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

Abstract

A two-step hydrothermal process was developed to synthesize hydrous 30RuO 2–70SnO 2 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 RuO 2 ( C S,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 (Ru δSn 1−δ)O 2· xH 2O onto partially dissolved SnO 2· xH 2O and the formation of (Ru δSn 1−δ)O 2· xH 2O crystallites in the second step. This effect significantly promotes the utilization of RuO 2 (i.e., very high C S,Ru). The excellent capacitive performances, very similar to that of RuO 2· xH 2O, suggest the deposition of RuO 2-enriched (Ru δSn 1−δ)O 2· xH 2O onto SnO 2· xH 2O seeds as well as the individual formation of (Ru δSn 1−δ)O 2· xH 2O crystallites in the second hydrothermal step.