A high-energy aqueous asymmetric electrochemical capacitor was developed using manganese dioxide (alpha-MnO2)/graphene oxide (GO) nanocomposites. The nanostructured alpha-MnO2 was prepared from micron-sized commercial electrolytic manganese dioxide (EMD) via a hydrothermal reaction in the presence and absence of sodium dodecylsulphate (SUS), alpha-MnO2(SDS) and alpha-MnO2, respectively. Unlike the as-prepared alpha-MnO2, the presence of SDS during the hydrothermal reaction conferred different morphologies on the intermediate precursors for the alpha-MnO2(SDS). Also, the XRD patterns showed that the alpha-MnO2(SDS) are more crystalline than the as-prepared alpha-MnO2. The superior electrochemical performance of the alpha-MnO2(SDS)/GO composite (280 F g(-1), 35 Wh kg(-1), and 7.5 kW kg(-1) at 0.5 A g(-1)) coupled with excellent long cycle life clearly indicates that this electrode system has the potential of being developed as an efficient aqueous asymmetric electrochemical capacitor. The high performance of the alpha-MnO2(SDS)/GO composite was interpreted in terms of the enhanced crystallinity of the alpha-MnO2(SDS). Interestingly, the electrochemical performance is comparable to or even better than those reported for the more conductive graphene/MnO2 composites. (C) 2013 Elsevier Ltd. All rights reserved.