The design of active acidic oxygen evolution reaction (OER) catalysts is of paramount importance to achieve efficient large-current-density industrial hydrogen fuel production via water electrolysis. Herein, we develop a Pt based catalyst with high electrochemical activity for the OER in acidic conditions under a large current. We achieve this by modulating the electronic structure of Pt into a high-valence, electron-accessible Pt1(2.4+δ)+ (δ = 0-0.7) state during the reaction. This electron-accessible Pt1(2.4+δ)+ single-site catalyst can effectively maintain a large OER current density of 120 mA cm-2 for more than 12 h in 0.5 M H2SO4 at a low overpotential of 405 mV, and it shows a high mass activity of ~3350 A gmetal-1 at 10 mA cm-2 current density and 232 mV overpotential. Using in situ synchrotron radiation infrared and X-ray absorption spectroscopies, we directly observe in an experiment that a key (*O)-Pt1-C2N2 intermediate is produced by the potential-driven structural optimization of square pyramidal Pt1–C2N2 moieties; this highly favors the dissociation of H2O over Pt1(2.4+δ)+ sites and prevents over-oxidation and dissolution of the active sites.
