Cobalt-free fluorine doped Bi_0.7Sr_0.3FeO_3-δ oxides for energetic cathodes of low-temperature solid oxide fuel cells

Cobalt-free fluorine doped Bi_0.7Sr_0.3FeO_3-δ-xF_x oxides are presented here as cathode catalysts of low-temperature solid oxide fuel cells (LT-SOFCs). Their crystal structure, thermal behaviors, catalytic activity, oxygen transport properties, oxygen reduction kinetics, long-term stability and CO₂ tolerance are systematically exploited. At 600 °C, the optimized Bi_0.7Sr_0.3FeO_2.9-δF_0.10 (BSFF0.10) cathode possesses outstanding ORR activity with a low polarization resistance of 0.12 Ω cm². When tested under realistic conditions, the anode-supported fuel cell with BSFF0.10 cathode delivers a high output of 837 mW cm⁻² with stable operation for 110 h. Such outstanding performance can be ascribed to the improved oxygen transport kinetics, larger oxygen vacancy concentration, increased oxygen surface adsorption energy, and O 2p-band centers close to the Fermi level. In addition, the oxygen reduction process on BSFF0.10 electrode interface is mainly determined to be the dissociation of adsorbed molecule oxygen processes. The BSFF0.10 cathode exhibits superior CO₂ tolerance because of the lower basicity resulting from the presence of Bi³⁺, Fe²⁺/Fe³⁺/Fe⁴⁺ and F⁻. Although the BSFF0.10 cathode is not reactive with CO₂ to form carbonate species, post-characterizations reveal that the CO₂ atmosphere can induce the migration of Bi ions to the surface, which in turn affects the performance of the cathode. These results show that fluorine doping is an effective approach to develop highly active and robust cathode for SOFCs.