Preparation and characterization of bamboo-based activated carbons as electrode materials for electric double layer capacitors Yong-Jung Kim a, * , Byoung-Ju Lee b , Hiroaki Suezaki b , Teruaki Chino b , Yusuke Abe b , Takashi Yanagiura b , Ki Chul Park b , Morinobu Endo a,b a Institute of Carbon Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan b Department of Electric and Electronic Engineering, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan Received 9 November 2005; accepted 8 February 2006 Available online 29 March 2006 Keywords: Char; Activation; BET surface area; Microporosity; Electrochemical properties The everyday use of disposable bamboo-based products has prompted us to recycle their waste into carbon materi- als for adsorbents [1,2] and electrodes for electric double layer capacitors (EDLCs). Hitherto diverse carbon precur- sors including abundant biomass materials have been investigated to prepare activated carbons (ACs) as elec- trode materials with high surface area, large porosity and homogeneous pore size [3–5]. In general, the pore structure and size of ACs strongly depend on the types of their pre- cursor and activation process [6]. In this work, the activa- tion of bamboo-based carbons was examined at various ratios of KOH. Furthermore, the specific capacitance of the bamboo-based AC electrodes was evaluated using two different kinds of (aqueous and non-aqueous) electro- lytes with different ion sizes, whereby detailed information about the structure and size of the developed pores has been obtained. As a typical procedure, a pristine bamboo was frag- mented into pieces (without dehydration), and heated at 700 °C for 2 h in an argon (Ar) flow (500 ml/min). After drying at 120 °C overnight, the bamboo char mixed with KOH was heated at 800 °C for 1 h in an Ar flow (800 ml/ min). The heating rate was 10 °C/min in all experiments. The resulting ACs was washed well with deionized water, and dried in vacuo at 80 °C for 24 h. According to the mix- ing ratios (bamboo-based char/KOH), the samples were designated as OBC (no addition of KOH), OBK1 (1/1), OBK2 (1/2), OBK3 (1/3) and OBK4 (1/4). The details of the capacitance measurement have been described in the previous report [7]. The fundamental properties of the bamboo-based char and ACs are summarized in Table 1. The relative mesopore fraction volume (V meso /V micro ) and area (S meso /S micro ) exhibited the highest values in the OBK3 sample. In addi- tion, the apparent density (q) of the OBK3 sample was the lowest due to the high percentage of the large pore volume (V tot –V micro ). The resistance of the OBK3 electrode is com- paratively low, which would result from the low diffusion resistivity of the electrolyte through the relatively large pores (meso/macropores) of the electrode. In general, a pore widening is caused by a relatively high concentration of activating agent in the presence of many open-pores, through which the activating agent penetrates into the structure. Therefore, pore structure and macroscopic mor- phology appear to drastically change in the OBK3 sample. Fig. 1 shows the variation of the pore-size distribution (PSD) evaluated by a density functional theory (DFT) method. The OBC sample provided the simple peak of 1.86 nm, accompanied by the large pores of 30–200 nm. After KOH activation, the dominant peak was shifted to the smaller range, providing two peaks at 0.7 and 1.5 nm. The variation manner of the PSD is well consistent with a concept of pore-width development, which results from the repetition of collapse and recombination of pores by KOH activation. The two dominant peaks (0.6 and 1.4 nm) observed in OBK1 were slightly shifted to the lar- ger values in OBK2, accompanied by the increase in the incremental pore volume. In OBK3, however, the peak at 0.7 nm has remarkably decreased with the increase in the 1.5-nm peak and the macropores (>50 nm). This indicates that the small pores grow into the large pores by the coa- lescence with each other and/or by the collapse of the small pores themselves. In OBK4, the peak at 0.7 nm increased again with the decrease in the 1.5 nm peak. It is likely to be due to the re-creation of pores on the morphology sta- bilized after the collapse over the whole structure. Based on these results, it is concluded that the development of micropores is nearly completed at 2-fold amount of KOH addition, and further addition of KOH forms larger size of pores. 0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.02.011 * Corresponding author. Tel.: +81 26 269 5655; fax: +81 26 269 5208. E-mail address: yjk@endomoribu.shinshu-u.ac.jp (Y.-J. Kim). 1592 Letters to the Editor / Carbon 44 (2006) 1581–1616