Geometric structure distribution and oxidation state demand of cations in spinel NixFe1-xCo2O4 composite cathodes for solid oxide fuel cells

Abstract

Slow oxygen reduction reaction (ORR) is a key obstacle of intermediate-temperature solid oxide fuel cells (IT-SOFCs). In this study, we prepared a complex spinel oxide (Fe3+Co2+)(Fe2+Fe3+Co3+)2O4 as good ORR catalyst for IT-SOFCs. Inducing oxidation state optimization of Fe and Co ions, the strategy of Ni doping at octahedral sites of FeCo2O4 with narrow band gap can effectively facilitate small polaron hopping of valence electrons, resulting in high electrical conductivity. In the p-type semiconductor NixFe1-xCo2O4 (NFCOx), the strong oxidant Ni3+ and highly electronegative Ni2+ would capture electrons to increase hole concentration and decrease resistance. The substitution of Ni that is smaller in size for Fe of bigger size at B-sites would cause lattice distortion and structure defects, which are beneficial to the promotion of oxygen transport capacity. The analysis of ORR kinetics based on the distribution of relaxation time (DRT) method reveals that the migration of adsorbed oxygen from NFCOx surface to the triple-phase boundary (TPB) is the rate-determining step. Owing to high electronic conductivity and fast oxygen transport, power density as high as 1.93 W/cm2 at 800 °C could be obtained when NFCO2-60GDC composite was used as cathode for IT-SOFCs, which is superior to (75.5% higher) that of using commercial LSM-based cathode (1.10 W/cm2 at 800 °C). The electrochemical performance of NFCO2-based IT-SOFCs is better than that of the other SOFCs with cathodes reported.