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

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 60 mu m 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 0.40 Omega cm(2), while that of the asymmetric capacitor with a positive alpha-Co(OH)(2)/CNT electrode measures 0.45 Omega cm(2), much less than the as-grown CNT capacitor. Electrodeposited alpha-Co(OH)(2) enhances the cell capacitance significantly, especially in a wide potential window. When operated at current density 20 A g(-1) and window 1.8 V, the cell capacitance of asymmetric capacitor measures 62.6 F g(-1), much higher than that of symmetric capacitor 8.7 F g(-1). But the symmetric CNT capacitor displays a better power performance because of a lower ESR. At current density 30 A g(-1), its power density reaches 20.6 kW kg(-1) with energy density 2.2 Wh kg(-1). In contrast, the asymmetric capacitor stores more energy, at 30 A g(-1), it exhibits power density 11.4 kW kg(-1) and energy density 7.8 Wh kg(-1). (C) 2012 Elsevier B.V. All rights reserved.