Methylene blue (MB+) (BDH, England), KCl and KNO3 (BDH, England), were used without further purification in our experiments. All of the solutions were prepared using Millipore Milli-Q water (resistivity > 18.2 MΩ−1) and total organics was lower than 3 ppb. An electrochemical workstation (CH Instrument series) was used to carry out all the electrochemical measurements. All the experiments were conducted at room temperature (∼23 °C) and under N2-atmosphere in a conventional three-electrode cell.