Owing to the low Pt loading (1.0 wt% Pt), the current value for H + adsorption was insufficient for accurate ECSA calculation. Therefore, we prepared a catalyst with a higher Pt loading (20.0 wt% Pt) for further investigation. Pt17/CB(20.0 wt% Pt) was prepared by loading [Pt17(CO)12(PPh3)8] z at 20.0 wt% Pt followed by calcination using a similar method to that used to prepare Pt17/CB(1.0 wt% Pt). The calcination conditions were more severe (250 °C, 120 min) than for Pt17/CB(1.0 wt% Pt), considering that the number of ligands present on the support also increased. For the obtained Pt17/CB(20.0 wt% Pt), a slight increase in average particle size based on the aggregation of Pt17 NCs was observed (Fig. ). This is thought to be because Pt17 NCs were mainly loaded on CB by physisorption, different from the case of Pt17/CB(1.0 wt% Pt). Because of such a weak interaction between Pt17 NCs and CB, a part of Pt17 NCs aggregated during the calcination process. Electrochemical cleaning was performed on the obtained Pt17/CB(20.0 wt% Pt), followed by electrochemical measurements. A commercial catalyst (TEC10E50E; Pt NPs/CB(46.9 wt% Pt)) was used as a comparison (Fig. ). HER mass activity at different Pt loadings was also calculated and compared. As a result, Pt17/CB(20.0 wt% Pt) showed 3.59-times the HER mass activity (1.76 vs. 0.49 A mg - 1 Pt at -0.055 V vs. RHE; Fig. and Fig. ) than Pt NPs/CB(46.9 wt% Pt).