Capacitors are devices that store electric charge. Supercapacitors, more accurately known as electric double-layer capacitors or electrochemical capacitors, can store much more charge thanks to the double layer formed at an electrolyte-electrode interface when voltage is applied.

Thanks to their large surface area, excellent electrical conductivity, electrochemical stability and mechanical flexibility, CNTs are promising advanced electrode materials for flexible supercapacitors. Existing techniques to fabricate flexible CNT electrodes are generally indirect, however, and involve processes such as slurry casting, ink-jet printing, vacuum filtering or electrophoretic deposition, to name a few. What is more, the electrodes produced using such technqiues typically consist of densely packaged CNT films with limited available surface area.

Entangled CNTs

Now, researchers at Central China Normal University have put forward a new, straightforward method to synthesize flexible CNT electrodes that involves directly growing CNT films on flexible carbon cloth through a chemical vapour deposition (CVD) process using nickel as a catalyst. As shown in the image, the as-grown CNTs are slightly entangled, forming a well developed network with many open pores. This porous 3D architecture allows ions from a surrounding electrolyte to rapidly cover the entire surface of the CNT electrode so that it can subsequently be used as an electric double layer.

As expected, an AFSC device constructed by the researchers using the CNT network films as symmetric electrodes and H3PO4/poly(vinyl alcohol) (PVA) polymer gel as the electrolyte/separator had a high specific capacitance, ultralong cycle life of 100,000 cycles, good rate capability (it can scan at 1000 mV s–1), high energy density (of 2.4 µWh cm–2) and high power density (of 19 mW cm–2).

And that is not all: the device keeps its excellent electrochemical attributes even when it is bent or folded, even if it is subjected to high mechanical pressure (of 63 kPa) and over a wide temperature window (up to 100 °C). After charging for only five seconds, three such AFSC devices connected in series can efficiently power a red round LED for 60 seconds.

The researchers say that the performance of the AFSC could be further improved by using an ionic-liquid-based gel electrolyte with a wider operation potential, by increasing CNT mass loading, and by adding other pseudocapacitive materials to the device. The work could help in the design of practical AFSCs that could find use in various flexible portable and wearable electronic devices in the future.

More information about the research can be found in the journal Nanotechnology (in press).

Further reading

Super-aligned CNTs replace metal in supercapacitor upgrade (Sep 2010)
Out-of-plane growth of CNTs on graphene for supercapacitor applications (Dec 2011)
Aligned carbon nanotubes and non-aqueous electroytes ramp up performance of supercapacitors (May 2012)