Oxygen permeability and electrochemical performance of Ca-doped NdBaCo2O5+δ in IT-SOFC

Abstract

The layered perovskites NdBa1−xCaxCo2O5+δ (NBCax, where x = 0.0, 0.1, 0.2, 0.3, and 0.4) have been synthesized through a conventional solid-state reaction and examined as potential cathode candidates for intermediate-temperature solid oxide fuel cells (IT-SOFC; 600°C–800°C). The Ca-substituted oxides exhibit a P4/mmm tetragonal structure and a phase transition to simple perovskite at x ≥ 0.3. The electrical conductivity in air at 300°C–800°C and oxygen permeation flux of NBCax oxides are influenced by Ca doping content. The activation energy for the oxygen migration process indicates that bulk diffusion is predominant at temperatures below 800°C. X-ray photoelectron spectroscopy indicates the coexistence of Co3+ and Co4+ and oxygen-active sites on the surface of the NBCax oxides. Variations in Co4+ and adsorbed oxygen contents are related to the number of oxygen vacancies and are influenced by Ca content. Among the oxides examined, NdBa0.9Ca0.1Co2O5+δ exhibits good electrical conductivity, high amounts of Co4+ and adsorbed oxygen, high maximum power density of 1286 mW·cm−2 at 800°C in H2 fuel in the La0.9Sr0.1Ga0.8Mg0.2O3 electrolyte–supported cell, and low polarization resistance of the cell, as determined using AC impedance spectroscopy. These findings suggest that Ca incorporation can improve the electrochemical performance of NdBaCo2O5+δ, rendering it a promising candidate for use as a cathode material in IT-SOFCs.