Voltammetric characterization of ruthenium oxide-based aerogels and other RuO2 solids: The nature of capacitance in nanostructured materials

Ruthenium dioxide is an important electrode material for applications in el ectrocatalysis and power sources. High surface areas are achieved in hydrou s RuO2 precipitates and in mixed ruthenium oxide-titanium oxide, (Ru-Ti)O-x , aerogels (in which nanoscale domains are networked to form a highly porou s structure). The electrochemical properties of(Ru-Ti)O-x aerogels, RuO2, a nd hydrous RuO2 are examined by direct pressing of sub-milligram quantities of the solid onto the surface of a conductive carbon/wax composite. Voltam metric measurements in acidic electrolyte confirm a pseudocapacitive respon se for all the RuOx-based materials. Despite an improvement in BET surface area, as compared with other RuO2 materials, the (Ru-Ti)O-x aerogel display s a low specific capacitance, which correlates to the high degree of crysta llinity of the aerogel. Nanocrystallites of rutile RuO2, formed during ther mal treatment of the sol-gel Ru/Ti precursors, deleteriously affect the spe cific capacitance of the material; however, all RuOx domains in the aerogel are voltammetrically addressable. The influence of proton-donating species on the observed capacitance for the (Ru-Ti)O-x aerogel is evident from the dependence of the voltammetric charge in acidic electrolyte on the potenti al scan rate.