Two-step hydrothermal synthesis of Ru–Sn oxide composites for electrochemical supercapacitors

A two-step hydrothermal process was developed to synthesize hydrous 30RuO2–70SnO2 composites with much better capacitive performances than those fabricated through the normal hydrothermal process, co-annealing method, or modified sol–gel procedure. A very high specific capacitance of RuO2 (CS,Ru), ca. 1150 F g−1, was obtained when this composite was synthesized via this two-step hydrothermal process with annealing in air at 150 °C for 2 h. The voltammetric currents of this annealed composite were found to be quasi-linearly proportional to the scan rate of CV (up to 500 mV s−1), demonstrating its excellent power property. From Raman, UV–vis spectroscopic and TEM analyses, the reduction in mean particulate size is clearly found for this two-step oxide composite, attributable to the co-precipitation of (RuδSn1−δ)O2·xH2O onto partially dissolved SnO2·xH2O and the formation of (RuδSn1−δ)O2·xH2O crystallites in the second step. This effect significantly promotes the utilization of RuO2 (i.e., very high CS,Ru). The excellent capacitive performances, very similar to that of RuO2·xH2O, suggest the deposition of RuO2-enriched (RuδSn1−δ)O2·xH2O onto SnO2·xH2O seeds as well as the individual formation of (RuδSn1−δ)O2·xH2O crystallites in the second hydrothermal step.