O─O Radical Coupling in Ultrathin Reconstructed Co6.8Se8 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 Co6.8Se8 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 Co6.8Se8 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 Co6.8Se8 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 Co6.8Se8 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 Co6.8Se8 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.