A NiCo₂O₄ Shell on a Hollow Ni Nanorod Array Core for Water Splitting with Enhanced Electrocatalytic Performance

Abstract Nickel cobaltite (NiCo₂O₄) nanostructures have been considered as promising bifunctional electrocatalyst materials for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline electrolyte. However, the activity of these materials toward overall water splitting is still unsatisfactory because a larger voltage is needed to drive the reactions. This study shows that more‐efficient bifunctional catalysts based on NiCo₂O₄ materials can be obtained by adopting an integrated composite electrode with robust hollow structures. This novel electrode is constructed by the growth of hollow Ni@NiCo₂O₄ nanorod arrays on three‐dimensional (3D) Ni foam. The interlayer of Ni hollow nanorods is conformally covered by NiCo₂O₄ nanosheets, providing large surface area and fast electron transport. Benefiting from these integrated characteristics, the hollow Ni@NiCo₂O₄ electrode as an oxygen evolution electrocatalyst can deliver a low overpotential of 270 mV at 10 mA cm⁻² and present excellent electrochemical durability. Meanwhile, the HER performance also impressive, with an overpotential of about 87 mV at 10 mA cm⁻². As a result, the hollow Ni@NiCo₂O₄ electrode as a bifunctional catalyst can achieve a current density of 10 mA cm⁻² with a voltage of only 1.58 V, showing promise for high performance yet cheap electrocatalysts for the overall water splitting reactions.