In terms of the ECSAs estimated by H + adsorption, Pt17/CB(20.0 wt% Pt) has 1.07-times (101.3 vs. 94.6 m 2 g -1 Pt; Fig. ) that of Pt NPs/CB(46.9 wt% Pt). Thus, the activity enhancement of Pt17/CB(20.0 wt% Pt) compared with Pt NPs/CB(46.9 wt% Pt) is higher than that of ECSA. Consequently, when specific activity (SA), the activity per surface active site, is calculated by dividing ECSA by HER mass activity, Pt17/CB(20.0 wt% Pt) shows 3.39-times the SA than that of Pt NPs/CB(46.9 wt% Pt) (Fig. ). Although it is difficult to accurately calculate the active site by ECSA because, unlike Pt NPs, no specific crystalline planes are exposed in Pt17NCs, our estimation implies that the Pt atoms of Pt17 NCs might have better HER properties than those of Pt NPs. Therefore, it is reasonable to expect that further improvement in activity can be achieved if it becomes possible to prepare catalysts with a higher Pt loading ratio while suppressing the aggregation of Pt17NCs.