Tuning d Orbital of Ni Single Atom by Encapsulating Ni Nanoparticle in Carbon Nanotube for Efficient Oxygen Evolution Reaction

Single atom catalysts (SACs) have received considerable attention due to their high-atomic-utilization efficiency and tunable activity and selectivity. Here, in combination of experiments and calculations, we demonstrated that the electronic structures and the oxygen evolution reaction (OER) activity of the confined Ni SAC in a nitrogen-doped carbon nanotube are modulated by the encapsulated Ni nanoparticle (Ni@NiNCNT). The synergistic interaction between Ni SAC and Ni nanoparticle endows the Ni@NiNCNT with a satisfactory OER performance of 358 mV to achieve 10 mA cm–2 current density and a Tafel slope of 89 mV dec–1, superior to the control samples and commercial RuO2. In addition, when employed as an air-cathode catalyst for rechargeable zinc–air batteries (ZABs), a Ni@NiNCNT modified battery outperformed a Pt/C+RuO2 modified battery, with a higher power density and superior constant current charge–discharge cycle stability for 40 h. Theoretical simulations further revealed that the Ni nanoparticle can remarkably optimize the adsorption strength of oxygen atom on Ni SAC, leading to a small overpotential of 0.22 V for the rate-limiting step of *O formation. Furthermore, the charge transfer from Ni nanoparticle to Ni SAC, which handles Ni-d orbital characters of Ni SAC and accordingly the adsorption strength toward oxygenates, is responsible for the origin of the OER activity. Our results provide a new way to tune electronic structures of the SAC and thus to tune its catalytic activity and should be insightful for designing new type electrocatalysts based on SAC.