Fabrication and Basic Investigation of Flat Lignocellulosic Carbon Material for Self-Supporting Electrodes in Electric D
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Fabrication and Basic Investigation of Flat Lignocellulosic Carbon Material for Self-Supporting Electrodes in Electric Double-Layer Capacitors Tsubasa Funabashi1, Masamichi Sato2, Masao Kitajima3, Shuichi Shoji1, and Jun Mizuno4 1
School of Advanced Science and Engineering, Waseda University, Shinjuku, Tokyo 169-8555, Japan 2 Durarth Company, Tokorozawa, Saitama 359-1141, Japan 3 Waseda Research Institute for Science and Engineering, Shinjuku, Tokyo 169-8555, Japan 4 Institute for Nanoscience and Nanotechnology, Waseda University, Shinjuku, Tokyo 162-0041, Japan ABSTRACT A novel flat, wood-based carbon material with heterogeneous pores, referred to as flat lignocellulosic carbon material (FLCM), was successfully fabricated by carbonizing samples of the softwood Picea jezoensis (Ezomatsu or Jezo spruce, a Japanese conifer). Simultaneous improvements of the specific surface area of the FLCM and the affinity of electric double-layer capacitor (EDLC) for electrolyte solvents were achieved by vacuum ultraviolet/ozone (VUV/O3) treatment. The specific surface area of the VUV/O3-treated FLCM showed a 50% increase over that of the original FLCM. The spectra measured by X-ray photoelectron spectroscopy (XPS) indicated that the number of O-C=O (carboxyl or ester) bonds increased, whereas the number of C-C bonds decreased. Additionally, the feasibility of using the FLCM as a self-supporting electrode in EDLCs was examined by measuring the electrochemical properties in a threeelectrode system. The FLCM was confirmed as an appropriate self-supporting EDLC electrode material without warps and cracks. In addition, the FLCM can be used without any binder. Realization of FLCM-based EDLC electrodes with bendability, an area of several tens of square centimeters, and no risk of warp or crack formation, were indicated. Thus, FLCMs present a fascinating class of self-supporting carbon electrode materials for EDLCs. INTRODUCTION Recently, lignocellulosic carbon materials have been used in various devices. A typical device in which they are used is the electric double-layer capacitor (EDLC) [1]. In particular, the demand for EDLCs is growing rapidly because they are unique electrical storage devices that can provide a large amount of energy in a short time, have a high power density, and have a long cycle life as compared to that of conventional batteries [2]. EDLCs find applications in engine startup acceleration for electric vehicles [3], backup power supply for electric devices [4], load leveling [5], and storage of electricity generated from solar or wind energy [6]. Activated carbon materials are widely used to fabricate electrodes for EDLCs because they are inexpensive and have a large specific surface area. However, the use of a binder in activated carbon electrodes is unavoidable, and this degrades the EDLC performance [7,8]. Although many carbon materials made of lignocellulosic resources have been reported, there are very few cases of self-supporting carbon material for EDLC electrodes [9]. Even in those studies, the conventiona
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