(1) Charging period corresponded to the period where electrons transferred from the fixed electrode to the flow electrode at an applied voltage of −0.45 V (vs. Ag/AgCl), and (2) the discharging period was that the electrons transferred from the flow electrode to the fixed electrode at an applied voltage of −0.10 V (vs. Ag/AgCl). The charging step and discharging step were alternately changed every 30 s.