Transparent and Flexible Supercapacitors with Single Walled Carbon Nanotube Thin Film Electrodes Recep Yuksel, † Zeynep Sarioba, † Ali Cirpan, ‡ Pritesh Hiralal, § and Husnu Emrah Unalan* ,†,∥ † Department of Micro and Nanotechnology, Middle East Technical University, Ankara 06800, Turkey ‡ Department of Chemistry, Middle East Technical University, Ankara 06800, Turkey § Department of Engineering, University of Cambridge, Cambridge CB3 0FA, England ∥ Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara 06800, Turkey ABSTRACT: We describe a simple process for the fabrication of transparent and ﬂexible, solid-state supercapacitors. Symmetric electrodes made up of binder-free single walled carbon nanotube (SWCNT) thin ﬁlms were deposited onto polydimethylsiloxane substrates by vacuum ﬁltration followed by a stamping method, and solid-state supercapacitor devices were assembled using a gel electrolyte. An optical transmittance of 82% was found for 0.02 mg of SWCNTs, and a speciﬁc capacitance of 22.2 F/g was obtained. The power density can reach to 41.5 kW·kg −1 and shows good capacity retention (94%) upon cycling over 500 times. Fabricated supercapacitors will be relevant for the realization of transparent and ﬂexible devices with energy storage capabilities, displays and touch screens in particular. KEYWORDS: supercapacitors, single walled carbon nanotubes, transparent, ﬂexible, polydimethylsiloxane, thin ﬁlm 1. INTRODUCTION Supercapacitors have received a lot of attention due to their high speciﬁc power and moderate energy densities. They have a wide range of applications spanning from electric vehicles and pulse power systems to portable devices. 1−3 Signiﬁcant eﬀorts have been spent in the development of the basic components of many ﬂexible and transparent electronic devices, opening possibilities for new device concepts and form factors; however, energy and power sources in those devices have retained their classic form factor. Therefore, it is of great interest to develop both transparent and ﬂexible supercapacitors. 4−9 Single walled carbon nanotube (SWCNT) thin ﬁlm electro- des are a highly appealing candidate material for this purpose. 6,8 Owing to their high conductivity, permeability (resulting in high power density) and chemical inertness (long cycle lifetime), SWCNT thin ﬁlms are promising candidates for active supercapacitor electrode materials. 7,11,12 For practical device applications, these characteristics together with the fact that carbon typically forms a purely double layer make SWCNT thin ﬁlms unique. Other alternatives like pseudocapacitive materials, such as metal oxides (e.g., RuO 2 , IrO 2 and MnO 2 ) and conducting polymers (polyaniline and polypyrrole), tend to suﬀer from reduced cyclability and power densities. 7,10,12 SWCNT thin ﬁlm electrodes, in fact, have already been successfully demonstrated in prototype devices, such as solar cells, 13 photodetectors 14 and organic light emitting diodes. 15 An alternative transparent electrode candidate, conducting polymers, revealed high capacitance but showed rapid degradation, volumetric changes and limited cycle life. 16 In addition, conducting polymer electrodes are limited to work in a strict potential window. Possible overcharging and discharging during operation could easily damage the conducting polymers. 12 Chemical routes, on the other hand, have diﬃculties in the synthesis of conducting polymers. 12,17 Many conducting polymers have more than one oxidation step. Moreover, some polymers should be doped to increase their conductivities. 5,18 Charged state and the doping nature of the conducting polymers aﬀect their electrochemical performance. Because of this, they are suited only for particular electrode and/or electrolyte systems. Therefore, it is diﬃcult to use conducting polymers as electrodes in a symmetric super- capacitor assembly. 19 Metal-oxide supercapacitors have a high speciﬁc capacity, but they are not inherently conducting and necessitate the use of metallic or conductive ﬁllers in a composite structure. 20,21 In addition, the optoelectronic properties and especially ﬂexibility of thin ﬁlm crystalline metal oxide (e.g., MnO 2 , 13 RuO 2 , 22 ITO 23 ) supercapacitors are far from that of the SWCNT thin ﬁlm supercapacitors. 7,24 SWCNT thin ﬁlm electrodes can be made highly transparent and can be simply deposited onto polydimethylsiloxane (PDMS) and polyethylene terephthalate (PET) substrates for the realization of ﬂexible electrodes. The sheet resistance of SWCNT ﬁlms can simply be tuned with the Received: June 22, 2014 Accepted: August 15, 2014 Published: August 15, 2014 Research Article www.acsami.org © 2014 American Chemical Society 15434 dx.doi.org/10.1021/am504021u | ACS Appl. Mater. Interfaces 2014, 6, 15434−15439