An aqueous asymmetric electrochemical capacitor was developed using manganese dioxide / multiwalled carbon nanotubes (MnO2/MWCNT) nanocomposites. The nanostructured MnO2 was generated from micron-sized commercial electrolytic manganese dioxide (EMD) via a hydrothermal reaction, with the addition of the surfactant sodium dodecylsulphate (SDS), and acid treatment. The capacitance of the nano-sized MnO2 was improved. The MnO2/MWCNT hybrid showed excellent specific capacitance (170 F g(-1)), energy density (18 Wh kg(-1)), power density (4 kW kg(-1)), satisfactory cycle life, and a high frequency response (8 kHz).