Electrochemical capacitors of miniature size with patterned carbon nanotubes and cobalt hydroxide

Abstract

Two miniature electrochemical capacitors, of high power and sufficient energy capacity, are prepared using photolithography and transferred to a plastic substrate. The preparation procedure involves, inverting a 60μm thick interdigital pattern of multi-walled carbon nanotubes (CNT) with a Scotch tape, such that the pre-sputtered gold layer at the bottom effectively collects interfacial charge. The equivalent series resistance (ESR) of the symmetric CNT capacitor measures 0.40Ωcm2, while that of the asymmetric capacitor with a positive α-Co(OH)2/CNT electrode measures 0.45Ωcm2, much less than the as-grown CNT capacitor. Electrodeposited α-Co(OH)2 enhances the cell capacitance significantly, especially in a wide potential window. When operated at current density 20Ag−1 and window 1.8V, the cell capacitance of asymmetric capacitor measures 62.6Fg−1, much higher than that of symmetric capacitor 8.7Fg−1. But the symmetric CNT capacitor displays a better power performance because of a lower ESR. At current density 30Ag−1, its power density reaches 20.6kWkg−1 with energy density 2.2Whkg−1. In contrast, the asymmetric capacitor stores more energy, at 30Ag−1, it exhibits power density 11.4kWkg−1 and energy density 7.8Whkg−1.