US researchers have developed a scalable process to produce continuous ribbons of aligned carbon nanotubes (CNTs), for the next generation of double-layer capacitors.
In the last decade, there has been a considerable growth in the wide-spread use of carbon nanomaterials across a range of industries. But the most common bottleneck to any further development is the scalability of their production. Although CNTs can be synthesised in large quantities, present processes for the growth of vertically-aligned CNTS – particularly of interest to the electronics market - are limited to a small range of substrate materials.
But a group of researchers from Clemson University in the US have developed a relatively low-cost roll-to-roll method – their system can grow vertically-aligned CNTs (VACNTs) directly onto aluminium foil ribbons that are continuously draw through a reactor. Their process produces high density, high capacity (~50 F/g) forests of aligned CNTs that outperform commercial CNTs. The team also used these ribbons of aligned CNTs as the electrodes in a range of high-performance supercapacitor cells.
Today’s supercapacitors tend to use carbon materials in their electrodes, with their performance related to the electrode’s surface area. So, considerable research effort has focussed on using CNTs as supercapacitor electrodes. But issues of substrate preparation and high operating temperatures have rendered the system complex and inefficient. What the Clemson team have done is develop a system that negates these issues – by adapting a standard Chemical Vapour Deposition (CVD) system, they have managed to decrease the growth temperature to 600 °C, which is below the melting temperature of aluminium. This means that it can be used to directly synthesise VACNTs onto a current collector substrate – in this case, aluminium foil ribbons.
The work, recently published in Nano Energy 8 (2014) 9-16 [DOI: 10.1016/j.nanoen.2014.05.004], also reports on the direct assembly of these VACNT ribbons into supercapacitors. When compared with capacitors made with buckypaper and CNT forests from a stationary CVD set-up, the roll-to-roll electrodes performed well, with a charge capacity of 24.8 mAh/g. But their discharge time (630 ms), energy density (11.5 Wh/kg) and power density (1270 W/kg) all vastly outperformed the other electrodes. The roll-to-roll devices also showed excellent cycle stability, with no loss of performance over more than a thousand cycles.
These results demonstrate the real potential for this technique, and the team believe that it offers a viable process for the production of supercapacitor electrodes.
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