One-step hydrothermal synthesis of NiCo2S4–rGO as an efficient electrocatalyst for the oxygen reduction reaction

Abstract

In this work, for the first time, we have developed a one-step hydrothermal method to synthesize a hybrid material consisting of NiCo₂S₄ nanocrystals grown on reduced graphene oxide as an efficient nonprecious electrocatalyst for the oxygen reduction reaction (ORR) in alkaline medium. Our synthetic process here is quite simple, straightforward and environmentally benign. NiCo₂S₄–rGO shows significantly enhanced catalytic activity over rGO and NiCo₂O₄–rGO, and close reduction activity but much superior methanol tolerance and better durability than the commercial Pt/C catalyst. The half wave potential (E_1/2) for the NiCo₂S₄–rGO hybrid is only about 62 mV more negative than that of the commercial Pt/C catalyst but 81 mV more positive than that of NiCo₂O₄–rGO, 116 mV than that of rGO. The superior performance of NiCo₂S₄–rGO is presumably attributed to a combination effect of mixed valence in transition metal composites favorable for O₂ to be absorbed/reduced and a synergetic effect resulting from NiCo₂S₄ and rGO. The advantages over NiCo₂O₄–rGO might be ascribed to the inverse spinel crystal structure of NiCo₂S₄, its relative higher conductivity, and the fact that Na₂S serves both as the S source and reducing agent facilitating an increase in the hybrid catalytic activity.