5.98 mA cm -2 ; Fig. ) of Pt NPs/CB(1.0 wt% Pt). Additionally, Pt17/CB(1.0 wt% Pt) provided Tafel slopes similar with theoretical one for the Volmer step of 120 mV/dec assuming that H + adsorption on the catalyst surface serves as the rate determining step for the two-step HER. The reduction potential at which a current of -10 mA cm -2 is obtained, which is commonly used for HER activity comparisons, is -0.075 V vs. RHE for Pt17/CB(1.0 wt% Pt), indicating a significantly decreased overvoltage for the HER compared with -0.103 V vs. RHE for Pt NPs/CB(1.0 wt% Pt). Furthermore, the obtained mass activity of Pt17/CB(1.0 wt% Pt) at -0.05 V vs. RHE is significantly higher than the HER mass activities reported in previous experimental studies using Pt NCs or single-atom Pt catalysts (Table ).