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Available at www.sciencedirect.com
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Nanodiamond particles/reduced graphene oxide composites as efficient supercapacitor electrodes Qi Wang a,b, Nareerat Plylahan c,d, Manjusha V. Shelke e, Rami Reddy Devarapalli e, Musen Li b, Palaniappan Subramanian a, Thierry Djenizian c,d, Rabah Boukherroub a,*, Sabine Szunerits a,* a Institut de Recherche Interdisciplinaire (IRI, USR 3078), Universite´ Lille 1, Parc de la Haute Borne, 50 Avenue de Halley, BP 70478, 59658 Villeneuve d’Ascq, France b Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Shandong University, Jinan 250061, China c Aix Marseille University, CNRS, LP3 UMR 7341, 13288 Marseille, France d ALISTORE-ERI, CNRS, FR3104, France e Physical and Materials Chemistry Division, National Chemical Laboratory (CSIR-NCL), Pune 411 008, India
A R T I C L E I N F O
A B S T R A C T
Article history:
The paper reports on the preparation of reduced graphene oxide (rGO) modified with nan-
Received 1 August 2013
odiamond particles composites by a simple solution phase and their use as efficient elec-
Accepted 28 October 2013
trode in electrochemical supercapacitors. The technique relies on heating aqueous
Available online 13 November 2013
solutions of graphene oxide (GO) and nanodiamond particles (NDs) at different ratios at 100 C for 48 h. The morphological properties, chemical composition and electrochemical behavior of the resulting rGO/NDs nanocomposites were investigated using UV/vis spectrometry, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, transmission electron microscopy (TEM) and electrochemical means. The electrochemical performance, including the capacitive behavior of the rGO/NDs composites were investigated by cyclic voltammetry and galvanostatic charge/discharge curves at 1 and 2 A g1 in 1 M H2SO4. The rGO/ND matrix with 10/1 ratio displayed the best performance with a specific capacitance of 186 ± 10 F g1 and excellent cycling stability. 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
With the growing demand for portable systems and hybrid electrical vehicles requiring high power in short-time pulses, the electrochemical capacitors are gaining increasing interest. Carbon materials such as activated carbon, mesoporous carbon and carbon nanotubes usually display good stability, but the capacitance values are limited by the microstructures in the materials [1,2]. Graphene, a two-dimensional all sp2 hybridized carbon with unique electronic and mechanical properties, has received a rapidly growing interest as material in supercapacitors [3]. To exploit the potential of graphene-
based materials for supercapacitor applications, different approaches have been considered. Chemically reduced graphene oxide (rGO), synthesized through hydrazine reduction of graphene oxide (GO), has been the first reported graphenebased electrochemical double-layer capacitor with specific capacitance values of 135 F g1 in aqueous electrolytes [4]. Chen et al. investigated the capacitive properties of partially reduced graphene oxide, prepared by the reaction of GO with hydrobromic acid. The presence of oxygen functional groups on the rGO facilitated the penetration of the electrolyte, introducing additional pseudo-capacitive effects. As a result, specific capacitance values of 348 and 158 F g1 have been
* Corresponding authors. E-mail addresses:
[email protected] (R. Boukherroub),
[email protected] (S. Szunerits). 0008-6223/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.10.077
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measured in 1 M H2SO4 and 1-butylimidazolium hexafluorophosphate (BMIPF6), respectively at a current density of 0.2 A g1 [5]. Graphene-based electrodes prepared simply through chemical and/or thermal reduction still do not have sufficient large pores due to rGO sheets agglomeration and thus do not facilitate the access of the electrolyte. Consequently, high specific capacitance values were in most cases only achievable by charging/discharging at current densities below 1 A g1 [6,7]. Less-agglomerated graphene-based electrodes with suitable pore sizes are still highly demanded. Self-assembled graphene hydrogels formed by hydrothermal reduction of GO has shown to possess well-defined and cross-linked 3D porous structure with specific capacitance of 175 F g1 at a discharge current density of 10 mV/s in 1 M H2SO4 [7]. To inhibit agglomeration of rGO through electrostatic interactions and providing at the same time open nanochannels, incorporation of intercalating spacers is a promising strategy. Organic molecules such as 1-pyrenecarboxylic acid [33] or tetrabutylammonium hydroxide [9] as well as metallic nanoparticles [10] were used as spacers and resulted in specific capacitances of 120 F g1 (in 6 M KOH) [8], 194 F g1 (at 1 A g1 in 2 M H2SO4) [9] or 269 F g1 in 0.5 M H2SO4 [10]. The use of carbon nanotubes as spacer in graphene-based hybrid films has shown to increase the specific capacitance to 385 F g1 at a scan rate of 10 mV s1 in 6 M KOH [11]. Diamond nanoparticles (also referred to as nanodiamonds, NDs) have received considerable interest for applications in tribology and nanobiotechnology owing to their chemical inertness, biocompatibility and high specific area [12]. The average diamond particle size in typical detonation diamond is 10 nm, but depends strongly on the surface functionalization [13,14]. As NDs have shown electrochemical activity, we investigated the possibility to use them as intercalating material into GO nanosheets as well as their ability to reduce GO into rGO [15]. In this paper, we report on the preparation of reduced graphene oxide/nanodiamonds (rGO/NDs) composites using a solution phase process. The direct reaction of aqueous solutions of GO and NDs at different GO/NDs ratios at 100 C for 48 h gave the corresponding composites with enhanced properties. The GO matrix was partially reduced to rGO under these conditions, while the NDs particles were intercalated into the rGO sheets. Incorporation of NDs into rGO matrix resulted in a significant improvement of the dispersibility of the rGO/NDs composite in solvents such as ethanol and DMF with the suspensions being stable for several weeks. The rGO/NDs composites have been successfully used as electrodes in supercapacitors with a maximum specific capacitance of 186 ± 10 F g1 in 1 M H2SO4 at a current density of 1 A g1.
2.
Experimental part
2.1.
Materials
Graphite powder ( 1.0) [26], suggesting that the rGO/NDs nanocomposite have relatively little defects. Scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) images of the different rGO/ NDs composites are displayed in (Fig. 4A and B). Increasing the NDs concentration results in the formation of highly porous nanocomposites. TEM images show that the NDs particles are well dispersed in the rGO matrix.
3.2. Electrochemistry of nanodiamonds composites
reduced
graphene
oxide/
To investigate the usefulness of the rGO/NDs composites as electrode materials, their electrochemical properties were evaluated. While GO is an insulating material, partial restoration of the sp2 network (as indicated above from XPS and UV– vis results) of the different rGO/NDs matrices together with the reported electrochemical activity of NDs [15] should provide rGO/NDs with electrochemical activity. Fig. 5 shows cyclic voltammograms of the different rGO/NDs matrices after drop-casting 15 mg/mL of the composite material onto glassy carbon electrode using Fe(CN)63/4 (Fig. 5A) and Ru(NH3)63+ (Fig. 5B) as redox couples. There was a remarkable difference of the electrochemical activity between the GC electrodes with and without coating with rGO/NDs nanocomposites. In the case of Fe(CN)63/4, the GC electrode showed a peak separation, DE of 108 mV, which increased upon coating with rGO/NDs to 168 mV (1/1), 153 mV (4/1) and 190 mV (10/1). The electrochemical behavior of the rGO/NDs (20/1) nanocomposite is comparable to that of 4/1 nanocomposite. This behavior is independent of the redox mediator and did not qualitatively change using Ru(NH3)63+. The results clearly suggest that a GC electrode coated with rGO/NDs (0.5 mg cm2 mass loading) is more conducting compared to that coated with GO, where no electrical current could be detected under similar conditions.
rGO-NDs (10/1) rGO-NDs (4/1) rGO-NDs (1/1) GO sp
3
ND x5
1200
1600 2000 2400 -1 wavenumber / cm
2800
Fig. 3 – (A) UV/vis and (B) Raman spectra of GO (black), NDsOH (grey), rGO/NDs (1/1) (green), rGO/NDs (4/1) (blue), rGO/ NDs (10/1) (red) and rGO/NDs (20/1) (violet). (A colour version of this figure can be viewed online.)
3.3.
Supercapacitor behavior
To check if the observed electrochemical conductivity is elevated enough to enable current collection, we investigated the supercapacitor behavior of the rGO/NDs composite electrodes. In contrast to conventional high surface area materials such as conducting polymers and nonporous electrode materials like carbon nanotubes, the effective surface area of graphene and its derivatives is not solely dependent on the distribution of pores in the solid state [4]. The chemical state is an additional important parameter to take into consideration. The presence of oxygen functional groups
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(A)
(B)
Fig. 4 – (A) SEM images and (B) typical HR-TEM images of rGO/NDs matrices. The HR-TEM image of ND-OH is also included.
(B) 0.3
(A) 0.3 0.2
0.2 0.1
i / mA
i / mA
0.1 0 -0.1 glassy carbon GO rGO/NDs (1/1) rGO/NDs (4/1) rGO/NDs (10/1) rGO/NDs (20/1)
-0.2 -0.3 -0.4 -0.4
-0.2
0
0.2
0.4
0.6
0.8
E / V vs. Ag/AgCl
0 -0.1
glassy carbon GO rGO/NDs (1/1) rGO/NDs (4/1) rGO/NDs (10/1) rGO/NDs (20/1)
-0.2 -0.3
-0.6
-0.4 -0.2 0 E / V vs. Ag/AgCl
0.2
Fig. 5 – Cyclic voltammograms of a bare glassy carbon electrode before (doted black) and after coating with GO (grey), rGO/NDs (1/1) (green), rGO/NDs (4/1) (blue), rGO/NDs (10/1) (red) and rGO/NDs (20/1) (violet) recorded in (A) Fe(CN)63/4 (5 mM)/PBS (0.1 M) at a scan rate of 100 mV s1 and in (B) Ru(NH3)63+ (5 mM)/PBS (0.1 M) at a scan rate of 50 mV s1. (A colour version of this figure can be viewed online.)
enhances generally the wettability and the capacitance values of graphene-based supercapacitors [27]. More specifically, quinone type functions provide pseudo-capacitance through Faradic redox reactions, which are pH dependent and more pronounced when the pH of the electrolyte is below 3 [28]. In the case of rGO/NDs, quinone type functions are present on the NDs surface due to oxidation of C–OH to C@O during the reduction of GO to rGO. They are also present on rGO and both might be responsible for a pseudocapacitance contribution. Fig. 6A shows the cyclic voltammograms of the different interfaces in 1 M H2SO4. Beside the rGO/NDs (1/1) composite, rectangle-like shaped curves characteristic of capacitive behavior are observed with specific capacitance varying between 9 and 186 F g1, depending on the rGO/NDs nanocomposite (Table 1). The rectangular shape indicates that the main contribution to the capacitance is the charge and discharge of the double layer, which is enhanced by the presence of NDs working probably as spacers. The rGO/NDs (10/1) matrix shows in addition a pseudocapacitance contri-
bution, mostly likely due to the presence of quinone-functions in the matrix. Similar results have been reported for rGO prepared by electrochemical reduction of GO in N2purged PBS [27] or through reaction with hydrobromic acid [5]. The best performance is observed indeed for rGO/NDs (10/1) with a specific capacitance of 186 F g1. Matrices with a rGO/NDs ratio of 20/1 resulted in a lower capacitance of 120 F g1, indicating that maximal values are achieved with a 10/1 ratio. Highly NDs loaded rGO matrices (1/1) display low specific capacitance values of 9 F g1, increasing to 114 F g1 for the 4/1 matrices (Table 1). The better performance of the rGO/NDs matrix with a ratio of 10/1 is not completely clear yet. The supercapacitor behavior of the different nanocomposites was further studied by galvanostatic tests at different current densities. Due to the poor capacitance of rGO/NDs (1/1), this sample was not investigated further. Fig. 6B shows galvanostatic charge/discharge profiles at current densities of 1 or 2 A g1. The discharge curves somewhat deviate from
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rGO/ NDs 4/1 rGO/ NDs10/1 rGO/ NDs 20/1
0.8 E / V vs Ag/AgCl
-1
1
1
(B)
rGO/ NDs 1/1 rGO/ NDs 4/1 rGO/ NDs 10/1 rGO/ NDs 20/1
1.5
current densities / A g
6 8 ( 2 0 1 4 ) 1 7 5 –1 8 4
0.5 0 -0.5 -1
0.6 0.4 0.2
-1.5 -2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0
0.8
40
80 120 time / s
E / V vs Ag/AgCl
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(C)
Current densities / A g
-1
2
1
0
-1
-2 0
0.2
0.4
0.6
0.8
E / V vs Ag/AgCl
Fig. 6 – (A) Cyclic voltammograms of rGO/NDs nanocomposites in 1 M H2SO4; scan rate = 10 mV s1; (B) galvanostatic charge/ discharge profile of the different rGO/NDs nanocomposites at current density of 1 A g1 (full line) and 2 A g1 (dotted line) in the potential window ranging between 0 and 0.8 V; (C) cyclic voltammograms of rGO/NDs (10/1) nanocomposites, prepared by heating GO and NDs at 100 C for 48 h in absence (red) and in presence (blue) of hydrazine, in 1 M H2SO4; scan rate = 10 mV s1. (A colour version of this figure can be viewed online.)
Table 1 – Characteristics of NDs, graphene oxide (GO) and reduced graphene oxide (rGO)/NDs nanocomposites. Specific capacitance (F/g) in 1 M H2SO4 electrolyte
Product
NDs GO rGO/NDs rGO/NDs rGO/NDs rGO/NDs
(1/1) (4/1) (10/1) (20/1)
Size (nm)
f (mV)
CV (10 mV/s)
Galvanostatic discharge 1 A g1
Galvanostatic discharge 2 A g1
31 ± 6 151 ± 1 225 ± 2 239 ± 7 289 ± 18 298 ± 9
17 ± 1 41 ± 4 30 ± 1 34 ± 2 37 ± 2 42 ± 2
– – 9±2 114 ± 8 186 ± 10 120 ± 5
– – – 137 ± 15 169 ± 13 122 ± 10
– – – 125 ± 11 143 ± 11 111 ± 10
linearity due to the ohmic drop. The specific capacitance of each sample was calculated from the linear part of the discharge curves (0–0.3 V). As shown in Table 1, the values of the specific capacitance obtained by the galvanostatic discharge curves agree with the values obtained by the CV curves. The decrease in specific capacitance at higher current densities is due to fast kinetics. Furthermore, we investigated the capacitive properties of rGO/NDs reduced through hydrazine. In fact, it is well estab-
lished that GO reduction with hazardous hydrazine gives the best performance in terms of electrical conductivity. The reduction of GO/NDs (ratio = 10/1) in the presence of hydrazine under otherwise similar conditions afforded rGO/ NDs composite with a specific capacitance of 241 F g1 (Fig. 6C). This is higher than the capacitance value (186 F g1) recorded for GO/NDs (ratio = 10/1) although the shape of the voltammogram is very similar with the presence of a pseudocapacitive component. This result suggests that
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200
183
specific capacitance / F g
-1
R E F E R E N C E S
150
100
50
0
0
200
400 600 800 cycle number
1000
Fig. 7 – Specific capacitance of rGO/NDs 10/1 versus cycle number at a current density of 2 A g1.
even a strong reducing agent such as hydrazine is not capable of removing all electrochemically active oxygen-containing groups even though the electrical properties of the composite have been improved. It has to be however noted that the specific capacitances calculated for thin films with low mass loading, as performed in this work, cannot be directly compared with capacitance values reported for other graphenebased [4,29–31] with often more than an order of magnitude higher mass loading [32]. Due to the most promising performance of rGO/NDs 10/1, the stability of this sample in a long term galvanostatic cycling was tested. Fig. 7 depicts the specific capacitance of rGO/NDs 10/1 versus cycle number. The sample was cycled at 2 A g1 for 1000 cycles showing a stable performance upon cycling.
4.
Conclusion
Conducting reduced graphene oxide matrices with intercalated nanodiamond particles of different densities were prepared by simple mixing GO with nanodiamonds and heating at 100 C for 48 h. Electrochemical investigation of the different rGO/NDs nanocomposites showed that the electrochemical behavior depends on the initial GO/NDs ratio used for the formation of the nanocomposites. A GO/NDs ratio of 10/1 resulted in nanographene matrices with a specific capacitance of 186 F g1 with excellent long term stability. These findings may have important consequences for technological applications in the field of supercapacitors.
Acknowledgements R.B. and S.S. gratefully acknowledge financial support from the Centre National de Recherche Scientifique (CNRS), the University Lille 1 and Nord Pas de Calais region. S.S. thanks the Institut Universitaire de France (IUF) for financial support. We thank Emmanuelle Pichonat for helping with the Raman measurements. Financial support through the P7-PEOPLEIRSES funded MATCON project is acknowledged. Q.W. thanks the Natural Science Foundation of China, under Grant No. 50972078 and the Chinese government for the China Scholarship Council Award. T.D. and N.P. thank ANR No. 2010 JCJC 910 01 MICROLIONH for financial support.
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