Positive carbon electrodes (mass = 3.0 mg) were charged in a two-electrode cell with oversized counterelectrode (mass = 34.0 mg) by voltage sweep (2.0 mV s -1 ) to a cell voltage of 1.0 V. After charging, the positive electrodes were taken out, washed for 1 min with deionized water and placed for 3 h in three different solutions: