TaeYoung
Kim
a,
Hyun
Chang Kang
b,
Tran
Thanh Tung
c,
Jung
Don Lee
b,
Hyeongkeun
Kim
a,
Woo
Seok Yang
a,
Ho
Gyu Yoon
*b and
Kwang S.
Suh
*b
aElectronic Materials & Device Research Center, Korea Electronics Technology Institute, Seongnam 463-816, Korea
bDepartment of Materials Science and Engineering, Korea University, 5-1 Anam-dong, Seongbuk-gu, Seoul 136-713, Korea. E-mail: hgyoon@korea.ac.kr; suhkwang@korea.ac.kr; Fax: 82 2 929 4408; Tel: 82 2 927 4546
cSmart Plastics Group, European University of Brittany, LIMATB-UBS, Lorient, France
First published on 16th August 2012
We present a facile and scalable method for the rapid production of reduced graphene oxide (RG–O) by ionic liquid-assisted microwave chemistry. Microwave irradiation of graphite oxide (GO) in an ionic liquid (IL) enables the rapid reduction of GO within 15 s, producing RG–O containing the IL within their porous structures. The reduced graphene oxide prepared by IL-assisted microwave irradiation (mRG–O) electrodes exhibited a high specific capacitance of ∼135 F g−1, which can be attributed to the open architecture of the mRG–O filled with IL moieties. A supercapacitor made with mRG–O in an IL electrolyte operated at a voltage of 3.5 V and showed a high energy density (∼58 W h kg−1) and power density (∼246 kW kg−1).
Graphene, an atomically thin two-dimensional (2D) carbon structure14–16 with a theoretical specific surface area of 2630 m2 g−1 and high electrical conductivity, may be an ideal carbon electrode for EDL supercapacitors as both the surfaces of a graphene sheet are readily accessible to electrolytes.17–20 It has been previously demonstrated that supercapacitors based on chemically modified graphene electrodes exhibit specific capacitance values of 135 and 99 F g−1 in aqueous KOH and organic electrolytes,18 respectively. A variety of graphene-based electrode materials derived from graphite oxide (GO) have been reported to show capacitance values of ∼200 F g−1 in an aqueous electrolyte,18,21 ∼120 F g−1 in organic electrolytes,21,22 and ∼75 F g−1 in an ionic liquid electrolyte.21,23 We have previously demonstrated that reduced graphene oxide (RG–O) exhibits a high capacitance and energy density in an ionic liquid electrolyte when their surfaces are modified with poly(ionic liquids).24
The key challenges associated with the practical use of supercapacitors are finding the cost effective and large-scale production of graphene-based materials, as well as proper control over the effective surface area of the electrodes, which is accessible to electrolyte ions. The most common route towards bulk quantities of graphene-based materials involves the oxidation of graphite to graphite oxide, followed by exfoliation and reduction with chemical reducing agents, such as hydrazine, hydroquinone, sodium borohydride or ascorbic acid.25–28 It has been shown that GO can be reduced by rapid thermal heating under inert gas.22 As a rapid heating source, microwave irradiation has also been used to create exfoliated and reduced GO with loosely stacked platelets.29 The microwave-assisted irradiation provides a facile route to the production of RG–O electrode materials for supercapacitors, but with limited compatibility with high-voltage withstanding electrolytes, such as ionic liquids. More rapid and high yield production methods for RG–O, with an improved compatibility with electrolyte ions, are highly desirable for such applications as electrode materials for energy storage devices. From the perspective of microwave chemistry, ionic liquids (ILs) have received much attention as a new reaction media because they consist entirely of ions with high polarizability and, therefore, absorb microwave radiation efficiently to increase the yields of products with higher reaction rates.30,31 Other practical advantages of ILs include low vapour pressure evolution upon microwave irradiation ensuring safe operation.
By combining the advantages of both ILs and microwave heating, we describe here a facile and scalable route toward the production of RG–O based electrode materials for EDL capacitors. The IL-assisted microwave process leads to the rapid reduction of GO within 15 s. An additional advantage associated with this approach is that the IL, which is used as a reaction medium, is retained in the resulting RG–O structures, allowing for their direct use as electrolytes for supercapacitors. Since ILs are promising solvent-free electrolytes that greatly broaden the electrochemical window during device operation, the approach described herein enables the production of microwave-reduced GO (mRG–O) containing IL electrolytes and thus allows for high voltage supercapacitors with enhanced energy densities.
Electrochemical measurements were performed with an Eco Chemie Autolab PGSTAT100 potentiastat equipped with the FRA2 frequency response analyzer module. Electrical impedance spectroscopy (EIS) was done with an amplitude of 10 mV over a frequency range of 0.05 Hz to 1 MHz.
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Fig. 1 (a) Schematic representation of IL-assisted microwave reduction of GO and (b) photographs of microwave-reduced graphene oxide (mRG–O) prepared by microwave irradiation of GO powder with a small volume of the ionic liquid (left photo, GO/IL = 1/1, w/w) and the GO suspension in the ionic liquid (right photo, GO/IL = 0.8 mg ml−1) |
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Fig. 2 (a) SEM image of mRG–O platelets prepared by 15 s microwave irradiation of GO powder in an IL showing a 3D porous structure, (b) comparison of electron diffraction patterns of GO and mRG–O, (c) XRD patterns of GO and mRG–O. |
Fig. 2b compares the SAED patterns of GO and a mRG–O film. While GO shows clear diffraction spots of hexagonal patterns, mRG–O shows a ring pattern rather than a spot pattern indicating that the sample consists of randomly oriented layers. The relative intensities of the inner {100} type and outer {110} reflections indicates that the mRG–O platelets consist of large numbers of monolayers with disordered stacking.20
Fig. 2c compares the XRD pattern of GO and mRG–O. GO shows a XRD peak at 10.6° corresponding to a d-spacing of ∼0.83 nm. In contrast, this XRD peak is absent for mRG–O, except for a broad peak, which is due to mRG–O being largely exfoliated and reduced by microwave radiation.
To investigate the chemical functional groups attached to the graphene sheet, the GO and mRG–O samples were further characterized by FT-IR (Fig. 3a). The spectrum of GO indicates the presence of oxygen functional groups, including carbonyls CO (1727 cm−1), carboxyls C–O (1414 cm−1), epoxys C–O (1222 cm−1), and alkoxys C–O (1047 cm−1), which is consistent with the previously reported data.33–36 The feature at 1625 cm−1 is assigned to an aromatic C
C stretch of the underlying graphitic domains. After microwave treatment, the intensity of the absorption band due to the C
O groups (1727 cm−1) decreases significantly. Instead, new absorption bands appear at 1571, 1346, 1170, 1125, 791, 741 and 648 cm−1, which are attributed to the presence of EMIM-NTf2 in the mRG-O sample.37–42
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Fig. 3 (a) FT-IR spectra of GO and mRG–O. (b) C 1s and N 1s XPS spectra of GO and mRG–O with the peak deconvolution shown. (c) TGA plots of as-prepared GO versus mRG–O. |
Fig. 3b compares the XPS spectra of GO and mRG–O. The C 1s core-level spectrum of pristine GO indicates the presence of oxygen-based functional groups on the platelets with deconvoluted peaks at 286.1 (C–OH), 287.6 (CO), and 298 eV (COOH). The C 1s XPS spectrum of mRG–O indicates that these oxygen-based functional groups have been significantly removed by exposure to microwave radiation, which was evidenced by a significantly increased C/O ratio (from <1 to >3).
The TGA curve (Fig. 3c) of the GO sample showed a significant weight loss (∼50% relative to the starting materials) from 30 to 250 °C, mainly due to the evaporation of residual water molecules and the decomposition of the labile oxygen functional groups of GO.25 In contrast, mRG–O has a comparatively small amount of weight loss (∼10%) below 300 °C, indicating substantial removal of the oxygen-containing groups. Instead, mRG–O showed a weight loss from 350 to 450 °C due to the decomposition of EMIM-NTf2 in the mRG–O sample.
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Fig. 4 (a) Schematic of the assembled structure of the supercapacitor cell. (b) Cyclic voltammograms of the supercapacitors made with the mRG–O electrodes in EMIM-NTf2. The cycled potential ranges are 0 to 3.5 V with a scan rate of 120, 140 and 160 mV s−1. (c) Galvanostatic charge/discharge curves measured at different current densities. (d) Nyquist plot over the frequency range of 0.05 Hz∼1 MHz. |
The CV curves of the mRG–O electrodes were measured with various scan rates in the range of 120–160 mV s−1 for voltage sweeps between 0 and 3.5 V. As shown in Fig. 4b, the mRG–O electrode exhibits nearly rectangular CV traces even at a high scan rate, indicating good capacitive characteristics.17 The non-existence of any spurious peaks in the CV curves suggest that the pseudo-capacitance contribution arising from the presence of the functional groups on the surface of mRG–O is nominal and our device is primarily non-faradic within this voltage window.17,19 The supercapacitor assembled with the mRG–O electrode and IL electrolyte was stable up to 3.5 V, which is attributable to the good electrochemical stability of the IL electrolyte (see ESI†, Fig. S1). We note that the cell operated even at 3.8 V, but with limited cycle life. Hence, we used 3.5 V as a maximum cell voltage for further testing.
The galvanostatic charge/discharge curves for mRG–O (Fig. 4c) show good symmetry and fairly linear slopes with small IR drops, confirming the formation of an efficient electrical double layer and good charge propagation across the two electrodes.17
Assuming a symmetric capacitor, the specific capacitance was calculated by measuring the slope of the galvanostatic discharge curve according to the following equation:43
The mRG–O electrodes soaked in the EMIM-NTf2/AN electrolyte exhibited a specific capacitance (Csp) of 135 F g−1 at a current density of 2 A g−1. We note that the control supercapacitor cell with activated carbon (AC) and chemically reduced RG–O electrodes showed specific capacitance values less than 50 F g−1 in the same electrolytes (see ESI†, Fig. S2 and S3), which were significantly lower than for the cell with mRG–O electrodes. The charge storage ability of supercapacitors is directly linked to the wettability of the electrode materials with IL electrolytes. The IL, used as the medium for the microwave reaction, was retained within the mRG–O structures and thus improved the wettability between mRG–O and the IL electrolytes, which partly explains the substantial improvement in the capacitive current.
Fig. 4d shows the impedance curves of mRG–O electrodes as measured over the frequency range of 0.05 Hz–1 MHz. While a depressed semicircle in the high frequency region is modelled by a parallel combination of an interfacial charge transfer resistance and the double layer capacitance, a vertical curve in the low frequency region indicates a nearly ideal capacitive behaviour of the cell.44 By extrapolating the vertical portion of the plot to the real axis gives an equivalent series resistance (ESR) of ∼9 ohms, which is contributed to by both the electrolyte resistance and the electronic resistance of the mRG–O electrodes. A straight line with a slope of 45° in the low frequency range is represented by Warburg impedance, resulting from the frequency dependence of ion diffusion and transport in the electrolyte.44 The mRG–O electrode shows a short Warburg region, indicating the enhanced access of electrolyte ions in the porous structure of the electrodes.
To evaluate further the performance of the supercapacitors based on the mRG–O electrodes, the energy density (E) and power density (P) were calculated from the galvanostatic charge/discharge curves, considering the total mass (M) of active materials from both electrodes.40 The total resistance based on the internal resistance (IR) drop from the charge–discharge curves was used to calculate P and E for the assembled device, of which the value (∼8.6 Ω) was close to the value (∼9 Ω) obtained from the EIS measurement.
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Fig. 5 (a) Ragone plot of energy density versus power density. The energy density and power density were normalized to the mass of mRG–O from both electrodes (blue dots) and the total mass of the two electrodes, including the active materials, electrolyte, and current collectors. (b) Cycling performance of our supercapacitor, showing a capacitance retention of ∼97% after 1000 cycles of charging and discharging at a current density of 2 A g−1. |
Footnote |
† Electronic Supplementary Information (ESI) available: additional data for the supercapacitors with various electrode materials, including activated carbon, chemically-reduced graphite oxide and microwave-reduced graphite oxide. See DOI: 10.1039/c2ra21400h/ |
This journal is © The Royal Society of Chemistry 2012 |