Nanotubes of NiCo 2 S 4 /Co 9 S 8 Heterostructure: Efficient Hydrogen Evolution Catalyst in Alkaline Medium.

The most important issue in water splitting is the development of efficient, abundant, and cost-effective hydrogen and oxygen evolution catalysts. The development of an efficient electrocatalyst for the hydrogen evolution reaction (HER) under alkaline conditions is described here following a simple hydrothermal route. Here, a method for the synthesis of NiCo2 S4 /Co9 S8 , Co9 S8 , and NiCo2 S4 nanotubes has been developed. The NiCo2 S4 /Co9 S8 heterostructure has been introduced as an efficient electrocatalyst towards HER under alkaline conditions (1.0 m KOH). The vertically aligned nanotube heterostructure (NiCo2 S4 /Co9 S8 ) shows the most efficient activity as compared to bare Co9 S8 and NiCo2 S4 nanotubes. The heterostructure of NiCo2 S4 and Co9 S8 shows a significant anodic shift in the onset potential compared to the bare counterpart. NiCo2 S4 /Co9 S8 can generate a current density of 10 mA cm-2 upon application of only -0.172 V vs. RHE, whereas Co9 S8 and NiCo2 S4 require -0.293 V and -0.239 V vs. RHE, respectively. The heterostructure formation and the nanotube morphology of Co9 S8 and NiCo2 S4 facilitates a fast charge transportation which results in higher electrocatalytic activity. The hydrogen gas evolution rate of the NiCo2 S4 /Co9 S8 heterostructure was determined to be 2.29 μmol min-1 .