A group of Chinese and American scientists have demonstrated a supercapacitor made from freestanding films of carbon nanotubes which could be used to power the next generation of portable electronics.

In today’s world, there is a growing demand for portable and, increasingly, wearable electronics – but the search for reliable power sources for these applications goes on. Batteries and supercapacitors have been used to power electronic devices for decades, but for this new generation of electronics, energy storage needs to be flexible, lightweight, and long-lasting.

Supercapacitors (SCs) have high power densities, can be charged/discharged quickly and have long lifetimes, but their liquid electrolytes have limited their potential use in portable electronics. Now, an international team of researchers have used films of functionalised carbon nanotubes (CNT) to produce a solid-state supercapacitor, which could bring flexible energy storage to the market [doi:10.1016/j.nanoen.2014.02.014].

The starting point for their prototype device was a freestanding film of functionalised CNTs, produced using vacuum filtration. By varying the annealing temperature of the CNT films, the researchers could alter the functional groups on the film, and thus optimise their supercapacitor. Each of the films were characterised using Fourier transform IR-absorption spectroscopy, thermogravimetric analysis, and x-ray photoelectron spectroscopy. This analysis showed that the film annealed at 200 °C showed the highest capacitance, along with excellent conductivity and long-term stability. This film was then used as the positive electrode in their supercapacitor, with a film of CNT/MoO3-x as the cathode, and a flexible gel electrolyte of PVA/LiCl.

Despite having a volume of only 0.015 cm3, this device outperformed a range of other solid-state supercapacitors currently in the literature – in terms of both energy density (1.5 mWhcm-3) and power density (4.2 W cm-3). To provide a “real-world” test of their capacitor, the team, led by Jun Zhou from Huazhong University in China, built a wireless transport system consisting of nine CNT-SCs. This prototype processed a sound signal and transmitted it through an antenna, and its performance matched well with the simulated results. It was also found that this SC array could maintain 90% of its initial capacity beyond 10,000 charge discharge cycles, further highlighting its potential for use as an energy storage system for portable electronics.

 Nano Energy (2014) doi:10.1016/j.nanoen.2014.02.014

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