Powdery Mn3O4 and Mn2O3 electrodes with carbon and binding polymer were electrochemically stimulated and activated by successive potential cycles in a mild aqueous electrolyte containing alkali sulfate. The activation of the manganese oxides is affected by the electrode material, milling treatment, potential region, and electrolyte solution. It is found that the ball-milled Mn3O4 electrode demonstrated the highest specific capacitance, 190Fg−1, in 1moldm−3 Na2SO4 aqueous solution due to the phase transition from Mn3O4 to electrochemically active birnessite, Na y MnO2·nH2O. The increase in capacitance originated from the formation of birnessite possessing highly porous morphology. The nano-structured birnessite demonstrated long cycle life of about 2000 cycles with acceptable capacitance retention of 190–160Fg−1. The birnessite was applied as positive electrode of the asymmetric electrochemical capacitor with activated carbon negative electrode in the mild aqueous solution.