Multi-element doped SrFeO₃-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 developed a multielement 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 has a remarkable synergistic effect that substantially enhances both proton and oxygen transport kinetics. Compared with the SrFe_0.9X_0.1O₃ cathode, the ME-SFO cathode has superior reaction kinetics, 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 incorporated the negative thermal expansion oxide NdMnO₃ into the cathode, effectively counterbalancing the thermal expansion of ME-SFO. The optimized composition, ME-SFO (80 wt%) + NdMnO₃ (20 wt%), significantly improved cycling stability while maintaining high performance. This modification enhances the cathode/electrolyte interfacial condition, further increasing the 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 the 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.