Enhancing chloride evolution kinetics on Fe-Co₃O₄ by engineering spin state transitions

The chlorine evolution reaction (CER) is pivotal for advancing the chlor-alkali industry, which is limited by the sluggish kinetics. Transition metal catalysts offer highly tunable electronic structures, where regulating the spin state of metal centers is crucial but challenging for enhancing CER performance. Herein, Fe-doped spinel oxides (Fe-Co₃O₄) were synthesized via a simple sol-gel method. The strategic incorporation of Fe ions introduces geometric confinement, which modulates octahedral distortion at the surface and thereby regulating the spin state of Co centers, ultimately optimizing CER activity and stability. Combined experimental and theoretical analyses reveal that Fe doping triggers a spin-state transition in Co ions from high-spin towards intermediate-spin or low-spin configurations. The enhanced electronic interaction between Co and Fe 3d orbitals in Fe-Co₃O₄ leads to superior CER selectivity compared to pure Co₃O₄, even under unfavorable conditions such as high pH and lower Cl^- concentrations. Furthermore, Fe-Co₃O₄ nanocubes exhibit excellent durability, maintaining stable performance for over 50 h, which is significantly better than that of Co₃O₄. Theoretical calculations confirm that octahedral Co and Fe sites in Fe-Co₃O₄ serve as the active centers for chloride ion release, in contrast to the tetrahedral Co sites in pristine Co₃O₄. The spin state modulation of Co not only facilitates chloride ion adsorption but also optimizes the adsorption energetics, elucidating the fundamental mechanism behind the enhanced CER activity. This work highlights the critical role of spin-state regulation at active sites and proposes a chitosan gel-mediated cross-linking strategy for fabricating high-performance CER catalysts.