課題番号 ：S-16-NM-0068
利用形態 ：機器利用
利用課題名（日本語） ：グラフェン複合材料による高エネルギー密度スーパーキャパシターの開発
Program Title (English) ：Graphene SupercapacitorsStudy（ on Advanced Graphene Materials for Electrode in Supercapacitors with high energy density）
利用者名（日本語） ：
Username (English) ：Li Jing
所属名（日本語） ：筑波大学大学院数理物質科学研究科化学専攻
Affiliation (English) ： Graduate School of Pure and Applied Sciences, University of Tsukuba

１．概要（Summary）
The graphene material prepared by the chemical reduction method usually has oxygenic functional groups in it and such functional groups often result in interactions between the graphene electrode and the electrolyte in supercapacitors. We have examined the existential form of interactions between graphene as the electrode and three kinds of ionic liquids, EMI-TFSI and EMI-BF4. Fourier transform infrared spectroscopy (FT-IR) analyses confirmed that the residual hydroxyl groups in graphene were transferred to EMI+ and TFSI- lost oxygen atoms to graphene, while little reaction took place in BF4- during the process of charging. The chemical reactions are suggested to contribute to the device capacitance while it is also one of the reasons for the decreased electrochemical stability window. Graphene-based supercapacitors in EMI-TFSI electrolyte showed the higher energy density (156 Wh/kg) than that in EMI-BF4 under the same operating voltage.

２．実験（Experimental）
【利用した主な装置】
Fourier transfer infrared spectroscopy (FT-IR)
Laser Raman Microscope
Platform staff provided sufficient support to use these equipments.

【実験方法】
Firstly, we charge the graphene supercapacitors to a specific voltage where reactions will happen. Afterwards, we disassemble the cell and take out electrodes. Then we characterize the variations on graphene electrode by FT-IR, which is the reflection of reactions.

３．結果と考察 （Results and Discussion）
According to results of FT-IR (Figure 1), there are three kinds of oxygenous functional groups on GOs. They are hydroxyl, carboxyl and epoxy groups. The peak in the range from 3500 cm-1 to 3000-1 corresponds to the single bond between oxygen and hydrogen (O-H), indicating the existence of hydroxyl groups. At the same time, the peak around 1750 cm-1 results from the double bond between carbon and oxygen (C=O), which is the characteristic of carboxyl groups. Moreover, peaks in the range from 1200 cm-1 to 900 cm-1 reflect the single bond between carbon and oxygen, which may be from hydroxyl groups or epoxy groups.

Fig. 1 FT-IR of graphene electrodes before (black) and after (red) charging in EMI-TFSI.
After charging in EMI-TFSI, the peak of O-H on electrodes have an obvious decrease, indicating that hydroxyl groups are removed from graphene and connect to ions in EMI-TFSI. At the same time, the peak of C=O decreases after charging, which implies that oxygens in TFSI- connect to graphene.
When the anion of electrolyte is changed from TFSI- to BF4-, there is a similar phenomenon on the peak of O-H (Fig 2). It shows that hydroxyl groups connect to EMI, rather than BF4 or TFSI. However, the spectrum doesn’t reflect little changed on the peak of C=O. It is supposed that BF4- has little reactions with graphene electrode under charging.

Fig 2 FT-IR of graphene electrodes before (black) and after (red) charging in EMI-BF4..

Combining with the two spectra, the decreased peak of O-H is due to that hydroxyl groups are connected to EMI-TFSI. At the same time, the lost oxygen atoms from TFSI- connect to carbon atoms on graphene by double bond, which results in the increase of corresponding peak. Besides, BF¬4- has little reactions with graphene electrode under charging.

４．その他・特記事項（Others）
The research is financial supported by Japan Science and Technology Agency and Advanced Low Carbon Technology Research and Development Program.

５．論文・学会発表（Publication/Presentation）
(1) J. Li, J. Tang, J. Yuan, K. Zhang, Q. Shao, Y. Sun and L.-C. Qin, “Interactions between Graphene and Ionic Liquid Electrolyte in Supercapacitors” Electrochimica Acta 197 (2016) 84–91

(2) Jing Li, Jie Tang, Jinshi Yuan, Kun Zhang, Qingguo Shao, Yige Sun, Lu-Chang Qin, “A Study on Interaction between Ionic Liquids and Electrodes in Graphene Supercapacitors”, 2015 MRS Fall Meeting, 2015, Nov 29 – Dec 4.

６．関連特許（Patent）