We have investigated the degradation behavior of a typical asymmetric supercapacitor cell operating in an aqueous solution of Na2SO4 with a voltage window of 2 V, where alpha-MnO2 and activated carbon (AC) were chosen as the active materials for positive and negative electrodes, respectively. In this study, potentials of alpha-MnO2 and AC electrodes of the asymmetric cell were monitored with respect to an Ag/AgCl reference electrode during repetitive charge/discharge cycling. Since the charges passed at positive and negative electrodes are the same, a change in a certain parameter should be compensated by changes in the other parameters. An irreversible current due to reduction of water on AC electrode triggered capacitance fade in the initial charge/discharge cycling. This phenomenon made the lowest potential more positive to be deviated from the potential region where water reduction can occur, resulting in the following stable operation. Then, alpha-MnO2 facilitated the oxidation of water, which deactivated alpha-MnO2 itself probably due to an irreversible increase in reduced Mn sites. The deactivation of alpha-MnO2 was compensated by expansion of its potential window in the cathodic scan, leading to further capacitance fade due to dissolution. (C) 2015 The Electrochemical Society.