Fast Modulation of d ‐Band Holes Quantity in the Early Reaction Stages for Boosting Acidic Oxygen Evolution

Synthesis of highly active and durable oxygen evolution reaction (OER) catalysts applied in acidic water electrolysis remains a grand challenge. Here, we construct a type of high‐loading iridium single atom catalysts with tunable d ‐band holes character ( h ‐HL−Ir SACs, ∼17.2 wt % Ir) realized in the early OER operation stages. The in situ X‐ray absorption spectroscopy reveals that the quantity of the d ‐band holes of Ir active sites can be fast increased by 0.56 unit from the open circuit to a low working potential of 1.35 V. More remarkably, in situ synchrotron infrared and Raman spectroscopies demonstrate the quick accumulation of *OOH and *OH intermediates over holes‐modulated Ir sites in the early reaction voltages, achieving a rapid OER kinetics. As a result, this well‐designed h ‐HL−Ir SACs exhibits superior performance for acidic OER with overpotentials of 216 mV @10 mA cm −2 and 259 mV @100 mA cm −2 , corresponding to a small Tafel slope of 43 mV dec −1 . The activity of catalyst shows no evident attenuation after 60 h operation in acidic environment. This work provides some useful hints for the design of superior acidic OER catalysts.