Preparation and Electrochemical Properties of NiCo 2 O 4 Nanospinels Supported on Graphene Derivatives as Earth-Abundant Oxygen Bifunctional Catalysts.

This work reports on the facile synthesis and characterisation of a non-precious-metal bifunctional catalyst for oxygen reduction and evolution reactions (ORR and OER). A few-layer reduced graphene oxide-supported NiCo2 O4 catalyst is prepared using a rapid and easy two-step method of synthesis. It consists of the solvothermal poyl(vinylpyrrolidone)-assisted assembly of metal complexes onto few-layer graphene followed by a calcination step aiming at converting metal complexes into the spinel phase. Using this synthesis approach, the most active material demonstrates an outstanding activity towards the OER and ORR, making it one of the best bifunctional catalysts of these reactions ever reported. This composite catalyst exhibits improved bifunctional behaviour with a low reversibility criterion of 746 mV. The ORR process follows a four-electron pathway and the hydroxyl selectivity is higher than those with pure reduced graphene oxide or NiCo2 O4 materials, showing the synergistic effect between the two phases. Moreover, the high activity of this composite catalyst is confirmed by comparing its performance with those obtained on other cobaltite catalysts prepared using a different synthesis method, or those obtained using a different graphene-based support.