RuO2·xH2O/NiO composites as electrodes for electrochemical capacitors: Effect of the RuO2 content and the thermal treatment on the specific capacitance

RuO2·xH2O/NiO composites having RuO2 contents in the range 0–100 wt.% have been prepared by a co-precipitation method. Structural, microstructural and textural transformations after heating the as-prepared composites at 200 and 600 °C have been followed by X-ray diffraction, scanning electron microscopy (SEM) and nitrogen adsorption/desorption isotherms. At 200 °C the composites are made of micrometric particles in which nanometric crystallites of the two oxides are aggregated. The composites show microporosity (0.02–0.10 cm3/g), mesoporosity (0.07–0.12 cm3/g) and relatively high specific surface area (62–309 m2/g). At 600 °C the composites are fully dehydrated and RuO2 has crystallized and segregated. Microporosity and mesoporosity as well as specific surface area are strongly decreased. Specific capacitance and specific surface area of the composites heated at 200 and 600 °C have been measured and discussed on the basis of the RuO2 content. For comparison the specific capacitance and specific surface area of mixtures of NiO and RuO2·xH2O (or RuO2) have been taken as references. The higher specific capacitance of the 200 °C-heated composites compared to the 600 °C-heated ones is due to the higher specific surface area of the former and the higher pseudocapacitance of RuO2·xH2O compared to RuO2. The discussion reported in this work can be applied to other composites such as RuO2·xH2O/carbon and RuO2·xH2O/other oxides.