Preparation and Evaluation of Proton Conductive Membranes for Medium Temperature Fuel Cells

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Solid state proton-conductive membranes with high conductivity in the medium temperature range (100-200ーC) even under low humidity are highly required as the electrolytes for PEFCs. Since the operation of PEFCs in the medium temperature range improves the utilization of total electric power generated in the cells. and depresses the poisoning of Pt catalysts with CO in the fuel gases. In addition, working of PEFCs under low humidity permits to reduce the weight and volume of humidifiers. In the present work, proton conducting membranes based on sol-gel derived phosphosilicate (P_2O_5-SiO_2) gel have been fabricated for the application to medium temperature fuel cells as an electrolyte.Two types of proton conductive membranes, which show high conductivities even at medium temperatures with low humidity, have been successfully prepared. One is a composite sheet composed of phosphosilicate gel powder and organic polymer such as heat resistant polyimide. The other is a hybrid sheet in which inorganic and organic components derived from an organosilane such as 3-Glycidoxypropyltrimethoxysilane and phosphoric acid are covalently bonded.Fuel cells using a composite sheet composed of phosphosilicate (P_2O_5-SiO_2, P/Si=1 in mole ratio) gel powder and polyimide operated up to 150ーC under a relatively low water vapor, pressure. Maximum power density of the fuel cell increased with increasing operation temperature and was 28 mW cm^<-2> at 150 ℃ and 18 %RH. With respect to the hybrid type, fuel cells using a hybrid sheet derived from 3-Glycidoxypropyltrimethoxysilane and phosphoric, acid also continuously operated at 130 ℃.It was found that the addition of tin and tantalum as well as aluminum improved the chemical durability of phosphosilicate gels by depressing the leaching of phosphoric acid from the gels. At the same time, the improvement of durability of the gels resulted in a decrease in the proton conductivity.