Platinum Metals Rev., 2002, 46, (3), 105
Nanocrystalline Ruthenium Supercapacitor Material
Electrochemical supercapacitors (1) can be power sources for a large range of equipment. They store large quantities of charge and can be repeatedly charged/discharged either quickly or slowly over tens of thousands of cycles. The high energy density is due to an electric double layer, while the rapid charge/discharge ability comes from being made of materials having either rectangular capacitive cyclic voltammo-grams (CVs) or pseudocapacitive behaviour, such as high surface-area activated carbons or conducting polymers. both can be used in aqueous or nonaqueous media.
Ruthenium oxide has a very high charge storage capacity when used in aqueous solutions. In attempts to optimise its capacitive properties, prior work has looked at the hydration of Ru oxide, its crystallinity and particle size. As Ru is costly, other elements have often been mixed with it.
Now scientists from the Université du Québec à Montréal, ESIRS-Energie et Matériaux and Institut de recherche d'Hydro-Quebec, Canada, have produced nanocrystalline TixFeyRuzOn powders by ball-milling, and have examined their structures and electrochemical characteristics (P. soudan, J. Gaudet, D. Guay, D. Bélanger and R. Schulz, Chem. Mater., 2002, 14, (3), 1210-1215). When the O:Ti ratio was > 1, the Ru atoms were in an hexagonal phase. Electrodes made from the powders had an increase in capacitance from ∼ 5 to ∼ 50 F g-1 on cycling in H2SO4 or NaOH — due to the growth and modification of a surface layer, RuO2·xH2O. When O:Ti < 1, Ru was in a cubic phase and after cycling or (preferably) leaving the electrode in 1 M NaOH, the maximum capacitance was near 50 to 60 F g-1, also due to the growth and modification of a surface oxide layer.
The Ru material produced by ball-milling had a specific surface area of only a few m2 g-1 and agglomerated into larger grains, so reducing the electrochemical surface area. However, a leaching process (milling the nanocrystalline material with Al, then removing the Al) increased the specific area 10-fold. CVs of this nanocrystalline Ti2FeRuO2 in NaOH had 110 F g-1 capacitance, even when returned to H2SO4.
It is concluded that the crucial factors for improving the capacitance of Ru-containing materials are: the electrochemical behaviour of the matrices used to dissolve the Ru and preventing the Ru agglomeration.
- 1S. Trasatti and P. Kurzweil, Platinum Metals Rev., 1994, 38, ( 2 ), 46