Reduced Graphene Oxide Supported Nickel–Manganese–Cobalt Spinel Ternary Oxide Nanocomposites and Their Chemically Converted Sulfide Nanocomposites as Efficient Electrocatalysts for Alkaline Water Splitting

The electrolysis of water provides a powerful pathway for the storage and conversion of clean and renewable energy. Therefore, the development of earth-abundant, inexpensive, highly efficient electrocatalysts contributes a great deal to the overall efficiency of a water electrolytic system. Here, inspired by the low charge transfer resistance of mixed-valence cations, the favorable H atom binding energy of cobalt, and high electrical conductivity of graphene, we report a facile synthesis strategy to synthesize a spinel ternary oxide material consisting of nickel, manganese, and cobalt supported on reduced graphene oxide (rGO/NMC) with further conversion into a spinel ternary sulfide via a gaseous sulfurization protocol. The rGO/NMC-312 oxide material is found to be an efficient OER electrocatalyst with an overpotential as low as 320 mV for a current density of 10 mA cm–2, which is comparable to that of the state of the art OER catalysts. In addition, when used as HER electrocatalysts, the as-converted rGO/NMC-312 sulfide materials exhibit a low overpotential of 151 mV to reach a current density of 10 mA cm–2, a small Tafel slope of 52 mV/decade, and a remarkable long-term stability. Impressively, a voltage of 1.56 V is required to achieve a current density of 20 mA cm–2 in an alkaline medium at room temperature by applying rGO/NMC-312 oxide and sulfide as an alkaline water electrolysis anode and cathode, respectively. Our work offers a strategy to apply spinel ternary oxides and sulfides as electrocatalysts in water electrolysis.