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Abstract

The electric double layer (EDL) formed by ions at the electrode-electrolyte interface is a fundamental component of any electrochemical system, ranging from batteries to fuel cells and electrolyzers. At the atomic level, the behavior of the EDL varies greatly with the choice of electrode, electrolyte, and conditions. However, it is thought to follow one general rule analogous to a capacitor: low electrode potentials lead to a negative electrode surface charge which attracts cations to the EDL, whereas high potentials lead to a positive surface charge and anion attraction. Here, we show that this most basic rule does not apply for the oxide-electrolyte interface. Using an interface-sensitive X-ray absorption spectroscopy (XAS) approach, we show that increasing the electrode potential attracts cations and repels anions at the IrO₂-electrolyte interface. We show that this behavior is driven by the surface redox chemistry of the oxide, which dictates the surface charge of the electrode by modulating the surface electrophilicity. Our findings rationalize the cation-dependent performance trends observed for oxide electrodes, paving the way to exploit them in electrolyte engineering.