Soft X-ray absorption spectroscopic investigation of MnO2/graphene nanocomposites used in supercapacitor

• Influence of growth time on electronic structure of MRGO studied by soft-XAS. • Charging/discharging mechanism of MnO2/RGO studied by ex situ soft-XAS. • RGO involves an oxygen-functionality-related pseudocapacitance. • MRGO involves Mn(III)/Mn(IV) transition in charging/discharging. Electrochemistry and X-ray absorption spectroscopy (XAS) of MnO 2 /reduced graphene oxide (RGO) were made to evaluate capacitive performance and electronic structure as a supercapacitor. MnO 2 was deposited on the surface of RGO by a spontaneous redox reaction. At low current density (1 A g −1 ), the specific capacitance of MnO 2 /RGO (MRGO) greatly increased with increasing growth time from 281 to 462 F g −1 . Electrochemical results show that the specific capacitance of RGO and MRGO arises from the combined contributions of electrochemical double-layer capacitance (EDLC) and pseudo-capacitance. To gain insights into the charge storage mechanism of RGO and MRGO, it was characterized using ex-situ soft XAS techniques at Mn L-edge and C, O K-edges in the charging/discharging process. The ex-situ soft XAS results provide evidence that the contribution of RGO to specific capacitance involves an oxygen-functionality-related contribution of pseudocapacitance and EDLC. In addition, the specific capacitance of MRGO reveals pseudocapacitive contributions from the redox processes that involve the Mn(III)/Mn(IV) redox reaction in MnO 2 . This work utilizes electrochemical and ex-situ XAS techniques to elucidate charge storage mechanism for supercapacitor applications.