CARBON
5 6 ( 2 0 1 3 ) 1 4 6 –1 5 4
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
High rate performance activated carbons prepared from
ginkgo shells for electrochemical supercapacitors
Liang Jiang a,b,c, Jingwang Yan
Baolian Yi a,b
a
b
c
a,b,*
,
Lixing Hao
a,b
, Rong Xue
a,b
, Gongquan Sun
a,b
,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Partially graphitized ginkgo-based activated carbon (GGAC) is fabricated from ginkgo shells
Received 1 August 2012
by pyrolysis, KOH activation and heat treatment using cobalt nitrate as graphitization cat-
Accepted 29 December 2012
alyst. The graphitization temperature is 900 °C. The GGAC has a microporous structure and
Available online 7 January 2013
its specific surface area is 1775 m2 g1. XRD patterns show that the carbon becomes more
graphitic after heat treatment. The specific capacitance of the GGAC reaches to 178 F g1 at
a potential scan rate of 500 mV s1, which is superior to that of commercial activated carbons and ordered mesoporous carbons. The high electrochemical performance of the GGAC
is attributed to its good electronic conductivity and high surface area. Partially graphitized
activated carbon is a promising electrode material for electrochemical supercapacitors
with high rate performance.
Ó 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Electric double-layer capacitors (EDLCs), which are also
known as electrochemical supercapacitors, are recognized
as a new alternative to the present power sources due to their
high power density, long cycle life and fast charge capability
[1–4]. Research and development of EDLCs has become a global hotspot in recent years. For example, supercapacitors
have been considered to be a promising power source for
the electric vehicles. EDLCs have been successfully applied
in the emergency doors (16 per plane) on Airbus A380 and
have been running for several years in Shanghai [4]. Among
the researches in EDLCs, it is an important field to develop
electrode materials with large specific capacitance and high
electrochemical stability [4]. Activated carbons (ACs) satisfy
the basic requirements for the electrode materials of EDLCs
due to its low cost, high surface area and large porosity [5–9].
Improving the graphitization degree of activated carbons is
beneficial to increasing their electrical conductivity, which is
vital to the rate capability of supercapacitors [10,11]. Conventional activation methods can lead to high surface areas but
without graphitization [12]. High temperature treatment is
an effective way to form well-developed graphitic structure,
but high energy consumption is unavoidable during the high
temperature treatment [11,13]. Besides, high temperature will
decrease the surface area and pore volume of activated carbons [14].
Catalytic graphitization by means of transition metals is
another effective way to obtain graphitic structure at relatively low temperatures [15–18]. Several researchers have prepared graphitized activated carbons for the electrode
materials of supercapacitors. Wang et al. reported a co-gelation route to synthesize porous graphitized carbons by using
teraethylorthosilicate (TEOS) as a template, metal nitrates as
* Corresponding author at: Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023,
China. Fax: +86 411 84379685.
E-mail address: [email protected] (J. Yan).
0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carbon.2012.12.085
700
600
V/cm g
3 -1
500
400
300
200
100
GAC
GGAC
0
0.0
0.2
0.4
0.6
0.8
1.0
-1
Relative press/p p0
Fig. 2 – Nitrogen (77 K) adsorption isotherms of the GAC and
GGAC.
(a) 0.30
GAC
GGAC
-1
0.25
3
catalyst and furfuryl alcohol (FA) as precursor [19]. Gao et al.
synthesized graphitized carbons with an ordered mesoporous
structure using ferric oxide as catalyst, which can even
achieve a specific capacitance of 155 F g1 at a scan rate of
200 mV s1 [20]. Sevilla et al. obtained graphitized porous carbons by catalytic graphitization of porous carbons with silica
xerogel as a template and phenolic resin as carbon precursor.
The graphitized carbons exhibited superior electronic conductivities up to two orders larger than non-graphitized carbons [15]. However, such synthetic methods reported are
time-consuming and not cost effective due to the utilization
of expensive raw materials.
The biomass resources for energy storage applications are
abundant. Preparation of graphitized activated carbons by
using cellulose as raw material is rarely reported. Ginkgo is
one of the oldest living fossils on the earth and ginkgo
shell-based activated carbons have not been studied as the
electrode materials for EDLCs to our knowledge. In this paper,
the ginkgo shell was used as the starting material to fabricate
partially graphitized ginkgo shell-based activated carbons
(GGAC). The GGACs are prepared by pyrolysis and KOH activation, and metal salt catalysts are used to create graphitic domains in activated carbons. Wide-angle XRD analysis
demonstrates the degree of graphitization is enhanced markedly. The GGACs are applied in supercapacitors and excellent
rate capability is obtained compared to the non-graphitized
carbons.
2.
147
5 6 (2 0 1 3) 1 4 6–15 4
Pore Volume/cm g
CARBON
Experimental
0.20
0.15
0.10
0.05
0.00
partially
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Pore Size/nm
(b)
GAC
GGAC
-1
0.006
3
The activated carbons were prepared from ginkgo shells. After
dried at 60 °C for 24 h, the ginkgo shells were pyrolyzed at
600 °C with a temperature ramp rate of 10 °C min1 and kept
for 1 h under nitrogen atmosphere in a tubular furnace. Then
the char and KOH (mass ratio 1:2) were thoroughly mixed and
heat treated at 700 °C for 1 h in N2. After activation, the carbonized sample was neutralized with HCl (2 mol L1) and
rinsed in deionized water for several times until the pH
reached to 7. Finally, the sample was dried in an oven at
60 °C for 24 h. The activated carbon was denoted as GAC
Pore Volume/cm g
2.1.
Preparation and characterization of
graphitized ginkgo shell-based activated carbons
0.004
0.002
0.000
0
110
100
90
-2
-1
70
-4
60
-6
50
40
DTG / %⋅min
TG/%
80
-8
30
20
0
100
200
300
400
500
600
700
800
5
10
15
20
25
30
Pore Size/nm
0
-10
900
Temperature/°C
Fig. 1 – TG and DTG curves of the ginkgo shells.
Fig. 3 – Pore size distribution of the GAC and GGAC.
Micropore size distribution by HK method (a); mesopore size
distribution by BJH method (b).
(ginkgo shell-based activated carbon). The GAC was impregnated with a dilute solution of cobalt nitrate (1 wt.%) for
12 h and then heat treated at 900 °C for 2 h with a ramp rate
of 10 °C min1 under nitrogen gas flow. Finally, the sample
was treated with HCl (2 mol L1) to remove the residual metal
impurities and washed with deionized water until the pH
reached 7. The final product is referred as GGAC.
148
CARBON
5 6 ( 2 0 1 3 ) 1 4 6 –1 5 4
Table 1 – Structure parameter of the GAC and GGAC. Smic and
Sext are micropore and external surface area, respectively.
Material
BET surface
area/m2 g1
Smic/m2 g1
2067
1775
2009
1506
GAC
GGAC
Sext/m2 g1
58
269
carbon pellet. DC currents of 100, 500 and 1000 mA were
applied to the pellet and the terminal voltages were recorded.
The resistance of the carbon pellet then was calculated by
dividing the recorded voltages by the current density applied.
The resistance was measured by applying three different currents in order to increase the experiment accuracy. The electrical conductivity of the carbon powder was calculated using
the following formula,
d ¼ L=RS
Intensity/a.u.
(a)
GGAC
GAC
20
40
60
80
2 Theta/°
(b)
•
where d is the electrical conductivity, R is the resistance of the
carbon pellet, L is the thickness of the carbon pellet and S is
the cross sectional area of the carbon pellet.
The tap density of the sample was determined by the following procedure: carbon powder was filled into a measuring
cylinder of 10 ml and tapped for 200 times. Then the tap density was calculated by dividing the mass of the sample by its
volume [21].
The surface information was characterized by XPS with an
Escalab 250 Xi X-ray photoelectron spectrometer (Thermo
Scientific).
The content of cobalt in the carbon was determined by
inductively coupled plasma spectrometer (ICP-AES) on an IRIS
intrepid II XSP instrument (Thermo Electron Corporation).
Co
Intensity/a.u.
2.2.
• (111)
• (200)
• (220)
85 wt.% Carbons, 10 wt.% acetylene black and 5 wt.% polytetrafluoroethylene (PTFE) were mixed and ground with a mortar
and pestle. Then the sample was pressed to form an electrode
membrane of 0.15 mm in thickness. Nickel foam was used as
current collectors.
2.3.
20
40
60
Fabrication of the electrode
Electrochemical characterization
80
2 Theta/°
Fig. 4 – XRD patterns of the GAC and GGAC (a); XRD pattern of
unwashed GGAC (b).
The specific surface area and the porous texture of activated carbons were characterized by nitrogen adsorption at
77 K with a Quantachrome Autosorb-1 system (Quantachrome). The specific surface area was calculated by the conventional BET (Brunauer–Emmet–Teller) method. The pore
size distributions of micropores and mesopores were analyzed by HK (Horvath–Kawazoe) method and BJH (Barrett–Joyner–Halenda) method, respectively. Wide-angle XRD patterns
were collected on a PANalytical X Pert PRO diffractometer
using Cu Ka radiation at 40 mA and 40 kV.
Morphologies and surface species of the samples were
characterized by a transmission electron microscope (TEM)
equipped with an energy dispersive X-ray spectroscope
(EDX) (FEI Tecnai G2) at 120 kV.
The electrical conductivity of the carbon powders was
measured by a direct voltampere method with an electrochemical workstation (PARSTAT 2273, Princeton Applied Research, USA). The sample was put in a hollow plexiglass
tube, and a pressure up to 2 MPa was applied to form a dense
For the electrochemical characterizations, a three-electrode
system was employed in which platinum plate and Ag/AgCl
were used as counter and reference electrode, respectively.
KOH aqueous solution (6 mol L1) was used as electrolyte.
Cyclic voltammetry (CV) tests were performed on an electrochemical workstation (PARSTAT 2273, Princeton Applied Research, USA) in the potential range of 1.0 to 0 V vs. Ag/
AgCl. The sweep rates varied from 2 to 1000 mV s1. The tests
of cycling performance of the electrodes were carried out by
using a three-electrode system on a CHI 760D electrochemical
testing system (CH Instruments, China). The electrochemical
impedance spectroscopy (EIS) tests were carried out in a frequency range of 0.01 Hz–100 kHz on an electrochemical workstation (PARSTAT 2273, Princeton Applied Research, USA).
3.
Results and discussion
Fig. 1 shows the thermogravimetric analysis result of the
ginkgo shells. It reveals that the major thermal decomposition takes place at 200–400 °C. The slight weight loss occurs
at room temperature to 200 °C is due to the dehydration of
the ginkgo shell. The obvious weight loss around 200–400 °C
is attributed to the degradation of cellulose and hemicelluloses. Degradation of lignin takes place slowly in a wide
CARBON
5 6 (2 0 1 3) 1 4 6–15 4
149
(b)
(a)
(c)
Fig. 5 – (a) TEM image of GAC; (b) and (c) TEM image of GGAC.
temperature range and lasts to higher temperatures [22–24].
The global yields of GAC and GGAC are about 24 and
17 wt.% respectively in our experiment.
Fig. 2 shows the N2 adsorption–desorption isotherms at
77 K for GAC and GGAC. Both nitrogen isotherms for GAC
and GGAC (Fig. 2) exhibit typical characteristics of type I isotherms according to the IUPAC classification, which implies
the existence of well-developed micropores. Compared to
the nitrogen isotherm of GAC, there is a hysteresis loop at relatively high pressures on the isotherm of GGAC, which may
be associated with the capillary condensation of N2 in mesopores. The corresponding pore size distribution (Fig. 3) obtained by HK method in microporous region (<2 nm) and
BJH method in mesoporous region (2–50 nm) further illustrates the microporous characteristics of the samples. The
diameters of the micropores are distributed from 0.4 to
2 nm in all the samples. The BET surface area and the relevant data about the samples are shown in Table 1. The GGAC
has a lower surface area compared with GAC, but the external
area to total surface area ratio of GGAC reaches to 15.2% (only
2.8% for GAC). It suggests that the reaction of cobalt oxide
with activated carbons gives birth to more mesopores, which
facilitates the mass diffusion in the electrodes during the
practical applications of supercapacitors [25].
XRD patterns of the samples are shown in Fig. 4. No diffraction peaks are found in the pattern of GAC. However, intense XRD peaks are observed at around 2h = 26°, 42°, 44°
and 78° after the graphitization treatment with catalyst,
which correspond to the diffraction of (0 0 2), (1 0 0), (1 0 1)
and (1 1 0) plane of the graphitic lattice, respectively. It indicates that the graphitic structures are formed during the heat
treatment in the presence of cobalt. In order to further investigate the graphitization process, the unwashed GGAC was
characterized by XRD and the result is shown in Fig. 4b. Diffraction peaks at around 44°, 51°, 76° corresponds to the diffraction of (1 1 1), (2 0 0) and (2 2 0) plane of the metallic
cobalt. This result indicates that the graphitic structures were
formed in the presence of metallic cobalt. In general, cobalt
oxide is formed firstly due to the decomposition of cobalt nitrate when the impregnated carbonaceous materials are heattreated under inert atmosphere. Then, cobalt oxide is further
reduced by carbon to metallic cobalt. The nanoparticles of
metallic cobalt or cobalt oxide inside the carbon matrix act
as catalysts for the transformation of amorphous carbon to
partially graphitized carbon [16]. The graphitic structures
around the metallic cobalt were formed through dissolution
of amorphous carbon into catalyst particles followed by the
precipitation of graphitic carbon.
150
CARBON
5 6 ( 2 0 1 3 ) 1 4 6 –1 5 4
40
(a)
30
Current density/A g
Intensity/a.u.
-1
(a)
C-O
C=O
O=C-O
H2O
20
10
0
-10
-20
5mV/s
10mV/s
20mV/s
50mV/s
100mV/s
-30
-40
-50
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Voltage/V (vs. Ag/AgCl)
544
540
536
532
528
Binding Energy/eV
30
(b)
20
Intensity/a.u.
C-OH
Current density/A g
-1
(b)
C=O
O=C-O
H2O
10
0
-10
5mV/s
10mV/s
20mV/s
50mV/s
100mV/s
-20
-30
-1.0
540
536
532
528
Binding Energy/eV
Fig. 6 – XPS spectra of the O1s region for the samples (a) GAC
and (b) GGAC.
Morphologies of the samples were investigated by TEM
and the results are shown in Fig. 5. As shown in Fig. 5a, only
amorphous carbon structure can be observed in GAC. In contrast, obviously graphitic structure can be observed after the
graphitization treatment with Co as catalyst as shown in
Fig. 5b. A large number of graphite ribbons in GGAC can be observed, which indicates that the graphitization level is increased compared with GAC. The thickness of graphite
ribbons ranges from 5 to 10 nm.
The ash content of GGAC was measured by heating the
sample at 800 °C in air for 1 h and it was found that the ash
content of GGAC is 0.8 wt.%. The ICP was employed to investigate the cobalt content in GGAC. It was found that the cobalt
content is 0.07 wt.%. This result suggests that a small amount
of cobalt wrapped by carbon could not be removed by HCl.
The residual cobalt particles can be observed by TEM as
shown in Fig. 5c.
Fig. 6 shows the XPS analysis results for GAC and GGAC.
The O1s spectra of GAC and GGAC were fitted to four peaks
according to the literatures [26,27], which include oxygen in
carbonylic groups (531.2–531.6 eV), oxygen in phenolic and
lactone groups (532.7–533.0 eV), noncarbonyl oxygen in carboxylic groups (533.8–534.4 eV) and oxygen in H2O (535.5–
536.3 eV). The content of the oxygen in carbonylic groups of
GGAC is 35.2% which is higher than 30.9% of GAC. The existence of the extra carbonylic groups can further improve the
wettability of the carbon surface and reduce the charge trans-
-0.8
-0.6
-0.4
-0.2
0.0
Voltage/V (vs. Ag/AgCl)
Fig. 7 – (a) Cyclic voltammograms of the electrodes of GAC at
different scan rate in 6 mol L1 KOH aqueous solution; (b)
cyclic voltammograms of the electrodes of GGAC at different
scan rate in 6 mol L1 KOH aqueous solution.
fer resistance, which are beneficial to the improvement of the
electrochemical performance of GGAC [28].
The electrical conductivities for the carbon powders are
measured under a pressure of 2 MPa. The electrical conductivities of GAC and GGAC are 1.4 and 3.7 S cm1, respectively.
These results indicate that the graphitization treatment significantly improve the electrical conductivities of the carbon
materials. Due to the formation of the graphitic structures,
the tap density of the carbon materials was improved. The
tap density of GGAC is 0.49 g cm3, which is higher than that
of GAC (0.47 g cm3). Higher tap density of an electrode material is beneficial to increase its loading on the electrodes, thus
increasing the power density and energy density of a
supercapacitor.
Fig. 7a and b illustrate the cyclic voltammograms of GAC
and GGAC electrodes at different scan rates from 5 to
100 mV s1. The current densities in CVs are normalized with
respect to the mass of the active electrode material. It can be
seen that the both electrodes exhibit nearly ideal EDLC behavior with rectangular CV curves at lower potential scan rate.
However, at high scan rate the shape of the CV curves for
the GAC electrode deviates from an ideal rectangular shape
markedly while the CV curves of the GGAC electrode still keep
a rectangular shape. It reveals that the GGAC have better ionic
accessibility than GAC, which suggests that GGAC is more
suitable for quick charge/discharge operations [29]. In order
CARBON
6
40
(a)
30
-1
Current density/A g
Current density/A g
-1
4
2
0
-2
-4
GAC
GGAC
-6
151
5 6 (2 0 1 3) 1 4 6–15 4
(b)
20
10
0
-10
-20
-30
GAC
GGAC
-40
-50
-1.0
-0.8
-0.6
-0.4
-0.2
-1.0
0.0
(c)
-0.4
-0.2
0.0
40
20
0
-20
-40
-60
GAC
GGAC
-80
-1.0
(d)
100
Current density/A g-1
-1
Current density/A g
-0.6
150
80
60
-0.8
Voltage/V (vs. Ag/AgCl)
Voltage/V (vs. Ag/AgCl)
-0.8
-0.6
-0.4
-0.2
50
0
-50
-100
GAC
GGAC
-150
-1.0
0.0
-0.8
-0.6
-0.4
-0.2
0.0
Voltage/V (vs. Ag/AgCl)
Voltage/V (vs. Ag/AgCl)
Fig. 8 – Cyclic voltammograms for GAC and GGAC electrodes at a scan rate of 10 mV s1 (a); 100 mV s1 (b); 200 mV s1 (c) and
500 mV s1 (d).
Table 2 – The relationship of specific capacitances with potential scan rates for GAC and GGAC electrode in 6 mol L1 KOH.
Cg at different voltage scan rates (F g1)
Electrode
GAC
GGAC
2
365
237
5
328
224
10
309
215
20
292
208
50
263
197
200
190
191
500
87
178
1000
33
108
100
400
GAC
GGAC
Kurary
350
300
250
200
150
GAC
GGAC
Kurary
80
Retention(%)
Specific Capacitance/F g -1
100
232
197
60
40
100
20
50
(b)
(a)
0
0
0
200
400
600
Scan rate/mV s-1
800
1000
0
200
400
600
Scan rate/mV s
800
1000
-1
Fig. 9 – Specific capacitance as a function of the potential scan rates of GAC, GGAC and Kurary activated carbon (a); capacitance
retention ratio as a function of the potential scan rates of GAC, GGAC and Kurary activated carbon (b).
152
CARBON
(a)
1.0
350
20
Zim/Ω
0.6
0.4
15
Zim/Ω
(b)
GAC
GGAC
0.8
Specific Capanitance/F g-1
25
5 6 ( 2 0 1 3 ) 1 4 6 –1 5 4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Zre/ Ω
10
GAC
GGAC
5
300
250
200
150
100
50
0
0
0
5
10
15
20
25
0.01
0.1
1
Zre/ Ω
Imaginary Capacitance/F g-1
160
10
100
1000
10000 100000
Frequency/Hz
(c)
0.73Hz
GAC
GGAC
140
120
1.93Hz
100
80
60
40
20
0
0.01
0.1
1
10
100
1000
10000 100000
Frequency/Hz
Fig. 10 – Nyquist plot for GAC and GGAC electrodes (a); the inset shows the magnification of the high-frequency region of the
impedance spectra. The correlation of specific capacitances with frequency for the GAC and GGAC electrodes (b); the
correlation of imaginary parts of the specific capacitances with frequency for the GAC and GGAC electrodes (c).
series resistance (ESR), which is beneficial for the rapid charging/discharging operation. The excellent performance of
GGAC gives the credit to the graphitization process by improving the electronic conductivity and the relatively high ratio of
mesopores favors the charge transfer processes during the rapid charge/discharge operation [29].
The specific capacitance is calculated according to the following equation,
Specific Capacitance/F g-1
200
180
160
140
120
100
80
60
C¼
40
Q
2DVm
ð1Þ
20
0
0
1000
2000
3000
4000
5000
Cycles
Fig. 11 – The capacitance vs. the cycle number at a scan rate
of 100 mV s1.
to prove this point of view, CV measurements at higher scan
rate from 10 to 500 mV s1 were carried out and the results for
GAC and GGAC electrode are compared and shown in Fig. 8.
The CV curves of GAC become distorted with increasing the
scan rate. At the scan rate of 500 mV s1, the CV curve of
GAC electrode becomes spindle-shaped and that of GGAC still
possesses the characteristics of capacitive electrodes, i.e.
rectangular-shaped. It is concluded that GGAC electrode
exhibits an excellent ionic response and a small equivalent
where Q is the charge integrated from the whole voltage
range, DV is the whole voltage difference, and m is the mass
of active material in a single electrode [30]. The specific capacitances at different scan rates are listed in Table 2.
The dependence of specific capacitances and capacitance
retention ratios on the scan rates are plotted in Fig. 9. The performance of a commercial activated carbon (Kurary) is also
shown in Fig. 9 as a reference. The specific capacitance of
GAC is 365 F g1 at 2 mV s1, which is quite higher than that
of the commercial activation carbons. But GAC exhibits a poor
rate performance as most of the activated carbons reported in
the literature [31]. The specific capacitance of GAC is also
higher than that of GGAC at low scan rate. However, at high
scan rate, the capacitance retention ratio of GGAC is higher
than that of GAC. GGAC maintains a specific capacitance of
178 F g1 at 500 mV s1, whereas GAC decreases to 87 F g1.
CARBON
5 6 (2 0 1 3) 1 4 6–15 4
Furthermore, the specific capacitance of GGAC remains at
108 F g1 at 1000 mV s1 and that of GAC decreases to
33 F g1. The capacitance retention of Kurary carbon is only
12% at 1000 mV s1, which is much lower than that of GGAC
(46%). It demonstrates that GGAC is more suitable for high
current density application as well. The rate performance of
GGAC is close to the HPGC that with excellent rate performance [29].
The EIS analysis is applied to analyze the electrochemical
properties of the supercapacitors with GAC and GGAC as electrode materials. A nearly vertical line in the low-frequency region can be observed on the Nyquist plot of the GGAC
electrode (Fig. 10a). The existence of a nearly vertical line on
a Nyquist plot reflects a good capacitive performance of an
electrode material [32]. The plot does not have a semicircle
at high frequencies, implying the fast ion diffusion in the
GGAC electrode. The gravimetric capacitance C and imaginary
part of gravimetric capacitance Cim at different frequencies
are calculated according to Eqs. (2) and (3) [33,34]. The detailed
results are shown in Fig. 10.
1
C¼
2pfZim m
Cim ¼
4.
153
Conclusions
High rate performance partially graphitized ginkgo shellsbased activated carbons can be prepared by pyrolysis, KOH
activation and heat treatment using cobalt nitrate as the catalyst. The porous partially graphitized activated carbons exhibit superior capacitive behavior over a wide range of scan
rates and surpass the conventional activated carbons especially at high scan rate. The CV curves of GGAC still maintain
rectangular shape at 500 mV s1 with the specific capacitance
of 178 F g1. It also shows excellent cycle stability with 92.6%
capacity retention for 5000 cycles. The high performance of
GGAC ascribes to the improved electronic conductivity and
high specific area. Catalytic graphitization is an effective
method to prepare partially graphitized activated carbon, a
promising electrode material for electrochemical capacitors
with high rate performance.
Acknowledgements
ð2Þ
Zre
2pfmðjZre j2 þ jZim j2 Þ
ð3Þ
where Zre and Zim are the real part and the imaginary part of
the impedance, C is the specific capacitance, Cim is the
imaginary part of the capacitance, f is the operating
frequency, and m is the mass of the active material in the
electrodes.
It is found that the specific capacitance decreases gradually with the increasing of frequency. The specific capacitance
of GGAC approaches the saturation state below 1 Hz and
achieves 230 F g1 at 0.01 Hz. The imaginary part of capacitance reaches the maximum at f0, which defines the time
constant by s = 1/f0. The time constant is described as a characteristic relaxation time of the whole system (the minimum
time to discharge all of the energy from a device with an efficiency of more than 50%) [33]. The relaxation time of GAC and
GGAC can be analyzed from Fig. 10c. It is obvious that GGAC
electrode exhibits a faster frequency response with
f0 = 1.93 Hz (0.52 s), which is much quicker than that of GAC
electrode with 0.73 Hz (1.37 s). The rapid frequency response
of GGAC ascribes to its superior conductivity and appropriate
pore size distribution.
Cycle life of a supercapacitor is of great importance to its
practical applications. As shown in Fig. 11, the electrochemical supercapacitor with GGAC as electrode materials maintains a stable performance over 5000 cycles at a scan rate of
100 mV s1. The specific capacitance drops from 189 to
178 F g1 after the initial 3000 cycles and keeps above
175 F g1 during the latter 2000 cycles. Thus, GGAC electrode
achieves a capacity retention of 92.6% during the whole
5000 cycles. It indicates that GGAC is an outstanding electrode
material for supercapacitors with high power density. The
excellent performance is closely related to the superior conductivity of the GGAC due to the graphitization treatment
and the reasonable pore distribution as well.
This work was supported by the National Science Foundation
of China (51177156/E0712) and the Project to Support Xingjiang with Science and Technology from Chinese Academy of
Sciences (XBXJ-2011-041).
R E F E R E N C E S
[1] Srinivasan V, Weidner JW. Mathematical modeling of
electrochemical capacitors. J Electrochem Soc
1999;146(5):1650–8.
[2] Kotz R, Carlen M. Principles and applications of
electrochemical capacitors. Electrochim Acta 2000;45(15–
16):2483–98.
[3] Miller JR, Simon P. Materials science – electrochemical
capacitors for energy management. Science
2008;321(5889):651–2.
[4] Simon P, Gogotsi Y. Materials for electrochemical capacitors.
Nat Mater 2008;7(11):845–54.
[5] Frackowiak E, Beguin F. Carbon materials for the
electrochemical storage of energy in capacitors. Carbon
2001;39(6):937–50.
[6] Qu DY. Studies of the activated carbons used in double-layer
supercapacitors. J Power Sources 2002;109(2):403–11.
[7] Dash R, Chmiola J, Yushin G, Gogotsi Y, Laudisio G, Singer J,
et al. Titanium carbide derived nanoporous carbon for
energy-related applications. Carbon 2006;44(12):2489–97.
[8] Rufford TE, Hulicova-Jurcakova D, Zhu ZH, Lu GQ.
Nanoporous carbon electrode from waste coffee beans for
high performance supercapacitors. Electrochem Commun
2008;10(10):1594–7.
[9] Elmouwahidi A, Zapata-Benabithe Z, Carrasco-Marin F,
Moreno-Castilla C. Activated carbons from KOH-activation of
argan (Argania spinosa) seed shells as supercapacitor
electrodes. Bioresour Technol 2012;111:185–90.
[10] Lu AH, Li WC, Salabas EL, Spliethoff B, Schuth F. Low
temperature catalytic pyrolysis for the synthesis of high
surface area, nanostructured graphitic carbon. Chem Mater
2006;18(8):2086–94.
154
CARBON
5 6 ( 2 0 1 3 ) 1 4 6 –1 5 4
[11] Xia BY, Wang JN, Wang XX, Niu JJ, Sheng ZM, Hu MR, et al.
Synthesis and application of graphitic carbon with high
surface area. Adv Funct Mater 2008;18(12):1790–8.
[12] Xu B, Chen YF, Wei G, Cao GP, Zhang H, Yang YS. Activated
carbon with high capacitance prepared by NaOH activation
for supercapacitors. Mater Chem Phys 2010;124(1):504–9.
[13] Fuertes AB, Alvarez S. Graphitic mesoporous carbons
synthesised through mesostructured silica templates.
Carbon 2004;42(15):3049–55.
[14] Hanzawa Y, Hatori H, Yoshizawa N, Yamada Y. Structural
changes in carbon aerogels with high temperature
treatment. Carbon 2002;40(4):575–81.
[15] Sevilla M, Fuertes AB. Catalytic graphitization of templated
mesoporous carbons. Carbon 2006;44(3):468–74.
[16] Derbyshire FJ, Presland AEB, Trimm DL. Graphite formation
by dissolution-precipitation of carbon in cobalt, nickel and
iron. Carbon 1975;13(2):111–3.
[17] Oya A, Marsh H. Phenomena of catalytic graphitization. J
Mater Sci 1982;17(2):309–22.
[18] Qi J, Jiang LH, Wang SL, Sun GQ. Synthesis of graphitic
mesoporous carbons with high surface areas and their
applications in direct methanol fuel cells. Appl Catal BEnviron 2011;107(1–2):95–103.
[19] Wang ZL, Zhang XB, Liu XJ, Lv MF, Yang KY, Meng JA. Cogelation synthesis of porous graphitic carbons with high
surface area and their applications. Carbon 2011;49(1):161–9.
[20] Gao WJ, Wan Y, Dou YQ, Zhao DY. Synthesis of partially
graphitic ordered mesoporous carbons with high surface
areas. Adv Energy Mater 2011;1(1):115–23.
[21] Li H, Ding YH, Yuan JL, Wang ZY. Study of the electrochemical
performance of nickel hydroxide. J Power Sources 1995;57(1–
2):137–40.
[22] Sricharoenchaikul V, Pechyen C, Aht-ong D, Atong D.
Preparation and characterization of activated carbon from
the pyrolysis of physic nut (Jatropha curcas L.) waste. Energy
Fuels 2008;22(1):31–7.
[23] Yang HP, Yan R, Chen HP, Lee DH, Zheng CG. Characteristics
of hemicellulose, cellulose and lignin pyrolysis. Fuel
2007;86(12–13):1781–8.
[24] Sanchez-Silva L, Lopez-Gonzalez D, Villasenor J, Sanchez P,
Valverde JL. Thermogravimetric–mass spectrometric analysis
of lignocellulosic and marine biomass pyrolysis. Bioresour
Technol 2012;109:163–72.
[25] Lee J, Yoon S, Oh SM, Shin CH, Hyeon T. Development of a
new mesoporous carbon using an HMS aluminosilicate
template. Adv Mater 2000;12(5):359–62.
[26] Zielke U, Huttinger KJ, Hoffman WP. Surface-oxidized carbon
fibers. 1. Surface structure and chemistry. Carbon
1996;34(8):983–98.
[27] Figueiredo JL, Pereira MFR, Freitas MMA, Orfao JJM.
Modification of the surface chemistry of activated carbons.
Carbon 1999;37(9):1379–89.
[28] Li LX, Li F. The effect of carbonyl, carboxyl and hydroxyl
groups on the capacitance of carbon nanotubes. New Carbon
Mater 2011;26(3):224–8.
[29] Wang DW, Li F, Liu M, Lu GQ, Cheng HM. 3D aperiodic
hierarchical porous graphitic carbon material for high-rate
electrochemical capacitive energy storage. Angew Chem Int
Ed 2008;47(2):373–6.
[30] Jiang HL, Liu B, Lan YQ, Kuratani K, Akita T, Shioyama H,
et al. From metal-organic framework to nanoporous carbon:
toward a very high surface area and hydrogen uptake. J Am
Chem Soc 2011;133(31):11854–7.
[31] Wang JC, Kaskel S. KOH activation of carbon-based materials
for energy storage. J Mater Chem 2012;22(45):23710–25.
[32] Yuan DS, Chen JX, Tan SX, Xia NN, Liu YL. Worm-like
mesoporous carbon synthesized from metal-organic
coordination polymers for supercapacitors. Electrochem
Commun 2009;11(6):1191–4.
[33] Huang CH, Zhang Q, Chou TC, Chen CM, Su DS, Doong RA.
Three-dimensional hierarchically ordered porous carbons
with partially graphitic nanostructures for electrochemical
capacitive energy storage. ChemSusChem 2012;5(3):563–71.
[34] Frichet A, Gimenez P, Keddam M. An impedance
investigation of thin oxide layers on pure aluminum and of
their water-content. Electrochim Acta 1993;38(14):1957–60.