Alleviated surface calcium segregation and improved electrocatalytic properties of La0.3Ca0.7Fe0.7Cr0.3O3-δ cathode: A demonstration of A-site deficiency effect

Abstract

The crystal structure, electrical conductivity, thermal expansion behavior and electrocatalytic properties for oxygen reduction reaction of A-site deficient (La0.3Ca0.7)1−xFe0.7Cr0.3O3-δ (x = 0.01–0.05) perovskites were investigated in comparison with their stoichiometric counterpart (x = 0). The tolerance limit of the perovskite structure for A-site deficiency was x = 0.04 or so. The unit cell parameters decreased and oxygen nonstoichiometry (δ) increased with increasing A-site deficiency. The electrocatalytic activity was appreciably improved by introducing an A-site deficiency of x = 0.04, concomitant with a slight decrease of the electrical conductivity and thermal expansion coefficient. The porous electrode with x = 0.04 exhibited a polarization resistance of 0.044 Ω·cm2 at 800 °C, lowered by about 42% compared with the electrode with x = 0. Moreover, the introduction of the A-site deficiency enhanced the stability of the perovskite structure and electrocatalytic activity relative to cathodic polarization. The electrode with x = 0 showed segregation of calcium onto the surface and an increase of the polarization resistance by 0.106 Ω·cm2 after being cathodically polarized under 250 mA·cm-2 at 800 °C for 6 h. In contrast, surface calcium segregation was undetected for the electrode with x = 0.04 after being likewise polarized and the electrocatalytic activity of the polarized A-site deficient electrode was virtually undegraded. The improved stability of the structure and electrocatalytic activity of the A-site deficient electrode was tentatively explained with respect to its defect chemistry.