Self-Composition of Hierarchical Core–Shell-Structured NiCo2O4@NiCo2O4 Microspheres with Oxygen Vacancies for Efficient Oxygen Evolution Electrocatalysis

The reactivity of oxygen evolution reaction (OER) electrocatalysts can be effectively controlled by the subtle design of their morphologies, composition, and surface defects. Herein, the hierarchical core–shell-structured NiCo2O4@NiCo2O4 microspheres (NCO-2) are synthesized by a "self-composition" strategy in the one-pot hydrothermal process followed by calcination. The unique nanoarchitecture of NCO-2 has NiCo2O4 solid microspheres as the core and ultrathin porous NiCo2O4 nanosheets as the shell, acting synergistically with the surface oxygen vacancies, which can afford more efficient active sites and shorten the transportation/diffusion path for electrons and electrolyte ions, thus evidently optimizing the OER kinetics. Consequently, the obtained NCO-2 exhibits significant electrocatalytic activity with a low overpotential of ∼350 mV and long-term stability of 30 h at 10 mA cm–2 in a 1.0 M KOH electrolyte, which outperforms those of NiCo2O4 with diverse morphologies (solid sphere, urchin-like, and hollow structures) and commercial IrO2. This approach provides a promising route to design intricate catalyst structures for OER applications.