Self-Standing Carbonaceous Sheets Composed of Micro and Nanometer Fibers Derived Bamboo and Application to PEFC and Edlc
Authors: Miyoshi Haruka , Kinumoto Taro , Matsumura Takuya , Matsuoka Miki , Arai Yasuhiko , Tsumura Tomoki , Toyoda Masahiro 
Published: February 2020

Bamboo is a sustainable natural resource and the forests are considered to act as large carbon sinks, giving bamboo an advantage over other sustainable bioresources. However, it concerned that the rapid growth of bamboo may lead to biodiversity loss. This could be considered a serious disadvantage of bamboo in terms of its environmental impact. Bamboo has a fibrous microstructure and is composed mainly of cellulose, lignin, and hemicellulose, giving the isolation of cellulose fiber with relative ease. Owing to the unique microstructure of bamboo, we attempted to prepare self-standing carbonaceous fiber sheet by using bamboo as a starting material and have been investigated its application to polymer electrolyte fuel cells (PEFC) as the gas diffusion layer (GDL) [1].
The Bamboo (Phyllostachys edulis, moso bamboo) cut in Japan was employed in this study. The basic preparation procedure of the self-standing carbonaceous bamboo fiber sheet (CBFS) was reported in [1]. In brief, CBFS was prepared viadelignification, defibration, molding, and carbonization. The bamboo fiber was obtained after the defibration process and then it was subjected to molding by using a plastic mold. After molding, the resultant fiber sheet was immersed in polyvinyl alcohol (PVA) aqueous solution for addition of PVA into the sheet. We also prepared CBFS without PVA. Finally, the sheet was subjected to carbonization under argon atmosphere. Here, the CBFS added PVA is denoted as CBFS w/ PVA. The microstructure of the CBFS was observed by field emission scanning electron microscope (FE-SEM) and was analyzed by X-ray diffraction (XRD) and Raman spectroscopy to elucidate the carbonization degree. The both of the in-plane and the through-plane of electrical conductivity were measured and the contact angle for a water droplet was measured to evaluate hydrophobicity of CBFS.
A gas diffusion electrode was prepared by using CBFS, CBFS w/ PVA, the commercially available Pt-loaded carbon catalyst (Pt/C, TEC10E50E, Tanaka Kikinzoku Kogyo K. K., Japan) and Nafion ionomer solution by general manner. A single PEFC operation test was carried out by using Nafion117 membrane as the electrolyte and JARI’s standard PEFC single cell (the geometric electrode surface area was restricted to 1 cm × 1 cm).
FE-SEM observation image shows CBFS is composed by aggregation of the carbonaceous fibers. The average diameter of fiber in the CBFS is ca. 6 μm irrespective to PVA addition, which is very close to that in commercially available carbon paper-based GDL as TGP-H-120 (Toray). In the case of CBFS w/ PVA, thin films derived from PVA were observed between each fibers and the surface of the fiber was seemed to be smooth, suggesting that PVA was partially covered with the bamboo fiber. The electrical conductivity was surely increased by addition of PVA.
Figure 1 shows PEFC operation test results by using TGP-H-120, CBFS, and CBFS w/ PVA as GDL. The maximum power of CBFS was 135 mW cm-2, which is lower than that of TGP-H-120. On the other hand, the value was increased by addition of PVA and reached at 155 mW cm-2. Thus, the bamboo is expected as a potential GDL material.
We also prepared cellulose nanofibrils (cellulose nanofibers; named as NFC or CFS) from the bamboo through chemical and physical treatments. Then, self-standing carbonaceous nanofiber sheet have been already obtained. The preparation procedure and its application to electric double layer capacitor (EDLC) will be also presented.
Reference
[1] T. Kinumoto et al., ACS Sustainable Chemistry & Engineering, Vol. 3(7), 1374−1380 (2015).

Acknowledgement
This study was supported by The Environment Research and Technology Development Fund (ERTDF) from The Ministry of Environment, JAPAN, and the Venture Business Support Program from Oita University. The authors express gratitude to Mr. Yasuji Imahori Mr. Hideki Nakamizo, Mrs. Mayumi Nakamizo, and Mr. Yasuki Imahori of Silverloy Co. Ltd for supplying the bamboo strips.

Figure 1