Capacitance Catalysis: Positive and Negative Effects of Capacitance of Mo2C in Photocatalytic H-2 Evolution

A non-noble metal cocatalyst with capacitance is cost-effective and has excellent photocatalytic H2evolution performance forwater splitting. But the intrinsic action of capacitance is unclear. Here,by controlling the capacitance of Mo2C, we reported a volcanic curverelationship between the photocatalytic activity of Mo2C/CdS and thecapacitance of Mo2C. At the middle capacitance of 18.95 F/g ofMo2C-a4, Mo2C-a4/CdS exhibits the best H2evolution activity of5560 mu mol/g/h. When Pt is introduced on the surface of Mo2C, thephotocatalytic activity of (Pt/Mo2C-a4)/CdS is further enhanced to8090 mu mol/g/h. Comparing the activities between (Pt/Mo2C)/CdSand Mo2C/CdS with the capacitance of Mo2C, the activity ratio anddifference values increase andfinally reach a plateau with theincreasing capacitance. Electrochemical and photoelectrochemicalstudies demonstrate that the capacitance of Mo2C has dual opposite effects (positive and negative) in photocatalytic reactions.The capacitance can capture electrons and store them, thereby enhancing the separation efficiency of photoexcited carriers of CdS.However, overlarge capacitance constrains the stored electrons and prevents them from releasing, thus decreasing the cocatalyticefficiency, which can be resolved in some degree by introducing some Pt on Mo2C. This work elucidates the dual effects of capacitance and puts forward the concept of capacitance catalysis