課題番号 ：S-19-SH—0009
利用形態 ：共同研究型支援
利用課題名（日本語） ：新規カーボン材料の開発
Program Title (English) ：Development of New Carbon Nanomaterials
利用者名（日本語） ：ウィシュダー・サッタヤラット1), 藤重雅嗣/2),竹内健司/2),
　　　　　　　　　　　　　ウィナッダー　ウォンウィリヤパン1)
Username (English) ：P.Ukkakimapan1), M. Fujishige2),K. Takeuchi2),W. Wongwiriyapan1)
所属名（日本語） ：1) キングモンクット工科大学ラドクラバン, 2) 信州大学
Affiliation (English) ：1) King Mongkut’s Institute of Technology Ladkrabang, 2) Shinshu University

１．概要（Summary ）
In this study, nitrogen self-doped activated carbons (ACs) obtained via the direct activation of Samanea saman green leaves (SSLs) for high energy density supercapacitors were investigated. The SSL-derived direct-activated carbons (hereinafter referred to SD-ACs) were synthesized by using NaOH as an activating agent and heating up to 720 °C without a hydrothermal carbonization or pyrolysis step. Surpassing the ACs derived from the two-step convention method, SD-ACs showed superior properties, including a higher surface area (2930 m2/g and nitrogen content (4.6 at%).

２．実験（Experimental）
Samanea saman leaves (SSLs) powder was directly activated (hereinafter referred to SD-AC) with NaOH at different ratios of 1:1.5, 1:1.75 and 1:2 (hereinafter referred to as SD1.5, SD1.75 and SD2, respectively). The activation was conducted at 720 °C at a heating rate of 5 °C/ min for 1 h under an argon at a flow rate of 500 mL/min. For comparison, AC derived from SSL was also prepared via a conventional process (hereinafter referred to as S-AC). The elemental and chemical compositions were characterized using X-ray photoelectron spectroscopy (XPS, PHI Quantera II). Nitrogen adsorption–desorption isotherm measurements were performed using a gas adsorption analyzer (Micromeritics ASAP 2020).

３．結果と考察（Results and Discussion）
Fig. 1a shows the isotherms of the SD- and S-ACs, revealing a steep curve at the low-pressure range, then parallel to the relative pressure axis at the high-pressure range. Hence, the SD- and S-ACs were identified as Type I isotherms (according to the IUPAC classification)[1], implying a microporous structure. A larger adsorption volume implies a larger surface area. Among the SD-ACs, SD2 showed the highest surface area and the highest total pore volume. The significant increase in surface area for SD2 compared that of S-ACs may be attributed to the synergistic effect of the organic matter decomposition and NaOH chemical activation.

Fig. 1 (a) Nitrogen adsorption and desorption isotherms at 77 K, and (b) pore size distribution of SD1.5, SD1.75, SD2 and S-AC.
The quantitative analysis of the elemental composition is summarized in Table 1. The nitrogen contents in SD1.5, SD1.75, SD2 and S-AC are 3.6, 4.2, 4.6 and 1.7 at%, respectively. The nitrogen content in SD2 was approximately 3 times higher than that in S-AC. Moreover, compared to the carbonized S-ACs, the nitrogen content in S-AC after activation decreased from 2.2 at% to 1.7 at%. During carbonization, chlorophyll may be decomposed with other organic compounds, resulting in a decrease in the nitrogen content. The source of nitrogen in SSLs may come from chlorophyll, which is related to green pigments found in the chloroplasts of plants, providing nitrogen self-doped ACs.
Moreover, the nitrogen content in the SSL-derived AC is approximately 4.6 at%. This value is comparable to the nitrogen-doped mesoporous by post treatment under an ammonia atmosphere. These results imply that the direct activation of nitrogen-containing agricultural waste, such as SSLs, is a promising approach to obtain nitrogen self-doped activated carbon.
The N1S spectra were further deconvoluted to analyze their nitrogenated structures, as shown in Fig.2. There are four main peaks of nitrogen in different chemical states in the carbon network located at approximately 398.3, 399.8, 401.1 and 403.6 eV, corresponding to pyridinic nitrogen (N-6), pyrrolic nitrogen (N-5), quaternary nitrogen (N-Q) and oxidized nitrogen (N-X), respectively.The content of each nitrogenated structure was calculated based on the peak area, as shown in Table 1. Interestingly, N-5 was the main component of all the samples, while other nitrogen configurations were in the following order: N-6 > N-Q > N-X. The origin of the dominant N-5 is depicted from the structure of chlorophyll [2].
The direct activation exhibited advantages in terms of higher surface area and higher nitrogen content. Moreover, the direct activation is simple and not time-consuming. The productivity of the SSL-derived ACs is comparable to other plant-based biomass-derived ACs by the conventional method, and higher than the ACs derived from animal sources, such as silkworm pupae.

Table 1 XPS peak analysis of the SD- and S-ACs

Fig. 2 XPS spectra of SD- and S-ACs.

４．その他・特記事項（Others）
[1] M. Thommes, K. Kaneko, A. V. Neimark, J. P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol and K. S. W. Sing, Pure Appl. Chem., 2015, 87, 9.
[2] H. Chen, F. Yu, G. Wang, L. Chen, B. Dai and S. Peng, ACS Omega, 2018, 3, 4724.

５．論文・学会発表（Publication/Presentation）
(1)V. Sattayarut, T. Wanchaem, P. Ukkakimapan, V. Yordsri, P. Dulyaseree, M. Phonyiem, M. Obata, M. Fujishige, K. Takeuchi, W. Wongwiriyapan and M. Endo: “Nitrogen self-doped activated carbons via the direct activation of Samanea saman leaves for high energy density supercapacitors” RSC Advances, 9, 21724 – 21732 (2019).

６．関連特許（Patent）
なし