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Fabrication and characterization of solution-processed carbon nanotube supercapacitors Suvi Lehtimäki1, Juho Pörhönen1, Sampo Tuukkanen1, Pasi Moilanen2, Jorma Virtanen3 and Donald Lupo1 1

Tampere University of Technology, Dept. of Electronics and Communications Engineering P.O. Box 692, FI-33101 Tampere, Finland 2

Morphona Ltd. Myllärintie 1, 40640 Jyväskylä, Finland 3

The University of Akron, Dept. of Polymer Engineering 250 South Forge Street, Akron, OH 44325-0301, USA ABSTRACT We report the fabrication and characterization of supercapacitors prepared on a flexible substrate using a printable, high-viscosity carbon nanotube (CNT) ink. The CNT-hemicellulose composite ink was prepared using ultrasonication and applied on the substrate with a doctor blade. Aqueous sodium chloride was used as electrolyte. The capacitance of the supercapacitors was 16 mF for a device size of 2 cm2. The measurements were carried out in accordance to an international standard for electric double layer capacitors. INTRODUCTION Supercapacitors have become an interesting alternative for batteries due to their long cycle lifetime and high output power [1]. In 2012, Keskinen et al. demostrated that supercapacitors can be fabricated with printing methods, using environmentally friendly materials [2]. The use of printing methods provides new possibilities for the economic mass production of disposable, low cost devices, where the supercapacitor could be integrated with other printed electronics components. A potential application for the printed supercapacitors could be as an energy storage in an autonomous system which harvests energy from ambient light, vibrations or RF fields, powering for example sensor nodes [3, 4]. Graphitic nanomaterials such as carbon nanotubes (CNT) are interesting applicants for supercapacitor electrodes due to their high surface area and high conductivity [5]. Using carbon nanotubes as the sole electrode material facilitates very simple processing, because the material is conductive enough to act as the current collector in addition to being the active, high surface area electrode. We present the fabrication of solution processable carbon nanotube supercapacitors on a flexible substrate. The conductive ink for the electrodes is a nanocomposite of multi-walled carbon nanotubes (MWNT) and a hemicellulose, xylan. Hemicelluloses are the second most abundant polymer in nature, constituting up to 50 % of annual and perennial plants [6]. The hemicellulose acts as a dispersing agent for the nanotubes in water, but it also increases the ink’s viscosity and thereby improves its printability. The electrolyte used here is aqueous NaCl, which means the supercapacitors consist of only plastic, carbon, hemicellulose, water and regular table salt. This makes the devices environmentally friendly and safe to use in any application of ubiquitous electronics.

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EXPERIMENT Supercapacitor electrodes were prepared on a poly(ethylene terephthalate) (PET) film. To promote ink wetting, a polyurethane primer layer (Edolan GS from Tana