Revisiting the ruthenium oxide-based water oxidation catalysts in acidic media: From amorphous to crystalline

Ruthenium oxides (RuO₂) are benchmark electrocatalysts for the oxygen evolution reaction (OER) in water splitting (2 H₂O → O₂ + 4 H⁺ + 4e^–), but their catalytic activity and stability vary significantly among amorphous, crystalline, and hydrated forms. In this work, we explored the structure-performance relationships of various RuO₂ electrocatalysts using a combination of techniques. Cyclic voltammetry revealed that the redox behavior and adsorption energies of key intermediates (*OH and *O) differ based on the crystallinity and particle size of RuO₂. Hydrogen peroxide (H₂O₂) as a probing molecule indicated that the interaction strength of the *OOH intermediate is stronger on amorphous and low-crystallinity RuO₂ than on highly crystalline RuO₂. Furthermore, in-situ techniques, including electrochemical attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), electrochemical-quartz crystal microbalance (QCM), and Raman spectroscopy, showed that amorphous RuO₂ hydrate exhibited higher OER activity but lower stability, while highly crystalline rutile RuO₂ was more stable up to 1.6 V but had weaker intermediate adsorption. These findings provide a clear understanding of how structural differences impact the catalytic performance and stability of RuO₂ electrocatalysts, offering guidance for optimizing OER catalysts.