Sr‐ and Ni‐Doped LaCoO3 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 ≥ 700°C and a thermal expansion matched to that of the electrolyte and interconnect. We report on the properties 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 NiIII/Ni2+ redox couple with the low‐spin/intermediate‐spin CoIV/Coiii and high‐spin Fe4+/Fe3+ redox couples. Motional enthalpies ΔHm = 0.03, 0.02, and 0.08 eV, respectively, were estimated for Ni2+, CoIV, and Fe4+ polarons. Optimal compositions have percolation pathways between dopants. Comparisons with transport data for the conventional cathode materials La1-xSrxCoO3-δ and La1-xSrxMnO3 indicate superior cathode performance can be expected.