Lattice Distortion Driven Spin‐State Engineering in Fe‐Based Electrodes for High‐Performance Reversible Proton‐Conducting Solid Oxide Cells

Abstract Reversible proton‐conducting solid oxide cells (R‐PSOCs) require bifunctional oxygen electrocatalysts that balance high activity with cost‐effectiveness. Here, a novel strategy is reported to enhance the catalytic performance of cobalt‐free Fe‐based electrodes by introducing Zn ²⁺ into SmBaFe ₂ O _5+δ (SBF), which induces lattice distortion and drives a spin‐state transition of Fe ³⁺ from high‐spin to low‐spin states. This distortion‐driven spin‐state modulation strengthens Fe–O covalency, promotes oxygen‐vacancy ordering, and enhances proton hydration, significantly improving both oxygen reduction and evolution reaction kinetics. The optimized Zn‐modified SBF electrode achieves a peak power density of 0.95 W cm ⁻² in the PCFC mode and a current density of 1.69 A cm ⁻² in the PCEC mode at 700 °C, demonstrating excellent durability and chromium resistance over 100 h of cycling. Density functional theory (DFT) calculations confirm that the Zn‐induced spin‐state transition reduces reaction energy barriers, ensuring efficient bifunctional catalysis. These findings establish spin‐state engineering as a powerful and cost‐effective approach for designing high‐performance, cobalt‐free Fe‐based air electrodes, providing a scalable solution for next‐generation R‐PSOCs.