Systematic evaluation of Co-free LnBaFe2O5+δ (Ln = Lanthanides or Y) oxides towards the application as cathodes for intermediate-temperature solid oxide fuel cells

Co-free oxides with a nominal composition of LnBaFe2O5+δ, where Ln = La, Pr, Nd, Sm, Gd, and Y, were synthesized and phase structure, oxygen content, electronic conductivity, oxygen desorption, thermal expansion, microstructure and electrochemical performance were systematically investigated. Among the series of materials tested, LaBaFe2O5+δ oxide showed the largest electronic conductivity and YBaFe2O5+δ oxide had the smallest thermal expansion coefficient (TEC) of 14.6 × 10−6 K−1 within a temperature range of 200–900 °C. All LnBaFe2O5+δ oxides typically possess the TEC values smaller than 20 × 10−6 K−1. The oxygen content, electronic conductivity and TEC values are highly dependent on the cation size of the Ln3+ dopant. The lowest electrode polarization resistance in air under open circuit voltage condition was obtained for SmBaFe2O5+δ electrode and was approximately 0.043, 0.084, 0.196, 0.506 and 1.348 Ω cm2 at 800, 750, 700, 650 and 600 °C, respectively. The SmBaFe2O5+δ oxide also demonstrated the best performance after a cathodic polarization. A cell with a SmBaFe2O5+δ cathode delivered peak power densities of 1026, 748, 462, 276 and 148 mW cm−2 at 800, 750, 700, 650 and 600 °C, respectively. The results suggest that certain LnBaFe2O5+δ oxides have sufficient electrochemical performance to be promising candidates for cathodes in intermediate-temperature solid oxide fuel cells.