Enhanced Low-Temperature Proton Conduction in Sr_0.02La_0.98NbO_4−δ by Scheelite Phase Retention

Abstract

The high-temperature scheelite phase of Sr0.02La0.98NbO4 materials has been retained to room temperature via vanadium substitution for the Sr0.02La0.98Nb1−xVxO4−δ compositional range of 0.25 ≤ x ≤ 0.325. Such structural stabilization avoids the characteristic break in the thermal expansion coefficient (TEC) that may otherwise be deleterious for the application of LaNbO4-based materials as proton-conducting electrolytes. The transport properties of composition Sr0.02La0.98Nb0.7V0.3O4−δ have been extensively characterized. The composition offers pure proton conduction with higher conductivity than the base composition Sr0.02La0.98NbO4−δ in the low-temperature range under wet oxidizing conditions. A wide ionic domain is observed, which increases as the temperature decreases. Suggested operational limits have been documented. Under dry oxidizing conditions, vanadium substitution is shown to suppress p-type conductivity, in comparison to the base composition Sr0.02La0.98NbO4−δ. In contrast, under wet conditions, Sr0.02La0.98Nb0.7V0.3O4−δ is observed to be a pure proton conductor in oxygen and argon atmospheres at temperatures lower than ∼700 °C. The total ionic conductivity equals that of the proton conductivity, within experimental error, suggesting negligible oxide-ion conduction in this material under these conditions.