Electrochemical properties of ruthenium-based nanocrystalline materials as electrodes for supercapacitors

Nanocrystalline TixFeyRuzOn materials were prepared by mechanical alloying using high-energy ball milling. The electrochemical properties of the materials were investigated in 1 M NaOH and 1 M H2SO4 aqueous solutions, using a composite electrode technology. The tested materials fall in two categories. On one hand, when the O/Ti ratio is larger than 1, Ru atoms are found in an hexagonal phase. Upon cycling in H2SO4 or NaOH, these materials exhibit a significant increase of their capacitance from similar to5 to similar to50 F/g. This is due to the progressive growth of a ruthenium oxide layer at the surface. On the other hand, when the O/Ti ratio is smaller than 1, Ru atoms are found in a cubic phase (CsCl), along with Ti and Fe atoms. In that case, the growth of a stable oxide phase at the surface of the material occurs only when it is cycled in basic electrolyte. The maximum attainable capacitance is also close to 50-60 F/g. The individual crystallites of as-milled nanocrystalline materials suffer from a strong tendency to agglomerate together. For example, it is shown that the electrochemically active surface area of nanocrystalline RuO2 is only increased by a factor of 2 when the crystallite size is decreased from 600 to 15 nm, which amounts to a 40-fold increase of the specific surface area. Thus, higher surface area materials were obtained by performing an additional milling step with Al, which is followed by a subsequent leaching of Al with a NaOH solution. With that procedure, the best performances were obtained with leached Ti2FeRuO2, with a maximum capacitance of 110 F/g.