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.