Unless otherwise specified, all the measurements were carried out in a 3- electrode configuration (disk working/reference/counter electrodes) in air- saturated 0.1 M KOH aqueous electrolytic solution (Potassium hydroxide concentrate, 0.1 M KOH in water (0.1 N), Supelco, product number 61699- 1L) with 2500 RPM rotation speed, and all potentials refer to the RHE.