Effect of grains and grain boundaries on concentrations and mobility of charge carriers in Ba3Ca1.18Nb1.82O9-δ

Abstract

Perovskite-based protonic conductors are currently the most promising electrolytes in medium and low-temperature hydrogen energy devices. Herein, the Ba3Ca1.18Nb1.82O9-δ double perovskite oxide protonic conductor is prepared, and the influence of bulk and grain boundaries on transport properties is systematically investigated via defect chemistry. The Ba3Ca1.18Nb1.82O9-δ exhibits a hexagonal distortion, and the calculated standard molar hydration enthalpies of Ba3Ca1.18Nb1.82O9-δ, bulk, and grain boundaries are -138 kJ/mol, -131 kJ/mol, and -144 kJ/mol, respectively. The migration activation energies of protons, oxygen vacancies, and hole conduction in bulk Ba3Ca1.18Nb1.82O9-δ under oxygen are 0.29 eV, 1.51 eV, and 0.63 eV, respectively. The proton concentration in grains is slightly lower than in grain boundaries. In contrast, the proton mobility in bulk is three orders higher than in grain boundaries. The proton conduction of double perovskite oxide protonic conductor is dominant in Ar and reductive atmospheres at 500–800°C, where the conductivity (σh·) and holes transport number (th·) increase with oxygen partial pressure. Therefore, the conduction of Ba3Ca1.18Nb1.82O9-δ could be enhanced by improving carriers’ mobility in grain boundaries. The right grain boundaries block oxygen vacancies at low temperatures, increasing the predominance area diagrams for proton conduction. Hence, grain boundaries control the transport property of double perovskite oxide protonic conductor.