In-situ phase formation enables graded thermal expansion and structural stability in reversible protonic ceramic electrochemical cell cathodes

Proton-conducting solid oxide fuel cells (H-SOFCs) hold great promise for sustainable energy conversion, yet their commercialization is hindered by the poor balance between electrochemical activity and long-term thermal-mechanical stability of conventional cobalt-based cathodes. In this work, we propose a self-derived secondary phase-induced thermal expansion gradient strategy to overcome this dilemma. Taking La 0.6 Sr 0.4 Ce x Fe 1- x O 3- δ ( x =0, 0.2, 0.3) as a model system, we demonstrate that Ce doping thermodynamically drives the in-situ formation of La 0.6 Sr 0.4 Ce x Fe 1- x O 3- δ , La x Ce 1- x O 2 , and LaSrFeO 4 secondary phases, which are tightly coupled with the parent perovskite lattice. Owing to their distinct thermal expansion coefficients (TECs), these phases establish an intrinsic graded thermal expansion structure that effectively mitigates thermal mismatch, alleviates interfacial stress, and prevents delamination during thermal cycling. Microstructural analyses confirm that the gradient buffering phases form coherent interfaces and construct multi-channel ion/electron transport networks, thereby preserving structural integrity. Electrochemical measurements reveal that La 0.6 Sr 0.4 Ce 0.2 Fe 0.8 O 3- δ exhibits significantly reduced polarization resistance and accelerated oxygen reduction reaction (ORR) kinetics compared with undoped La 0.6 Sr 0.4 FeO 3- δ . The optimized composition delivers an ohmic resistance of 0.06 Ω cm 2 at 700 °C and a peak power density of 1.73 W cm -2 at 700 °C, highlighting simultaneous enhancements in activity and durability enabled by the secondary-phase-engineered TEC gradient.