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Ionic liquid-induced three-dimensional macroassembly of graphene and its applications in electrochemical energy storage Chengzhou Zhu, Junfeng Zhai and Shaojun Dong* Ionic liquid (IL)-induced three-dimensional macroassembly of graphene (GN) has been achieved through one-step hydrothermal treatment. Significantly, the three-dimensional GN–IL (TGN–IL) nanostructures
Received 4th May 2014 Accepted 17th June 2014
provide ideal electrode materials for supercapacitors because they combine the unique properties of GN
DOI: 10.1039/c4nr02400a
and IL in overcoming the restacking of GN, enlarging the specific surface area, improving the GN conductivity and ensuring the high electrochemical utilization of GN as well as the open channels
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provided by 3D nanostructures.
Introduction Graphene (GN), a single-layer sheet of sp2 hybridized carbon atoms, is recognized as the basic building block of all dimensional graphitic materials.1 The unique structure endows GN with various superior properties such as high electrical and thermal conductivities, good transparency, great mechanical strength, inherent exibility, and huge specic surface area (SSA).2–4 Therefore, GN has been proven to nd wide applications during recent years in the eld of sensing,5,6 electronics and optoelectronics,7,8 electrochemical energy storage,9,10 efficient catalysis,11,12 nanocomposites,13,14 etc. Signicantly, the emergence of GN provides an excellent alternative to electrode materials, and great efforts have been made to utilize GN-based nanomaterials as promising supercapacitor electrode materials with high performance in energy devices due to high electrical conductivity, large SSA, profuse interlayer structure and abounding functional groups involved.15–17 Specically, GNbased nanomaterials derived from graphene oxide (GO) can be manufactured on a ton scale at low cost, making them potentially cost-effective materials for supercapacitors.18 A supercapacitor based on chemically-modied GN electrodes exhibited specic capacitances of 135 and 99 F g1 with aqueous and organic electrolytes, respectively.19 Thereaer, there are many reports on GN-based supercapacitors, while the specic capacitances of which were about 200 F g1.20–24 Although these GN-based supercapacitors have shown good power density and life-cycle stability, their specic capacitances still fall far below the theoretical value of 550 F g1 calculated for a single-layer GN.19,25 A reduction in the SSA of GN due to restacking during its processing as a result of strong van der State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. E-mail: [email protected]; Fax: +86-431-85689711; Tel: +86-431-85262101
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Waals interactions accounts for the overall low capacitance. In this regard, great contributions have been made to design highperformance GN-based nanomaterials aiming to enlarge the SSA, improve the conductivity, and control the pore size, layer stacking and distribution of GN. For example, Ruoff et al. have synthesized a novel GN by simple activation with KOH, achieving high SSA values of up to 3100 m2 g1. A specic capacitance of above 166 F g1 was obtained at a current density of up to 5.7 A g1.26 In addition, many researchers have focused on introducing stabilizers or spacers, such as poly(sodium 4styrensulfonate),27 water28 and carbon nanotubes,9 into GN layers to inhibit the restacking of GN, improve the electrolyte– electrode accessibility in supercapacitors and thus enhance specic capacitances of the obtained GN materials. Compared with 2D lms, less-agglomerated, self-supported, and binder free GN-based 3D nanostructures with suitable pore sizes are highly needed in constructing high-performance electrochemical energy devices. To date, several approaches have been developed to construct 3D graphene-based nanomaterials, such as self-assembly,29,30 hydrothermal treatment,31,32 leavening strategy,33 and so on.34,35 Shi's group has prepared a series of self-assembled GN hydrogels.32,36 The unique structure of these GN nanostructures endowed them with high mechanical strength and a well-dened 3D porous network, which offers an opportunity to optimize the ionic diffusion in GN-based electrodes for supercapacitors. Duan et al. have synthesized functionalized GN hydrogels through a convenient one-step chemical reduction of GO using hydroquinones as reducing and functionalizing molecules simultaneously, which showed an impressive specic capacitance of 441 F g1 at 1 A g1 in the 1 M H2SO4 aqueous electrolyte.37 Although assembling GN into freestanding 3D hydrogel nanostructures with a controlled microstructure was regarded to be an effective strategy for increasing their electrochemical performances, their specic capacitances are still unsatisfactory and needed to be further improved.
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Herein, we reported the ionic liquid (IL)-induced 3D macroassembly of GN through a one-step hydrothermal treatment and used them as advanced electrode materials for supercapacitors. The introduction of IL can not only effectively reduce GO, but also plays a key role in the formation of 3D GN (TGN–IL) nanostructures. The obtained TGN–IL nanostructures provide ideal electrode materials for supercapacitors because they combined the unique properties of GN and IL in overcoming the restacking of GN, improving the conductivity, enlarging the SSA and ensuring the high electrochemical utilization of GN as well as the open channels provided by 3D nanostructures. Signicantly, the specic capacitance was evaluated as 373 F g1 for TGN–IL nanostructures, which was much higher than that of 3D GN (TGN) nanostructures (241 F g1). At the same time, they also exhibited large energy density and good stability aer 1000 charge/discharge cycles. Considering the high electrochemical performance, the TGN–IL nanostructures may nd use in advanced electrochemical energy storage devices.
Experimental section
Paper
GO aqueous dispersion (2 mg mL1) was obtained and used for further synthesis of TGN–IL nanostructures. Synthesis of TGN–IL nanostructures 1-(3-Aminopropyl)-3-methylimidazoliumbromide (IL–NH2) was rst prepared according to a previously reported method.39 The obtained IL–NH2 (120 mg) was added into 30 mL of a GO dispersion (2 mg mL1), and then the aggregation of the GO occurred due to the salt effect induced by IL–NH2. Then, NaOH (60 mg) was added into the above turbid mixture, and the mixture was subjected to sonication for 30 min. Aer sonication, the turbid mixture was transformed into a homogeneous and transparent dispersion. The obtained dispersion was transferred into the container of a Teon-lined stainless steel autoclave, sealed and maintained at 100 C for 12 h. Then, the as-prepared TGN–IL hydrogel was dialyzed against deionized water. The TGN–IL hydrogel was freeze-dried to obtain the nal product. As a control, pure TGN hydrogel was also prepared using the same method at 180 C with the exception of IL–NH2 and NaOH.32
Chemicals
Electrochemical measurements
Graphite was purchased from Alfa Aesar. 3-Bromopropylamine hydrobromide (98%) was obtained from Aldrich. 1-Methylimidazole (98%, Linhai Kaile Chemicals, China) was distilled under a reduced pressure before use. Unless otherwise stated, water used throughout all experiments was puried with a Millipore system.
A suspension of the as-prepared materials (TGN–IL or TGN) at a concentration of 1 mg mL1 was prepared by ultrasonically dispersing them in water. Prior to the surface coating, GCE (3 mm in diameter) was polished with 1.0 and 0.3 mm alumina slurry sequentially and then washed ultrasonically with water and ethanol for a few minutes. The suspension (5 mL) was then dropped onto the GCE and dried at room temperature. Then, 5 mL of Naon (0.2%) was placed on the surface of the above modied GCE and dried before use in electrochemical experiments.
Instrumentation Infrared spectra were collected on a VERTEX 70 Fourier transform infrared (FTIR) spectrometer (Bruker). X-ray diffraction (XRD) spectra were obtained by using a D8 ADVANCE (Germany) ˚ radiation source. X-ray diffractometer with a Cu Ka (1.5406 A) photoelectron spectroscopy (XPS) analysis was carried out on an ESCALAB MK II X-ray photoelectron spectrometer. The electrochemical impedance (EIS) experiments were performed in 1 M H2SO4. The spectra were measured by using Autolab with PGSTAT 30 (Eco Chemie B.V., Utrecht, The Netherlands) and with the aid of a frequency response analysis system soware using an oscillation potential of 5 mV over a frequency range of 100 kHz to 0.1 Hz. Electrochemical measurements including cyclic voltammetry and galvanostatic charge/discharge were performed with a CHI 660 electrochemical analyzer (CH Instruments, Inc., USA). A conventional three-electrode cell was used, which consisted of an Ag/AgCl (saturated KCl) electrode as the reference electrode, a platinum wire as the counter electrode, and the TGN–IL and TGN modied glassy carbon electrodes (GCE) as working electrodes. Preparation of GO Graphite oxide was synthesized from the natural graphite powder based on a modied Hummers method.38 Then, exfoliation of graphite oxide to GO was achieved by ultrasonication for 40 min (1000 W, 20% amplitude). Finally, a homogeneous
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Results and discussion IL has attracted an increasing amount of interest in electrochemical sensing,40,41 electrocatalysis42,43 and electrochemical energy storage,44,45 owing to its high conductivity, chemical and thermal stability, and wide electrochemical window. It is expected that combining GN with IL will provide fertile opportunities for the construction of GN-based hybrid nanocomposites and their potential applications. As mentioned above, the construction of TGN nanostructures with high mechanical strength and a well-dened 3D porous network is favorable for the ionic diffusion and thus the enhancement of the electrochemical performance of GN-based electrodes for supercapacitors. However, there are few reports on the synthesis TGN–IL nanostructures and their use in supercapacitors.38 Towards this aim, we synthesized an IL-induced 3D macroassembly of GN nanostructures. Briey, a mixture of GO and IL–NH2 was sealed in a Teon-lined autoclave and maintained at 100 C for 12 h. Then, the TGN–IL nanostructures were obtained. In this process, a nucleophilic ring-opening reaction between the epoxy groups of GO and the amine groups of IL–NH2 takes place in an alkaline medium.39 Simultaneously, an efficient self-assembly driven by hydrophobic and p–p
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stacking interactions between the conjugated structures of the IL-modied GN could occur at a high GO concentration (2 mg mL1), as shown in Scheme 1. Besides, the cross-effect induced by IL may also play a critical role in the assembly of GN.29 It is noted that when the GO concentration was low (e.g., 0.5 mg mL1), only a black powdery material was produced aer the hydrothermal treatment. The mechanism of the hydrogel formation was similar to that using water,32 ascorbic acid21 and NaHSO3 (ref. 46) reported previously. As shown in Fig. 1A, in the beginning (a) a homogeneous and transparent suspension was formed aer mixing GO and IL– NH2 in NaOH. A regular hydrogel (b) with a cylindrical morphology was formed aer the hydrothermal treatment. The morphology of TGN–IL nanostructures (c) can be well maintained aer freeze-drying. Accordingly, the weight content of water in the TGN–IL hydrogel was calculated to be about 99.2%, illustrating the low mass density of the nal product. It should be noted that the GN–IL hydrogel could be effectively formed in the presence of IL–NH2 at 100 C, while the pure TGN hydrogel could not be obtained because GO could not be reduced, and therefore the simultaneous self-assembly could not be initiated. Fig. 1B and C show the typical SEM images of the as-prepared TGN–IL nanostructures. Close observation of the structure of TGN–IL nanostructures suggested that there are some typical features that are worth mentioning. It is found that the freeze-
Scheme 1
The procedure for preparing TGN–IL nanostructures.
Fig. 1 (A) Photographs of a 2 mg mL1 GO aqueous dispersion mixed with IL–NH2 before (a) and after (b) hydrothermal treatment at 100 C for 12 h. (c) A photograph of the obtained TGN–IL nanostructures. SEM (B and C) and TEM (D) images of the TGN–IL nanostructures.
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dried TGN–IL nanostructures displayed a macroporous structure with large sheets, which was different from the GN aerogel reported previously.47,48 Furthermore, the small sized GO ( TGN/GCE (0.070 cm2). The larger value of the effective area observed at the TGN–IL/GCE will be benecial for the specic capacitance of TGN–IL nanostructures. We further proceeded to investigate the rate-dependent cyclic voltammograms (CVs) in 1 M H2SO4 for GN–IL over a range of scan rates of 2–500 mV s1 (Fig. 4A). It was evident that the shape of CVs changed slightly even when the scan rate was increased to 500 mV s1, indicating a quick charge propagation capability of both double layer capacitance and pseudo-capacitance. To explore the advantages of TGN–IL nanostructures as an electrode material for supercapacitors, their electrochemical properties were analyzed by cyclic voltammetry and galvanostatic charge/discharge techniques using a three-electrode system. Fig. 4B compares the CVs of TGN–IL and TGN-based electrode materials at a scan rate of 50 mV s1. Similar to TGN, the anodic peak observed at 0.45 V and cathodic peak at
Fig. 3 (A) Cyclic voltammograms (CVs) obtained at TGN-IL/GCE and TGN/GCE interfaces in 0.10 M KCl solution containing 5 mM [Fe(CN)6]3/4. Scan rate: 50 mV s1. (B) The plot of the square root of scan rate vs cathodic peak current.
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0.36 V (vs. Ag/AgCl) are related to faradaic redox reactions between surface oxygen-containing groups and the aqueous electrolyte, which were responsible for the pseudocapacitance of GN.51 It is clear that the CV of the TGN–IL based electrode material presents a quasirectangular shape with a larger encircling area than that of TGN/GCE, suggesting good capacitive behavior. This feature of TGN–IL was also conrmed by galvanostatic charge/discharge curves at a current density of 1 A g1, as shown in Fig. 4C. The almost symmetric charge/ discharge curves revealed the capacitive behavior of electrical double layer capacitance and pseudocapacitance. The specic capacitance can be calculated by evaluating the charge/ discharge curve according to eqn (2) (ref. 48 and 52), Cs ¼ IDt/(DVm)
(2)
where I is the applied current, t is the discharge time, V is the potential drop during discharge, and m is the mass of the electrode (5 mg). According to eqn (2), the capacitance of TGN–IL was calculated to be 373 F g1 at a current density of 1 A g1, which was remarkably higher than that of TGN (241 F g1) under the same conditions. It should be noted that the specic capacitance of TGN–IL also outperformed those of GN-based nanostructures described previously.48,51,53,54 Furthermore, the rate performance of TGN–IL was also evaluated by galvanostatic charge/discharge under different current densities (Fig. 4D). Along with an increase in the current density, the capacitance was gradually reduced (Fig. 4E). Signicantly, the specic capacitance of TGN–IL nanostructures still remained as high as 208 F g1 even at a high discharge current density of 40 A g1, about 56% of their specic capacitance at 1A g1. In contrast, the specic capacitance of GN remained at 84 F g1 at a current density of 40 A g1, which is a decrease of more than 65% compared with their initial specic capacitance at 1 A g1. Such excellent specic capacitance and good rate capability of TGN–IL can be attributed to the following aspects. First, the signicantly improved electronic conductivity due to the introduction of IL plays a signicant role in enhancing the electrochemical performance of TGN–IL. This result was consistent with the previous report on improving the conductivity of a GN aerogel to enhance their capacitance via further chemical reduction.32,36 Such improvement can also be ascribed to its 3D porous nanostructures and high surface areas, which facilitate fast ion and electron transport during the charge/discharge processes. We further investigated the frequency responses of a supercapacitor by electrical impedance spectroscopy. The Nyquist plots of TGN–IL and TGN in the frequency range of 0.1 Hz to 1 MHz are shown in Fig. 5. Both of them exhibited two distinct parts including a semicircle in the high frequency region and a slope line in the low frequency region.54,55 The Nyquist plots show a more vertical line in the low-frequency region of TGN–IL nanostructures, proving that the TGN–IL electrode behaves like a nearly ideal capacitor. In addition, the charge transfer resistance (Rct) was signicantly reduced when GN was modied with IL. The reduced Rct of TGN–IL is consistent with its signicantly high electronic conductivity mentioned above.
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Electrochemical characterization. (A) CVs of the obtained TGN–IL nanostructures in 1 M H2SO4 at various scan rates. CVs (B) at 50 mV s1 and galvanostatic charge/discharge curves (C) at 1 A g1 of TGN–IL and TGN in 1 M H2SO4. (D) Galvanostatic charge/discharge curves of TGN–IL at different current densities. (E) Specific capacitances of TGN and TGN–IL at different current densities. Fig. 4
Nyquist plots of TGN–IL and TGN nanostructures. Inset shows the magnified high-frequency regions.
Fig. 5
(F g1), Q is the total charge delivered (C), V is the potential window of discharge (V), t is the discharge time (s), and E is the average energy density (W h kg1). As shown in Fig. 6A, the energy density of the TGN–IL electrode was calculated to be much higher than that of TGN at the same power density. TGN– IL has a maximum energy density of 51.8 W h kg1 at a power density of 500 W kg1, which is much higher than the values for GN-based nanomaterial supercapacitors declared in the literature.53,56,57 Signicantly, the energy density of TGN–IL nanostructures still remained at as high as 28.9 W h kg1 even at a power density of 20 kW kg1. Finally, the cycle stability of the TGN–IL nanostructured electrode based supercapacitors is examined at a current density of 40 A g1 and the results are shown in Fig. 6B. As can be seen, the specic capacitance of TGN–IL nanostructures still remained at 208 F g1 (92%) aer 1000 charge/discharge cycles, reecting that the TGN–IL
To evaluate the device performance of the supercapacitor based on TGN–IL nanostructures, the energy density (E) and power density (P) were calculated from galvanostatic charge/ discharge curves by the following equations:53,56 E ¼ (CDV2)/2
(3)
P ¼ QDV/2t ¼ E/t
(4)
where P is the average power density (W kg1), C is the specic capacitance based on the mass of the electroactive material
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(A) Ragone plots of TGN and TGN–IL nanostructures in 1 M H2SO4 solution. (B) Cycle stability of the TGN–IL nanostructures in 1 M H2SO4 (at a current density of 40 A g1). Fig. 6
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electrode material has good electrochemical stability and a high degree of reversibility in the repetitive charge/discharge cycling test. The excellent electrochemical performance as well as long cycle stability paves the way for TGN–IL nanostructures as a promising electrode material for supercapacitors in electrochemical energy storage devices in the future.
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Conclusions In summary, we have successfully synthesized TGN–IL nanostructures with the help of IL–NH2. The introduction of IL can not only effectively reduce GO, but also plays a key role in the formation of 3D GN nanostructures. Signicantly, the IL–functionalized 3D GN nanostructures provide ideal electrode materials for supercapacitors because of the combined unique properties of GN and IL in overcoming the restacking of GN, enlarging the SSA, improving GN's conductivity and ensuring the high electrochemical utilization of GN as well as the open channels provided by 3D nanostructures. The specic capacitance was determined as 373 F g1 for the obtained TGN–IL nanostructures, which was much higher than those of controlled TGN nanostructures (241 F g1) and GN materials reported previously. At the same time, they also exhibited large energy density and good stability even aer 1000 charge/ discharge cycles. Taking the higher capacitance and longer cycle stability into consideration, TGN–IL nanostructures may be extremely promising nanomaterials for the advanced electrochemical energy storage devices.
Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 20935003 and 21075116) and the 973 Project (no. 2009CB930100 and 2010CB933600).
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Ionic liquid-induced three-dimensional macroassembly of graphene and its applications in electrochemical energy storage Chengzhou Zhu, Junfeng Zhai and Shaojun Dong* Ionic liquid (IL)-induced three-dimensional macroassembly of graphene (GN) has been achieved through one-step hydrothermal treatment. Significantly, the three-dimensional GN–IL (TGN–IL) nanostructures
Received 4th May 2014 Accepted 17th June 2014
provide ideal electrode materials for supercapacitors because they combine the unique properties of GN
DOI: 10.1039/c4nr02400a
and IL in overcoming the restacking of GN, enlarging the specific surface area, improving the GN conductivity and ensuring the high electrochemical utilization of GN as well as the open channels
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provided by 3D nanostructures.
Introduction Graphene (GN), a single-layer sheet of sp2 hybridized carbon atoms, is recognized as the basic building block of all dimensional graphitic materials.1 The unique structure endows GN with various superior properties such as high electrical and thermal conductivities, good transparency, great mechanical strength, inherent exibility, and huge specic surface area (SSA).2–4 Therefore, GN has been proven to nd wide applications during recent years in the eld of sensing,5,6 electronics and optoelectronics,7,8 electrochemical energy storage,9,10 efficient catalysis,11,12 nanocomposites,13,14 etc. Signicantly, the emergence of GN provides an excellent alternative to electrode materials, and great efforts have been made to utilize GN-based nanomaterials as promising supercapacitor electrode materials with high performance in energy devices due to high electrical conductivity, large SSA, profuse interlayer structure and abounding functional groups involved.15–17 Specically, GNbased nanomaterials derived from graphene oxide (GO) can be manufactured on a ton scale at low cost, making them potentially cost-effective materials for supercapacitors.18 A supercapacitor based on chemically-modied GN electrodes exhibited specic capacitances of 135 and 99 F g1 with aqueous and organic electrolytes, respectively.19 Thereaer, there are many reports on GN-based supercapacitors, while the specic capacitances of which were about 200 F g1.20–24 Although these GN-based supercapacitors have shown good power density and life-cycle stability, their specic capacitances still fall far below the theoretical value of 550 F g1 calculated for a single-layer GN.19,25 A reduction in the SSA of GN due to restacking during its processing as a result of strong van der State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. E-mail: [email protected]; Fax: +86-431-85689711; Tel: +86-431-85262101
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Waals interactions accounts for the overall low capacitance. In this regard, great contributions have been made to design highperformance GN-based nanomaterials aiming to enlarge the SSA, improve the conductivity, and control the pore size, layer stacking and distribution of GN. For example, Ruoff et al. have synthesized a novel GN by simple activation with KOH, achieving high SSA values of up to 3100 m2 g1. A specic capacitance of above 166 F g1 was obtained at a current density of up to 5.7 A g1.26 In addition, many researchers have focused on introducing stabilizers or spacers, such as poly(sodium 4styrensulfonate),27 water28 and carbon nanotubes,9 into GN layers to inhibit the restacking of GN, improve the electrolyte– electrode accessibility in supercapacitors and thus enhance specic capacitances of the obtained GN materials. Compared with 2D lms, less-agglomerated, self-supported, and binder free GN-based 3D nanostructures with suitable pore sizes are highly needed in constructing high-performance electrochemical energy devices. To date, several approaches have been developed to construct 3D graphene-based nanomaterials, such as self-assembly,29,30 hydrothermal treatment,31,32 leavening strategy,33 and so on.34,35 Shi's group has prepared a series of self-assembled GN hydrogels.32,36 The unique structure of these GN nanostructures endowed them with high mechanical strength and a well-dened 3D porous network, which offers an opportunity to optimize the ionic diffusion in GN-based electrodes for supercapacitors. Duan et al. have synthesized functionalized GN hydrogels through a convenient one-step chemical reduction of GO using hydroquinones as reducing and functionalizing molecules simultaneously, which showed an impressive specic capacitance of 441 F g1 at 1 A g1 in the 1 M H2SO4 aqueous electrolyte.37 Although assembling GN into freestanding 3D hydrogel nanostructures with a controlled microstructure was regarded to be an effective strategy for increasing their electrochemical performances, their specic capacitances are still unsatisfactory and needed to be further improved.
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Herein, we reported the ionic liquid (IL)-induced 3D macroassembly of GN through a one-step hydrothermal treatment and used them as advanced electrode materials for supercapacitors. The introduction of IL can not only effectively reduce GO, but also plays a key role in the formation of 3D GN (TGN–IL) nanostructures. The obtained TGN–IL nanostructures provide ideal electrode materials for supercapacitors because they combined the unique properties of GN and IL in overcoming the restacking of GN, improving the conductivity, enlarging the SSA and ensuring the high electrochemical utilization of GN as well as the open channels provided by 3D nanostructures. Signicantly, the specic capacitance was evaluated as 373 F g1 for TGN–IL nanostructures, which was much higher than that of 3D GN (TGN) nanostructures (241 F g1). At the same time, they also exhibited large energy density and good stability aer 1000 charge/discharge cycles. Considering the high electrochemical performance, the TGN–IL nanostructures may nd use in advanced electrochemical energy storage devices.
Experimental section
Paper
GO aqueous dispersion (2 mg mL1) was obtained and used for further synthesis of TGN–IL nanostructures. Synthesis of TGN–IL nanostructures 1-(3-Aminopropyl)-3-methylimidazoliumbromide (IL–NH2) was rst prepared according to a previously reported method.39 The obtained IL–NH2 (120 mg) was added into 30 mL of a GO dispersion (2 mg mL1), and then the aggregation of the GO occurred due to the salt effect induced by IL–NH2. Then, NaOH (60 mg) was added into the above turbid mixture, and the mixture was subjected to sonication for 30 min. Aer sonication, the turbid mixture was transformed into a homogeneous and transparent dispersion. The obtained dispersion was transferred into the container of a Teon-lined stainless steel autoclave, sealed and maintained at 100 C for 12 h. Then, the as-prepared TGN–IL hydrogel was dialyzed against deionized water. The TGN–IL hydrogel was freeze-dried to obtain the nal product. As a control, pure TGN hydrogel was also prepared using the same method at 180 C with the exception of IL–NH2 and NaOH.32
Chemicals
Electrochemical measurements
Graphite was purchased from Alfa Aesar. 3-Bromopropylamine hydrobromide (98%) was obtained from Aldrich. 1-Methylimidazole (98%, Linhai Kaile Chemicals, China) was distilled under a reduced pressure before use. Unless otherwise stated, water used throughout all experiments was puried with a Millipore system.
A suspension of the as-prepared materials (TGN–IL or TGN) at a concentration of 1 mg mL1 was prepared by ultrasonically dispersing them in water. Prior to the surface coating, GCE (3 mm in diameter) was polished with 1.0 and 0.3 mm alumina slurry sequentially and then washed ultrasonically with water and ethanol for a few minutes. The suspension (5 mL) was then dropped onto the GCE and dried at room temperature. Then, 5 mL of Naon (0.2%) was placed on the surface of the above modied GCE and dried before use in electrochemical experiments.
Instrumentation Infrared spectra were collected on a VERTEX 70 Fourier transform infrared (FTIR) spectrometer (Bruker). X-ray diffraction (XRD) spectra were obtained by using a D8 ADVANCE (Germany) ˚ radiation source. X-ray diffractometer with a Cu Ka (1.5406 A) photoelectron spectroscopy (XPS) analysis was carried out on an ESCALAB MK II X-ray photoelectron spectrometer. The electrochemical impedance (EIS) experiments were performed in 1 M H2SO4. The spectra were measured by using Autolab with PGSTAT 30 (Eco Chemie B.V., Utrecht, The Netherlands) and with the aid of a frequency response analysis system soware using an oscillation potential of 5 mV over a frequency range of 100 kHz to 0.1 Hz. Electrochemical measurements including cyclic voltammetry and galvanostatic charge/discharge were performed with a CHI 660 electrochemical analyzer (CH Instruments, Inc., USA). A conventional three-electrode cell was used, which consisted of an Ag/AgCl (saturated KCl) electrode as the reference electrode, a platinum wire as the counter electrode, and the TGN–IL and TGN modied glassy carbon electrodes (GCE) as working electrodes. Preparation of GO Graphite oxide was synthesized from the natural graphite powder based on a modied Hummers method.38 Then, exfoliation of graphite oxide to GO was achieved by ultrasonication for 40 min (1000 W, 20% amplitude). Finally, a homogeneous
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Results and discussion IL has attracted an increasing amount of interest in electrochemical sensing,40,41 electrocatalysis42,43 and electrochemical energy storage,44,45 owing to its high conductivity, chemical and thermal stability, and wide electrochemical window. It is expected that combining GN with IL will provide fertile opportunities for the construction of GN-based hybrid nanocomposites and their potential applications. As mentioned above, the construction of TGN nanostructures with high mechanical strength and a well-dened 3D porous network is favorable for the ionic diffusion and thus the enhancement of the electrochemical performance of GN-based electrodes for supercapacitors. However, there are few reports on the synthesis TGN–IL nanostructures and their use in supercapacitors.38 Towards this aim, we synthesized an IL-induced 3D macroassembly of GN nanostructures. Briey, a mixture of GO and IL–NH2 was sealed in a Teon-lined autoclave and maintained at 100 C for 12 h. Then, the TGN–IL nanostructures were obtained. In this process, a nucleophilic ring-opening reaction between the epoxy groups of GO and the amine groups of IL–NH2 takes place in an alkaline medium.39 Simultaneously, an efficient self-assembly driven by hydrophobic and p–p
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stacking interactions between the conjugated structures of the IL-modied GN could occur at a high GO concentration (2 mg mL1), as shown in Scheme 1. Besides, the cross-effect induced by IL may also play a critical role in the assembly of GN.29 It is noted that when the GO concentration was low (e.g., 0.5 mg mL1), only a black powdery material was produced aer the hydrothermal treatment. The mechanism of the hydrogel formation was similar to that using water,32 ascorbic acid21 and NaHSO3 (ref. 46) reported previously. As shown in Fig. 1A, in the beginning (a) a homogeneous and transparent suspension was formed aer mixing GO and IL– NH2 in NaOH. A regular hydrogel (b) with a cylindrical morphology was formed aer the hydrothermal treatment. The morphology of TGN–IL nanostructures (c) can be well maintained aer freeze-drying. Accordingly, the weight content of water in the TGN–IL hydrogel was calculated to be about 99.2%, illustrating the low mass density of the nal product. It should be noted that the GN–IL hydrogel could be effectively formed in the presence of IL–NH2 at 100 C, while the pure TGN hydrogel could not be obtained because GO could not be reduced, and therefore the simultaneous self-assembly could not be initiated. Fig. 1B and C show the typical SEM images of the as-prepared TGN–IL nanostructures. Close observation of the structure of TGN–IL nanostructures suggested that there are some typical features that are worth mentioning. It is found that the freeze-
Scheme 1
The procedure for preparing TGN–IL nanostructures.
Fig. 1 (A) Photographs of a 2 mg mL1 GO aqueous dispersion mixed with IL–NH2 before (a) and after (b) hydrothermal treatment at 100 C for 12 h. (c) A photograph of the obtained TGN–IL nanostructures. SEM (B and C) and TEM (D) images of the TGN–IL nanostructures.
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dried TGN–IL nanostructures displayed a macroporous structure with large sheets, which was different from the GN aerogel reported previously.47,48 Furthermore, the small sized GO ( TGN/GCE (0.070 cm2). The larger value of the effective area observed at the TGN–IL/GCE will be benecial for the specic capacitance of TGN–IL nanostructures. We further proceeded to investigate the rate-dependent cyclic voltammograms (CVs) in 1 M H2SO4 for GN–IL over a range of scan rates of 2–500 mV s1 (Fig. 4A). It was evident that the shape of CVs changed slightly even when the scan rate was increased to 500 mV s1, indicating a quick charge propagation capability of both double layer capacitance and pseudo-capacitance. To explore the advantages of TGN–IL nanostructures as an electrode material for supercapacitors, their electrochemical properties were analyzed by cyclic voltammetry and galvanostatic charge/discharge techniques using a three-electrode system. Fig. 4B compares the CVs of TGN–IL and TGN-based electrode materials at a scan rate of 50 mV s1. Similar to TGN, the anodic peak observed at 0.45 V and cathodic peak at
Fig. 3 (A) Cyclic voltammograms (CVs) obtained at TGN-IL/GCE and TGN/GCE interfaces in 0.10 M KCl solution containing 5 mM [Fe(CN)6]3/4. Scan rate: 50 mV s1. (B) The plot of the square root of scan rate vs cathodic peak current.
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0.36 V (vs. Ag/AgCl) are related to faradaic redox reactions between surface oxygen-containing groups and the aqueous electrolyte, which were responsible for the pseudocapacitance of GN.51 It is clear that the CV of the TGN–IL based electrode material presents a quasirectangular shape with a larger encircling area than that of TGN/GCE, suggesting good capacitive behavior. This feature of TGN–IL was also conrmed by galvanostatic charge/discharge curves at a current density of 1 A g1, as shown in Fig. 4C. The almost symmetric charge/ discharge curves revealed the capacitive behavior of electrical double layer capacitance and pseudocapacitance. The specic capacitance can be calculated by evaluating the charge/ discharge curve according to eqn (2) (ref. 48 and 52), Cs ¼ IDt/(DVm)
(2)
where I is the applied current, t is the discharge time, V is the potential drop during discharge, and m is the mass of the electrode (5 mg). According to eqn (2), the capacitance of TGN–IL was calculated to be 373 F g1 at a current density of 1 A g1, which was remarkably higher than that of TGN (241 F g1) under the same conditions. It should be noted that the specic capacitance of TGN–IL also outperformed those of GN-based nanostructures described previously.48,51,53,54 Furthermore, the rate performance of TGN–IL was also evaluated by galvanostatic charge/discharge under different current densities (Fig. 4D). Along with an increase in the current density, the capacitance was gradually reduced (Fig. 4E). Signicantly, the specic capacitance of TGN–IL nanostructures still remained as high as 208 F g1 even at a high discharge current density of 40 A g1, about 56% of their specic capacitance at 1A g1. In contrast, the specic capacitance of GN remained at 84 F g1 at a current density of 40 A g1, which is a decrease of more than 65% compared with their initial specic capacitance at 1 A g1. Such excellent specic capacitance and good rate capability of TGN–IL can be attributed to the following aspects. First, the signicantly improved electronic conductivity due to the introduction of IL plays a signicant role in enhancing the electrochemical performance of TGN–IL. This result was consistent with the previous report on improving the conductivity of a GN aerogel to enhance their capacitance via further chemical reduction.32,36 Such improvement can also be ascribed to its 3D porous nanostructures and high surface areas, which facilitate fast ion and electron transport during the charge/discharge processes. We further investigated the frequency responses of a supercapacitor by electrical impedance spectroscopy. The Nyquist plots of TGN–IL and TGN in the frequency range of 0.1 Hz to 1 MHz are shown in Fig. 5. Both of them exhibited two distinct parts including a semicircle in the high frequency region and a slope line in the low frequency region.54,55 The Nyquist plots show a more vertical line in the low-frequency region of TGN–IL nanostructures, proving that the TGN–IL electrode behaves like a nearly ideal capacitor. In addition, the charge transfer resistance (Rct) was signicantly reduced when GN was modied with IL. The reduced Rct of TGN–IL is consistent with its signicantly high electronic conductivity mentioned above.
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Electrochemical characterization. (A) CVs of the obtained TGN–IL nanostructures in 1 M H2SO4 at various scan rates. CVs (B) at 50 mV s1 and galvanostatic charge/discharge curves (C) at 1 A g1 of TGN–IL and TGN in 1 M H2SO4. (D) Galvanostatic charge/discharge curves of TGN–IL at different current densities. (E) Specific capacitances of TGN and TGN–IL at different current densities. Fig. 4
Nyquist plots of TGN–IL and TGN nanostructures. Inset shows the magnified high-frequency regions.
Fig. 5
(F g1), Q is the total charge delivered (C), V is the potential window of discharge (V), t is the discharge time (s), and E is the average energy density (W h kg1). As shown in Fig. 6A, the energy density of the TGN–IL electrode was calculated to be much higher than that of TGN at the same power density. TGN– IL has a maximum energy density of 51.8 W h kg1 at a power density of 500 W kg1, which is much higher than the values for GN-based nanomaterial supercapacitors declared in the literature.53,56,57 Signicantly, the energy density of TGN–IL nanostructures still remained at as high as 28.9 W h kg1 even at a power density of 20 kW kg1. Finally, the cycle stability of the TGN–IL nanostructured electrode based supercapacitors is examined at a current density of 40 A g1 and the results are shown in Fig. 6B. As can be seen, the specic capacitance of TGN–IL nanostructures still remained at 208 F g1 (92%) aer 1000 charge/discharge cycles, reecting that the TGN–IL
To evaluate the device performance of the supercapacitor based on TGN–IL nanostructures, the energy density (E) and power density (P) were calculated from galvanostatic charge/ discharge curves by the following equations:53,56 E ¼ (CDV2)/2
(3)
P ¼ QDV/2t ¼ E/t
(4)
where P is the average power density (W kg1), C is the specic capacitance based on the mass of the electroactive material
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(A) Ragone plots of TGN and TGN–IL nanostructures in 1 M H2SO4 solution. (B) Cycle stability of the TGN–IL nanostructures in 1 M H2SO4 (at a current density of 40 A g1). Fig. 6
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electrode material has good electrochemical stability and a high degree of reversibility in the repetitive charge/discharge cycling test. The excellent electrochemical performance as well as long cycle stability paves the way for TGN–IL nanostructures as a promising electrode material for supercapacitors in electrochemical energy storage devices in the future.
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Conclusions In summary, we have successfully synthesized TGN–IL nanostructures with the help of IL–NH2. The introduction of IL can not only effectively reduce GO, but also plays a key role in the formation of 3D GN nanostructures. Signicantly, the IL–functionalized 3D GN nanostructures provide ideal electrode materials for supercapacitors because of the combined unique properties of GN and IL in overcoming the restacking of GN, enlarging the SSA, improving GN's conductivity and ensuring the high electrochemical utilization of GN as well as the open channels provided by 3D nanostructures. The specic capacitance was determined as 373 F g1 for the obtained TGN–IL nanostructures, which was much higher than those of controlled TGN nanostructures (241 F g1) and GN materials reported previously. At the same time, they also exhibited large energy density and good stability even aer 1000 charge/ discharge cycles. Taking the higher capacitance and longer cycle stability into consideration, TGN–IL nanostructures may be extremely promising nanomaterials for the advanced electrochemical energy storage devices.
Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 20935003 and 21075116) and the 973 Project (no. 2009CB930100 and 2010CB933600).
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