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

Manganese oxide (MnO x) 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 MnO x polymorphs towards improvement of its capacitance remains to be thoroughly investigated. In this paper, the results from electrodeposition of mesoporous MnO x 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 MnO x did not manifest any capacitance behavior in 1.0 M Na 2 SO 4 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 Na 2 SO 4 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 × 106 Ω for the pristine MnO x electrode to 39.13 Ω for the treated one. Based on structural and surface chemistry evolution of the MnO 2 films after annealing, the electrochemical storage behavior of annealed MnO x is discussed. Image 1 • Electrodeposited mesoporous MnO 2 films were treated for rapid thermal annealing. • XPS analysis confirmed Mn2+/Mn3+ increase besides hydroxyl groups attachment. • RTA treatment decreased the charge transfer resistance from 0.678 MΩ to 39.13 Ω. • EIS indicated excellent pseudocapacitive behavior due to large electroactive area. • High areal capacitance and cyclic stability up to 5000 cycles were achieved. [ABSTRACT FROM AUTHOR]