S-Edge-rich MoxSy arrays vertically grown on carbon aerogels as superior bifunctional HER/OER electrocatalysts.

Molybdenum disulfide (MoS2) is a potential earth-abundant electrocatalyst for the hydrogen evolution reaction (HER), but the lack of in-depth understanding of its intrinsic activity still impedes the further optimization and design of MoS2-based electrocatalysts. Herein, we report a facile in situ hydrothermal synthetic method to prepare vertical MoxSy arrays grown on guar gum-derived carbon aerogels (GCA), termed MoxSy@GCA. The obtained well-assembled MoxSy@GCA architectures consist of uniform, few-layered and S-edge-rich MoxSy nanoflakes with a length of approximately 100 nm, which effectively prevent the inherent stacking among MoxSy layers and connect the charge transfer path between interlayers, thus endowing MoxSy@GCA with a huge number of active sites and high conductivity. Benefitting from all these advantages, the optimal Mo4S16@GCA exhibited extraordinary HER/OER performances, including a low onset potential for both the HER (24.28 mV) and OER (1.53 V), and a low overpotential at 10 mA cm-2 for the HER (54.13 mV) and OER (370 mV), which are both extremely close to that of the noble Pt/C. Furthermore, a series of operando Raman spectroscopy measurements on Mo4S16@GCA were conducted to identify the intrinsic HER/OER-active sites during the HER and OER process. The results show that the S-H bond is generated simultaneously as HER/OER excitation, indicating the rich S-edge may be the intrinsic active site, which will accelerate the HER/OER kinetic process. Density functional theory (DFT) calculations revealed that the observed superb HER/OER activity can be attributed to the synergistic effect of rich S-edge of MoxSy and confinement effect of GCA, which collaboratively promote the proton adsorption and electrocatalytic kinetics. Reasonably, this study will have profound guiding value for the rational tailoring of the microstructure and size of transition metal electrocatalysts via hierarchical porous carbon aerogels.