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

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.