O−O Radical Coupling in Ultrathin Reconstructed Co_6.8Se₈ Nanosheets for Effective Oxygen Evolution and Zinc‐Air Batteries

Designing ultrathin transition metal electrocatalysts with optimal surface chemistry state is crucial for oxygen evolution reaction (OER). However, the structure-dependent electrochemical performance and the underlying catalytic mechanisms are still not clearly distinguished. Herein, we synthesize ultrathin Co_6.8Se₈ nanosheets (NSs) with subnanometer thickness by incorporating catalytically inactive selenium (Se) into ultrathin Co(OH)₂, thereby switching the OER reaction pathway from adsorbate evolution mechanism (AEM) to oxide path mechanism (OPM). The prepared ultrathin Co_6.8Se₈ NSs exhibit an overpotential of 253 mV at 10 mA/cm², outperforming the mostly reported Co-based electrocatalysts. Advanced operando synchrotron spectroscopies and X-ray absorption spectroscopy reveal the ultrathin Co_6.8Se₈ NSs, whose surface is reconstructed into Se-doped Co(OH)₂ during the OER process, could trigger direct O*-O* radical coupling rather than OOH* intermediates within AEM pathway thus lowering the energy input. Density functional theory calculations confirm that Co_6.8Se₈ NSs with shorter Co-Co bond length and stable Co-Se bond could optimize the rate-determining step barrier via OPM pathway. Besides, rechargeable zinc-air batteries based on Co_6.8Se₈ NSs exhibit excellent stability for more than 500 h of continuous charge-discharge cycles at 4 mA/cm². The present study highlights the structural-dependent switch of OER pathways and provides valuable insights for further development of ultrathin OER catalysts.