Revisiting the Pseudocapacitance of Hydrous Ruthenium Oxide Using 3D Bode Analysis

Hydrous ruthenium oxide (RuO2·xH2O) remains a benchmark material for pseudocapacitive energy storage. In this study, we revisit its electrochemical behavior using a recently adapted three-dimensional (3D) Bode analysis to investigate how the hydration content influences surface-controlled redox activity. The RuO2 electrodes with varying hydration levels are prepared via controlled heat treatments and analyzed using cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and the 3D Bode analysis. We confirm that intermediate hydration levels─particularly RuO2·xH2O heated at 150 °C─yield the highest specific capacitance (1075 F g–1 at 10 mV s–1) and optimal rate performance. The 3D Bode analysis resolves overlapping electrochemical processes by mapping various impedance parameters across both potential and frequency. At low heat treatments, the phase-angle component of the 3D Bode analysis finds that pseudocapacitance is localized in a potential window, but for the materials with optimized specific capacitance, the response is pseudocapacitive across the entire potential range. This work demonstrates the utility of the 3D Bode analysis as a powerful, generalizable tool for deconvolving electrochemical mechanisms in energy-storage materials.