A new cheap asymmetric supercapacitor based on activated carbon (AC) and NaMnO2 as electrodes and aqueous Na2SO4 solution as electrolyte was assembled. It shows an energy density of 19.5Whkg−1 at a power density of 130Wkg−1 based on the total mass of the active electrode materials and an excellent cycling behavior. This asymmetric aqueous AC//NaMnO2 capacitor is promising for practical applications due to its low price, easy preparation of NaMnO2 and friendliness to environment.