Unveiling the Origin of Unusual Pseudocapacitance of RuO₂·nH₂O from Its Hierarchical Nanostructure by Small-Angle X-ray Scattering

Hydrous ruthenium oxide (RuO2·nH2O) 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.