Various alpha-MnO2 nanostructures have been successfully synthesized by a simple hydrothermal method based on the redox reactions between the MnO4- and H2O in mixture containing KMnO4 and HNO3. The effect of varying the hydrothermal time to synthesize MnO2 nanostructures and the forming mechanism of alpha-MnO2 nanorods were investigated by using XRD, SEM and TEM. The results revealed an evolvement of morphologies ranging from brushy spherical morphology to nanorods depending upon the hydrothermal time. The surface area of the synthesized nanomaterials varied from 89 to 119 m(2)/g. Electrochemical properties of the products were evaluated using cyclic voltammetry and galvanostatic charge-discharge studies, and the sample obtained by hydrothermal reaction for 6 h at 120 degrees C showed maximum capacitance with a value of 152 F/g. In addition, long cycle life and excellent stability of the material were also demonstrated. (C) 2009 Elsevier Ltd. All rights reserved.