Understanding the Role of Nanoscale Heterointerfaces in Core/Shell Structures for Water Splitting: Covalent Bonding Interaction Boosts the Activity of Binary Transition-Metal Sulfides

The appropriate catalyst model with a precisely designed interface is highly desirable for revealing the real active site at the atomic level. Herein, we report a proof-of-concept strategy for creating an exposed and embedding interface model by constructing a unique Co₉S₈ core with a full WS₂ shell (Co₉S₈/FWS₂) and a half WS₂ shell (Co₉S₈/HWS₂) to uncover the synergistic effect of heterointerfaces on the catalytic performances. Tailoring the heteroepitaxial growth of WS₂ shell, Co₉S₈/HWS₂ with exposed Co-S-W interfaces leads to the exceptional electron density changes on edged-S atoms with large amounts of lone-pair electrons. Meanwhile, the unique Co₉S₈/HWS₂ could accelerate the kinetic adsorption of hydrogen- and oxygen-containing intermediates. Such Co₉S₈/HWS₂ electrocatalysts show extremely low overpotentials of 78 and 290 mV at a current density of 10 mA cm^-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction, respectively. Using Co₉S₈/HWS₂ as both the cathode and anode, an alkali electrolyzer delivers a current density of 10 mA cm^-2 at a quite low cell voltage of 1.60 V. The results of both operando Raman spectroscopy and electron spin resonance indicate the presence of S-S terminal and S-S bridging with unsaturated S atoms during the HER process. The present work reveals the synergistic effects of nanoscale interfaces on overall electrocatalytic water splitting.