1) The HOMO-to-LUMO transition in [Pt17(CO)12(PPh3)8] 2+ is forbidden, with nearby orbitals primarily comprising the s, p, and d orbitals of Pt. 2) Pt17/CB(20.0 wt% Pt) exhibits 3.59-times the HER mass activity of the commercial Pt NPs/CB(46.9 wt% Pt) catalyst. 3) Pt17/graphite generates surface Pt atoms more favourable for the HER than the Pt(111) surface in Pt NPs/CB, leading to higher HER activity. These findings offer clear design guidelines for understanding the electronic structure of atomically precise Pt NCs, developing highly active HER catalysts using Pt NCs, and reducing Pt usage in PEMWE applications. It is also expected that alloying these NCs will lead to the creation of even more active catalysts.