Use of Vertically-Aligned Carbon Nanotube Array to Enhance the Performance of Electrochemical Capacitors

In the domain of energy storage, electrochemical capacitors have numerous applications ranging from hybrid vehicles to consumer electronics, with very high power density at the cost of relatively low energy storage. Here, we report an approach that uses vertically aligned carbon nanotube arrays as electrodes in electrochemical capacitors. Different electrolytes were used and multiple parameters of carbon nanotube array were compared: carbon nanotube arrays were shown to be two to three times better than graphite in term of specific capacitance, while the surface functionalization was demonstrated to be a critical factor in both aqueous and nonaqueous solutions to increase the specific capacitance. We found that a maximum energy density of 21 Wh/kg at a power density of 1.1 kW/kg for a hydrophilic electrode, could be easily achieved by using tetraethylammonium tetrafluoroborate in propylene carbonate. These are encouraging results in the path of energy-storage devices with both high energy density and power density, using only carbon-based materials for the electrodes with a very long lifetime, of tens of thousands of cycles.

Vertically aligned carbon nanotubes for supercapacitor and the effect of surface functionalization to its performance

Mélanie Guittet

As reliable mobile energy sources became more and more needed and alternative energy demands became more prevalent, the research in advanced energy storage technologies turned into a topic of utmost importance in today's society. Innovative electrode materials that are able to provide increased energy densities and long lifetime must be then be developed. Nanomaterials such as carbon nanotubes are of tremendous prospects in that context. This master thesis aims at realizing an electrochemical capacitor using a vertically grown carbon nanotube array, which is used as electrode in dierent sets of experiments to evaluate its electrical energy storage capability. Cyclic voltammetry and electrochemical impedance spectroscopy are used to determine the specic capacitance of the device, which is composed of two nanotubes electrodes separated by a polypropylene lter, in an adequate electrolyte. The surface chemistry was modied by adding hydroxyl groups onto the nanotubes surface, thus making them hydrophilic and providing an ecient way to increase their storage ability by a factor of two to three. Dierent electrolytes were compared, both aqueous and non aqueous, and the important parameter in choosing an appropriate electrolyte for a high storage ability was shown to be the polarity of the solution. The carried out performance studies gave encouraging results with a maximum energy density of 21 Wh/kg at a power density of 1.1 kW/kg for a hydrophilic electrode using tetraethylammonium tetrafluoroborate in propylene carbonate. Respectively, a maximum power density was found to be 22 kW/kg at energy density of 2 Wh/kg for a hydrophobic sample. This device can be claimed to be entirely carbon-based, with a relatively small ecological print compared to most lithium-based capacitors and inexpensive. The lifetime is also extremely long, indeed more than hundreds of thousands cycles without failure have been achieved.

2011

Vertically aligned carbon nanotubes for supercapacitor and the effect of surface functionalization to its performance

Mélanie Guittet

2011