From Water Oxidation to Reduction: Homologous Ni–Co Based Nanowires as Complementary Water Splitting Electrocatalysts

Abstract

A homologous Ni–Co based nanowire system, consisting of both nickel cobalt oxide and nickel cobalt sulfide nanowires, is developed for efficient, complementary water splitting. The spinel-type nickel cobalt oxide (NiCo2O4) nanowires are hydrothermally synthesized and can serve as an excellent oxygen evolution reaction catalyst. Subsequent sulfurization of the NiCo2O4 nanowires leads to the formation of pyrite-type nickel cobalt sulfide (Ni0.33Co0.67S2) nanowires. Due to the 1D nanowire morphology and enhanced charge transport capability, the Ni0.33Co0.67S2 nanowires function as an efficient, stable, and robust nonnoble metal electrocatalyst for hydrogen evolution reaction (HER), substantially exceeding CoS2 or NiS2 nanostructures synthesized under similar methods. The Ni0.33Co0.67S2 nanowires exhibit low onset potential of −65, −39, and −50 mV versus reversible hydrogen electrode, Tafel slopes of 44, 68, and 118 mV dec−1 at acidic, neutral, and basic conditions, respectively, and excellent stability, comparable to the best reported non-noble metal-based HER catalysts. Furthermore, the homologous Ni0.33Co0.67S2 nanowires and NiCo2O4 nanowires are assembled into an all-nanowire based water splitting electrolyzer with a current density of 5 mA cm−2 at a voltage as 1.65 V, thus suggesting a unique homologous, earth abundant material system for water splitting.