Working electrodes were mechanically polished with MicroPolish TM alumina and silica slurry suspension (0.3 µm, Buehler) on a MicroCloth polishing cloth (Buehler). The polished electrodes were then thoroughly rinsed with Milli-Q water and sonicated for 15 min. Counter and reference electrodes were rinsed with MilliQ water prior to use. Reference electrodes were calibrated with respect to the reversible hydrogen electrode, following the procedure described in reference .