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The oxygen evolution reaction has an important role in many alternative-energy schemes because it supplies the protons and electrons required for converting renewable electricity into chemical fuels 1-3 . Electrocatalysts accelerate the reaction by facilitating the required electron transfer 4 , as well as the formation and rupture of chemical bonds 5 . This involvement in fundamentally different processes results in complex electrochemical kinetics that can be challenging to understand and control, and that typically depends exponentially on overpotential 1,2,6,7 . Such behaviour emerges when the applied bias drives the reaction in line with the phenomenological Butler-Volmer theory, which focuses on electron transfer 8 , enabling the use of Tafel analysis to gain mechanistic insight under quasi-equilibrium 9-11 or steady-state assumptions 12 . However, the charging of catalyst surfaces under bias also affects bond formation and rupture 13-15 , the effect of which on the electrocatalytic rate is not accounted for by the phenomenological Tafel analysis 8 and is often unknown. Here we report pulse voltammetry and operando X-ray absorption spectroscopy measurements on iridium oxide to show that the applied bias does not act directly on the reaction coordinate, but affects the electrocatalytically generated current through charge accumulation in the catalyst. We find that the activation free energy decreases linearly with the amount of oxidative charge stored, and show that this relationship underlies electrocatalytic performance and can be evaluated using measurement and computation. We anticipate that these findings and our methodology will help to better understand other electrocatalytic materials and design systems with improved performance.