Enhanced supercapacitive behaviors of birnessite

The birnessite type manganese dioxide electrode was prepared by the electrochemical stimulation as we recently described. It showed 190 F g(-1) in a Na2SO4 aqueous solution between -0.1 and 0.9 V versus Ag/AgCl at 1 A g(-1). The specific capacitance of birnessite was decreased by the manganese dissolution when the reduction and oxidation were repeated. By adding small amounts of Na2HPO4 or NaHCO3 into the electrolyte, the capacitance increased to 200-230 F g(-1) and the manganese dissolution was successfully suppressed. Thanks to the additives, the birnessite demonstrated the much improved cycleability over > 1800 cycles. (c) 2008 Elsevier B.V. All rights reserved.