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K. Cai, M. F. Foda, X. Lei and H. Han, Nanoscale, 2014, DOI: 10.1039/C4NR05473C.

1

Synthesis of functionalized 3D porous graphene using both ionic 1

liquid and SiO2 spheres as “spacers” for high-performance 2

supercapacitors application 3

Tingting Li, Na Li, Jiawei Liu, Kai Cai, Mohamed F. Foda, Xiaomin Lei and Heyou 4

Han∗ 5

State Key Laboratory of Agricultural Microbiology, College of Science, College of Food Science 6

and Technology, Huazhong Agricultural University, 1 Shizishan Street, Wuhan 430070, PR China 7

Abstract 8

In this paper, a high-capacity supercapacitor material based on functionalized three-dimensional 9

(3D) porous graphene was fabricated by low temperature hydrothermal treatment of graphene 10

oxide (GO) using both ionic liquid (IL) and SiO2 spheres as “spacers”. In the synthesis, the 11

introduction of dual “spacers” effectively enlarged the interspace between graphene sheets and 12

suppressed their re-stacking. Besides, the IL also acted as structure-directing agent played a 13

crucial role in inducing the formation of unique 3D architecture. Consequently, fast electron/ion 14

transport channels were successfully constructed and numerous oxygen-containing groups on 15

graphene sheets were effectively reserved, which had unique advantages in decreasing ion 16

diffusion resistance and providing additional pseudocapacitance. As expected, the obtained 17

material exhibited superior specific capacitance and rate capability compared to singly “spacer” 18

designed electrodes, and simultaneously maintained excellent cycling stability. Specifically, there 19

were nearly no loss of its initial capacitance after 3000 cycles. In addition, we further assembled a 20

symmetric two-electrode device using the material which showed outstanding flexibility and low 21

equivalent series resistance (ESR). More importantly, it was capable of yielding a maximum 22

power density of about 13.3 kW kg–1 with an energy density of about 7.0 W h kg–1 at a voltage of 23

1.0 V in 1 M H2SO4 electrolyte. All these impressive results demonstrate that the material obtained 24

by this approach is greatly promising for high-performance supercapacitors application. 25

∗To whom correspondence should be addressed. Fax: +86-27-87288505; Tel:

+86-27-87288505; E-mail: hyhan@mail.hzau.edu.cn.

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Keywords: 3D porous graphene, Dual “spacers”, Pseudocapacitance, Rate capability, 26

Supercapacitors 27

1. Introduction 28

Supercapacitors with high power density, excellent charging/discharging rate capability, and long 29

life-cycles have become one of the most intense research focuses in the electrical energy storage 30

field.1–4 They commonly store energy using either ion adsorption (electrochemical double layer 31

capacitors, EDLCs) or fast surface redox reactions (pseudocapacitors).3,5,6 It is well accepted that 32

the textural properties of electrode materials play a dominant role in the development of 33

supercapacitors. Naturally, it would be very interesting to develop a hybrid type of electrode 34

material with unique architecture, where EDLCs and pseudocapacitors can concurrently combine 35

to contribute to the high power property and better energy storage performances. In this point of 36

view, recent efforts have been focused on the preparation of high-capacity electrode materials, 37

which may be achieved both by providing desired electro-active species and by creating open 38

porous channels with enhanced specific surface area to improve the accessibility of the ions from 39

the electrolyte to the active regions of electrode materials. 40

As a promising electrode material, graphene, an atom-thick two-dimensional nanostructure, is 41

receiving growing attention due to their excellent electronic conductivity, good electrochemical 42

stability, high surface area and flexibility.4,7,8 Various morphologies of graphene or 43

graphene-based composites have been developed as electrode materials for supercapacitors.9–15 44

Nevertheless, easily and efficiently reducing of graphene oxide (GO) to reduced graphene oxide 45

(rGO) is still a key topic in this research field. Among all the reduction strategies, thermal 46

exfoliation of GO is conceived to be simple and environmentally friendly in which no hazardous 47

reductant is used. But this process usually requires a rapid heating (>2000 °C min–1) up to high 48

temperature, which means large energy consumption and critical treatment conditions.16 Recently, 49

hydrothermal treatment of GO has attracted more and more attention because of its outstanding 50

advantages, such as high yield, simple manipulation, easy control, environmentally friendly and so 51

on.17–19 Particularly, low temperature hydrothermal treatment can remain desired 52

oxygen-containing groups on the surface of graphene which not only enhance the surface 53

wettability of graphene electrodes but also significantly increase the specific capacitance by the 54

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