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

RuO(2)(.)xH(2)O/NiO composites having RuO2 contents in the range 0-100wt.% have been prepared by a co-precipitation method. Structural, microstructural and textural transformations after heating the as-prepared composites at 200 and 600 degrees C have been followed by X-ray diffraction, scanning electron microscopy (SEM) and nitrogen adsorption/desorption isotherms. At 200 degrees C the composites are made of micrometric particles in which nanometric crystallites of the two oxides are aggregated. The composites show microporosity (0.02-0.10cm(3)/g), mesoporosity (0.07-0.12 cm(3)/g) and relatively high specific surface area (62-309 m(2)/g). At 600 degrees 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 degrees 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 RuO(2)(.)xH(2)O (or RuO2) have been taken as references. The higher specific capacitance of the 200 degrees C-heated composites compared to the 600 degrees C-heated ones is due to the higher specific surface area of the former and the higher pseudocapacitance of RuO2.xH(2)O compared to RuO2. The discussion reported in this work can be applied to other composites such as RuO(2)(.)xH(2)O/carbon and RuO(2)(.)xH(2)O/other oxides. (c) 2006 Elsevier Ltd. All rights reserved.