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Exfoliation of Graphite into Graphene in Aqueous Solutions
of Inorganic Salts
Journal:
Journal of the American Chemical Society
Manuscript ID:
ja-2014-017156.R1
Manuscript Type:
Article
Date Submitted by the Author:
27-Mar-2014
Complete List of Authors:
Parvez, Khaled; Max Planck Institute for Polymer Research,
Wu, Zhong-Shuai; Max Planck Institute for Polymer Research,
Li, Rongjin; Max Planck Institute for Polymer Research,
Liu, Xianjie; Linköping University,
Graf, Robert; Max Planck Institute for Polymer Research,
Feng, Xinliang; Max Planck Institute for Polymer Research,
Müllen, Klaus; Max-Planck-Institute for Polymer Research,
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Exfoliation of Graphite into Graphene in Aqueous
Solutions of Inorganic Salts
Khaled Parvez,
†
Zhong-Shuai Wu,
†
Rongjin Li,
†
Xianjie Liu,
‡
Robert Graf,
†
Xinliang Feng,
†,§,*
Klaus Müllen
†,*
†
Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz 55128, Germany
‡
Department of Physics, Chemistry and Biology, Linköping University, SE-58183 Linköping,
Sweden
§
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 200240
Shanghai, P. R. China
*Address correspondence to muellen@mpip-mainz.mpg.de; feng@mpip-mainz.mpg.de
ABSTRACT:
Mass production of high-quality graphene sheets is essential for their practical application in
electronics, optoelectronics, composite materials and energy-storage devices. Here we report a
prompt electrochemical exfoliation of graphene sheets into aqueous solutions of different
inorganic salts ((NH
4
)
2
SO
4
, Na
2
SO
4
, K
2
SO
4
, etc). Exfoliation in these electrolytes leads to
graphene with a high yield (>85%, ≤3 layers), large lateral size (up to 44 µm), low oxidation
degree (a C/O ratio of 17.2), and a remarkable hole mobility of 310 cm
2
V
-1
s
-1
. Further, highly
conductive graphene films (11 Ω
sq.
-1
) are readily fabricated on an A4-size paper by applying
brush painting of a concentrated graphene ink (10 mg/mL, in N,N’-dimethylformamide (DMF)).
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All-solid-state flexible supercapacitors manufactured based on such graphene films deliver a
high area capacitance of 11.3 mF cm
-2
and an excellent rate capability of 5000 mV s
-1
. The
described electrochemical exfoliation shows great promise for the industrial-scale synthesis of
high-quality graphene for numerous advanced applications.
KEYWORDS: electrochemical exfoliation, high-quality graphene, electrolyte, graphene-ink,
supercapacitor
INTRODUCTION
Graphene, a two-dimensional honeycomb sp
2
carbon lattice, has received immense attention
for its potential application in next-generation electronic devices,
1,2
composite materials,
3,4
energy storage devices,
5
etc., due to its intriguing electrical, mechanical, and chemical
properties.
6,7
Mass production of high-quality, solution-processable graphene via a simple low-
cost method, however, remains a major challenge. Several graphene preparation methods have
been developed since its discovery.
8
Among them, mechanically exfoliated and epitaxially
grown graphene provides high-quality material, but in only limited quantities, for fundamental
research
8,9
. Chemical vapour deposition (CVD) using catalytic metal substrates such as Ni or Cu,
produces large-area high-quality graphene.
9,10
The major obstacles to cost-effective industrial-
scale production of CVD-grown graphene, however, are the requirements of high temperature, a
sacrificial metal, and multi-step transfer processes onto the desired substrates.
Chemical
exfoliation of graphite based on the Hummers method is an appealing route to produce solution-
processable graphene oxide (GO) in bulk-scale, but requires thermal or chemical reduction to
partially restore the electronic properties of graphene.
11
Several other methods have been
developed to overcome these limitations, such as solvent- and/or surfactant-assisted liquid-phase
exfoliation,
12
electrochemical expansion,
13
and formation of graphite intercalated compounds.
14
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Nevertheless, extensive sonication processes are indispensable for these methods, which limit the
size and yield of thin graphene layers.
Recently, electrochemical exfoliation of graphite has attracted attention due to its easy, fast,
and environmentally friendly nature to produce high-quality graphene.
15-18
Electrochemical
exfoliation of graphite has been performed mainly in two different types of electrolytes, i.e.,
ionic-liquids
17,19
and aqueous acids (e.g., H
2
SO
4
or H
3
PO
4
).
15,18,20
Exfoliation in ionic liquids
results in only a low yield of graphene with a small lateral size (<5 µm), and is often
functionalised with the ionic liquids, which disrupt the electronic properties of graphene.
17,21
On
the other hand, exfoliation in acidic electrolytes can yield graphene with a better quality and a
larger lateral size, but a significant amount of oxygen-containing functional groups cannot be
avoided due to the over-oxidation of graphite by the acid.
15,18,20
Therefore, a proper electrolyte
system that can balance the high-quality and large-quantity synthesis of exfoliated graphene (EG)
is highly in demand.
In this work, we demonstrate a highly efficient electrochemical exfoliation of graphite in
aqueous inorganic salts, such as ammonium sulphate ((NH
4
)
2
SO
4
), sodium sulphate (Na
2
SO
4
),
and potassium sulphate (K
2
SO
4
). Under neutral pH conditions for electrochemical exfoliation,
graphene sheets with the highest C/O ratio of 17.2 (i.e., oxygen content of 5.5 atomic% [at%])
and lowest defect density were obtained. The EG sheets were readily produced on a scale of tens
of grams, with ~80% of the flakes larger than 5 µm, and ~ 85% of flakes having 1 to 3 layers.
Moreover, single-layer graphene sheets had a high hole mobility of 310 cm
2
V
-1
s
-1
with a sheet
resistance of 1.96 kΩ sq.
-1
, which is superior to the chemically reduced graphene oxide (rGO).
The high solution-processability of EG further allowed for the preparation of concentrated
graphene ink in N,N’-dimethylformamide (DMF) without any additional surfactants. A simple
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paintbrush application of EG-ink to A4-sized paper yielded highly conductive (~ 11 Ω sq.
-1
with
0.74 mg cm
-2
EG loading), mechanically stable, and large-area graphene films. All-solid-state
flexible supercapacitors fabricated based on such graphene paper exhibited a high-area capacity
of 11.3 mF cm
-2
(at a scan rate of 1 mV s
-1
) and a high rate capability (up to 5000 mV s
-1
).
RESULTS AND DISCUSSION
Graphene preparation by electrochemical exfoliation. Electrochemical exfoliation of graphite
was performed in a two-electrode system using platinum as the counter electrode and a graphite
flake as the working electrode. Different types of aqueous inorganic salt electrolyte solutions
were examined and among them sulphate-containing salts such as (NH
4
)
2
SO
4
exhibited the best
exfoliation efficiency. Electrolyte solutions were prepared by dissolving (NH
4
)
2
SO
4
in water
(concentration of 0.1 M and pH ~6.5-7.0). When a direct current (DC) voltage of +10 V was
applied to a graphite electrode, the graphite flakes began to dissociate and disperse into the
electrolyte solution (Fig. 1a). The voltage was kept constant for 3 to 5 min to complete the
exfoliation process. Afterwards, the exfoliated product was collected by vacuum filtration and
repeatedly washed with water to remove any residual salts. The yield of the exfoliated EG flakes
was more than 75% relative to the total weight of the starting graphite electrode. The collected
powder was then dispersed in DMF by sonication for 10 min. Thus, a dispersion of ~ 2.5 mg/mL
was obtained, which was stable for 3 weeks without apparent agglomeration (Fig. 1b).
Remarkably, the exfoliation process could be readily scaled up depending on the type and size of
the graphite electrode used (Fig. S1). For example, in a series of electrochemical experiments,
~16.3 g of graphene sheets was obtained (Fig. 1c) within 30 min using three graphite foils (each
with a dimension of 11.5 cm × 2.5 cm) simultaneously (Fig. S1a).
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