Partial Surface Selenization of Cobalt Sulfide Microspheres for Enhancing the Hydrogen Evolution Reaction

Electrocatalysis of water is a scalable and easily available source of the production of hydrogen (H ₂ ), the future energy carrier. This drive for clean energy inspired us to develop an inexpensive, readily producible, highly active, and stable catalyst to replace current state of the art platinum catalysts. Building on the promising hydrogen evolution reaction (HER) activity of many pyrites, their structural tuning by different metals and nonmetals has been found to be effective in several instances. We present here one such effort by partial surface selenization of mesoporous cobalt sulfide material, which displayed long-term operational stability (for at least 25 h) besides attaining a current density of 100 mA cm ^-2 at an overpotential of 160 mV versus the reversible hydrogen electrode (RHE) (in acidic media). A low Tafel slope (of 52 mV dec ^-1 ) and high exchange current density (j ₀ ) (of 70 μA cm ^-2 ) make our catalyst better to most existing systems. More importantly, using a variety of analytical techniques, electrochemical measurements, and theoretical calculations, we have analyzed the morphology of the material and rationalized the key to the enhanced intrinsic activity (as compared to the meso-CoS ₂ ) per active site. This study is expected to explain similar systems and modify approaches to enhancing the electrochemical activity of metal chalcogenides.