Electrochemical doping of single-walled carbon nanotubes in double layer capacitors studied by in situ Raman spectroscopy P.W. Ruch, L.J. Hardwick 1 , M. Hahn 2 , A. Foelske, R. Ko ¨tz * , A. Wokaun General Energy Research Department, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland ARTICLE INFO Article history: Received 6 May 2008 Accepted 24 August 2008 Available online 2 September 2008 ABSTRACT The electrochemical doping of single-walled carbon nanotubes (SWCNTs) in 1 M Et 4 NBF 4 in acetonitrile was investigated by in situ Raman spectroscopy. The capacitance was deter- mined to be 82 F/g for the positive and 71 F/g for the negative SWCNT electrode, respec- tively, which approaches the typical values for microporous activated carbons used in supercapacitors. The changes in the Raman intensities and shifts of the D and G + bands as well as of the radial breathing modes (RBMs) during electron and hole injection were studied as a function of the electrode potential. For the D and G + bands, hole doping leads to strong upshifts which can be attributed to a stiffening of C–C bonds and the correspond- ing phonon modes. Electron doping results in much less pronounced changes in the band positions. The intensity attenuation of the RBM bands was found to be markedly different for semi-conducting and metallic SWCNTs, whereby sufficiently high doping leads to a loss of Raman intensity due to bleaching of electronic transitions. The main RBM bands upshift upon both electron and hole doping, which is attributed to changes in the chemical envi- ronment of individual SWCNTs upon charging and discharging of the electrochemical dou- ble layer within SWCNT bundles. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Electrochemical double layer capacitors (EDLCs) consisting of activated carbon electrodes and non-aqueous electrolytes rely mainly on the charging of the electrochemical double layer for energy storage and its fast discharge for high power capability [1,2]. Upon charging, the injection of electrons or holes into the electronic band structure of the electrodes is compensated on the electrolyte side by the accumulation of the corresponding counter-ions within the electrochemical double layer. Ideally, no electron transfer occurs across the electrode/electrolyte interface, which implies that charge storage is exclusively electrostatic and only takes place within the electrochemical double layer. If Faradaic processes do contribute to the charge storage, a so-called pseudocapaci- tance can occur in addition to the double layer capacitance [1,2]. The more generic terms electrochemical capacitors, supercapacitors and ultracapacitors can be used to describe either mode of capacitive charge storage. The charging of an EDLC essentially corresponds to a simultaneous electrochemical doping of the negative and the positive electrode. Considering that the active material of the electrodes usually consists of highly porous carbon which is sp 2 -hybridized and p-electron conducting [1], two different phenomena are expected to occur upon doping. First, the injection of charge into the electrode leads to a change in the number of mobile electronic charge carriers, which affects the electronic conductivity and the space 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.08.023 * Corresponding author: Fax: +41 56 310 44 15. E-mail address: ruediger.koetz@psi.ch (R. Ko ¨ tz). 1 Present address: Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. 2 Present address: Honeywell Specialty Chemicals, D-30926 Seelze, Germany. CARBON 47 (2009) 38 – 52 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon