In situ Surface Chemistry Engineering of Cobalt-Sulfide Nanosheets for Improved Oxygen Evolution Activity

Large surface area and unique surface chemical environment are two critical factors for a catalyst, both of which can highly influence the catalytic activity. In this study, we present a small molecular amine mediated one-step approach to directly synthesize thin cobalt sulfide nanosheets by employing sulfur powder and CoCl2 as the reaction precursor. The surface chemical environment including cobalt oxidation state of the obtained cobalt sulfide materials can be in situ finely tuned by sulfur dosage. The as-prepared and optimized cobalt sulfide nanosheets with thickness of 5–12 nm have a favorable surface chemical environment including amounts of highly active Co(III) species, easy transform of Co(II) to Co(III), richness of defects, and higher electrochemical surface area, which induce enhanced catalytic activity for oxygen evolution reaction (OER) in alkaline media. When used as an electrocatalyst, the optimized cobalt sulfide catalyst offers an overpotential of 312 mV at a current density of 10 mA cm–2 and mass loading of 0.37–0.45 mg cm–2, which can remain constant for at least 36 h of OER operation in 1 M KOH electrolyte. This electrocatalytic activity is better than most of reported monometallic cobalt sulfide materials and commercial RuO2.