Proton and Electron Conductivity in Hydrous Ruthenium Oxides Evaluated by Electrochemical Impedance Spectroscopy: The Origin of Large Capacitance

Abstract

Electrochemical impedance spectroscopy was conducted on a series of hydrous ruthenium oxides, RuO₂·xH₂O, (x = 0.5, 0.3, 0) and a layered ruthenic acid hydrate (H_0.2RuO_2.1·nH₂O) in order to evaluate their protonic and electronic conduction. The capacitor response frequency was observed at lower frequency for RuO₂·xH₂O with higher water content, which was suggested to be due to electrolyte exhaustion within the film and/or utilization of hydrated interparticle micropores that have high ionic resistance. Analysis of the impedance data indicated that the charge-transfer resistance through the film is not significantly affected by the water content in RuO₂·xH₂O, and the capacitor frequency response is dominated by the protonic conduction. The capacitor response frequency of layered H_0.2RuO_2.1·nH₂O was comparable to RuO₂·0.5H₂O. The high specific capacitance at low frequency for layered H_0.2RuO_2.1·nH₂O is attributed to the utilization of the expandable hydrous interlayer, which accounts for the ionic conduction. The present results demonstrate the importance of hydrous regions (either interparticle or interlayer) to allow appreciable protonic conduction for high energy and high power electrochemical capacitors.