The HER generally proceeds through two main pathways. Initially, H + is adsorbed and reduced as Hads on the electrode surface (the Volmer step). Subsequently, the reaction can proceed through either: 1) the combination of two Hads to form H2, which then desorbs (the Volmer-Tafel mechanism), or 2) the incorporation of another H + and electron onto Hads to form H2, which then desorbs (the Volmer-Heyrovsky mechanism). Here, the mechanism by which the reaction proceeds depends largely on the electrode material. In the case of the Pt(111) surface, which is considered to be present in Pt NPs/CB, the HER is likely to proceed by the Volmer-Tafel mechanism with Tafel step being the rate-limiting step. Thus, we have calculated the reaction when it proceeds by the Volmer-Tafel mechanism (Fig. ). The free-energy change in the Tafel process in Pt17(iii)/graphite and Pt17(v)/graphite was smaller than that of Pt(111). From the slope of the current-potential curve obtained in the experiment (Fig. ), the rate-limiting step of Pt17/CB was found to be Volmer step, different from the case of Pt(111). Probably, there are reaction pathways with a low Tafel barrier (such as site iii and v) in Pt17/graphite, and these reaction sites are presumed to have changed the rate-limiting step to Volmer step. This HER energy diagram implies that the smaller H adsorption energy on Pt17(X)/graphite comparted to that on a large Pt nanoparticle causes the high HER activity of Pt17/CB(20.0 wt% Pt), although it is difficult to demonstrate this computationally. Note that the rate-limiting step of Pt NPs/CB was also Volmer step in our experiment (Fig. ). It is presumed that in the Pt NPs/CB, small and highly active NCs also exist and they mainly contribute to the activity of Pt NPs/CB shown in Figure .