22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Synthesis of carbon nanowalls by plasma-enhanced chemical vapor deposition
and their rapid intercalation behaviour as lithium ion capacitor anode material
A.D.C. Permana, A. Kameyama, M. Suzuki and S. Mori
Department of Chemical Engineering, Tokyo Institute of Technology, Tokyo, Japan
Abstract: The author reported the deposition of carbon nanowalls (CNWs) by plasmaenhanced CVD in CO/H 2 microwave gas discharge system without any catalyst. The
electrochemical properties and morphology are analyzed. Deposited CNWs sample
perform better electrochemical performance at high current density than graphitic material
sample. The discharge capacity was measured 80.27, 64.31, 45.47, and 32.36 mAh/g at 6,
10, 15, and 20 C-rate respectively. This electrochemical performance stimulates the usage
of CNWs as Li-ion capacitor (LIC) material for fast-charging device.
Keywords: microwave plasma CVD, CNWs, LIC
1. Introduction
Development of the capacitor technology has reached a
certain point to improve the energy density which is the
main drawback compared to Li-ion batteries (LIB)
technology. The hybrid system of supercapacitor and LIB,
or so-called Li-ion capacitor (LIC), is expected to fill the
gap between those two earlier mentioned devices in a
Ragone plot. In the development of LIC technology, it
employs graphite for anode material and activated carbon
as for cathode. Graphite was selected as an anode material
of LIB because it has more negative potential than
when
intercalated
by Li-ion and
Li 4 Ti 5 O 12
environmentally benign. However, the SEI formation
prevents cathode to fully discharged resulting irreversible
capacity loss. To solve this problem, anode then predoped by Li-ion to reduce the capacity loss.
Decreasing particle size into nano-scale may establish
the transport kinetics between Li-ion and electron and
enhance the electrode-electrolyte interphase area which
also improves electrochemical properties. But even so,
some drawbacks such as low electrode integrity and
volumetric energy hold the system to achieve the highest
performance. In the recent years, many groups have
developed some approaches to use unordinary material
for the electrode. The purpose of course is to improve the
performance of the LIC, particularly by means of the
energy density to be more like LIB [1-5].
Carbon nanowalls (CNWs) is well known as 2dimensional network of stacked grapheme layer on a
substrate with several nanometers of thickness. Their
unique properties, i.e. large surface area, chemical
stability, mechanical strength, and high conductivity [6],
draw many attentions for researches which are now
developed for applications of field emission electron [7],
sensor [8], fuel cell [9], capacitor [10], and LIB [11].
Through many studies, CNWs can be fabricated by
various methods such as microwave plasma [12], radio
frequency (RF) plasma [13], hot filament chemical vapor
deposition (CVD) [9], sputtering [14], and so on. Most of
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them, the gas plasma used is CH 4 and H 2 . Among those
methods, CVD assisted by microwave plasma is found
very attractive because the operating temperature can be
lowered and the possibility of no-catalyst process is very
high [15].
In our group, CNWs growth by microwave plasma
CVD without any catalyst and substrate pre-treatment has
been developed by using CO as carbon source and H 2 gas
plasma [16]. This study describes first use of CNWs as
anode material for LIC application.
2. Experimental
CNWs were prepared by a microwave plasma CVD
system utilizing CO/H 2 gas plasma discharge similar with
our previous studies [17].
Fig. 1. Experimental apparatus for microwave plasma
CVD.
Commercial LIC-grade Cu foil with 1 cm2 area is
placed inside of a 15 mm inner diameter quartz discharge
tube without any catalyst or pre-treatment flowed by CO
and H 2 gas plasma connected by a modified ASTeX
DPA25 plasma applicator. The operating condition of
CNWs deposition process were as follows: total gas flow
rate, 50 sccm: CO flow rate, 46 sccm; H 2 flow rate, 4
1
sccm; total pressure, 250 Pa; microwave plasma discharge
power, 80 W; deposition time, 7 min.
Electrochemical properties of all the samples were
measured by using a Swagelok type two-electrode test
cell system. Sample was placed at the bottom as the anode
followed by monolayer polypropylene separator of
Celgard 2400, and 16 mm φ Li foil purchased from Honjo
Metal Co. Ltd. as the cathode. Electrolyte used in this
study is 1M LiPF 6 in ethylene carbonate/dimethyl
carbonate (EC/DMC, 1:1 by volume) purchased from Ube
Industries, Ltd. VersaSTAT3 potentiostat/galvanostat
electrochemical system was used to observe cyclic
voltammetry (CV) and constant-current constant-voltage
(CCCV) charge-discharge.
The morphology of deposited CNWs film was
characterized using scanning electron microscope (SEM,
Hitachi S-4500) and transmission electron microscope
(TEM, JEOL JEM-2010F).
3. Results and Discussion
Deposited CNW on Cu foil was produced by
microwave plasma system written above at deposition
time 7 min is compared to graphitic carbon material
(labelled as CNW and C).
CCCV performs the charging of Li ion into CNW
sample at 37.2 mAh/g (0.1 C-rate) then having discharge
curves as the responds at 2.23, 3.72, 5.58, and 7.44 A/g
(correspond to 6, 10, 15, and 20 C-rate) in the potential
range of 0.1-2 V vs. Li+/Li as shown in Fig. 2a. The
comparison CNW sample to C sample measured was
shown in Fig. 2b. It can be deducted that C sample has
better performance at low C-rate as 92.81 mAh/g at 0.1C
than CNW as 85.10 mAh/g. When it comes to high Crate, however, the discharging performance of C sample
decreases as the C-rate increases to 6 C (44.25 mAh/g)
and 20 C (15.95 mAh/g). On the other side, CNW sample
can maintain their performance up to 80.27 and 32.36
mAh/g as it was measured at 6 C and 20 C respectively.
The effect of surface area increase by means of CNWs
shape can enhance the electrochemical capacitance [10,
18]. This CNWs sample could be suitable material for fast
charge-discharge device because of this superior high Crate behaviour.
In Fig. 3a and 3b, the SEM and TEM images of CNW
layer by deposition time of 7min after charge-discharge
process was presented. The height of the deposited wall
measured is about ~ 17 μm and the thickness is about ~
18 nm. As it can be seen from the images, the wall shapes
are maintained even after many times of charge-discharge
cycling process which can be compared to our previous
result [17].
Fig. 2. (a) Discharge profile of CNW sample at various
C-rate, and (b) Discharge capacity of all samples at
various C-rate.
Fig. 3. (a) SEM and (b) TEM images of CNW sample
after n cycle.
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4. Conclusion
CNWs deposited by microwave plasma CVD were
successfully measured their electrochemical performance
for capacitor application, in this study, LIC. Deposited
CNWs suggested to be very good materials for rapidcharging device referring to its discharge capacity at high
C-rate (20 C) by 32.36 mAh/g when compared to
graphitic material sample of 2.12 mAh/g.
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