Sulfur Vacancy-Rich Carbonaceous Co9S8-ZnS Nanotubes for the Oxygen Evolution Reaction

Metal sulfide electrocatalysts with high activity toward the oxygen evolution reaction (OER) are crucial for renewable energy technologies. However, it remains challenging to rationally design and synthesize metal sulfides integrated with high conductivity and rich porosity to achieve a superior activity. Herein, we report a brand-new, carbonaceous Co 9 S 8 -ZnS nanotube (Co 9 S 8 -ZnS/NTC) electrocatalyst synthesized via a two-step procedure including zinc-trimesic acid (ZnBTC) fiber nanocrystallization and its assembling with ZIF-67 before solid-state transformation (including sulfuration, gas-phase ion exchange, and carbonization). It is found that rich sulfur vacancies (point defect) and a hollow cavity (3-dimensional defect) are integrated into the resulting carbonaceous Co 9 S 8 -ZnS nanotube, originated from the non-equilibrium interdiffusion, which could facilitate electron transfer and OH – transport during the oxygen evolution. As expected, the designed Co 9 S 8 -ZnS/NTC delivers a low overpotential of 290 mV, a Tafel slope of 69 mV –1, an electrical resistance of 44 Ω for OER at 10 mA cm –2 in alkaline media, and a high electrochemically active surface area and turnover frequency of 12.2 mF cm –2 and 0.70 O 2 s –1, respectively, at 1.50 V, superior to single-component electrocatalysts of Co 9 S 8 and ZnS anchored on N-doped carbon. Density functional theory calculation demonstrates that the sulfur vacancy in Co 9 S 8 -ZnS/NTC delivers the decreased theoretical overpotential (1.29 V) and the enhanced activity of its neighboring Co sites, which was also beneficial to OER kinetics. Sulfur vacancy reparation results in a much lower electrocatalytic activity (overpotential, 465 mV) for Co 9 S 8 -ZnS/NTC, indicative of its critical role in OER. The concept demonstrated in this study paves the avenue to design other high-performance non-noble electrocatalysts for OER.