Electrochemical properties of MH2PO4/SiP2O7-based electrolytes (M = alkaline metal) for use in intermediate-temperature fuel cells

The proton-conductive MH2PO4/SiP2O7-based composites (M = K, Rb, Cs) were synthesized, and their structure and proton conductivity were investigated at intermediate temperatures. For all MH2PO4/SiP2O7-based composites, the chemical reaction between MH2PO4 and SiP2O7 has proceeded during the heat treatment at 220 °C for 1 h, resulting in the formation of MH5(PO4)2. The heat-treated composites exhibited higher conductivity than MH2PO4 due to the new conduction phase of MH5(PO4)2, regardless of the alkaline metal species. The electrolytes of MH5(PO4)2/SiP2O7 composites (M = K, Rb, Cs) were also fabricated so as to compare the conductivity with almost the same charge carrier concentration. It is noted that their conductivity is independent of the alkaline metal species.