NiCo<inf>2</inf>O<inf>4</inf>/rGO hybrid nanostructures for efficient electrocatalytic oxygen evolution

We report the synthesis of NiCo2O4/reduced graphene oxide (NiCo2O4/rGO) hybrid hierarchical structures with unique nanonet and microsphere morphologies by organic polar solvent-assisted solvothermal method. The electrocatalytic oxygen evolution reaction (OER) activity of these materials is studied by cyclic voltammetry, linear sweep voltammetry and chronoamperometry methods in O2-saturated 0.1 M KOH solution. The NiCo2O4/rGO hybrid nanocomposite materials are found to be highly active electrocatalysts for OER at lower overpotentials. The nanonet and microsphere-like NiCo2O4/rGO catalysts require overpotentials of 0.450 and 0.530 V at a current density of 10 mA cm−2, and their corresponding Tafel slopes are 53 and 62 mV dec−1, which are much lower than values reported for non-precious electrocatalysts. Further, both NiCo2O4/rGO catalysts show good catalytic stability with current retention more than 92 % over long period of 15,000 s determined by chronoampirometry and at the end of 1000th cycle determined by linear sweep voltammetry. The enhanced OER activity of nanostructured NiCo2O4/rGO hybrid catalysts is attributed to synergistic interaction between rGO and NiCo2O4, which seems to be essential for maintaining the large contact area at the electrode-electrolyte interface, better mass, and charge transport and to minimize the aggregation of NiCo2O4 nanoparticles.