Although the theoretical capacitance of MnO2 is 1370 F g-1 based on the Mn3+/Mn4+ redox couple, most of the reported capacitances in literature are far below the theoretical value even when the material goes to nanoscale. To understand this discrepancy, in this work, the electrochemical behavior and charge storage mechanism of K+-inserted α-MnO2 (or KxMnO2) nanorod arrays in broad potential windows are investigated. It is found that electrochemical behavior of KxMnO2 is highly dependent on the potential window. During cyclic voltammetry cycling in a broad potential window, K+ ions can be replaced by Na+ ions, which determines the pseudocapacitance of the electrode. The K+ or Na+ ions cannot be fully extracted when the upper cutoff potential is less than 1 V vs Ag/AgCl, which retards the release of full capacitance. As the cyclic voltammetry potential window is extended to 0-1.2 V, enhanced specific capacitance can be obtained with the emerging of new redox peaks. In contrast, the K+-free α-MnO2 nanorod arrays show no redox peaks in the same potential window together with much lower specific capacitance. This work provides new insights on understanding the charge storage mechanism of MnO2 and new strategy to further improve the specific capacitance of MnO2-based electrodes.
Keywords: MnO2; charge storage mechanism; nanorod; pseudocapacitance; supercapacitors.