For a faradaic electrochemical capacitor, bimessite-based manganese dioxide electrode, prepared by the electrochemical activation of ball-milled Mn3O4 composite electrodes at we recently reported, delivers 190 F g(-1) in a Na2SO4 aqueous solution in the potential range of -0.1 and 0.9 V vs. Ag/AgCl at a current rate of 1.0 A g(-1). However, the specific capacitance of birnessite gradually decreases during successive charge and discharge cycle, and the capacity decrease is accompanied by the gradual loss of net weight of manganese oxide because of manganese dissolution from electrode into electrolyte solution. By adding and dissolving small amounts of Na2HPO4, NaHCO3, and Na2B4O7 into the Na2SO4 solution, the capacitance of birnessite increases to 200-230 F g(-1) with much improved cyclability over >1000 cycles, resulting from suppression of the manganese dissolution because of pH buffer action by the additives. The improvement of cyclability and capacitance is due to the formation of protection layer on the birnessite and the optimized pH buffer capacity of aqueous electrolyte. (C) 2013 The ElectrOchemical Society. All rights reserved.