Size and Synergy Effects of Ultrafine 2.6 nm CoNi Nanoparticles Within 3D Crisscross N‐Doped Porous Carbon Nanosheets for Efficient Water Splitting

Abstract A non‐precious metal‐based catalyst for water electrolysis provides great promise for cost‐effective and highly efficient sustainable hydrogen production. It herein rationally synthesizes uniform superminiature CoNi nanoparticles (2.6 nm) embedded in 3D N‐doped randomly oriented and erected porous carbon nanosheets (CoNi@N‐PCNS). Taking advantage of the large specific surface area, expedited intermediate transport, and effectively exposed active sites of the hierarchical architecture, located CoNi nanoparticles yield a high atom utilization efficiency. Density functional theory calculations indicate that synergetic and cooperative interactions inside CoNi alloy modulate the d‐band center, leading to a moderate adsorption and desorption energy of reaction intermediates, further accelerating both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) kinetics. Accordingly, the as‐synthesized CoNi@N‐PCNS catalyst establishes superb catalytic activities for HER and OER, revealing overpotentials of 71.2 and 263.8 mV at 10 mA cm⁻², respectively. Remarkably, when assembled as a two‐electrode electrolyzer, a satisfying cell voltage of 1.59 V at 10 mA cm⁻², and superior stability are demonstrated, highlighting great promise toward water electrolysis.