Li2SO4-Al2O3 ceramic electrolytes are both cationic (lithium) and proton conducting and have been used as electrolytes in intermediate temperature fuel cells. The proton conductivity is of the order of 10(-2) S/cm at 500-degrees-C and a fuel cell operating at this temperature had a current density of more than 100 mA/cm2 for a terminal voltage of 0.4 V.