Development of High Proton-conducting Glasses and their application to fuel cell electrolyte

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Low temperature fuel cell system has attracted much attention because of its ability to produce high energy without the emission of harmful pollutants. For this purpose, the membranes with high proton conductivity and high chemical and thermal durability are required as the electrolytes of the fuel cell. Our interest focuses on the preparation of high proton-conducting glasses, and using a sol-gel method we succeeded in the preparation of P_2O_5-containing glasses exhibiting conductivities of 10^<-4>〜10^<-2> S/cm at room temperature.Two porous 5P_2O_5-95SiO_2 (mol%) glasses having different pore properties were prepared by the sol-gel method using Si(OC_2H_5)_4, PO(OCH_3)_3 and POCl_3, which are designated as No.1 and 3, respectively. It was found that the sample No.1 exhibits narrow pore size distribution with an average radius of 1.5 nm, whereas the sample No.3 has wide distribution ranging from 5 to 20 nm and an average radius of 8 nm. The conductivities were measured as a function … More of a ambient humidity and temperature. We found that the conductivity increases with increasing the relative humidity, but their humidity dependence is substantially different in two samples. In the sample No.1, the conductivity is 〜1xlO^<-6> S/cm at 30 %RH, and rapidly increases with increasing humidity, reaching a constant value of 〜1x10^<-2> S/cm above 〜70%. Of further interest was that the sample No.1 keeps high conductivity during decreasing humidity, though it gradually decreases at the humidity below 〜50%. This result indicates that the glass exposed once in high humidity holds the water in its pores and exhibits high conductivity irrespective of the humidity change. This finding is important for the practical application of this glass, because it enables the water management simple, consequently resulting in significant decrease of cost. On the other hand, the sample No.3 exhibits gradual increase in the conductivity with increasing the humidity up to 〜80%, at which the conductivity rapidly increases and reaches to 〜5xlO^<-2> S/cm. The conductivity in sample No.3 reversibly changes with the changed the humidity. It also was found that our glasses have potential as the electrolyte of fuel cell. Less