black was used as the catalyst in order to ensure many active sites per unit area of the joint interface. Using this platinum black, an ion-exchange membrane-electrode assembly was formed in which each electrode of the anode and the cathode, comprising a gas diffusion electrode substrate one surface of which is coated with the platinum black, is so joined that the catalyst side of each electrode comes into contact with the ion-exchange membrane. This membrane-electrode assembly used platinum in an amount of 4 to 10 mg/cm
2
per unit area of joint interface.
After that, a method was developed in which a catalyst comprised of platinum or a platinum alloy supported on conductive carbon and impregnated with an ion-exchange resin is used in an electrode, and this electrode is joined to an ion-exchange membrane by hot pressing. Thus, it became possible to make the whole catalyst layer with a stated thickness participate in the electrode reaction. The platinum or platinum alloy that this membrane-electrode assembly uses per unit area of joint interface was reduced to an amount of 0.1 to 1.0 mg/cm
2
, bringing about an improvement in the utilization efficiency of platinum.
In order to attain necessary performance, in such electrode catalysts, the metal component such as platinum or platinum alloys must be supported in an amount of 20 to 70% by mass based on the mass of the catalyst. However, in conventional processes for producing electrode catalysts, the metal component may agglomerate when the platinum or platinum alloy is supported in an amount of 20% by mass or more, so that the crystallite diameter of the metal component may grow. Hence, the performance can not be so much improved for the metal component supported in a large quantity, and no satisfactory performance has been achievable.
In recent years, as an electrode catalyst for solid polymer electrolyte fuel cells, it has been reported that, in respect of catalysts comprised of platinum supported on conductive carbon Vulcan XC-72R (trade name; available from Cabot Corp.) each in an amount of 10, 20, 40, 50 or 60% by mass, the platinum has crystallite diameter of 19, 21, 35, 40 or 55 Å (angstrom), respectively, as measured by X-ray powder diffractometry (J. Electrochem. Soc., Vol.144, No.11, pp.3845-3857, 1997).
It has also been disclosed that, as a process for producing catalyst used for the same purpose, a process in which platinum is reduced by adding sodium thiosulfate to an aqueous chloroplatinic acid solution having conductive carbon suspended therein, followed by treatment at 400° C. in an atmosphere of nitrogen containing 50% by volume of hydrogen, can make platinum have a crystallite diameter of 35 Å, the platinum being that of a catalyst comprised of 30% by mass of platinum supported on conductive carbon having a specific surface area of 300 m
2
/g (Japanese Laid-open Publication (Kokai) No. 8-117598).
In general, the higher specific surface area the conductive carbon has, the higher degree of dispersibility the platinum has at the same support percentage (the platinum has a smaller crystallite diameter). With an increase in specific surface area, however, the conductive carbon tends to cause electrochemical oxidative corrosion. It is known to graphitize conductive carbon in order to control such oxidative corrosion from occurring. However, the graphitization of conductive carbon causing the reduction of specific surface area of the resulting carbon, involves a disadvantage that the platinum crystallite diameter grows with an increase in the amount of platinum supported.
In addition to the platinum, platinum alloys are also used as active metal components of electrode catalysts. In the case of reformed-gas fuel cells which generate electricity by feeding to the anode a reformed gas obtained by reforming oxygen-containing hydrocarbons or other hydrocarbons, the platinum alloys are used as metal components for keeping platinum from being poisoned by carbon monoxide inevitably contained in a reformed gas. In the case of direct-methanol fuel cells which generate electricity by feeding to the anode a mixture of methanol and water, the platinum alloys are used as metal components for improving electrochemical oxidation activity to methanol. Also, it has been said that, in the case where platinum is used alone, there is a limit on the improvement of oxygen reduction activity in the cathode even if the platinum is made to have a crystallite diameter as small as possible. Accordingly, it has been studied to use a platinum alloy as a metal component in the cathode, as well.
When the platinum alloy is made supported on a carrier, methods are available in which the platinum and the counterpart metal component comprising the alloy are made supported simultaneously or in which they are made supported one by one. In order to accurately control the amount of platinum supported, a method may be used in which platinum is first made supported and then the counterpart metal component is made supported and thereafter these are alloyed. When the platinum to be first made supported in this method is supported in an amount more than 20% by mass, the platinum crystallite diameter tends to grow and hence the crystallites of the platinum alloy formed by subsequent alloying can not avoid having still larger diameter, bringing about an insufficient improvement in activity.
Thus, in conventional supported platinum electrode catalysts for solid polymer electrolyte fuel cells, the platinum crystallite diameter may greatly grow with an increase in the amount of platinum supported, in the case where the platinum is supported in an amount more than 20% by mass. As the result, the activity can not be so much improved for the platinum supported in a large quantity, bringing about unsatisfactory results especially on the oxygen reduction activity in the cathode.
In conventional supported platinum alloy electrode catalysts, too, there has been a problem that enhancing the amount of platinum supported may make the platinum alloy crystallite diameter greatly grow and any sufficient activity can not be attained.
SUMMARY OF THE INVENTION
The present invention was made in order to solve the above problems. Accordingly, an object of the present invention is to provide an electrode catalyst for electrochemical devices, such as solid polymer electrolyte fuel cells which is able to attain a high activity because the platinum crystallite diameter is kept small even when the platinum is supported in a large quantity in an amount more than 20% by mass.
Another object of the present invention is to provide a precursor for production of an electrode catalyst for electrochemical devices, e.g., solid polymer electrolyte fuel cells, the electrode catalyst being comprised of, as a metal component, a platinum alloy whose crystallite diameter is kept small.
To achieve the above objects, the present invention provides an electrode catalyst which comprises a conductive carbon, platinum supported on the conductive carbon in an amount of from 20% by mass to 70% by mass based on the mass of the catalyst, and oxygen bonded chemi

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