Ultrasmall VMnMoS _x Clusters with Bifunctional OER/HER Performance for Highly Efficient Water Splitting

Electrochemical water splitting is a promising technology for green hydrogen production, and it is important to develop advanced catalysts for the anode of oxygenevolution reaction (OER) and cathode of hydrogen evolution reaction (HER). Herein, we demonstrate a hydrothermal sulfidation method with polyoxometalatesas precursors to obtain S-doped VMnMo-based cluster (s-VMnMoS_x) of ultrasmall size (∼0.9 nm). s-VMnMoS_x catalystshows excellent bifunctional OER/HER activity, requiring ultralowoverpotentials of 290 mV for OER and 181 mV for HER at the current density of 500 mA cm^-2. For overall water splitting applications, s-VMnMoS_x catalyst achieves a low cell voltage of 1.72 V to reach an industrial-scalecurrent density of 500 mA cm^-2, and shows negligible deactivationfor 1000 hours. Interestingly, our work demonstrates that S-doped VMnMo-based catalyst of large particle size (l-VMnMoS_x) shows poor OER/HER performance. Experimental and theoretical calculation results indicate that, bytuning the size of S-doped VMnMo-based catalysts down to the cluster regime, the primary OER active sites are changed from Mn to V, while the primary HER active sites (Mo) are not changed, and both the OER/HER energy barriers of s-VMnMoS_x are significantly lower than those of l-VMnMoSx, leading to the excellent catalytic performance of s-VMnMoS_x.