MnOx thin film based electrodes: Role of surface point defects and structure towards extreme enhancement in specific capacitance

Manganese oxide (MnOx) is a low cost and environment-friendly electrode material for supercapacitors. Although most efforts have been devoted to increase the surface area of this material, the role of surface defects in different MnOx polymorphs towards improvement of its capacitance remains to be thoroughly investigated. In this paper, the results from electrodeposition of mesoporous MnOx thin film electrodes and the effect of rapid thermal annealing (RTA) on their electrochemical storage behavior are presented and discussed. X-ray photoelectron spectroscopy (XPS) analyses showed an increase of the Mn2+/Mn3+ ratio as well as hydroxyl group defects on the surface of the annealed electrodes. The as-deposited MnOx did not manifest any capacitance behavior in 1.0 M Na2SO4 electrolyte solution. However, after RTA treatment, the specific capacitance was tremendously enhanced. An areal capacitance as high as 110 mF cm(-2) at 5 mV s(-1) was recorded in 1.0 M Na2SO4 electrolyte with a capacitance retention of 74% after 5000 charge-discharge cycles. Furthermore, the RTA treatment of electrodes significantly decreased the charge transfer resistance value from 0.678 x 10(6) Omega for the pristine MnOx electrode to 39.13 Omega for the treated one. Based on structural and surface chemistry evolution of the MnO2 films after annealing, the electrochemical storage behavior of annealed MnOx is discussed.