Criteria for the application of high temperature proton conductors in SOFCs

A critical evaluation of high temperature proton conductors (HTPCs) which might be considered in SOFC applications is presented. A comparison of the conductivities of the best oxygen ion conductors with those of prominent HTPCs shows that a few of the latter may possibly compete in the temperature range 500-700 degrees C. The HTPCs considered here were (a) the classical doped SrCeO3, (b) doped BaCeO3, (c) doped Sr and Ba zirconates, and (d) complex perovskites of type Ba3Ca1.18Nb1.82O9-delta. The compounds favored are those with a high enthalpy of solution of water. Generally, the following materials parameters are desirable: deviation from exact oxygen stoichiometry delta approximate to 0.05; enthalpy of solution of water -Delta H-s greater than or equal to 1.5 eV; effective activation energy for proton transport E-H(eff) < = 0.55 eV and proton transport number t(H) approximate to 1.