Asymmetrical supercapacitors with aqueous electrolytes were fabricated from carbon nanotubes (CNTs) individually coated with SnO2 (CNTs/SnO2) and MnO2 (CNTs/MnO2) as the negative and positive electrodes, respectively. The CNTs/SnO2 nanocomposite is used as the negative electrode material in an asymmetrical supercapacitor. The physicochemical properties of the CNTs/SnO2 and CNTs/MnO2 nanocomposites were examined by X-ray diffraction, scanning and transmission electron microscopy, cyclic voltammetry, and galvanostatic charge-discharge. Individually, the supercapacitors were tested for charge and discharge to a cell voltage of 1.70 V in 2.0 M KCl without noticeable water decomposition. The asymmetrical cell could reach the specific energy of 20.3 Wh/kg, which is comparable to that obtained from electric double-layer supercapacitors using organic electrolytes (17-18 Wh/kg). The maximum specific power of the cell, 143.7 kW/kg, is perhaps the highest among all reported aqueous asymmetrical supercapacitors. It also shows an exceptional stability of over 1000 cycles, with the capacity loss being less than 8%. A 10 V stack was also constructed with nine individual supercapacitors connected through bipolar electrodes of the nanocomposites and porous separators containing 1.0 M Na2SO4. The stack exhibited remarkable capacitive behavior resulting from the individual cells. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3205482] All rights reserved.