Currently, Pt metals, which are precious and expensive, are the only materials that meet these requirements. Consequently, reducing the amount of Pt used in PEMWE electrocatalysts is critical for making this technology more economically viable and environmentally sustainable. Using [Pt17(CO)12(PPh3)8] z (Fig. ) as a precursor to deposit Pt17 NCs on carbon black (CB; Pt17/CB) is a promising strategy to enhance the activity of HER catalysts while decreasing overall Pt usage. Using ~1 nm-Pt NCs increases the number of active sites for the reaction, potentially leading to catalysts that are more efficient than typical CB catalysts loaded with larger (~2-3 nm) Pt nanoparticles (Pt NPs) commonly used in PEMWEs. Additionally, DFT calculations can provide insight into the electronic structure, high stability, and catalytic activity of these small Pt NCs, guiding the future design of highly active and stable electrocatalysts for the HER. Accordingly, in the present study, we aimed to elucidate the electronic structure of [Pt17(CO)12(PPh3)8] z using DFT calculations (Fig. ); develop an HER catalyst with enhanced activity compared with commercial Pt NPs/CB by using [Pt17(CO)12(PPh3)8] z as a precursor (Fig. ); and determine the origin of the high HER activity exhibited by Pt17/CB (Fig. ). Our results revealed that the transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) in [Pt17(CO)12(PPh3)8] 2+ is forbidden and that the orbitals neighbouring them primarily comprise the s, p, and d orbitals of Pt. Additionally, we successfully created a Pt17/CB catalyst with an HER mass activity 3.59-times that of commercially available Pt NPs/CB. The DFT calculations also strongly suggested that the high HER activity of Pt17/CB arises from the presence of Pt atoms with an optimal electronic structure for the HER.