Capacitance Catalysis: Positive and Negative Effects of Capacitance of Mo<sub>2</sub>C in Photocatalytic H<sub>2</sub> Evolution
Li Luo, Dekang Li, Yuying Dang, Wei Wang, Guangsuo Yu, Jin Li, Baojun Ma
Abstract
A non-noble metal cocatalyst with capacitance is cost-effective and has excellent photocatalytic H2 evolution performance for water splitting. But the intrinsic action of capacitance is unclear. Here, by controlling the capacitance of Mo2C, we reported a volcanic curve relationship between the photocatalytic activity of Mo2C/CdS and the capacitance of Mo2C. At the middle capacitance of 18.95 F/g of Mo2C-a4, Mo2C-a4/CdS exhibits the best H2 evolution activity of 5560 μmol/g/h. When Pt is introduced on the surface of Mo2C, the photocatalytic activity of (Pt/Mo2C-a4)/CdS is further enhanced to 8090 μmol/g/h. Comparing the activities between (Pt/Mo2C)/CdS and Mo2C/CdS with the capacitance of Mo2C, the activity ratio and difference values increase and finally reach a plateau with the increasing capacitance. Electrochemical and photoelectrochemical studies 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 cocatalytic efficiency, 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.