Electrochemical Processing and Chemical Modification of Carbon Cloth for Capacitive Applications

Abstract

1. Introduction The market for portable and/or wearable electronics is continually expanding, owing to the rapid worldwide consumer adoption of smart phones. For this reason, electrode materials with lightweight and flexibility have attracted considerable attention as their power sources. Among them, we focus carbon cloth (CC), composed of numerous uniform activated carbon fibers, as the lightweight and inexpensive electrode material in EDLCs and the current collector to support pseudocapacitive materials. Basically, commercial or untreated CC (uCC) exhibits only poor electrochemical activity. However, the EDLC capacitance of CC can be enhanced by oxidizing it, followed by heating at high temperature under reductive gas atomspheres.1,2 In the present study, we doped nitrogen into CC, simultaneously with reduction, through a hydrothermal reaction of oxidized CC for the first time, and their electrochemical properties were explored. 2. Experimental A section of commercial CC, denoted hereafter as uCC, was oxidized in a solution of HNO3/H2SO4 mixed acids containing KMnO4. The thus-obtained CC (oxidized CC, oCC) was transferred to an autoclave with aqueous hydrazine/ammonia solution and hydrothermally treated at 160 °C, referring the procedure employed to reduce graphene oxide to graphene.3 The CC samples subjected to each process were characterized by XPS and other techniques. Electrochemical tests were made in aqueous electrolytes with and without K4[Fe(CN)6]. 3. Results and discussion XPS spectra in C 1s region of the CC samples presented four typical peaks assignable to C=C (284.6 eV), C-OH (286.2 eV), C=O (287.3 eV), O-C=O (288.7 eV). uCC consisted only of C=C (sp2 carbon). In oCC, the signal due to C=C remarkably decreased in intensity, while those related to the oxygen-containing groups (C-OH and C=O) were revealed. These components decreased again after the hydrothermal treatment with hydrazine/ammonia at 160 °C. At this time, the involvement of nitrogen was evidenced by the signals in N 1s region; i.e., graphitic-N (401.6 eV), pyrrolic-N (400.3 eV), and pyridinic-N (398.6 eV). To the best of our knowledge, this is the first report on the preparation of N-doped carbon cloth via a hydrothermal reaction. The product is denoted as N160-CC. The atomic ratio of N to C was estimated to be 0.11. The dominant species was pyridinic-N whose content increased with an increase in hydrothermal temperature from 25 °C to 160 °C. In CV measurements with K4[Fe(CN)6], the redox peaks of the [Fe(CN)6]4-/[Fe(CN)6]3- couple were detected with oCC and N160-CC, whereas they were hardly observed with uCC. The peak separation was narrower with N160-CC than with oCC, indicating faster electron transfer kinetics resulting from the simultaneous reduction and N-doping of oCC during the hydrothermal treatment. In galvanostatic charge/discharge tests in a 1.0 M H2SO4 solution. The areal capacitance (capacitance normarized by the unit area of the electrode) of N160-CC was estimated to be 136 mF cm-2 at 0.5 mA cm-2, being larger than that (76 mF cm-2) of the oCC piece before hydrothermal treatment. At the talk we will provide information regarding the effects of other strategies including electrochemical processing and chemical modification on the capacitance of CC. References (1) G. Wang, H. Wang, X. Lu, Y. Ling, M. Yu, T. Zhai, Y. Tong, Y. Li, Adv. Mater, 26, 2676 (2014). (2) S. Jiang, T. Shi, X. Zhan, H. Long, S. Xi, H. Hu, Z. Tang, J. Power Sources, 272, 16 (2014). (3) D. Long, W. Lei, L. Ling, J. Miyawaki, I. Mochida, S.-H. Yoon, Langmuir, 26, 16096 (2010).