2.1. Electrode pre-treatment and protocol
The electrolyte, 0.1 mol L−1 NaCl (pH 6.5), was deaerated by purging with nitrogen gas for 10 min. Then the electrode was cycled 10 times between −0.6 V and 0.6 V at a scan rate of 0.1 V s−1. The baseline of the differential pulse voltammetry (DPV) response was recorded between −0.39 V and 0.6 V. After this procedure, the working electrode was removed from the electrochemical cell and dried carefully, with a white fine towel, without subsequent washing, to retain the adsorbed ions from the electrolyte. In this way on the electrode surface remain residues of NaCl.