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Three-dimensional paper-like graphene framework with highly orientated laminar structure as binder-free supercapacitor electrode
Accepted Manuscript
Three-dimensional paper-like graphene framework with highly orientated laminar structure as binder-free supercapacitor electrode
http://www.journals.elsevier.com/ journal-of-energy-chemistry/
Yidan Gao , Yaoyao Zhang , Yong Zhang , Lijing Xie , Xiaoming Li , Fangyuan Su , Xianxian Wei , Zhiwei Xu , Chengmeng Chen , Rong Cai PII: DOI: Reference:
S2095-4956(15)00129-1 10.1016/j.jechem.2015.11.011 JECHEM 78
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
29 July 2015 11 September 2015 15 September 2015
Please cite this article as: Yidan Gao , Yaoyao Zhang , Yong Zhang , Lijing Xie , Xiaoming Li , Fangyuan Su , Xianxian Wei , Zhiwei Xu , Chengmeng Chen , Rong Cai , Three-dimensional paper-like graphene framework with highly orientated laminar structure as binder-free supercapacitor electrode, Journal of Energy Chemistry (2015), doi: 10.1016/j.jechem.2015.11.011
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ACCEPTED MANUSCRIPT
Three-dimensional paper-like graphene framework with highly orientated laminar structure as binder-free supercapacitor electrode
Yidan Gaoa,b, Yaoyao Zhanga,c, Yong Zhanga, Lijing Xiea, Xiaoming Lia, Fangyuan
a
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Sua, Xianxian Weia,d, Zhiwei Xuc,*, Chengmeng Chena,, Rong Caia,e
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy
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of Sciences, Taiyuan 030001, Shanxi, China b
University of Chinese Academy of sciences, Beijing 100049, China
c
Key Laboratory of Advanced Braided Composites, Ministry of Education, School of
Textiles, Tianjin Polytechnic University, Tianjin 300387, Shanxi, China Taiyuan University of Science and Technology, Taiyuan 030024, Shanxi, China
e
Academy of Opto-Electronics, Chinese Academy of Sciences, Beijing 100094, China
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d
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Abstract
A free-standing paper-like three-dimensional graphene framework (3DGF) with
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orientated laminar structure and interconnected macropores, was obtained by the hard
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template-directed ordered assembly. As the sacrificial templates, polystyrene (PS) latex spheres were assembled with graphene oxide (GO) to build up a sandwich type composite film, followed by heat removal of which with a simultaneous reduction of GO. The 3DGF exhibited high specific surface area of 402.5 m2/g, controllable pores and mechanical flexibility, which was employed as the binder-free supercapacitor
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electrode and show high specific gravimetric capacitance of 95 F/g at 0.5 A/g, with enhanced rate capability in 3 electrode KOH system.
Corresponding
author.
Fax:
+86-0351-4049061;
[email protected]. 3D
graphene
framework;
Polystyrene;
E-mail
address:
Sacrificial
template;
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Keywords:
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Manuscript received July 29, 2015; revised September 11, 2015, Accepted September 15, 2015
Supercapacitor 1. Introduction
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Supercapacitors are attracting increasing attention from industry due to their
electrical
systems
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potential in load cranes, forklifts, electric vehicles, consumer electronics and mobile [1–3].
Comparing
with
batteries,
supercapacitors
show
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irreplaceable properties such as high power density, fast charging/discharging rate,
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long cycle life (>100 000 cycles) and high reliability [1,4–8]. Recently, graphene is highly concerned for application in electrochemical energy storage, due to its large
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theoretical surface area (2630 m2/g), good electronic conductivity, and high electrochemical stability [9–11]. However, the graphene nanosheets tend to re-stack into irreversible agglomerates due to their strong interlayer van der Waals forces [12– 14]. Obviously, this π-π stacking of graphene will decrease the accessibility of surface to the electrolyte so as to affect the performance of the electrode [7,13–15]. In a typical process when preparing an electrode of powder form graphene, the insulating 2
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polymer binder is usually involved. The binder will increase the electronic resistance and wrap the surface of the electrode material, so as to impede the electron transportation and ion diffusion, as well as complicate the fabrication of the device [1,2,16]. To address these challenges, the binder-free electrodes should be developed
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to utilize the ultimate property of graphene for high performance supercapacitors. In the past few years, self-supported three-dimensional (3D) architectures with continuously interconnected macropores (such as aerogels [17–19], foams [20] and
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sponges [21], and hierarchical porous hybrid paper [22–25]) have been demonstrated to be effective in preventing graphene from aggregating [26]. The energy storage performances of the materials are significantly improved, which are attributed to the
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large surface area, high accessibility to reaction sites, facilitated ion transport, increased mechanical integrity, and/or synergistic effects among multicomponents
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[27–31]. These macroscopic materials are randomly assembled with graphene as the
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basic building blocks in a sol-gel or hydrothermal process. Without the assistance of templates, pores in the materials are randomly distributed and uncontrollable. Besides,
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the absent of external orientation during assembly will result in isotropic
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inter-connection of graphene to affect the macroscopic performance of the resulting material. Therefore, to assemble the graphene sheets in a precisely controllable manner for smarter energy storage still remain a challenge. Recently, the template-directed method using polymer microspheres as sacrificial templates has been developed for fabricating a pore-structure and porosity controlled 3D macroporous graphene architecture. In previous works, Choi et al. [32,33] have
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fabricated the 3D hierarchical metal oxide/chemical modified graphene hybridized films using PS spheres (2 μm) as structural guiding templates. During the process, the hydrazine is employed to reduce graphene oxide followed by removal of template by toluene. The method involved is considered to be toxic and complex. In our previous
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report, polymethyl methacrylate latex spheres was used, followed by one step thermal reduction and template removal, to obtain a self-standing macroporous graphene film for binder-free electrode. However, the surface area of the resulting material is only
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128.2 m2/g, which is due to the overlarge diameter (as high as 500 nm) of the templates [34]. Afterwards, Wang et al. [35] used PS microspheres templates (Φ280 nm) to prepare the macroporous graphene structure by a similar approach. However,
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the resulting material is still in powder form, and thus polymer binders is inevitable during the subsequent electrode preparation.
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In this contribution, a free-standing paper-like 3DGF with higher surface area,
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smaller pore size and highly orientated laminar structure was fabricated by a facile template method with PS microspheres as the sacrificial templates followed by
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thermal annealing. After systematical characterization, the material was evaluated as
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binder-free supercapacitor electrode which shows enhanced energy and power density. Besides, the electrochemical performance was further correlated to the microstructure of 3DGF.
2. Experimental 2.1. Synthesis of polystyrene nanospheres Polystyrene nanospheres with the size of 150 nm were synthesized by
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emulsifier-free emulsion polymerization in a 250 mL three-necked round bottom flask. In a typical process, the sodium dodecylbenzene sulfonate (0.07 g) and potassium persulfate (0.09 g) were firstly dissolved in 80 mL and 20 mL deionized water, respectively. Then, the solution and styrene monomer (5 mL) were poured into the
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three neck flask in Ar atmosphere. After about 30–40min, the obtained mixture was stirred at approximately 350 rpm and heated to 70 oC. After reaction for 8 h, the desired polystyrene nanospheres were obtained by centrifugation and washing with
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deionized water and ethanol several times. The resultant polystyrene nanospheres were re-dispersed in deionized water to obtain an aqueous suspension (30 mg/mL) for further application.
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2.2. Fabrication of the binder-free 3DGF
Graphite oxide was prepared by a modified Hummers’ method, followed by
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ultrasonication (100 W, 30 min) in deionized water to get the graphene oxide hydrosol
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(3 mg/mL). The graphene oxide hydrosol (10 mL) was mixed with polystyrene nanospheres suspension (3.5 mL) and sonicated for 15 min to get a homogeneous
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colloidal suspension, which was then filtrated on a vacuum millipore filter to realize
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the sandwich type assembly of polystyrene nanospheres and graphene oxide sheets. The above composite film was peeled off from the filter and air dried at 50 oC overnight, and the weight ratio of GO and PS in the film was 1:3.5. The composite films were annealed to 800 oC (heating rate 10 oC/min) for 0.5 h under Ar atmosphere in a tubular furnace. GO within the composite film was thermally reduced into graphene, while the polystyrene nanospheres templates were removed simultaneously,
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so as to get the 3DGF. At the same time, the control sample was prepared by a similar procedure but with no polystyrene nanospheres template, resulting in the compact graphene film (GF800). 2.3. Characterization
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The morphologies and microstructure of the samples were examined by field emission scanning electron microscopy (FESEM, JEOL JSM-7001F, Japan), transmission electron microscope (TEM, JEM-2010, Japan), X-ray diffraction
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measurements (XRD, Rigaku, D/Max-2400, Japan) and nitrogen adsorption and desorption experiments (BELSORP-max, Japan). The specific surface area was obtained using Brunauer Emmett Teller (BET) method and the pore size distribution
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was calculated from the adsorption branch of the nitrogen isotherm using the Barrett Joyner Halenda (BJH) model. Thermogravimetric analysis (TGA, STA 409PC,
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NETZSCH, Germany) was carried out under Ar purge at a heating rate of 10 oC/min
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from room temperature to 1000 oC. The reduction of GO was confirmed using an Elementar (vario Macro cube, Germany).
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2.4. Electrochemical measurements
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The electrochemical properties of 3DGF and GF800 were measured in a 6 M
KOH solution as electrolyte. All electrochemical measurements were carried out on a CHI760D electrochemical working station (CH Instrument). The prepared 3DGF was utilized as working electrodes without adding polymeric binders or conducting additives. Hg/HgO electrode and platinum plate were used as the reference electrode and counter electrode, respectively. The specific capacitance was calculated from the
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results of galvanostatic charge-discharge tests using equations:
Csp,galvanostatic
it mE
(1)
where C is the specific capacitance (F/g), i is the discharge or charge current, t is
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the time discharge time, m is the mass of a part of electrode, and E is the potential window (–0.9–0 V). 3. Results and discussion
The Schematic illustration for the fabrication of 3DGF was shown in Fig. 1. PS
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microspheres and GO nanosheets were assembled owing to electrostatic interaction, and then a homogeneous mixture suspension was formed after ultrasonication. Subsequent vacuum filtration followed to realize the sandwich type assembly of GO
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and PS. The removal of PS templates and the thermally reduction of GO were realized
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through the subsequent calcination at 800 oC under Ar atmosphere.
Fig. 1. Schematic illustration for the fabrication of 3DGF.
Both SEM and TEM were employed to observe the microstructures of 3DGF before and after high-temperature thermal process (Fig. 2). The SEM image in Fig. 2(a) clearly shows that the PS microspheres were uniformly wrapped by crinkled GO
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sheets after the subsequent vacuum filtration, which is attributable to a strong electrostatic interaction between the surface of PS microsphere and GO nanosheet. Compared with pristine PS microspheres (Fig. S1) and GF800 (Fig. S2), the graphene encapsulated spheres exhibit crinkled and rough textures, which are associated with
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the presence of flexible and ultrathin GO nanosheets. Fig. 2(b) exhibits the cross-section of GO/PS sandwich composites. This kind of assembly is beneficial for inhibiting the aggregation of graphene sheets due to the intercalation of PS templates
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and the closely distribution of GO nanosheets. After calcination at 800 oC, the highly orientated laminar and macroporous structure of the integral free-standing 3DGF sample was preserved (Fig. 2c and d) after the removal of PS templates. Moreover,
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the spherical structures and holes (Fig. 2e) distributed in surface of 3DGF further confirm that the macroporous network of 3DGF with continuous walls does not
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collapse, which is could be the excellent mechanical properties. Meanwhile, gaseous
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products are liberated when the PS templates are pyrolyzed under high temperature, which prevents the re-stacking of graphene sheets and leads to tremendous opened
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macropores in the 3DGF. The high magnification TEM image in Fig. 2(f) has shown
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the size of macropores through 3DGF controlled in the range from 100 to 180 nm, and the wall of 3DGF was a few flexible and wrinkled graphene sheets. These features strongly suggest that the PS spheres directed synthesis strategy could bring the merits of ordered and macroporous structures, the resulted 3DGF could be qualified for using as additive/binder-free electrodes.
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(b)
(c)
1µm
100nm
(d)
(f)
(e)
1µm
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100nm
1µm
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(a)
100nm
Fig. 2. SEM images of the (a) surface and (b) a cross-section of a GO/PS composite film; (c) low and (d) high magnification SEM images of the cross-section of a 3DGF; (e) low magnification
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SEM images of surface of a 3DGF; (f) high magnification TEM images of 3DGF.
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The transformation from GO film to the final 3DGF was examined by XRD measurements (Fig. 3a) and elemental analysis (Fig. 3b). It is clearly that there was
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only one sharp peak centered at 2=11.5° in the XRD pattern of the GO film,
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corresponding to a distance of 7.69 nm between the stacked GO sheets. This value indicates that the interlayer spacing of GO is larger than that of graphite (approximately 0.34 nm) owing to a large amount of oxygen groups being introduced on the graphite sheets [36]. Compared with GO film, the XRD patterns of GO/PS composites show a weakened (001) peak at 11.5° and a broader peak centered at 19.7° corresponds to PS appears [37,38]. After being annealed, the GF800 exhibits a
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broader peak (002) centered at 26.0°, corresponding to the interlayer spacing of 0.34 nm, which further verifies that most of the oxygen-containing groups intercalated into the interlayers of graphite have been removed and the conjugated structure of GF800 has been restored after the thermal process [39–41]. GO film shows C/O mole ratio of
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1.2 and this value increases up to 14 for 3DGF after thermal treatment at 800 oC, demonstrating the elimination of most oxygen-containing functional groups and the reduction of graphene sheets. Furthermore, the weaker and broader peak (002) of
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3DGF indicating the graphene sheets in the 3DGF are only loosely stacked, which confirms that gases released from the pyrolysis of the PS templates can promote the exfoliation of graphene sheets and facilitate the formation of opened macropores in the 3DGF.
GO GO/PS GF800 3DGF
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(002)
(b)
80
C O
60
C/O=1.2
20 0
GO
3DGF Samples
1000
(c)
C/O=14
40
40
2(degree)
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20
100
Elemental composition (wt%)
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(001)
Intensity(a.u.)
(a)
GO/PS PS GO
(d)
800
78.8%
60
600
3
Mass (%)
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Va (cm STP/g)
80
40
8.9%
20
400
200
0 200
400
600
o
800
0 0.0
1000
Temperature ( C)
0.2
0.4
0.6
0.8
1.0
P/P0
Fig. 3. (a) XRD patterns of GO film, GO/PS composite film, GF800 and 3DGF, (b) elemental analysis of GO film and 3DGF, (c) TG curves of GO film, GO/PS composite film, and PS 10
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microsphere, (d) nitrogen sorption isotherms of 3DGF.
TG (Fig. 3c) shows the typical thermal behavior and the structural evolution of all contents in PS/GO composite film during the calcination in the Ar atmosphere. In
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the TG curve of GO, the mass loss in the range of 100 to 230 oC is about 21.2%, related to the pyrolysis of oxygen-containing functional groups, suggesting GO can be derived into graphene through the thermal reduction. The PS microspheres were fully
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pyrolyzed and totally removed at 450 oC. In the case of the GO/PS composite, two stages of weight loss at 230 and 450 oC were attributed to the thermal reduction of GO and pyrolysis of PS, respectively. Moreover, the weight of GO/PS composite film is
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essentially unchanged when the temperature over 450 oC and the residual weight about 8.9% is observed, which confirms that the sacrificed PS templates have been
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removed completely and satisfactorily replicated after high temperature calcination.
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The pore-size characterization of the 3DGF was further verified by measuring the Hg penetration and nitrogen adsorption/desorption isotherms. The typical Type IV
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isotherm characteristic with an adsorption hysteresis was observed from the sorption
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curves (Circle curves Fig. 3d), suggesting the mesopores characteristic of 3DGF [42,43]. The obvious hysteresis loop can be observed at relative pressures ranging from 0.48 to 1.0. The H3 type hysteresis loop indicates slit-like pores distributed in the samples [38]. Its corresponding BET surface area is 402.5 m2/g, which is much larger than that of GF800 (30.99 m2/g) and among the highest for 3D graphene materials made from GO and PS spheres reported in the literature so far. These results
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clearly indicate that the addition of PS microspheres as the macropore structural-directing agent and the gaseous products released from the GO/PS composite during thermal process both alter the pore texture and effectively prevent the irreversible aggregation of individual graphene sheets for 3DGF. In addition, the
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Hg penetration result represents a very high porosity (91.49%) with an ultra-low bulk density of 0.0606 g/mL, indicating the 3DGF is very light. This paper-like 3DGF with higher surface area, smaller pore size and highly orientated laminar structure would
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reduce the ion diffusion distance and realize fast ion transport into the well interconnected porous networks of 3DGF.
200
(c)
50
-100 -150
0.2
0.4
Scan rate: 50 mV/s
0
-100
0.6
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0.0
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0 -50
3DGF GF800
100
Capacitance (F/g)
Capacitance (F/g)
100
3 mV/s 50 mV/s 100 mV/s 500 mV/s 1000 mV/s
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(a) 150
0.8
1.0
0.0
0.2
Potential (V)
0.8
1.0
(d) Capacitance (F/g)
100
AC
Potential (V)
-0.2
0.6
120
0.2 A/g 0.5 A/g 1 A/g 3 A/g
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(c)
0.0
0.4
Potential (V)
-0.4
-0.6
80
60
40
GF800 3DGF
20
-0.8 0
0
200
400
600
800
1000
0.0
0.5
1.0
1.5
2.0
Current density (A/g)
Time (s)
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3.0
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(e)
(f)
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Fig. 4. (a) CV evolution of 3DGF at different scan rates from 3 to 1000 mV/s, (b) CVs of 3DGF and GF800 at scan rate of 50 mV/s, (c) galvanostatic charge/discharge profiles of 3DGF at different current densities from 0.2 to 3A/g, (d) specific capacitance plotted of 3DGF and GF800
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as a function of charge/discharge current density, (e) a photograph of the dimention suited paper-like 3DGF and the symmetric coin-type supercapacitor, (f) a circuit constructed with three
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supercapacitors in series.
The corresponding cyclic voltammetric (CV) curves of 3DGF were measured in
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a three-electrode system in 6 M KOH at different scanning rates from 3 to 1000 mV/s.
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As shown in Fig. 4(a), the 3DGF electrode exhibits a lightly distorted rectangular-shaped CV curves at 3 mV/s and a slight hump at around 0.45 V is
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observed, which can be ascribed to the pseudocapacitive contribution from little
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oxygen-containing functional groups [43–46] through reversible Faradic redox reactions [24,34]. However, the profiles of CV shows an increased distortion from the typical rectangular shape, and turned into leaf-like shapes with the increase of scan rate from 3 to 1000 mV/s, which suggests that the CV behavior derived mainly from the electric double layer capacitance because of the coulombic interactions [35,47]. Conversely, seldom capacitive characteristic was observed for GF800, which only
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exhibit very narrow CV curve at 50 mV/s (Fig. 4b, Fig. S3), and this result further demonstrates that the 3D macroporous structure with higher surface area is favor for the increase of specific capacitance. The capacitive performance was further investigated with galvanostatic charge/discharge cycling experiments. Fig. 4(c) plots
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the galvanostatic charge/discharge profiles vs. time curves of 3DGF at different current densities from 0.2 to 3 A/g. All these profiles were typical triangle form with little distortion, indicating an excellent supercapacitive performance [44,45,47]. A
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remarkable specific capacitance of 95 F/g for 3DGF was obtained at a current density of 0.5 A/g, while GF800 was only 9 F/g. Moreover, the 3DGF at 3 A/g maintained 88% retention of its initial specific capacitance measured at 0.5 A/g, while GF800 film
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retained only 26% at the same current density (Fig. 4d). Compared to the GF800 film, the improved rate capability of 3DGF indicates again that the 3D macroporous
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structure allows for effective ion migration into the active sites, thereby generating
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reversible capacitive behavior even at high charging/discharging rates. The binder-free 3DGF electrodes with high rate capability, mechanical flexibility and
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enhanced gravimetric capacitance (Fig. 4e) can be cut to be dimensional suitable
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electrode for the symmetric coin-type supercapacitor to illuminate a light emitting diode (LED) bulb (Fig. 4f), so as to show the high perspective of which for future energy storage. 4. Conclusions In conclusion, the 3D paper-like graphene with higher surface area (402.5 m2/g), smaller pore size and highly orientated laminar structure was fabricated by a facile
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template method with PS microspheres as the sacrificial templates followed by thermal annealing in this paper. This hard template-directed synthesis strategy using PS microspheres as the sacrificial templates could bring the merits of highly orientated laminar and macroporous structure for 3DGF, which could be qualified for
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using as binder-free electrodes. A remarkable specific capacitance of 95 F/g for 3DGF was obtained at a current density of 0.5 A/g, while GF800 was only 9 F/g. Moreover, the 3DGF exhibit the superior rate capability, with 88% of capacitance retention at 3
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A/g, which indicates that the 3D interconnected porous structure significantly enhances ion diffusion and electron transport with a shortened ion-diffusion as well as reduced resistance. The 3DGF with a macroporous architecture fabricated by our
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approach is further optimized: i) the PS microspheres (150 nm) used as sacrificial templates are smaller than previous reports; ii) The resultant 3DGF has a large surface
area,
continuously
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specific
interconnected
macroporous
structures,
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controllable pore size and porosity; iii) it can be directly cut to be dimention suitable binder-free electrode, and a LED bulb can be illumined. Such an environment-friendly,
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facile fabrication process and controlled dimensional make the 3DGF electrodes
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developed in this study a promising candidate for next generation supercapacitors. Supporting Information The SEM imagines for PS microspheres and GF800. The photograph for GF800.
The CV evolution of GF800 at different scan rates from 3 to 100 mV/s. Acknowledgments We acknowledge the financial support from the Natural Science Foundation of
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China (51302281 and 51402324), Natural Science Foundation of Shanxi Province (2013011012–7). References [1] Wu Z S, Winter A, Chen L, Sun Y, Turchanin A, Feng X, Muellen K. Adv. Mater., 2012,24(37):5130 [3] Huang Y, Liang J, Chen Y. Small, 2012,8(12):1805
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[40] Chen X C, Wei W, Lv W, Su F Y, He Y B, Li B, Kang F, Yang Q H. Chem. Commun., 2012,48(47):5904
[41] McAllister M J, Li J L, Adamson D H, Schniepp H C, Abdala A A, Liu J, Herrera-Alonso M, Milius D L, Car R, Prud'homme R K, Aksay I A. Chem. Mater., 2007,19(18):4396 [42] Ryu Z Y, Zheng J T, Wang M Z, Zhang B J. Carbon, 1999,37(8):1257
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[43] Frackowiak E, Metenier K, Bertagna V, Beguin F. Appl. Phys. Lett., 2000,77(15):2421 [44] Kotz R, Carlen M. Electrochim. Acta, 2000,45(15-16):2483 [45] Frackowiak E, Beguin F. Carbon, 2001,39(6):937
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Graphical Abstract
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Three-dimensional paper-like graphene framework with highly orientated laminar structure as binder-free supercapacitor electrode
http://www.journals.elsevier.com/ journal-of-energy-chemistry/
Yidan Gao , Yaoyao Zhang , Yong Zhang , Lijing Xie , Xiaoming Li , Fangyuan Su , Xianxian Wei , Zhiwei Xu , Chengmeng Chen , Rong Cai PII: DOI: Reference:
S2095-4956(15)00129-1 10.1016/j.jechem.2015.11.011 JECHEM 78
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
29 July 2015 11 September 2015 15 September 2015
Please cite this article as: Yidan Gao , Yaoyao Zhang , Yong Zhang , Lijing Xie , Xiaoming Li , Fangyuan Su , Xianxian Wei , Zhiwei Xu , Chengmeng Chen , Rong Cai , Three-dimensional paper-like graphene framework with highly orientated laminar structure as binder-free supercapacitor electrode, Journal of Energy Chemistry (2015), doi: 10.1016/j.jechem.2015.11.011
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Three-dimensional paper-like graphene framework with highly orientated laminar structure as binder-free supercapacitor electrode
Yidan Gaoa,b, Yaoyao Zhanga,c, Yong Zhanga, Lijing Xiea, Xiaoming Lia, Fangyuan
a
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Sua, Xianxian Weia,d, Zhiwei Xuc,*, Chengmeng Chena,, Rong Caia,e
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy
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of Sciences, Taiyuan 030001, Shanxi, China b
University of Chinese Academy of sciences, Beijing 100049, China
c
Key Laboratory of Advanced Braided Composites, Ministry of Education, School of
Textiles, Tianjin Polytechnic University, Tianjin 300387, Shanxi, China Taiyuan University of Science and Technology, Taiyuan 030024, Shanxi, China
e
Academy of Opto-Electronics, Chinese Academy of Sciences, Beijing 100094, China
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d
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Abstract
A free-standing paper-like three-dimensional graphene framework (3DGF) with
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orientated laminar structure and interconnected macropores, was obtained by the hard
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template-directed ordered assembly. As the sacrificial templates, polystyrene (PS) latex spheres were assembled with graphene oxide (GO) to build up a sandwich type composite film, followed by heat removal of which with a simultaneous reduction of GO. The 3DGF exhibited high specific surface area of 402.5 m2/g, controllable pores and mechanical flexibility, which was employed as the binder-free supercapacitor
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electrode and show high specific gravimetric capacitance of 95 F/g at 0.5 A/g, with enhanced rate capability in 3 electrode KOH system.
Corresponding
author.
Fax:
+86-0351-4049061;
[email protected]. 3D
graphene
framework;
Polystyrene;
address:
Sacrificial
template;
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Keywords:
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Manuscript received July 29, 2015; revised September 11, 2015, Accepted September 15, 2015
Supercapacitor 1. Introduction
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Supercapacitors are attracting increasing attention from industry due to their
electrical
systems
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potential in load cranes, forklifts, electric vehicles, consumer electronics and mobile [1–3].
Comparing
with
batteries,
supercapacitors
show
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irreplaceable properties such as high power density, fast charging/discharging rate,
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long cycle life (>100 000 cycles) and high reliability [1,4–8]. Recently, graphene is highly concerned for application in electrochemical energy storage, due to its large
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theoretical surface area (2630 m2/g), good electronic conductivity, and high electrochemical stability [9–11]. However, the graphene nanosheets tend to re-stack into irreversible agglomerates due to their strong interlayer van der Waals forces [12– 14]. Obviously, this π-π stacking of graphene will decrease the accessibility of surface to the electrolyte so as to affect the performance of the electrode [7,13–15]. In a typical process when preparing an electrode of powder form graphene, the insulating 2
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polymer binder is usually involved. The binder will increase the electronic resistance and wrap the surface of the electrode material, so as to impede the electron transportation and ion diffusion, as well as complicate the fabrication of the device [1,2,16]. To address these challenges, the binder-free electrodes should be developed
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to utilize the ultimate property of graphene for high performance supercapacitors. In the past few years, self-supported three-dimensional (3D) architectures with continuously interconnected macropores (such as aerogels [17–19], foams [20] and
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sponges [21], and hierarchical porous hybrid paper [22–25]) have been demonstrated to be effective in preventing graphene from aggregating [26]. The energy storage performances of the materials are significantly improved, which are attributed to the
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large surface area, high accessibility to reaction sites, facilitated ion transport, increased mechanical integrity, and/or synergistic effects among multicomponents
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[27–31]. These macroscopic materials are randomly assembled with graphene as the
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basic building blocks in a sol-gel or hydrothermal process. Without the assistance of templates, pores in the materials are randomly distributed and uncontrollable. Besides,
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the absent of external orientation during assembly will result in isotropic
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inter-connection of graphene to affect the macroscopic performance of the resulting material. Therefore, to assemble the graphene sheets in a precisely controllable manner for smarter energy storage still remain a challenge. Recently, the template-directed method using polymer microspheres as sacrificial templates has been developed for fabricating a pore-structure and porosity controlled 3D macroporous graphene architecture. In previous works, Choi et al. [32,33] have
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fabricated the 3D hierarchical metal oxide/chemical modified graphene hybridized films using PS spheres (2 μm) as structural guiding templates. During the process, the hydrazine is employed to reduce graphene oxide followed by removal of template by toluene. The method involved is considered to be toxic and complex. In our previous
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report, polymethyl methacrylate latex spheres was used, followed by one step thermal reduction and template removal, to obtain a self-standing macroporous graphene film for binder-free electrode. However, the surface area of the resulting material is only
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128.2 m2/g, which is due to the overlarge diameter (as high as 500 nm) of the templates [34]. Afterwards, Wang et al. [35] used PS microspheres templates (Φ280 nm) to prepare the macroporous graphene structure by a similar approach. However,
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the resulting material is still in powder form, and thus polymer binders is inevitable during the subsequent electrode preparation.
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In this contribution, a free-standing paper-like 3DGF with higher surface area,
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smaller pore size and highly orientated laminar structure was fabricated by a facile template method with PS microspheres as the sacrificial templates followed by
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thermal annealing. After systematical characterization, the material was evaluated as
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binder-free supercapacitor electrode which shows enhanced energy and power density. Besides, the electrochemical performance was further correlated to the microstructure of 3DGF.
2. Experimental 2.1. Synthesis of polystyrene nanospheres Polystyrene nanospheres with the size of 150 nm were synthesized by
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emulsifier-free emulsion polymerization in a 250 mL three-necked round bottom flask. In a typical process, the sodium dodecylbenzene sulfonate (0.07 g) and potassium persulfate (0.09 g) were firstly dissolved in 80 mL and 20 mL deionized water, respectively. Then, the solution and styrene monomer (5 mL) were poured into the
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three neck flask in Ar atmosphere. After about 30–40min, the obtained mixture was stirred at approximately 350 rpm and heated to 70 oC. After reaction for 8 h, the desired polystyrene nanospheres were obtained by centrifugation and washing with
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deionized water and ethanol several times. The resultant polystyrene nanospheres were re-dispersed in deionized water to obtain an aqueous suspension (30 mg/mL) for further application.
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2.2. Fabrication of the binder-free 3DGF
Graphite oxide was prepared by a modified Hummers’ method, followed by
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ultrasonication (100 W, 30 min) in deionized water to get the graphene oxide hydrosol
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(3 mg/mL). The graphene oxide hydrosol (10 mL) was mixed with polystyrene nanospheres suspension (3.5 mL) and sonicated for 15 min to get a homogeneous
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colloidal suspension, which was then filtrated on a vacuum millipore filter to realize
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the sandwich type assembly of polystyrene nanospheres and graphene oxide sheets. The above composite film was peeled off from the filter and air dried at 50 oC overnight, and the weight ratio of GO and PS in the film was 1:3.5. The composite films were annealed to 800 oC (heating rate 10 oC/min) for 0.5 h under Ar atmosphere in a tubular furnace. GO within the composite film was thermally reduced into graphene, while the polystyrene nanospheres templates were removed simultaneously,
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so as to get the 3DGF. At the same time, the control sample was prepared by a similar procedure but with no polystyrene nanospheres template, resulting in the compact graphene film (GF800). 2.3. Characterization
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The morphologies and microstructure of the samples were examined by field emission scanning electron microscopy (FESEM, JEOL JSM-7001F, Japan), transmission electron microscope (TEM, JEM-2010, Japan), X-ray diffraction
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measurements (XRD, Rigaku, D/Max-2400, Japan) and nitrogen adsorption and desorption experiments (BELSORP-max, Japan). The specific surface area was obtained using Brunauer Emmett Teller (BET) method and the pore size distribution
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was calculated from the adsorption branch of the nitrogen isotherm using the Barrett Joyner Halenda (BJH) model. Thermogravimetric analysis (TGA, STA 409PC,
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NETZSCH, Germany) was carried out under Ar purge at a heating rate of 10 oC/min
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from room temperature to 1000 oC. The reduction of GO was confirmed using an Elementar (vario Macro cube, Germany).
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2.4. Electrochemical measurements
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The electrochemical properties of 3DGF and GF800 were measured in a 6 M
KOH solution as electrolyte. All electrochemical measurements were carried out on a CHI760D electrochemical working station (CH Instrument). The prepared 3DGF was utilized as working electrodes without adding polymeric binders or conducting additives. Hg/HgO electrode and platinum plate were used as the reference electrode and counter electrode, respectively. The specific capacitance was calculated from the
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results of galvanostatic charge-discharge tests using equations:
Csp,galvanostatic
it mE
(1)
where C is the specific capacitance (F/g), i is the discharge or charge current, t is
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the time discharge time, m is the mass of a part of electrode, and E is the potential window (–0.9–0 V). 3. Results and discussion
The Schematic illustration for the fabrication of 3DGF was shown in Fig. 1. PS
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microspheres and GO nanosheets were assembled owing to electrostatic interaction, and then a homogeneous mixture suspension was formed after ultrasonication. Subsequent vacuum filtration followed to realize the sandwich type assembly of GO
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and PS. The removal of PS templates and the thermally reduction of GO were realized
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through the subsequent calcination at 800 oC under Ar atmosphere.
Fig. 1. Schematic illustration for the fabrication of 3DGF.
Both SEM and TEM were employed to observe the microstructures of 3DGF before and after high-temperature thermal process (Fig. 2). The SEM image in Fig. 2(a) clearly shows that the PS microspheres were uniformly wrapped by crinkled GO
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sheets after the subsequent vacuum filtration, which is attributable to a strong electrostatic interaction between the surface of PS microsphere and GO nanosheet. Compared with pristine PS microspheres (Fig. S1) and GF800 (Fig. S2), the graphene encapsulated spheres exhibit crinkled and rough textures, which are associated with
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the presence of flexible and ultrathin GO nanosheets. Fig. 2(b) exhibits the cross-section of GO/PS sandwich composites. This kind of assembly is beneficial for inhibiting the aggregation of graphene sheets due to the intercalation of PS templates
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and the closely distribution of GO nanosheets. After calcination at 800 oC, the highly orientated laminar and macroporous structure of the integral free-standing 3DGF sample was preserved (Fig. 2c and d) after the removal of PS templates. Moreover,
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the spherical structures and holes (Fig. 2e) distributed in surface of 3DGF further confirm that the macroporous network of 3DGF with continuous walls does not
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collapse, which is could be the excellent mechanical properties. Meanwhile, gaseous
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products are liberated when the PS templates are pyrolyzed under high temperature, which prevents the re-stacking of graphene sheets and leads to tremendous opened
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macropores in the 3DGF. The high magnification TEM image in Fig. 2(f) has shown
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the size of macropores through 3DGF controlled in the range from 100 to 180 nm, and the wall of 3DGF was a few flexible and wrinkled graphene sheets. These features strongly suggest that the PS spheres directed synthesis strategy could bring the merits of ordered and macroporous structures, the resulted 3DGF could be qualified for using as additive/binder-free electrodes.
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(b)
(c)
1µm
100nm
(d)
(f)
(e)
1µm
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100nm
1µm
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(a)
100nm
Fig. 2. SEM images of the (a) surface and (b) a cross-section of a GO/PS composite film; (c) low and (d) high magnification SEM images of the cross-section of a 3DGF; (e) low magnification
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SEM images of surface of a 3DGF; (f) high magnification TEM images of 3DGF.
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The transformation from GO film to the final 3DGF was examined by XRD measurements (Fig. 3a) and elemental analysis (Fig. 3b). It is clearly that there was
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only one sharp peak centered at 2=11.5° in the XRD pattern of the GO film,
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corresponding to a distance of 7.69 nm between the stacked GO sheets. This value indicates that the interlayer spacing of GO is larger than that of graphite (approximately 0.34 nm) owing to a large amount of oxygen groups being introduced on the graphite sheets [36]. Compared with GO film, the XRD patterns of GO/PS composites show a weakened (001) peak at 11.5° and a broader peak centered at 19.7° corresponds to PS appears [37,38]. After being annealed, the GF800 exhibits a
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broader peak (002) centered at 26.0°, corresponding to the interlayer spacing of 0.34 nm, which further verifies that most of the oxygen-containing groups intercalated into the interlayers of graphite have been removed and the conjugated structure of GF800 has been restored after the thermal process [39–41]. GO film shows C/O mole ratio of
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1.2 and this value increases up to 14 for 3DGF after thermal treatment at 800 oC, demonstrating the elimination of most oxygen-containing functional groups and the reduction of graphene sheets. Furthermore, the weaker and broader peak (002) of
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3DGF indicating the graphene sheets in the 3DGF are only loosely stacked, which confirms that gases released from the pyrolysis of the PS templates can promote the exfoliation of graphene sheets and facilitate the formation of opened macropores in the 3DGF.
GO GO/PS GF800 3DGF
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(002)
(b)
80
C O
60
C/O=1.2
20 0
GO
3DGF Samples
1000
(c)
C/O=14
40
40
2(degree)
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20
100
Elemental composition (wt%)
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(001)
Intensity(a.u.)
(a)
GO/PS PS GO
(d)
800
78.8%
60
600
3
Mass (%)
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Va (cm STP/g)
80
40
8.9%
20
400
200
0 200
400
600
o
800
0 0.0
1000
Temperature ( C)
0.2
0.4
0.6
0.8
1.0
P/P0
Fig. 3. (a) XRD patterns of GO film, GO/PS composite film, GF800 and 3DGF, (b) elemental analysis of GO film and 3DGF, (c) TG curves of GO film, GO/PS composite film, and PS 10
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microsphere, (d) nitrogen sorption isotherms of 3DGF.
TG (Fig. 3c) shows the typical thermal behavior and the structural evolution of all contents in PS/GO composite film during the calcination in the Ar atmosphere. In
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the TG curve of GO, the mass loss in the range of 100 to 230 oC is about 21.2%, related to the pyrolysis of oxygen-containing functional groups, suggesting GO can be derived into graphene through the thermal reduction. The PS microspheres were fully
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pyrolyzed and totally removed at 450 oC. In the case of the GO/PS composite, two stages of weight loss at 230 and 450 oC were attributed to the thermal reduction of GO and pyrolysis of PS, respectively. Moreover, the weight of GO/PS composite film is
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essentially unchanged when the temperature over 450 oC and the residual weight about 8.9% is observed, which confirms that the sacrificed PS templates have been
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removed completely and satisfactorily replicated after high temperature calcination.
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The pore-size characterization of the 3DGF was further verified by measuring the Hg penetration and nitrogen adsorption/desorption isotherms. The typical Type IV
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isotherm characteristic with an adsorption hysteresis was observed from the sorption
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curves (Circle curves Fig. 3d), suggesting the mesopores characteristic of 3DGF [42,43]. The obvious hysteresis loop can be observed at relative pressures ranging from 0.48 to 1.0. The H3 type hysteresis loop indicates slit-like pores distributed in the samples [38]. Its corresponding BET surface area is 402.5 m2/g, which is much larger than that of GF800 (30.99 m2/g) and among the highest for 3D graphene materials made from GO and PS spheres reported in the literature so far. These results
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clearly indicate that the addition of PS microspheres as the macropore structural-directing agent and the gaseous products released from the GO/PS composite during thermal process both alter the pore texture and effectively prevent the irreversible aggregation of individual graphene sheets for 3DGF. In addition, the
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Hg penetration result represents a very high porosity (91.49%) with an ultra-low bulk density of 0.0606 g/mL, indicating the 3DGF is very light. This paper-like 3DGF with higher surface area, smaller pore size and highly orientated laminar structure would
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reduce the ion diffusion distance and realize fast ion transport into the well interconnected porous networks of 3DGF.
200
(c)
50
-100 -150
0.2
0.4
Scan rate: 50 mV/s
0
-100
0.6
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0.0
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0 -50
3DGF GF800
100
Capacitance (F/g)
Capacitance (F/g)
100
3 mV/s 50 mV/s 100 mV/s 500 mV/s 1000 mV/s
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(a) 150
0.8
1.0
0.0
0.2
Potential (V)
0.8
1.0
(d) Capacitance (F/g)
100
AC
Potential (V)
-0.2
0.6
120
0.2 A/g 0.5 A/g 1 A/g 3 A/g
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(c)
0.0
0.4
Potential (V)
-0.4
-0.6
80
60
40
GF800 3DGF
20
-0.8 0
0
200
400
600
800
1000
0.0
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1.0
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2.0
Current density (A/g)
Time (s)
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(e)
(f)
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Fig. 4. (a) CV evolution of 3DGF at different scan rates from 3 to 1000 mV/s, (b) CVs of 3DGF and GF800 at scan rate of 50 mV/s, (c) galvanostatic charge/discharge profiles of 3DGF at different current densities from 0.2 to 3A/g, (d) specific capacitance plotted of 3DGF and GF800
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as a function of charge/discharge current density, (e) a photograph of the dimention suited paper-like 3DGF and the symmetric coin-type supercapacitor, (f) a circuit constructed with three
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supercapacitors in series.
The corresponding cyclic voltammetric (CV) curves of 3DGF were measured in
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a three-electrode system in 6 M KOH at different scanning rates from 3 to 1000 mV/s.
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As shown in Fig. 4(a), the 3DGF electrode exhibits a lightly distorted rectangular-shaped CV curves at 3 mV/s and a slight hump at around 0.45 V is
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observed, which can be ascribed to the pseudocapacitive contribution from little
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oxygen-containing functional groups [43–46] through reversible Faradic redox reactions [24,34]. However, the profiles of CV shows an increased distortion from the typical rectangular shape, and turned into leaf-like shapes with the increase of scan rate from 3 to 1000 mV/s, which suggests that the CV behavior derived mainly from the electric double layer capacitance because of the coulombic interactions [35,47]. Conversely, seldom capacitive characteristic was observed for GF800, which only
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exhibit very narrow CV curve at 50 mV/s (Fig. 4b, Fig. S3), and this result further demonstrates that the 3D macroporous structure with higher surface area is favor for the increase of specific capacitance. The capacitive performance was further investigated with galvanostatic charge/discharge cycling experiments. Fig. 4(c) plots
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the galvanostatic charge/discharge profiles vs. time curves of 3DGF at different current densities from 0.2 to 3 A/g. All these profiles were typical triangle form with little distortion, indicating an excellent supercapacitive performance [44,45,47]. A
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remarkable specific capacitance of 95 F/g for 3DGF was obtained at a current density of 0.5 A/g, while GF800 was only 9 F/g. Moreover, the 3DGF at 3 A/g maintained 88% retention of its initial specific capacitance measured at 0.5 A/g, while GF800 film
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retained only 26% at the same current density (Fig. 4d). Compared to the GF800 film, the improved rate capability of 3DGF indicates again that the 3D macroporous
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structure allows for effective ion migration into the active sites, thereby generating
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reversible capacitive behavior even at high charging/discharging rates. The binder-free 3DGF electrodes with high rate capability, mechanical flexibility and
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enhanced gravimetric capacitance (Fig. 4e) can be cut to be dimensional suitable
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electrode for the symmetric coin-type supercapacitor to illuminate a light emitting diode (LED) bulb (Fig. 4f), so as to show the high perspective of which for future energy storage. 4. Conclusions In conclusion, the 3D paper-like graphene with higher surface area (402.5 m2/g), smaller pore size and highly orientated laminar structure was fabricated by a facile
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template method with PS microspheres as the sacrificial templates followed by thermal annealing in this paper. This hard template-directed synthesis strategy using PS microspheres as the sacrificial templates could bring the merits of highly orientated laminar and macroporous structure for 3DGF, which could be qualified for
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using as binder-free electrodes. A remarkable specific capacitance of 95 F/g for 3DGF was obtained at a current density of 0.5 A/g, while GF800 was only 9 F/g. Moreover, the 3DGF exhibit the superior rate capability, with 88% of capacitance retention at 3
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A/g, which indicates that the 3D interconnected porous structure significantly enhances ion diffusion and electron transport with a shortened ion-diffusion as well as reduced resistance. The 3DGF with a macroporous architecture fabricated by our
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approach is further optimized: i) the PS microspheres (150 nm) used as sacrificial templates are smaller than previous reports; ii) The resultant 3DGF has a large surface
area,
continuously
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specific
interconnected
macroporous
structures,
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controllable pore size and porosity; iii) it can be directly cut to be dimention suitable binder-free electrode, and a LED bulb can be illumined. Such an environment-friendly,
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facile fabrication process and controlled dimensional make the 3DGF electrodes
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developed in this study a promising candidate for next generation supercapacitors. Supporting Information The SEM imagines for PS microspheres and GF800. The photograph for GF800.
The CV evolution of GF800 at different scan rates from 3 to 100 mV/s. Acknowledgments We acknowledge the financial support from the Natural Science Foundation of
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China (51302281 and 51402324), Natural Science Foundation of Shanxi Province (2013011012–7). References [1] Wu Z S, Winter A, Chen L, Sun Y, Turchanin A, Feng X, Muellen K. Adv. Mater., 2012,24(37):5130 [3] Huang Y, Liang J, Chen Y. Small, 2012,8(12):1805
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Graphical Abstract
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