Miniature asymmetric ultracapacitor of patterned carbon nanotubes and hydrous ruthenium dioxide

Abstract

A symmetric ultracapacitor CNT_CNT and an asymmetric ultracapacitor CNT_hRuO2 of mini size have been prepared with patterned carbon nanotubes (CNT) and hydrous ruthenium dioxide. Galvanostatic charge/discharge results indicate that CNT_hRuO2 is the superior one in both power and energy densities. In a potential window 2.0 V, the CNT_hRuO2 cell displays an energy density of 24.0 W h kg−1 at a power density of 22.9 kW kg−1. Its power density can be raised to 41.1 kW kg−1 at the expense of the energy density, which drops to 6.8 W h kg−1. On the other hand, CNT_CNT performs at a lower level, delivering 5.2 W h kg−1 at 5.5 kW kg−1. The favorable charge/discharge performance of CNT_hRuO2 is attributed to hydrous RuO2, whose pseudocapacitance drives the other electrode of the vertical CNT array to work harder and makes more use of its double-layer capacitance. The analysis of individual electrode capacitance indicates that the high capacitance of hRuO2 also causes a disproportion in voltage partition, which restricts the low limit of cycling current in an extended potential window. On energy cycling, CNT_hRuO2 demonstrates sufficient stability in 10 000 cycles, after an initial 13% drop in capacitance.