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

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 (NiCo 2 O 4 ) nanowires are hydrothermally synthesized and can serve as an excellent oxygen evolution reaction catalyst. Subsequent sulfurization of the NiCo 2 O 4 nanowires leads to the formation of pyrite‐type nickel cobalt sulfide (Ni 0.33 Co 0.67 S 2 ) nanowires. Due to the 1D nanowire morphology and enhanced charge transport capability, the Ni 0.33 Co 0.67 S 2 nanowires function as an efficient, stable, and robust nonnoble metal electrocatalyst for hydrogen evolution reaction (HER), substantially exceeding CoS 2 or NiS 2 nanostructures synthesized under similar methods. The Ni 0.33 Co 0.67 S 2 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 Ni 0.33 Co 0.67 S 2 nanowires and NiCo 2 O 4 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.