Abstract
Fuel cells are attractive energy conversion devices with high efficiencies and low emissions, and many studies have been conducted so far. Among them, fuel cells operating at 200-600°C are promising technologies which combine the many advantages of high- and low-temperature fuel cells. However, they have not been developed due to the lack of good ionic-conductors with high thermal stability at intermediate temperatures. Recently, we have developed new proton-conductive electrolytes consisting of solid acid and pyrophosphate, and evaluated their electrochemical, structural and thermal properties at intermediate temperatures. For the composite based on CsH2PO4/SiP2O7, the interfacial chemical reaction between CsH2PO4 and SiP2O7 during heat-treatment gave rise to the formation of a new phase of CsH5(PO4)2. The temperature dependence of conductivity for this composite was different from that for pure CsH2PO4, and the maximum conductivity achieved was 44 mS·cm−1 at 266°C. Using potassium and rubidium salts, MH2PO4 (M = K, Rb), as the solid acids for the composite electrolytes, analogous phenomena were confirmed despite the alkaline metal. Operation of a fuel cell employing CsH2PO4/SiP2O7-based composite electrolyte (thickness: ca. 1.3 mm) was demonstrated at 200°C and generated electricity up to 220 mA·cm−2 at 0.2 V. CsH5(PO4)2 composites with SiP2O7 and SiO2 were fabricated, and the composite effects were investigated at intermediate temperatures based on conductivity measurement, thermal analysis, and wettability evaluation. The melting and dehydration processes of CsH5(PO4)2 in composites were different depending on the matrix species. The composite with SiP2O7 matrix showed the highest conductivity of all composites. The conductivity of the composites appears to correlate with the wettability between the components as examined by contact angle measurement. These findings should be attributed to the differences in the interfacial interactions between CsH5(PO4)2 and the matrix.
