Surface and bulk study of strontium-rich chromium ferrite oxide as a robust solid oxide fuel cell cathode

A novel Co-free cathode, La_0.3Sr_0.7Fe_0.7Cr_0.3O_3-δ (LSFCr-3), exhibiting the desired combination of high electrical conductivity, physical and chemical stability, and electrocatalytic activity, was systematically investigated for SOFC applications. Its excellent performance is attributed primarily to the presence of Cr, which was found to be predominant in the 3+ and 4+ oxidation states in the LSFCr-3 bulk, thus likely maintaining a 6-fold coordination with oxygen anions. This, in turn, causes disorder in the oxygen vacancy sub-lattice, stabilized by the Fe ion-oxygen tetrahedra. However, on the surface of the LSFCr-3 oxide, Cr is primarily in the 6+ state, together with some Cr³⁺/Cr⁴⁺, even at 700 °C. Cr⁶⁺ can only be tetrahedrally coordinated by oxygen anions, resulting in a large concentration of oxygen vacancies on the LSFCr-3 surface, with a surface exchange coefficient and oxygen ionic conductivity of ca. 10^-5 cm s^-1 and ca. 10^-2 S cm^-1, respectively, at 700-800 °C. The use of LSFCr-3 as the cathode in a Ni-Ce_0.8Sm_0.2O_2-δ (SDC) anode-supported single solid oxide fuel cell in 3% H₂O-H₂/air gave a maximum power density of 0.81 W cm² at 750 °C, which is superior to that of similar cells in which La_0.6Sr_0.4Fe_0.8Co_0.2O_3-δ, a previously well studied material, was used as the cathode.