Stabilizing the high-spin state of Co3Se4 via interfacial electron injection during the hydrogen evolution reaction process boosts catalytic activity.

Regulating the electronic spin state of electrocatalysts is critical for enhancing the hydrogen evolution reaction (HER). Unlike multi-step catalytic reactions that require dynamic spin-state transitions, HER demands a stable, high-activity spin state throughout the catalytic process. However, maintaining this specific spin state remains challenging. The intrinsic high-spin Co3+ sites of Co3Se4 provide excellent theoretical HER activity, but intermediate adsorption during the reaction drains local electron density, altering the crystal field and causing the high-spin state to degrade. Herein, we propose an interfacial electron injection strategy to stabilize the highly active spin state by constructing a CoSe@Co3Se4 heterostructure. The built-in electric field arising from the work-function difference promotes interfacial electron redistribution from CoSe toward the Co3Se4 active sites. In situ spectroscopy and theoretical calculations reveal that interfacial electronic redistribution substantially mitigates the local electron depletion induced by intermediate adsorption. Consequently, the interfacial electronic and structural buffering provides a favorable local environment for stabilizing the high-spin Co3+ state at the catalytically relevant Co sites, which optimizes orbital interactions with intermediates and significantly lowers the energy barrier for water dissociation. Benefiting from this stabilized high-spin Co3+ state, the CoSe@Co3Se4 catalyst achieves an ultralow overpotential of 12 mV at 10 mA cm-2 for HER. This work provides a fundamental understanding of spin-state instability and offers a promising interface-engineering pathway to stabilize the highly active spin-state for advanced electrocatalysis.