Multi-element doped SrFeO ₃ -based cathodes with balanced thermal expansion for proton-conducting solid oxide fuel cells

SrFeO₃-based oxides have emerged as promising cathodes for proton-conducting solid oxide fuel cells (H-SOFCs), yet their performance at intermediate temperatures remains unsatisfactory. To overcome this limitation, we develop a multi-element doping strategy, resulting in the synthesis of a novel oxide, SrFe_0.9Nb_0.025Ta_0.025Mo_0.025W_0.025O₃ (ME-SFO). Unlike conventional SrFeO₃ materials doped with single elements, ME-SFO exhibits a remarkable synergistic effect that substantially enhances both proton and oxygen transport kinetics. ME-SFO demonstrates superior cathode reaction kinetics compared to SrFe_0.9X_0.1O₃ cathodes, achieving the lowest polarization resistance and activation energy. This enhanced catalytic activity translates into outstanding performance in H-SOFC applications, delivering a peak power density of 1748 mW cm^-2 at 700 °C—surpassing not only single-doped SrFe_0.9X_0.1O₃ variants but also other SrFeO₃-based cathodes reported for H-SOFCs. Although good long-term stability is achieved at a fixed temperature, ME-SFO suffers from high thermal expansion, which compromises cycling stability, leading to noticeable current density degradation after 10 test cycles. To mitigate this issue, we incorporate the negative thermal expansion oxide NdMnO₃ into the cathode, effectively counterbalancing ME-SFO's thermal expansion. The optimized composition, ME-SFO (80 wt.%) + NdMnO₃ (20 wt.%), exhibits significantly improved cycling stability while maintaining high performance. This modification enhances the cathode/electrolyte interfacial condition, further boosting fuel cell output to 1888 mW cm^-2 at 700 °C. Coupling NdMnO₃ with ME-SFO represents a “one stone, two birds” strategy, simultaneously improving both power output and cycling stability. This advancement positions ME-SFO as a highly competitive cathode material for H-SOFCs, offering a balanced combination of electrochemical performance, durability, and cycling stability.