Oxygen Vacancy-Determined Highly Efficient Oxygen Reduction in NiCo₂O₄/Hollow Carbon Spheres

Rationally generating oxygen vacancies in electrocatalysts is an important approach to modulate the electrochemical activity of a catalyst. Herein, we report a remarkable enhancement in oxygen reduction reaction (ORR) activity of NiCo₂O₄ supported on hollow carbon spheres (HCS) achieved through generating abundant oxygen vacancies within the surface lattice. This catalyst exhibits enhanced ORR activity (larger limiting current density of ∼-5.8 mA cm^-2) and higher stability (∼90% retention after 40 000 s) compared with those of NiCo₂O₄/HCS and NiCo₂O₄. The results of X-ray photoelectron spectroscopy (XPS) characterizations suggest that the introduction of oxygen vacancies optimizes the valence state of active sites. Furthermore, we carried out density functional theory (DFT) calculations to further confirm the mechanism of oxygen vacancies, and results show that oxygen vacancies enhance the density of states (DOS) near the Fermi level, decrease work function, and lower the calculated overpotential of NiCo₂O₄.