Electrochemical Properties of Debris Free Reduced Graphene Oxide Synthesized from Natural and Synthetic Graphite

Abstract

Reduced graphene oxide (rGO) is a promising material for energy devices such as supercapacitors and lithium-ion batteries due to its high electronic conductivity and surface area. Since rGO is synthesized using GO as a precursor, the purity of GO will affect the properties of rGO. GO is known to possess impurities (often called debris), which is formed during the rigorous oxidation/exfoliation/reduction processes. Such debris are poorly conductive and are undesirable for electrode materials.1 Various studies have been conducted to understand the effect of debris on properties such as conductivity, chemical stability and catalytic activity.2-4 However, there is still no detailed investigation emphasizing the influence of debris on the electrochemical properties of rGO. In this study, we synthesized debris-free rGO by treating GO prepared from natural or synthetic graphite with alkaline solution and compared their electrochemical properties.GO was synthesized from natural graphite (Z-5F, Ito Graphite Co., Ltd.) or synthetic graphite (TIMAREX®KS4 GRAPHITE, Imerys Graphite & Carbon), by a modified Hummers method.5 The GO suspension was prepared by exfoliating in water with ultrasonication, and then refluxed in aqueous NaOH to remove debris. GO was re-dispersed in water and debris-free GO was re-dispersed in a 1:1 mixture of acetonitrile and water. rGO electrodes were prepared by casting the GO or debris-free GO dispersion on a glassy carbon (GC) electrode and subsequently reduced with hydrogen. Electrochemical measurements were conducted is 0.5 M H2SO4. Pt mesh was used as a counter electrode and Ag/AgCl was used as a reference electrode.Morphological observation and structural analysis, performed using scanning electron microscopy (SEM) and thermal gravimetric analysis (TGA), confirmed the removal of debris by the treatment with alkaline solution. Figure 1 shows SEM images of GO (A: natural GO, B: synthetic GO) and debris-free GO (C: debris-free natural GO, D: debris-free synthetic GO). Small fragments were observed on the surface of the natural and synthetic GO, whereas these fragments were absent on both surfaces after the treatment with alkaline solution. The TG results reveal that GO without alkaline treatment loses mass at around 200°C due to the loss of oxygen containing surface functional groups, whereas no significant changes were seen for both debris-free natural and synthetic GO. The electrochemical characteristics of the rGO and debris-free rGO, as well as GO and debris-free GO, were examined. Details of the electrochemical performance will be discussed at the meeting.This work was partially supported by an Advanced Low Carbon Technology Research Development Program (JST-ALCA, JPMJAL1008). P. Rourke, P. A. Pandey, J. J. Moore, M. Bates, I. A. Kinloch, R. J. Young and N. R. Wilson, Angew. Chem. Int. Ed., 50, 3173-3177 (2011).Guo, S. Wang, G. Wang, Z. Niu, J. Yang and W. Wu, Carbon N. Y., 76, 203–211 (2014).Faria, D. Ste, A. C. M. Moraes, M. E. H. Maia, E. B. Barros, A. G. S. Filho, A. J. Paula and O. L. Alves, Chem. Mater., 24, 4080-4087 (2012).Su, M. Acik, K. Takai, J. Lu, S. Hao, Y. Zheng, P. Wu, Q. Bao, T. Enoki, Y. J. Chabal and K. P. Loh, Nat. Commun., 71, 1–9 (2012).S. Hummers, J. Am. Chem. Soc., 80, 1339-1339 (1958). Figure 1