Thermodynamic Stabilities of SrCeO3 and BaCeO3 Using a Molten Salt Method and Galvanic Cells

Abstract

and , which are known to be excellent proton conductors, are potential candidates as electrolytes in hydrogen concentrators and fuel cells. The anticipated application temperature is in the range of ∼500 to ∼1000°C. Typical synthesis/densification temperature of these cerates using conventional processing methods is ∼1400 to 1600°C. The objective of the present work was to determine thermodynamic stabilities of and with respect to the individual oxides and , and and in the anticipated application temperature regime. Two approaches were selected: the molten salt method and galvanic cells. In the former, and or and were added in a 1:1 molar ratio to molten salt eutectics ( or ) at ∼400°C. No or formed. Subsequently, and , formed by solid state synthesis at ∼1100°C, were added to molten salt eutectics at ∼400°C. when added to molten eutectic either transformed into and or and depending upon whether the air circulated over the molten salt was ambient or was passed through soda lime and ascarite to remove . In molten salt eutectic, decomposed to form and regardless of whether the air circulated was or was not passed through soda lime. decomposed to form and in both salt eutectics in both ambient air and that passed through soda lime. This shows that over the temperature range of investigation, both and are thermodynamically unstable. Galvanic cells with alkaline earth fluoride ion conductors were used to determine standard Gibbs free energies of the reactions and . The results of these experiments were consistent with those of the molten salt method. Implications of these results from the standpoint of applications of these materials are discussed.