(Invited) Performance As Electrode for Electric Double-Layer Capacitor of Activated Carbon Derived from Bamboo Residue after Steam Treatment Activated By Alkaline Metal Hydroxide

Introduction Biomass-derived activated carbon can be the electrode material for high performance EDLCs, owing to its high surface area, porosity, and excellent conductivity1). The performance of EDLC electrode can be presumed to depend on pore size distribution from the theory of electric double layer. In Japan, the disorderly expansion of bamboo forest is serious problem because it causes economical damage. Therefore, we have already proposed “cascading usage of bamboo”, the extraction of saccharides by hydrolysis and the usage of solid residue as the precursor for EDLC electrode, as the promising candidate of the solution for the bamboo problem2). The alkaline metal hydroxide, such as KOH and NaOH, is known to be the activator for preparing activated carbon, and the pore size distribution could be presumed to depend on the kind of alkaline metal hydroxide from the proposed activation mechanism. In this study, the solid residue of steam-treated bamboo is used as a precursor for activated carbon for the activation by KOH or NaOH, and the performance as EDLC electrode of the activated carbons is investigated in detail. Experimental Moso bamboo powder and water were separately set in a container for autoclave (HU-100, Sanai Science Co., Ltd.). The container was heated at 200°C for 2.5 hours, which was chosen based on previous work3). The treated bamboo was dispersed in distilled water at room temperature for 1 hour for the extraction of sugars. The solid residue after filtration was heated at 500oC for 1 hour with N2 gas flowing for carbonization. The carbonized sample was mixed with KOH or NaOH, and then the mixture was heated at 800°C for 1 hour with N2 gas flowing. The activated sample washed with heated distilled water for several times and then dried. Raman Spectroscopy (NRS-5100, JASCO Corporation) utilizing a 532 nm laser for excitation was utilized to examine the degree of graphitization within the sample structures. The morphology of the samples was examined by Field Emission Scanning Electron Microscope (FE-SEM) from Japan Electronics Co., Ltd (Model JSM-6701F). The adsorption isotherms of the samples for N2 (at 77 K) and CO2 (at 298 K) was measured by a commercial apparatus (BELSORP-miniII, MicrotracBEL). The electrochemical performance, such as cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), and galvanostatic charge/discharge measurement, of the sample was evaluated by using two-electrode cell with 1 M (C2H5)4NBF4 (TEABF4)/PC as electrolyte. Results and Discussions The yield, which is (weight after activation)/(weight before activation), decreased at higher weight ratios of KOH or NaOH, and the yield for NaOH activation was lower than that for KOH activation. The Raman spectra of the samples indicated that the samples have amorphous structure. The SEM images showed that the activated samples kept the microstructure derived from plant tissue. The N2 adsorption isotherms at 77 K for all the activated samples exhibited type I, indicating a microporous structure. However, the NaOH-activated samples displayed gentler slopes at low relative pressure, suggesting wider micropore size distribution. That is, the micropore size distribution depended on the kind of alkaline metal hydroxide. The capacitance values calculated from discharge process at 10 mA g-1 from 2.5-0 V was ca. 25 F g-1, which is comparable to the reported values. The capacitance values calculated from CV curves of NaOH-activated sample, 37.6 F g-1 at 1 mV s-1, was slightly higher than that of KOH-activated sample. Moreover, the decrement of the capacitance value at higher scan rate was trend to be suppressed for NaOH-activated samples. The capacitive contribution calculated according to the Dunn method4) indicated that NaOH-activated carbon showed a higher degree of capacitive contribution (74 %). The wider pore size distribution of NaOH-activated sample could contribute the suppression of the decrement of the capacitance value at higher scan rate. Conclusions The chemical activation by hydroxide of alkaline metal, such as KOH and NaOH, was performed to the solid residue after the extraction of steam-treated bamboo. From the results of the capacitance value, the suppression of the decrement of the capacitance value, and the capacitive contribution, the performance as EDLC electrode of NaOH-activated sample was better than that of KOH-activated sample. The wider pore size distribution for NaOH-activated sample could be one of the reasons for the higher performance. References 1) K.Ö. Köse et al. (2018) International Journal of Hydrogen Energy, 43:18607–16. 2) T. Tsubota et al., (2018) Journal of Porous Materials, 25: 1541-1548. 3) D.A. Khuong et al. (2023) Materials Chemistry and Physics, 304:127853. 4) D. Dubey et al. (2023) Journal of Energy Storage, 58:106441. Figure 1