Investigation of aliovalent transition metal doped La_0.7Ca_0.3Cr_0.8X_0.2O_3-δ (X=Ti, Mn, Fe, Co, and Ni) as electrode materials for symmetric solid oxide fuel cells

The crystal phase, chemical stability, thermal expansion behavior, oxygen non-stoichiometry, electrical conductivity, and electro-catalytic activity of La_0.7Ca_0.3Cr_0.8X_0.2O_3-δ (X=Ti, Mn, Fe, Co, and Ni) electrode materials have been investigated. All the compositions are chemically stable with a single-phase perovskite structure in both reducing and oxidizing atmospheres at high temperature. The linear thermal expansion coefficients of La_0.7Ca_0.3Cr_0.8X_0.2O_3-δ are approximately 10×10^-6 K^-1. The total electrical conductivity of La_0.7Ca_0.3Cr_0.8X_0.2O_3-δ increases in the order of X=Ti<Ni<Fe<Co<Mn, which is correlated to the porosity, valence orbital ionization potential, and oxygen non-stoichiometry upon the reduction process. The overall reaction process of the hydrogen oxidation for the La_0.7Ca_0.3Cr_0.8X_0.2O_3-δ anodes is limited by the charge transfer process that requires higher activation energy. In a similar trend observed in electrical conductivity, La_0.7Ca_0.3Cr_0.8Mn_0.2O_3-δ exhibits the lowest polarization resistance value of 0.12 Ω cm² at 800 °C. Finally, the symmetric single cell with La_0.7Ca_0.3Cr_0.8Mn_0.2O_3-δ as both the cathode and anode shows a maximum powder density of 220 mW cm^-2 at 800 °C.