Sr- and Ni-doped LaCaO3 and LaFeO3 perovskites -: New cathode materials for solid-oxide fuel cells

An improved cathode material for a solid-oxide fuel cell would be a mixed electronic and oxide-ion conductor with a good catalytic activity for oxygen reduction at an operating temperature T-op greater than or equal to 700 degrees C and a thermal expansion matched to that of the electrolyte and interconnect. We report on the proper ties of Sr- and Ni-doped LaCoO3 and LaFeO3 perovskites that meet these criteria. Single-phase regions were determined by X-ray diffraction, and thermogravimetric analysis measurements were used to obtain the temperatures above which oxygen loss, and hence oxide-ion conductivity, occurs. The conductivity and Seebeck measurements indicate the coexistence of both p-type and n-type polaronic charge carriers resulting from an overlap of the Ni-III/Ni2+ redox couple with the low-spin/intermediate-spin Co-IV/Co-III and high-spin Fe4+/Fe3+ redox couples. Motional enthalpies Delta H-m = 0.03, 0.02, and 0.08 eV. respectively, were estimated for Ni2+, Co-IV, and Fe4+ polarons. Optimal compositions have percolation pathways between dopants. Comparisons with transport data for the conventional cathode materials La1-xSrxCoO3-delta and La1-xSrx MnO3 indicate superior cathode performance can be expected.