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.