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. A new type of highly loaded iridium single atom catalysts with tunable d‐band holes during the reaction process was designed as an efficient OER catalyst in acidic solutions. The in situ modulated d‐band holes and near‐steady XO*−Ir−N4 moieties at early working voltages are responsible for the observed high activity and stability.