High-Efficiency Oxygen Reduction to Hydrogen Peroxide Catalyzed by Nickel Single-Atom Catalysts with Tetradentate N2O2 Coordination in a Three-Phase Flow Cell

Carbon-supported Ni-II single-atom catalysts with a tetradentate Ni-N2O2 coordination formed by a Schiff base ligand-mediated pyrolysis strategy are presented. A Ni-II complex of the Schiff base ligand (R,R)-(-)-N,N '-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine was adsorbed onto a carbon black support, followed by pyrolysis of the modified carbon material at 300 degrees C in Ar. The Ni-N2O2/C catalyst showed excellent performance for the electrocatalytic reduction of O-2 to H2O2 through a two-electron transfer process in alkaline conditions, with a H2O2 selectivity of 96 %. At a current density of 70 mA cm(-2), a H2O2 production rate of 5.9 mol g(cat.)(-1) h(-1) was achieved using a three-phase flow cell, with good catalyst stability maintained over 8 h of testing. The Ni-N2O2/C catalyst could electrocatalytically reduce O-2 in air to H2O2 at a high current density, still affording a high H2O2 selectivity (>90 %). A precise Ni-N2O2 coordination was key to the performance.