Preparation and characterization of bamboo-based activated carbons as electrode materials for electric double layer capacitors

Abstract

1The everyday use of disposable bamboo-based products has prompted us to recycle their waste into carbon materials for adsorbents [1, 2] and electrodes for electric double layer capacitors (EDLCs). Hitherto diverse carbon precursors including abundant biomass materials have been investigated to prepare activated carbons (ACs) as electrode materials with high surface area, large porosity and homogeneous pore size [3][4][5]. In general, the pore structure and size of ACs strongly depend on the types of their precursor and activation process [6]. In this work, the activation 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 nonaqueous) electrolytes 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 fragmented into pieces (without dehydration), and heated at 700 ºC for 2 h in an argon (Ar) flow (500 ml/min). After drying at 120ºC overnight, the bamboo char mixed with KOH was heated at 800 ºC for 1 h in an Ar flow (800 ml/min). The heating rate was 10 ºC/min in all experiments. The resulting ACs was washed well with deionized water, and dried in vacuo at 80 ºC for 24 h. According to the mixing 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 addition, the apparent density (ρ) of the OBK3 sample was the lowest due to the high percentage of the large pore volume 2 (V tot -V micro ). The resistance of the OBK3 electrode is comparatively 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 morphology appear to drastically change in the OBK3 sample. were slightly shifted to the larger 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 (> 50nm). This indicates that the small pores grow into the large pores by the coalescence 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 1.5nm peak. It is likely to be due to the re-creation of pores on the morphology stabilized after the collapse over the whole structure. Base...

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