Effect of electronic resistance and water content on the performance of RuO2 for supercapacitors

Hydrous ruthenium oxide, RuO2H2O, was prepared according to a sol-gel process and annealed at different temperatures. The importance of high electronic conductivity for high capacity in aqueous 3 M H2SO4 was revealed through two approaches. The electronic resistivity of RuO2H2O measured in situ as a function of the electrode potential shows a marked increase toward low potentials. This trend is more pronounced for the low-temperature annealed oxide (T < or = 150 deg C) where it results in a limitation of the capacitance at E < 0.4 V vs reversible hydrogen electrode. This finding is in line with the steep rise of the electrochemical impedance in the same potential region. A possible way to overcome this limitation is to mix two differently heat treated oxides, one with high conductivity (T = 300 deg C, Z300), the other with optimum capacity (T= 150 deg C, Z150). The observed specific capacity increase of hydrous RuO2 in the mixture from 738 to 982 F/g is attributed to an improvement of the electronic pathway along the particles of high-temperature-treated RuO2 (Z300) toward the high-capacity Z150 particles.