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 electrocatalysis and power sources. High surface areas are achieved in hydrous RuO2 precipitates and in mixed ruthenium oxide-titanium oxide, (Ru-Ti)O-x, aerogels (in which nanoscale domains are networked to form a highly porous structure). The electrochemical properties of(Ru-Ti)O-x aerogels, RuO2, and hydrous RuO2 are examined by direct pressing of sub-milligram quantities of the solid onto the surface of a conductive carbon/wax composite. Voltammetric measurements in acidic electrolyte confirm a pseudocapacitive response 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 displays a low specific capacitance, which correlates to the high degree of crystallinity of the aerogel. Nanocrystallites of rutile RuO2, formed during thermal treatment of the sol-gel Ru/Ti precursors, deleteriously affect the specific 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 potential scan rate.