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A Review: Synthesis Methods of Graphene and Its Application in Supercapacitor Devices
NURHAFIZAH Md Disa1,2,a*, SURIANI Abu Bakar1,2,b, SUHUFA Alfarisa1,2,c, AZMI Mohamed1,3,d, ILLYAS Md Isa1,3,e, AZLAN Kamari1,3,f,
NORHAYATI Hashim1,3,g and MOHAMAD Rusop Mahmood4,5,h 1Nanotechnology Research Centre, Faculty of Science and Mathematics, Universiti Pendidikan
Sultan Idris, 35900 Tanjung Malim, Perak, Malaysia
2Department of Physics, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, 35900 Tanjung Malim, Perak, Malaysia
3Department of Chemistry, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, 35900 Tanjung Malim, Perak, Malaysia
4NANO-ElecTronic Centre, Faculty of Electrical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia
5NANO-SciTech Centre, Institute of Science, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia
[email protected], [email protected], [email protected],[email protected], [email protected],
[email protected], [email protected], [email protected]
Kewords: graphene, chemical vapour deposition, Hummers method, electrochemical exfoliation method, supercapacitor
Abstract. Graphene is a remarkable material with high electron mobility, good mechanical strength
and almost transparent. In this paper, we review the available methods which are chemical vapour
deposition, Hummers and electrochemical exfoliation method for the production of graphene.
Among the extensive studies in the application of graphene, supercapacitor has gained much
attention nowadays. Therefore, we also briefly review the application of graphene as electrodes for
the supercapacitor devices.
Introduction
Graphene has become a famous material in nanotechnology research since many decades
ago. This material has just been isolated in 2004 by Novoselov and his co-workers [1]. Its ability to
form a high-quality 2D crystal arranged in a single sheet hexagonal lattice [1] has brought a major
breakthrough in the world of graphene research. Theoretically, this nanostructure possesses an
extremely high surface area of about 2630 m2g
-1 [2] which is multiple times higher than carbon
nanotube (CNT). However, inter-layer separations between graphene sheets which is ~0.335 nm [3]
and weak van der Waals interaction between the layers have become serious challenges in graphene
production technology. High electron mobility of graphene [4] has brought out this material into a
lot of manufacturing electronic devices.
Lately, several methods that have been recognized for graphene production which were
mechanical exfoliation, epitaxial growth on SiC, chemical vapour deposition (CVD), arc discharge
and reduction of graphite oxide [2]. Among them, chemical synthesis of graphite which is
electrochemical exfoliation method has attracted worldwide attention as an effective approach to
produce graphene due to greener, simpler preparation method and low production cost. However,
the optimization of various parameters should be considered in order to produce a high quality
graphene. In this review, the methods to synthesize graphene which are (i) CVD, (ii) Hummers and
(iii) electrochemical exfoliation methods are reviewed. The recent applications in supercapacitor
Advanced Materials Research Vol. 1109 (2015) pp 40-44 Submitted: 2014-07-14© (2015) Trans Tech Publications, Switzerland Accepted: 2014-07-18doi:10.4028/www.scientific.net/AMR.1109.40 Online: 2015-06-10
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TransTech Publications, www.ttp.net. (ID: 141.211.4.224, University of Michigan Library, Media Union Library, Ann Arbor, USA-18/06/15,03:20:36)
devices were discussed further. This study is meant to inspire researchers in massive graphene
production to completely revolutionize entire energy storage industries.
Synthesis of Graphene
Generally, graphene can be produced through various approaches which are mechanical cleavage,
arc discharge, epitaxial growth and CVD, Hummers and electrochemical exfoliation methods. Here,
we review several methods for graphene production.
CVD method. Low defect graphene can be produced through CVD method either on substrate or
without substrate [5]. Park et al. [6] reported a few layers of graphene were grown on Ni film-
coated Si substrate. They found that as the growth time increases at synthesis temperature of 960-
970°C, a higher density of graphene was produced. Besides that, Guermoune et al. [7] reported the
quality of graphene grown on Cu foils can be improved by increasing the synthesis temperature
from 650 to 850°C. The use of low-cost carbon source from alcohol groups is favourable compared
to CH4 gas [8]. However, complex substrate preparation has restricted this technique for mass
production of graphene. For comparison, Borysiak [8] reported the production of graphene using
CH4 gas on Cu substrates. Higher temperatures ranged from 800 to 1050°C allowed the faster
growth of graphene. Longer reaction time introduced did not affect the graphene structures and
fewer defects were seen on the samples. However, the use of high temperature to oxidize graphite
has caused this method to be less efficient due to the high energy consumption. Furthermore,
growth of graphene on the substrates has a disadvantage in term of transferring process due to the
possibility of structural damage occurrence. Until now, optimal parameters to produce graphene
using CVD method are still not fully understood by researchers.
Hummers method. Hummers method introduced a rapid approach to oxidize graphite powder
assisted by sulphuric acid (H2SO4), potassium permanganate (KMnO4) and sodium nitrate (NaNO3).
As compared to the previous methods [9, 10], by replacing HNO3 [9] with NaNO3 and KClO3 [10]
with KMnO4, Hummers method offered a safer process. The advantages of this method are it
involves less reaction time and lower temperature consumption. Chen et al. [11] reported an
improved Hummers method by eliminating the use of NaNO3 that led to the eco-friendly synthesis
of graphene oxide (GO). Several benefits of this study are short reaction time, avoid the formation
of acid fog and safe from hazardous reactions. They found that the reaction assisted by KMnO4 in
concentrated H2SO4 has successfully formed a single-layer graphene which has the similar role as
NaNO3 which can also form separated graphite bisulphate [12]. This approach was a simpler and
safer process compared to the previous Hummers method. Hence, the role of NaNO3 and HNO3 is
reduced. Bykkam et al. [13] also reported that GO was successfully synthesized through modified
Hummers method. The dimensions of GO produced were 35 to 65 nm in size. However, high
chemicals consumption has partially damaged GO structures and made this breakthrough as an
unfriendly approach for mass production of GO.
In other hand, oxidized graphite powder through Hummers method caused few oxygen
functional groups such as hydroxyl, epoxy, carbonyl and carboxyl attached to GO structures
produced. Therefore, the solubility of GO in most solvents also become a problem which is highly
needed to be explored more in order to produce high quality single layer graphene. In previous
study, Paredes et al. [14] reported on the behaviour of GO in the different organic solvents,
including N, N-dimethylformamide, N-methyl-2-pyrrolidone, tetrahydrofuran and ethylene glycol.
Through the simple sonication, a stable dispersion of GO over a long time was achieved and fully
exfoliated into single-layer. However, this method has been noted as non cost-effective due to the
high consumption of strong acids and lack of controlled thickness for graphene sheets production.
In addition, high defects level was also left over on the samples and sometimes heavy metal
attached to the samples are difficult to remove. Therefore, further process should be conducted.
Advanced Materials Research Vol. 1109 41
Electrochemical exfoliation method. In recent years, electrochemical exfoliation method assisted
by surfactant has gained much attention to be used for GO or graphene sheets production. Various
surfactants have been introduced to improve the compatibility of graphene in water-based system.
Polar hydrophilic head of surfactant is attracted to water and its hydrophobic tail is adsorbed on the
surface of graphene. This nature of surfactant made graphene to be well dispersed in water [15].
The common commercial surfactants used are sodium dodecyl sulphate (SDS), poly (sodium 4-
styrenesulfonate) (PSS) and sodium dodecylbenzene sulphonate (SDBS). The attachment of
benzene ring such in SDBS has been reported increase the degree of carbon materials dispersion
due to the binding and surface coverage of this intercalated compound to graphite [16]. Currently,
Alanyahóglu et al. [17] reported electrochemical approach as an efficient route to produce graphene
flakes due to its simple preparation at low temperature and cost effective. This method used fewer
chemicals and provide thickness control by adjusting the electrode potential from external power
supply. Three-electrode system assisted by SDS as a stabilizer was used in this study. The average
size and thickness of graphene sheets produced were about 500 and 1 nm, respectively.
Furthermore, Stankovich et al. [18] also reported the stable aqueous dispersion of graphitic
nanoplatelets via the reduction of exfoliated graphite oxide using PSS as the electrolyte. A
sufficient concentration of PSS can prevent the agglomeration of the platelets. This study showed
that PSS was a good stabilizer due to its ability to produce a stable suspension. Wang et al. [19] also
used PSS to assist the synthesis of large-scale graphene through electrolytic exfoliation of two-
electrode systems in aqueous medium. They observed that the interaction between polystyrene
sulfonate ions with positive graphite electrode leading to the appearance of black precipitate in the
solution. Due to this, researchers have found a giant aromatic structure, graphene with good
dispersion stability in aqueous solution [20] in the presence of surfactant rather than in organic
solvents [14]. The advantage of producing graphene sheets through this method is the capability of
surfactant to stabilize and properly hold the sheets in the dispersion. Importantly, graphene
produced in liquid form are readily to be composited with other materials and polymers. However,
the lack of uniformity of the structure produced via this method due to the high degree of graphene
sheets agglomeration remains a problem to the researchers. Therefore, another improvement in this
method is currently needed in order to optimize the quality of graphene produced.
The Use of Graphene as Electrodes In Supercapacitor Devices
Supercapacitor is also well-known as electrochemical double-layer capacitor. This energy
storage device store electrical charges at a low rate of energy density. Carbon-based material, metal
oxides and conducting polymer are the fundamental electrode materials for supercapacitor
applications. Carbon-based materials are suitable to be applied in energy storage devices due to the
high conductivity, electrochemical stability and high surface area [2] with high porosity percentage.
CNT and activated carbon have been reported as electrodes materials in supercapacitor application.
However, the high cost of CNT production and low performance of activated carbon [21] have
brought graphene to this application. Numbers of researches on the potential applications of
graphene in energy storage were deeply explored [22-24]. Unique nanostructures with high
conductivity and porosity of graphene made it suitable to be used in supercapacitor devices.
Fan et al. [25] reported the use of CNT/graphene sandwich as electrode for supercapacitor
and found that an extremely high specific capacitance of 385 Fg-1
at 10 mVs-1
scan rate was
obtained. While, functionalized exfoliated GO from exfoliated graphite was reported to has the
specific capacitance of 146 Fg-1
and energy density of 20 Whkg-1
[26]. In addition, Jin et al. [27]
found that a better supercapacitor performance can be obtained by using sodium carbonate
(Na2CO3) as an efficient reducing agent in GO. Obviously, they obtained a large cyclic pattern
which indicated that the electrodes have good charge propagation. Moreover, the conductivity of
the sample obtained was 10 Sm-1
which is several times higher than GO alone. Through this review,
GO or graphene were seen to be promising electrode materials to enhance the capacitance
performance of supercapacitors.
42 Nanoscience, Nanotechnology, and Nanoengineering: Fundamentals andApplications
Conclusion
The production of graphene through the physical and chemical approaches including CVD,
Hummers and electrochemical exfoliation methods has been reviewed. Due to its exceptional
properties, graphene has been applied in various applications including supercapacitor device. The
use of graphene increases the performance of supercapacitor. For this purpose, the optimization of
parameters to produce high-quality and quantity graphene is required for the enhancement of
electrodes performances.
Acknowledgements
The authors wish to thank L'Oréal-UNESCO Malaysia for Women in Science Fellowships on
financial aid and special thanks to the Faculty of Science and Mathematics, Universiti Pendidikan
Sultan Idris (UPSI) and NANO-ElecTronic Centre, Faculty of Electrical Engineering with NANO-
SciTech Centre, Institute of Science, Universiti Teknologi MARA (UiTM) for facility support.
References
[1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al.,
Electric field effect in atomically thin carbon films, Science 306 (2004) 666-674.
[2] X. Chen, G. Wu, Y. Jiang, Y. Wang, X. Chen, Graphene and graphene-based nanomaterials:
the promising materials for bright future of electro analytical chemistry, Analyst 136 (2011)
4631-4641.
[3] E. Pop, V. Varshney, A.K. Roy, Thermal properties of graphene: fundamentals and
applications, MRS. Bulletin 37 (2012) 1273-1281.
[4] K.I. Bolotin, K.J. Sikes, J. Hone, H.L. Stormer, P. Kim, Ultrahigh electron mobility in
suspended graphene, Phys. Rev. Lett. 101 No. 9 (2008) 6802-6805.
[5] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, et al., Large-scale pattern
growth of graphene films for stretchable transparent electrodes, Nature 457 (2009) 706-716.
[6] H.J. Park, J. Meyer, S. Roth, V. Skakalova, Growth and properties of few layers graphene
prepared by chemical vapor deposition, Carbon 48 (2010) 1088-1094.
[7] A. Guermoune, T. Chari, F. Popescu, S.S. Sabri, J. Guillemette, H.S. Skulason, T. Szkopek,
M. Siaj, Chemical vapor deposition synthesis of graphene on copper with methanol, ethanol,
and propanol precursors, Carbon 49 (2011) 4204-4210.
[8] M. Borysiak. Graphene synthesis by CVD on copper substrates.The 2009 NNIN REU
Research Accomplishments (2009) 70-71.
[9] B.C. Brodie. On the atomic weight of graphite. Philos. Trans. R. Soc. London 14 (1859)
249-308.
[10] L. Staudenmaier. Method for representing the graphite acid. Ber. Dtsch. Chem. Ges. 31
(1898) 1481-1487.
[11] J. Chen, B. Yao, C. Li, G. Shi, An improved Hummers method for eco-friendly synthesis of
graphene oxide, Carbon 64 (2013) 225-229.
[12] N.E. Sorokina, M.A. Khaskov, V.V. Avdeev, I.V. Nikolskaya, Reaction of graphite with
sulphuric acid in the presence of KMnO4, Russ. J. Gen. Chem. 75 (2005) 162-169.
[13] S. Bykkam, V.K. Rao, S.C.H. Chakra, T. Thunugunta, Synthesis and characterization of
graphene oxide and its antimicrobial activity against klebseilla and staphylococus, Adv.
Biotechnol. Res. 4 (2013) 142-146.
[14] J.I. Paredes, S. Villar-Rodil, A. Martinez-Alonso, J.M.D. Tascon, Graphene oxide
dispersions in organic solvents, Langmuir 24 (2008) 10560-10564.
Advanced Materials Research Vol. 1109 43
[15] J.C. Goak, S.H. Lee, J.H. Han, S.H. Jang, K.B. Kim, Y. Seo, Y.S. Seo, N. Lee,
Spectroscopic studies and electrical properties of transparent conductive films fabricated by
using surfactant-stabilized single-walled carbon nanotube suspensions, Carbon 49 (2011)
4301-4313.
[16] M.F. Islam, E. Rojas, D.M. Bergey, A.T. Johnson, A.G. Yodh, High weight fraction
surfactant solubilisation of single-wall carbon nanotubes in water, Nano Lett. 3 (2003) 269-
273.
[17] M. Alanyahoğlu, J.J. Segura, J.O. Solė, N.C. Pastor, The synthesis of graphene sheets with
controlled thickness and order using surfactant-assisted electrochemical processes, Carbon
50 (2012) 142-153.
[18] S. Stankovich, R.D. Piner, X. Chen, N. Wu, S.T. Nguyen, R.S. Ruoff, Stable aqueous
dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the
presence of poly(sodium 4-strenesulfonate), J. Mater. Chem. 16 (2006) 155-158.
[19] G. Wang, B. Wang, J. Park, Y. Wang, B. Sun, J. Yao, Highly efficient and large-scale
synthesis of graphene by electrolytic exfoliation, Carbon 47 (2009) 3242-3246.
[20] Y. Si, E.T. Samulski, Synthesis of water soluble graphene, Nano Lett. 8 (2008) 1679-1682.
[21] H. Wang, Q. Hao, X. Yang, L. Lu, X. Wang, Graphene oxide doped polyaniline for
supercapacitors, Electrochemistry Communications 11 (2009) 1158-1161.
[22] H. Gomez, M.K. Ram, F. Alvi, P. Villalba, E. Stefanakos, A. Kumar, Graphene-conducting
polymer nanocomposites as novel electrode for supercapacitors, J. Power Sources 196
(2011) 4102-4108. [23] J.J. Yoo, K. Balakrishnan, J. Huang, V. Meunier, B.G. Sumpter, A. Srivastava, M. Conway,
A.L. Mohanna Reddy, J. Yu, R. Vatjai, P.M. Ajayan, Ultrathin planar supercapacitors, Nano
Lett. 11 (2011) 1423-1427.
[24] C.M. Gee, C.C. Tseng, F.Y. Wu, H.P. Chang, L.J. Li, Y.P. Hsieh, C.T. Lin, J.C. Chen,
Flexible transparent electrodes made of electrochemically exfoliated graphene sheets from
low-cost graphite pieces, Displays 34 (2013) 315-319.
[25] Z. Fan, J. Yan, L. Zhi, Q. Zhang, T. Wei, J. Feng, M. Zhang, W. Qian, F. Wei, A three-
dimensional carbon nanotube/graphene sandwich and its application as electrode in
supercapacitors, Adv. Mater. 22 (2010) 3723-3728.
[26] P. Karthika, N. Rajalakshmi, K.S. Dhathathreyan, Functionalizad exfoliated graphene oxide
as supercapacitor electrodes, Soft Nanosci. Lett. 2 (2012) 59-66.
[27] Y. Jin, S. Huang, M. Zhang, M. Jia, D. Hu, A green and efficient method to produce a
graphene for electrochemical capacitors from graphene oxide using sodium carbonate as a
reducing agent, App. Surf. Sci. 268 (2013) 541-546.
44 Nanoscience, Nanotechnology, and Nanoengineering: Fundamentals andApplications
Nanoscience, Nanotechnology, and Nanoengineering: Fundamentals and Applications 10.4028/www.scientific.net/AMR.1109 A Review: Synthesis Methods of Graphene and its Application in Supercapacitor Devices 10.4028/www.scientific.net/AMR.1109.40
DOI References
[1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al., Electric field effect
in atomically thin carbon films, Science 306 (2004) 666-674.
http://dx.doi.org/10.1126/science.1102896 [2] X. Chen, G. Wu, Y. Jiang, Y. Wang, X. Chen, Graphene and graphene-based nanomaterials: the
promising materials for bright future of electro analytical chemistry, Analyst 136 (2011) 4631-4641.
http://dx.doi.org/10.1039/c1an15661f [3] E. Pop, V. Varshney, A.K. Roy, Thermal properties of graphene: fundamentals and applications, MRS.
Bulletin 37 (2012) 1273-1281.
http://dx.doi.org/10.1557/mrs.2012.203 [4] K.I. Bolotin, K.J. Sikes, J. Hone, H.L. Stormer, P. Kim, Ultrahigh electron mobility in suspended
graphene, Phys. Rev. Lett. 101 No. 9 (2008) 6802-6805.
http://dx.doi.org/10.1016/j.ssc.2008.02.024 [5] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, et al., Large-scale pattern growth of graphene
films for stretchable transparent electrodes, Nature 457 (2009) 706-716.
http://dx.doi.org/10.1038/nature07719 [6] H.J. Park, J. Meyer, S. Roth, V. Skakalova, Growth and properties of few layers graphene prepared by
chemical vapor deposition, Carbon 48 (2010) 1088-1094.
http://dx.doi.org/10.1016/j.carbon.2009.11.030 [7] A. Guermoune, T. Chari, F. Popescu, S.S. Sabri, J. Guillemette, H.S. Skulason, T. Szkopek, M. Siaj,
Chemical vapor deposition synthesis of graphene on copper with methanol, ethanol, and propanol precursors,
Carbon 49 (2011) 4204-4210.
http://dx.doi.org/10.1016/j.carbon.2011.05.054 [9] B.C. Brodie. On the atomic weight of graphite. Philos. Trans. R. Soc. London 14 (1859) 249-308.
http://dx.doi.org/10.1098/rstl.1859.0013 [10] L. Staudenmaier. Method for representing the graphite acid. Ber. Dtsch. Chem. Ges. 31 (1898) 1481-
1487.
http://dx.doi.org/10.1002/cber.18980310237 [11] J. Chen, B. Yao, C. Li, G. Shi, An improved Hummers method for eco-friendly synthesis of graphene
oxide, Carbon 64 (2013) 225-229.
http://dx.doi.org/10.1016/j.carbon.2013.07.055 [12] N.E. Sorokina, M.A. Khaskov, V.V. Avdeev, I.V. Nikolskaya, Reaction of graphite with sulphuric acid
in the presence of KMnO4, Russ. J. Gen. Chem. 75 (2005) 162-169.
http://dx.doi.org/10.1007/s11176-005-0191-4 [14] J.I. Paredes, S. Villar-Rodil, A. Martinez-Alonso, J.M.D. Tascon, Graphene oxide dispersions in organic
solvents, Langmuir 24 (2008) 10560-10564.
http://dx.doi.org/10.1021/la801744a [15] J.C. Goak, S.H. Lee, J.H. Han, S.H. Jang, K.B. Kim, Y. Seo, Y.S. Seo, N. Lee, Spectroscopic studies and
electrical properties of transparent conductive films fabricated by using surfactant-stabilized single-walled
carbon nanotube suspensions, Carbon 49 (2011).
http://dx.doi.org/10.1016/j.carbon.2011.06.007 [16] M.F. Islam, E. Rojas, D.M. Bergey, A.T. Johnson, A.G. Yodh, High weight fraction surfactant
solubilisation of single-wall carbon nanotubes in water, Nano Lett. 3 (2003) 269273.
http://dx.doi.org/10.1021/nl025924u [17] M. Alanyahoğlu, J.J. Segura, J.O. Solė, N.C. Pastor, The synthesis of graphene sheets with controlled
thickness and order using surfactant-assisted electrochemical processes, Carbon 50 (2012) 142-153.
http://dx.doi.org/10.1016/j.carbon.2011.07.064 [19] G. Wang, B. Wang, J. Park, Y. Wang, B. Sun, J. Yao, Highly efficient and large-scale synthesis of
graphene by electrolytic exfoliation, Carbon 47 (2009) 3242-3246.
http://dx.doi.org/10.1016/j.carbon.2009.07.040 [20] Y. Si, E.T. Samulski, Synthesis of water soluble graphene, Nano Lett. 8 (2008) 1679-1682.
http://dx.doi.org/10.1021/nl080604h [21] H. Wang, Q. Hao, X. Yang, L. Lu, X. Wang, Graphene oxide doped polyaniline for supercapacitors,
Electrochemistry Communications 11 (2009) 1158-1161.
http://dx.doi.org/10.1016/j.elecom.2009.03.036 [22] H. Gomez, M.K. Ram, F. Alvi, P. Villalba, E. Stefanakos, A. Kumar, Graphene-conducting polymer
nanocomposites as novel electrode for supercapacitors, J. Power Sources 196 (2011) 4102-4108.
http://dx.doi.org/10.1016/j.jpowsour.2010.11.002 [23] J.J. Yoo, K. Balakrishnan, J. Huang, V. Meunier, B.G. Sumpter, A. Srivastava, M. Conway, A.L.
Mohanna Reddy, J. Yu, R. Vatjai, P.M. Ajayan, Ultrathin planar supercapacitors, Nano Lett. 11 (2011) 1423-
1427.
http://dx.doi.org/10.1021/nl200225j [24] C.M. Gee, C.C. Tseng, F.Y. Wu, H.P. Chang, L.J. Li, Y.P. Hsieh, C.T. Lin, J.C. Chen, Flexible
transparent electrodes made of electrochemically exfoliated graphene sheets from low-cost graphite pieces,
Displays 34 (2013) 315-319.
http://dx.doi.org/10.1016/j.displa.2012.11.002 [25] Z. Fan, J. Yan, L. Zhi, Q. Zhang, T. Wei, J. Feng, M. Zhang, W. Qian, F. Wei, A threedimensional
carbon nanotube/graphene sandwich and its application as electrode in supercapacitors, Adv. Mater. 22
(2010) 3723-3728.
http://dx.doi.org/10.1002/adma.201001029 [26] P. Karthika, N. Rajalakshmi, K.S. Dhathathreyan, Functionalizad exfoliated graphene oxide as
supercapacitor electrodes, Soft Nanosci. Lett. 2 (2012) 59-66.
http://dx.doi.org/10.4236/snl.2012.24011 [27] Y. Jin, S. Huang, M. Zhang, M. Jia, D. Hu, A green and efficient method to produce a graphene for
electrochemical capacitors from graphene oxide using sodium carbonate as a reducing agent, App. Surf. Sci.
268 (2013) 541-546.
http://dx.doi.org/10.1016/j.apsusc.2013.01.004
