Correlation of the charge storage and magnetic susceptibility of hydrous RuO2

Hydrous RuO2 is a mixed metallic-protonic conductor that is used as a charge storage material in electrochemical capacitors and as an electrocatalyst. Previous structural analysis by Dmowski, et. al. has shown that RuO2 is a composite of ordered RuO2 nanoparticles that are surrounded by hydrous grain boundaries. In this paper, magnetic susceptibility (MS) is used to show that the hydrous RuO2 has both localized and delocalized electrons. The localized electrons are attributed to Ru3+ defects that decrease in concentration with decreasing water content. The delocalized electrons are represented by a temperature independent paramagnetic (TIP) component of the MS. The magnetic data is consistent with a structure having metallic nanoparticles whose electrons become more itinerant with decreasing structural water. We conclude that hydrous RuO2 stores charge analogously to double-layer capacitors in which charge is stored at the interface of the hydrous grain boundaries and the metallic nanoparticles, and that there is effectively no difference in the charge storage mechanisms of hydrous RuO2 and carbon.