1. Field of the Invention
The present invention relates to catalysts preferably used in fuel cells, and fuel cell electrodes.
2. Description of the Related Art
Fuel cells convert chemical energy of fuel into electric energy directly by electrochemically oxidizing fuel such as hydrogen or methanol. The fuel cell has attracted extensive attention as a clean power source because there is no production of NOx, SOx, etc. as caused by thermal power generation. Particularly polymer electrolyte fuel cells has been developed as a power supply for spaceship use because the size, weight and thickness of the polymer electrolyte fuel cells can be reduced compared with those of other fuel cells. Recently the polymer electrolyte fuel cells have been studied extensively as a power supply for automobile and mobile use.
The related-art fuel cells have not been spread widely because the performance of the fuel cells is still insufficient at temperature range lower than 100° C.
Power generation by fuel cells is based on an oxidation reduction reaction generated on electrodes. High active catalysts are essential to improve the performance of the fuel cell. Particularly the catalysts are apt to be poisoned regardless of whether the fuel cell is a hydrogen type polymer electrolyte fuel cell (PEMFC) using a reformed gas as fuel or whether the fuel cell is a methanol type polymer electrolyte fuel cell using a methanol solution as fuel (DMFC) because CO is contained in the raw material or intermediate product. The CO poisoning suppressing ability of PtRu catalysts currently mainly used as the anode catalyst is more excellent than Pt catalysts but is still insufficient. For example, in the case of DMFCs, voltage loss caused by the PtRu anode catalyst is about 0.3 V which accounts for about 25% of the voltage 1.21 V theoretically allowed to be generated.
In addition, in the PtRu catalyst, dissolution of Ru and the migration of Ru to an oxidant electrode during power generation were observed. It is necessary to improve durability of the PtRu catalyst.
To improve activity and durability of anode catalyst, extensive investigation have been made such as adding transition metal to PtRu alloy and changing carrier material.
Particularly addition of Sn to noble metal-containing catalyst has attracted attention.
For example, in JP-A-2004-28117, a Pt60Ru15Sn20 alloy thin film produced by sputtering method has been evaluated by activity current measurement according to an electrochemical method using an aqueous solution of dilute sulfuric acid. In T. Kim et al, Chemistry Letters Vol.33 (4) (2004) pp.478-479, Pt3-5Ru2-4Sn alloy fine particles carried on carbon black produced by a liquid phase reduction method have been evaluated by activity current measurement according to an electrochemical method using an aqueous solution of dilute sulfuric acid. These documents have reported that the activity of the alloy is higher than that of PtRu alloy produced in the same manner.
However, higher activity and higher durability are expected in the catalyst including Pt, Ru and Sn.
The invention provides Pt—Ru—Sn catalyst with excellent activity and durability. The invention also provides fuel cells with high output and high durability.