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 μm 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 Ω cm2, while that of the asymmetric capacitor with a positive α-Co(OH)2/CNT electrode measures 0.45 Ω cm2, 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 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.

► A miniature electrochemical capacitor has been fabricated on a plastic substrate with 60 μm thick interdigital electrodes of vertically aligned carbon nanotubes.
► The CNT electrode with pre-sputtered gold as current collector exhibits low resistance and renders high power performance.
► The symmetric CNT capacitor and the asymmetric capacitor with α-Co(OH)2 show power density ∼20 kW kg−1, with energy density around 2 and 5 kWh kg−1; respectively.