Metal Phosphates as Intermediate Temperature Proton Conducting Electrolytes

A series of metal phosphates were synthesized and screened as potential proton conductor electrolytes for fuel cells and electrolysers operational at intermediate temperatures. Among the selected, niobium and bismuth phosphates exhibited a proton conductivity of 10^-2 and 10^-7 S cm^-1, respectively, under the anhydrous atmosphere at 250 °C, showing close correlation with the presence of hydroxyl groups in the phosphate phases. At the water partial pressure of above 0.6 atm, both phosphates possessed a proton conductivity to a level of above 3 x 10^-2 S cm^-1. Reasonable stability of the proton conductivity was observed under either a constant low water partial pressure or under a humidity cycling test within a period of more than 80 hours.