THERMODYNAMIC STABILITIES OF SRCEO3 AND BACEO3 USING A MOLTEN-SALT METHOD AND GALVANIC CELLS

SrCeO3 and BaCeO3, 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 approximately 500 to approximately 1000-degrees-C. Typical synthesis/densification temperature of these cerates using conventional processing methods is approximately 1400 to 1600-degrees-C. The objective of the present work was to determine thermodynamic stabilities of SrCeO3 and BaCeO3 with respect to the individual oxides SrO and CeO2, and BaO and CeO, in the anticipated application temperature regime. Two approaches were selected: the molten salt method and galvanic cells. In the former, SrO and CeO2 or BaO and CeO2 were added in a 1:1 molar ratio to molten salt eutectics (NaOH-KOH or LiCl-KCl) at approximately 400-degrees-C. No SrCeO3 or BaCeO3 formed. Subsequently, SrCeO3 and BaCeO3, formed by solid state synthesis at approximately 1100-degrees-C, were added to molten salt eutectics at approximately 400-degrees-C. SrCeO3 when added to molten LiCl-KCl eutectic either transformed into SrCO3 and CeO2 or SrO and CeO2 depending upon whether the air circulated over the molten salt was ambient or was passed through soda lime and ascarite to remove CO2. In NaOH-KOH molten salt eutectic, SrCeO3 decomposed to form SrCO3 and CeO2 regardless of whether the air circulated was or was not passed through soda lime. BaCeO3 decomposed to form BaCO3 and CeO2 in both salt eutectics in both ambient air and that passed through soda lime, This shows that over the temperature range of investigation, both SrCeO3 and BaCeO3 are thermodynamically unstable. Galvanic cells with alkaline earth fluoride ion conductors were used to determine standard Gibbs free energies of the reactions SrCO3 + CeO2 --> SrCeO3 + CO2 and BaCO3 + CeO2 --> BaCeO3 + CO2. 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.