Energetics and pathways of proton transport in CaFeO3: A first-principles study

Proton-conducting solid oxide fuel cells (P-SOFC) represent one of the most promising energy conversion technologies due to their lower operating temperatures and reduced costs. However, existing electrolytes struggle to achieve high conductivity. To address this limitation, a novel hydrogen incorporation strategy leveraging the multivalent characteristics of transition metals has recently been reported. As one of the candidate perovskites with transition metal on the B site, CaFeO₃ shows potential for the electrolyte of P-SOFC. Herein, we systematically investigate the properties of CaFeO₃ by first-principles calculation and find that it possesses ferromagnetic ground state, energetic and chemical stability, as well as high-concentration hydrogen incorporation due to the charge transfer from H to Fe. The phase HCaFeO₃ is thermodynamically stable with semiconductor nature which can suppress electronic conductivity. Seven possible proton migration pathways involving proton transfer and rotation are subsequently identified and rigorously compared, enabling the design of a viable long-range proton migration trajectory with maximum energy barrier of 0.35 eV. This maximum barrier belongs to the proton rotation process, contradicting the conventional understanding that proton transfer is the rate-limiting step. Meanwhile, the magnitude of lattice distortion is identified as the primary factor governing proton migration energy barriers. Our findings not only demonstrate the significant potential of CaFeO₃ as a high-performance P-SOFC electrolyte, but also provide critical design principles for next-generation electrolyte materials for P-SOFC applications.