Unveiling the Origin of Unusual Pseudocapacitance of RuO₂·<i>n</i>H₂O from Its Hierarchical Nanostructure by Small-Angle X‑ray Scattering

Hydrous ruthenium oxide (RuO₂·<i>n</i>H₂O) has inherent proton–electron mixed-conductive nature and offers huge pseudocapacitance (>700 F g^–1), having attracted the attention of many capacitor engineers. However, the origin of the anomalous pseudocapacitance, exhibiting a strong maximum at a specific narrow optimum annealing temperature of ca. 150 °C, has yet to be understood. Here we show a long-awaited explanation for this mystery based on its hierarchical nanostructure unveiled by small-angle X-ray scattering (SAXS). The striking contrast in X-ray atomic scattering factors enables SAXS to exclusively probe heavy RuO₂ in subnano- to nanoscale, dispersed in confined water. We demonstrate that the surface area of the first aggregate of subnano primary RuO₂ particles dominates the accessible number of proton and hence pseudocapacitance, providing critical insights into the nanoarchitectural design of high-performance electrodes for electrochemical capacitors