Multimodal Operando Characterization of Cation Effects at the Iridium Oxide–Electrolyte Interface for Alkaline Water Oxidation

Understanding the electrode/electrolyte interface is essential for tuning electrocatalyst activity. Here, we combine operando optical spectroscopy, laser-induced current transient (LICT) measurements, and surface-enhanced infrared absorption spectroscopy (SEIRAS) to investigate the origin of cation-dependent oxygen evolution reaction (OER) activity on electrodeposited iridium oxide in 0.1 M MOH (M = TMA⁺, K⁺, Na⁺, and Li⁺). We find that OER activity increases with increasing cation size (TMAOH > KOH > NaOH > LiOH). Operando optical spectroscopy reveals that the energetics of the redox transitions and the population of the redox-active species are independent of the electrolyte. Instead, the intrinsic turnover frequency varies strongly with the nature of the cation. LICT, SEIRAS, and quantum mechanics/molecular mechanics (QM/MM) simulations suggest that the interfacial solvent structure is the origin of this difference. With increasing cation size, the fraction of isolated water molecules and cation-coordinated water molecules increases, producing a more disordered interfacial environment. LICT measurements confirm that the potential of maximum entropy shifts closer to the water oxidation potential in the presence of larger cations in the electrolyte. We propose that a more disordered interface results in more isolated and reactive OH^- ions and faster reorganization of the interfacial solvent structure during the rate-determining O-O bond formation step, thereby accelerating the OER kinetics. Through our work, using multimodal operando spectroscopy and molecular simulations, we highlight how interfacial solvent structure, controlled by electrolyte cations, governs reactivity at complex electrochemical interfaces.