Two-dimensional Cobalt Oxy-hydrate Sulfide Nanosheets with Modified t_2g Orbital State of CoO_6–<i>x</i> Octahedron for Efficient Overall Water Splitting

Abstract

Searching for low-cost, scalable, highlly active electrocatalysts for clean energy conversion has become the main challenge presently. A two-dimensional cobalt oxy-hydrate sulfide (CoO(OH)1–xSx) nanosheet was synthesized for the first time via a facile electrochemical deposition method followed by an anion-exchange process. Owing to the modified t2g orbitals of octahedral Co–Ox, enhanced electrical conductivity, and abundant active sites, CoO(OH)1–xSx delivers high activity toward overall water splitting in alkaline media. The obtained CoO(OH)0.75S0.25 nanosheets deliver a low overpotential of 166 mV and a small Tafel slope of 93.4 mV dec–1 in HER, while the CoO(OH)0.75S0.25 catalyst delivers an extremely small overpotential of 378 mV (at 30 mA cm–2) and a suppressed Tafel slope of 106.3 mV dec–1 for oxygen evolution reaction. The X-ray absorption fine structure and first-principle calculations further demonstrate that the substitution of sulfur heteroatoms in CoOOH can result from the unoccupied t2g orbit state and hence contribute to enhanced electrical conductivity and reduce the reaction barrier, all of which favor electrocatalytic functions in water splitting. This work not only proves that the specific CoO(OH)1–xSx nanosheet is a promising catalyst for water splitting but also provides an facile route to synthesize new transition-metal oxide sulfides for electrocatalytic water splitting.