RDE tests were performed in O2 saturated 0.1 mol/L KOH solution with a scan rate of 10 mV/s between 1.1 V and 0.2 V at different rotating rates using PINE 636 rotating-disk electrode system and CHI650 workstation. Ag/AgCl and Pt were used as reference and counter electrodes, respectively. All potentials were converted to the reversible hydrogen electrode (RHE).