Morphology evolution of spinel NiCo2O4 catalyst and its crystal plane effect in oxygen evolution reaction

Abstract

The crystal plane effect of cobalt-based oxide has garnered much attention in the oxygen evolution reaction (OER) and other electrocatalytic applications. Herein, we synthesized octahedron, truncated octahedron, and cubic NiCo₂O₄ with successively exposing increased area ratio of (100)/(111) planes regulated by the NaOH adding amount, further explored their OER behaviors. The nano-sized NiCo₂O₄-5 truncated octahedron was prepared under 5 ml NaOH exposing a relatively large and clear (111) plane area due to good crystallinity. Density Functional Theory (DFT) calculations reveal that the (111) plane features higher dangling bond density, smaller d-band centers, and stronger M − O (M = Ni, Co) covalency compared to the (100) plane, resulting in enhanced electron transfer and adsorption capabilities. These advantages endow NiCo₂O₄-5 with favorable dynamic behaviors and adsorption capacity during the alkaline OER process at room temperature, that the NiCo₂O₄-5/NF displays a low overpotential of 338.4 mV at 100 mA/cm² and long-term stability of 25 h. This work reveals the crystal planes evolution mechanism of spinel NiCo₂O₄ crystal, and the crystal plane effect of NiCo₂O₄ oxide as a catalyst on OER performance promotion.