An asymmetric aqueous electric double-layer capacitor is presented, which consists of two activated carbon electrodes, a compact Janus membrane with sulfonated polystyrene and quaternized polystyrene, aqueous acidic and alkaline electrolyte solutions. The carbon-based aqueous supercapacitor can not only run at a high cell voltage of 2.2 V, but also deliver a specific energy of 20.06 Wh kg(-1). Moreover, such a supercapacitor with specific energy of 40-50 Wh kg(-1) can be predicted after further optimization.