Hierarchical Nanoassembly of MoS2/Co9S8/Ni3S2/Ni as a Highly Efficient Electrocatalyst for Overall Water Splitting in a Wide pH Range

Abstract

The design of low-cost while high-efficiency electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in wide pH is highly challenging. We now report a hierarchical co-assembly of interacting MoS2 and Co9S8 nanosheets attached on Ni3S2 nanorod arrays which are supported on nickel foam (NF). This tiered structure endows high performance towards HER and OER over a very broad pH range. By adjusting molar ratios of Co:Mo precursors, we have created CoMoNiS-NF-xy composites (x:y means Co:Mo molar ratios ranged 5:1 to 1:3) with controllable morphology and composition. The three dimensional composites own abundant active sites capable of universal pH catalytic HER and OER activity. The CoMoNiS-NF-31 demonstrates the best electrocatalytic activity, giving ultralow overpotentials (113, 103 and 117 mV for HER) and (166, 228, and 405 mV for OER) to achieve the current density of 10 mA cm−2 in alkaline, acidic and neutral electrolyte, respectively. It also shows a remarkable balance between electrocatalytic activity and stability. Based on distinguished catalytic performance of CoMoNiS-NF-31 towards HER and OER, we demonstrate a two-electrode electrolyzer performing water electrolysis over a wide pH range, with low cell voltages of 1.54, 1.45 and 1.80 V at 10 mA cm−2 in alkaline, acidic and neutral medias, respectively. First-principles calculations suggest high OER activity arises from electron transfer from Co9S8 to MoS2 at the interface, which alters binding energies of adsorbed species and decreases overpotentials. Our results demonstrate that hierarchical metal sulfides can serve as highly efficient all-pH (pH=0-14) electrocatalysts for overall water splitting.