Capacitive energy storage at the electrochemical double layer formed on a particle surface can enable efficient devices that deliver high power and exhibit excellent reversibility. However, even with state of the art nanocarbons with highly controlled morphology to maximize the specific surface area, the available energy density remains far below that of existing rechargeable batteries. Utilizing nanoparticles of transition metal oxides is a viable option to alleviate the conflict between energy and power densities by accommodating additional electrons around the surface transition metal sites, called "pseudocapacitance". However, an understanding of pseudocapacitive surfaces has been limited due to a lack of suitable analysis methodology. Here, we focus on the RuO2/water interface and elaborate on a reaction scheme including charge transfer into related surface orbitals using density functional theory calculations based on interfacial structures determined under a given electrode potential at a fixed pH of 0. The extensive contributions of the surface oxygen atoms and their surface-site dependence are revealed through the Ru-O orbital hybridization and O-H bond breaking/formation, largely deviating from the general explanation based only on the nominal valence states (penta-, tetra-, or trivalent) of Ru atoms. (Chemical Equation Presented).
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