Sn0.9In0.1P2O7-based organic/inorganic composite membranes application to intermediate-temperature fuel cells

An anhydrous proton conductor, 10 mol % In3+-doped SnP2O7 (Sn0.9In0.1P2O7), was composed by l,8-bis(triethoxysilyl)octane (TES-Oct) and 3-(trihydroxysilyl)-l-propanesulfonic acid ((THS)Pro-SO3H) and was characterized by structural and electrochemical analysis. The composite membrane with 90 wt % Sn0.9In0.1P2O7 showed high proton conductivities of 0.04 S cm-1 or more between 150 and 200 deg C in unhumidified air. The packing of the Sn0.9In0.1P2O7 particles in the matrix was relatively uniform, with no formation of pinholes observed. Fuel cell tests verified that the open-circuit voltage was maintained at a constant value of ~970 mV regardless of the electrolyte thickness (60-200 (Jim), while the Ohmic resistance was decreased to 0.24 ft cm2 by reducing the electrolyte thickness to 60 mum. The peak power densities achieved with unhumidified H2 and air were 109 mW cm-2 at 100 deg C, 149 mW cm-2 at 150 deg C, and 187 mW cm-2 at 200 deg C. Furthermore, fuel cell performance was improved by hot-pressing an intermediate layer consisting of Sn0.9In0.1P2O7, Pt/C, TES-Oct, and (THS)Pro-SO3H between the electrolyte and cathode.