75，No. 8（2007） 601 ― Communication― Polyfluorene/Carbon Nanocomposite for Electrochemical Capacitors Kenji MACHIDA, Shunzo SUEMATSU, and Kenji TAMAMITSU ＊ Research Center, Nippon Chemi-Con Corporation (363, Arakawa, Takahagi-shi, Ibaraki 318-8505, Japan) Received January 31, 2007 ; Accepted June 6, 2007 A nanocomposite material based on polyfluorene (PF) loaded with a carbon black, namely Ketjen Black (KB), was investigated electrochemically as a cathode material for high-energy electrochemical capacitors. The nanocomposite was prepared by a chemical oxidation of fluorene monomer dissolved in the KB suspension. From TEM observation, thin PF films with 5-15 nm in thickness were loaded onto the surface of the aggregated KB particles. The nanocom- posite based capacitor electrode exhibited high specific capacitance of 160 F g−1 (260 F g−1 per PF mass) in an elec- trolyte of 1 M tetraethylammonium tetrafluoroborate/propylene carbonate. More importantly, the charge was found to be stored at high potential ranged from 0.4 to 1.0 V vs. Ag/Ag+, which is higher than the redox potentials for con- ventional conducting polymers. Key Words : Electrochemical Capacitor, Polyfluorene, Nanocomposite, Ketjen Black 1 Introduction Conducting polymers have been studied widely as electrode material in energy devices and more particu- larly in electrochemical capacitors.1-13）Charge storage of the conducting polymer based electrochemical capacitors is based on redox reaction of π-conjugated system (p- Fig. 1 Structure of polyfluorene (PF). doping or n-doping) in the conducting polymer, while charge storage of conventional electric double layer capacitors is double layer charging/discharging at the tive material with the PF is required. surface of activated carbon electrode. Rudge et al.1） In this study, a nanocomposite material based on PF reported electrochemical capacitors based on only one loaded with a nanocarbon substrate, namely Ketjen type of conducting polymer, such as polyaniline, that can Black (KB) having high specific surface area (800 m2 g−1), be reversibly p-doped (Type I capacitors, cell voltage: ～ high electronic conductivity (～100 S cm−1) and low cost, 1 V). Also, capacitors based on p- and n-dopable polymer was prepared and was investigated for its electrochemi- such as polythiophene derivatives have been studied cal capacitor properties. (Type III capacitors, cell voltage: ～3 V).1,2,5） For higher energy density, hybrid-type electrochemi- 2 Experimental cal capacitors (HEC) have been proposed in recent years.6,9-12） The polyfluorene/Ketjen Black nanocomposite (PF/KB For instance, Mastragostino et al.9）reported a high ener- nanocomposite) powder was prepared by following pro- gy HEC (～24 Wh kg −1, cell voltage: ～3.6 V) composed cedures. Firstly, KB powder (Ketjen Black Interational of an activated carbon as the anode and poly(3-methylth- Co.) was crushed mechanically by use of mixer (general- iophene) [p3MeT] as the cathode. The high energy is purpose) for 1 minute. Then, obtained KB powder was attributed to high capacitance (100～230 F g−1) and high dispersed into 10 mmol of fluorene monomer (98％, redox potential (～0.7 V vs. Ag/Ag+) of the p3MeT cath- Wako Chemicals) dissolved in chloroform under ultrason- ode. To obtain more high redox potential, other polythio- ication for 30 minutes. The KB amounts (W) were 1.0, 3.0, phene derivatives (～0.9 V vs. Ag/Ag+) were investigat- 5.0, 10 and 20 wt％ for fluorene mass. Into the suspen- ed. 6,10,11） sion, 10 mmol of iron trichloride (98％, Aldrich) as an oxi- The authors have been studying a polyfluorene (PF, dizing agent was added. After stirring for 72 h, the solu- Fig. 1) as a cathode material for high energy HEC. The tion was filtered with a Teflon filter (200 nm of pore size, film shows reversible redox reaction at higher redox Millipore). The obtained deposit was washed with chloro- potential (～1.2 V vs. Ag/Ag+)14,15）compared with other form, methanol, and distilled water to remove inactive- polymers mentioned above. A first report for electro- oligomer species, monomers, iron ions and chlorides. chemical properties of the PF material for electrochemi- Thereafter, it was dried under reduced pressure at 60 cal capacitors was published in 2006. The report degree C for 6 hours to obtain a black colored nanocom- revealed that the pure PF material was found to show posite powder. Actual specific content of KB (Wre) in low electrochemical capacity because of low electronic the nanocomposite powder was calculated by following conductivity of the bulk PF.13）For enhancement of elec- equation; trochemical activity of the PF, addition of electroconduc- 602 Electrochemistry Wre ＝MKB/Ycom (1) Table 1 Specific contents of KB (Wre) in PF/KB nanocomposite powder contrasted with adding KB amount where MKB and Ycom are the added KB mass at prepara- (W) at nanocomposite preparation. tion and the nanocomposite powder mass, respectively. W/wt％ Wre/wt％ Chemical structure and nanostructure of the nanocom- posite powder were characterized by FTIR spectroscopy 1.07.9 and TEM observation, respectively. 3.016 The PF/KB nanocomposite capacitor electrode was 5.040 fabricated by mixing 85 wt％ PF/KB nanocomposite 10 45 powder, 10 wt％ carboxymethylcellulose and 5 wt％ 20 68 elastomer binder. The mixture was coated onto an Al current collector. Thickness and geometric area of the capacitor electrode were 20 µm and 1 cm2, with single ples were found to consist of electroactive PF with main electrode mass loading in the range from 0.5 to 0.6 mg bonding manner of 2,7 position like a structure in Fig. 1, (electrode density: 0.25−0.30 g cm−3). Electrochemical corresponding to the peaks at 820, 760, 730 cm−1 assigned characteristics of the capacitor electrode were evaluated to 1,2,4-tri-substituted aromatic ring,16）as well as the by cyclic voltammetry (HZ-3000 system, Hokuto Denko). pure PF powder. Specific contents of KB (Wre) in the A three-electrode cell was used, with the PF/KB elec- PF/KB nanocomposite powders were summarized in trode as the working electrode, an activated carbon Table 1. The Wre values were significantly different from sheet (commercially purchased, 6 cm2, 150 µm in thick- the added KB amounts (W) at the nanocomposite prepa- ness) as the counter electrode, and Ag/Ag+ electrode as ration. This indicates that polymerization efficiency the reference electrode (consisting of Ag wire in a solu- (obtained PF mass /added monomer mass) of the PF in −2 tion of 10 M AgNO3 in propylene carbonate). The the nanocomposite were very low (8～12 ％) of all the potential of this reference electrode was ＋0.30 V vs. preparations due to dissolution of soluble oligomer Ag/Ag+ reference electrode using acetonitrile solution species by the chloroform washing. with same electrolyte salt. In this study, the potential Figure 2 shows TEM image of the nanocomposite + was expressed as vs. Ag/Ag using the acetonitrile solu- powder (Wre ＝45 wt％ sample). The certain species per- tion for comparison with other reports. The counter elec- ceived as PF films (5−15 nm) formed onto the aggregat- trode and the working electrode were sandwiched by ed KB particles were observed, and also bare KB sur- two glass plates through a cellulose separator (60 µm in faces were confirmed. At the low Wre samples (or high thickness). As an electrolyte, 1 M tetraethylammonium PF content samples), the PF species were increasingly tetrafluoroborate dissolved in propylene carbonate observed in the respective SEM images. In the case of (TEABF4/PC) was used for the electrochemical measure- sample of Wre ＝7.9 wt ％, the bulk PF fragments ments. The cell assembles and the electrochemical mea- (micrometer size) were observed, leading to low specific surements were carried out in argon atmosphere. capacitance. Cyclic voltammogram recorded at a scan rate of 5 mV −1 3 Results and Discussion s in 1 M TEABF4/PC for the PF/KB nanocomposite From FTIR spectra, the nanocomposite powder sam- electrode (Wre ＝40 wt％ sample) is depicted in Fig. 3(a) and clearly indicates that the electrochemical redox reaction (p-doping/dedoping) occurred reversibly com- pared with the voltammogram for pure KB electrode (Fig. 3(b)). The redox potential is ranged from＋0.4 to ＋1.0 V vs. Ag/Ag+, which is higher than the conventional polymers such as p3MeT (～＋0.7 V vs. Ag/Ag+)9） or Fig. 3 Cyclic voltammograms obtained for (a) PF/KB Fig. 2 TEM image obtained for PF/KB nanocomposite nanocomposite electrode (Wre ＝40 wt％), (b) pure KB powder (Wre ＝45 wt％). electrode as a blank. 75，No. 8（2007） 603 Fig. 4 Specific capacitance (Csp) obtained for PF/KB Fig. 5 Specific capacitance per PF mass (Csp) obtained for nanocomposite electrodes as a function of specific contents PF/KB nanocomposite electrodes as a function of specific of KB (Wre) in PF/KB nanocomposites. contents of KB (Wre) in PF/KB nanocomposites. other polythiophene derivatives (～＋0.9 V vs. Electrochim. Acta, 39, 273 (1994). Ag/Ag+)6,10,11）studied as electrochemical capacitors pre- 2）J. P. Ferraris, M. M. Eisssa, I. D. Brotherson, and D. C. viously. Specific capacitance (Csp) obtained for the Loveday, Chem. Mater., 10, 3528 (1998). PF/KB nanocomposite electrodes as a function of the Wre 3）D. Belanger, X. Ren, J. Davey, F. Uribe, and S. Gottesfeld, J. Electrochem. Soc., 147, 2923 (2000). is shown in Fig. 4. It is revealed the Csp value strongly −1 4）M. Mastragostino, C. Arbizzani, R. Paraventi, and A. depends on the Wre, and the maximum value of 160 F g Zanelli, J. Electrochem. Soc., 147, 407 (2000). is obtained for the sample of Wre ＝40 wt％. Figure 5 5）K. Naoi, S.