Insight into the Superior Activity of Bridging Sulfur-Rich Amorphous Molybdenum Sulfide for Electrochemical Hydrogen Evolution Reaction

Abstract Hydrogen evolution reaction (HER) activity of the molybdenum sulfide-based electrocatalysts originates from diverse types of active sulfur (S) sites, different S configurations, the ratio of molybdenum to S, and the crystallinity. The bridging S 2 2− has been considered as the major active site for HER. However, to realize more efficient electrocatalysts, newer architectures based on molybdenum sulfide are required to allow more active S sites beyond the endemic structural limits. Hence, the facile aging approach is used to maximize the bridging S 2 2− in amorphous molybdenum sulfide by adding (NH 4 ) 2 S x , leading to the highest proportion of bridging S 2 2− (up to 67%). Additionally, the effect of S configuration on HER activity in molybdenum sulfide is systematically studied by controlling of the amount of bridging S 2 2− . The bridging S-rich electrocatalysts exhibits an excellent HER activity with low onset potential of −96 mV and Tafel slope of 46 mV dec −1 , and stability for 1000 cycles.