External-Shell Oxygen Enabling the Local Environment Modulation of Unsaturated NbN 3 for Efficient Electrosynthesis of Hydrogen Peroxide.

Single-atom catalysts with a tunable coordination structure have shown grand potential in flexibly altering the selectivity of oxygen reduction reaction (ORR) toward the desired pathway. However, rationally mediating the ORR pathway by modulating the local coordination number of the single-metal sites is still challenging. Herein, we prepare the Nb single-atom catalysts (SACs) with an external-shell oxygen-modulated unsaturated NbN 3 site in carbon nitride and the NbN 4 site anchored in nitrogen-doped carbon carriers, respectively. Compared with typical NbN 4 moieties for 4e - ORR, the as-prepared NbN 3 SACs exhibit excellent 2e - ORR activity in 0.1 M KOH, with the onset overpotential close to zero (9 mV) and the H 2 O 2 selectivity above 95%, making it one of the state-of-the-art catalysts in the electrosynthesis of hydrogen peroxide. Density functional theory (DFT) theoretical calculations indicate the unsaturated Nb-N 3 moieties and the adjacent oxygen groups optimize the interface bond strength of pivotal intermediates (OOH*) for producing H 2 O 2 , thus accelerating the 2e - ORR pathway. Our findings may provide a novel platform for developing SACs with high activity and tunable selectivity.