Characterization of the porous carbon prepared by using halloysite as template and its application to EDLC

Article abstract of DOI:10.1016/j.jpcs.2006.01.044

A porous carbon was prepared using halloysite as a template and sucrose as carbon source by means of template method. Nitrogen adsorption analysis showed that the obtained porous carbon material possess a wide pore size distribution and a large pore volume, especially in the range of mesopores. The application of the resultant carbon for electrodes of electric double layers capacitor (EDLC) in organic LiPF₆/PE+CE electrolyte indicated that the specific capacitance, as high as 232 F/g at a current density of 0.05 mA/cm², was larger than for commercially activated carbons. Cyclic voltammetry (CV) also showed that the performance of templated carbon in 1 mol/l aqueous solution of H₂SO₄ was better at a large scan rate than for the commercial activated carbons.

Full text of DOI:10.1016/j.jpcs.2006.01.044

Journal of Physics and Chemistry of Solids 67 (2006) 1186–1189
www.elsevier.com/locate/jpcs
Characterization of the porous carbon prepared by using halloysite
as template and its application to EDLC
b
a
c
Guiyang Liu a, Feiyu Kang a, , Baohua Li , Zhenghong Huang , Xiuyun Chuan
*
a Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
b Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China
c School of Earth and Space Science, Peking University, Beijing 100871, China
Abstract
A porous carbon was prepared using halloysite as a template and sucrose as carbon source by means of template method. Nitrogen adsorption
analysis showed that the obtained porous carbon material possess a wide pore size distribution and a large pore volume, especially in the range of
mesopores. The application of the resultant carbon for electrodes of electric double layers capacitor (EDLC) in organic LiPF6/PECCE electrolyte
indicated that the specific capacitance, as high as 232 F/g at a current density of 0.05 mA/cm2, was larger than for commercially activated carbons.
Cyclic voltammetry (CV) also showed that the performance of templated carbon in 1 mol/l aqueous solution of H2SO4 was better at a large scan
rate than for the commercial activated carbons.
q 2006 Elsevier Ltd. All rights reserved.
Keywords: A. Microporous materials; B. Chemical synthesis; D. Electrochemical properties
1. Introduction
On the other hand, for energy storage the use of porous
carbons in EDLC has been widely documented. Different kinds
of porous carbons used as the electrodes of EDLC [4] such as
activated carbon fabric [5], carbon aerogel [6], carbon
nanotube [7], templated carbon [8], and so on, displayed
dissimilar performances. The present paper mainly describes
the pore structure and electrochemical performance of the
halloysite templated carbon in LiPF6/PECCE and H2SO4
electrolytes. In order to close more tightly to the practical
usage, the resultant carbon was compared with commercial
activated carbon for electrodes performance of EDLC.
The template method to prepare carbons with well-
controlled nanostructures has been regarded as a unique and
versatile technique. Most of the nanostructures gained by the
template method may be infeasible using other methods.
Pioneering study on this method was done by Kyotani et al.
[1] using clay minerals as templates to prepare layer-like
carbons. And then, various novel structure carbons were
obtained by this process such as ordered mesoporous carbon
by using MCM-48 as a template [2], micropore carbon with
high specific surface area by using Y zeolite as a template [3]
and so on. However, the high cost of the template materials
reported by far leads to a higher cost of the template carbon
and limits its usage greatly. To solve this problem we tried to
use a natural porous mineral halloysite as a template. Usually
the framework of halloysite is made of number of rods with
the length of dozens to hundreds nanometers and with the
diameter of several nanometers. In each rod, there is a straight
hole with a diameter of 0.7 or 1 nm. Low cost as well as stable
structure gives us a potential to use holloysite as a template to
prepare porous carbons.
2. Experimental
2.1. Sample preparation
The process of preparation was described as follows: (i) the
pretreatment of halloysite: baked in a beaker at 150 K for 3 h;
(ii) impregnation of sucrose and sulfuric acid into the halloysite
pores: stirred 4 h in the solution with the weight ratio of
sucrose:H2O:H2SO4Z5:10:1; (iii) polymerization and carbon-
ization of sucrose: dried in air at 100 K for 4 h and heated at
150 K for 2 h for polymerization and then at a heating rate of
5 K/min to 800 K under a nitrogen flow 4 h for carbonization;
and (iv) removal of the halloysite template: treated with
multiple cycles of concentrated HF/HCl, and distilled water.
*
Corresponding author. Tel.: C86 10 6277 3752; fax: C86 10 62783749.
E-mail address: fykang@tsinghua.edu.cn (F. Kang).
0022-3697/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2006.01.044

G. Liu et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1186–1189
1187
were used as the electrodes of coin-style EDLC in LiPF6/PEC
CE electrolyte. The specific capacitance was calculated by the
following Eq. (1) [12]
2000
1800
1600
1400
1200
1000
800
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
-0.01
SHC
AC1
AC2
BJH
IDt
Cm Z 2
(1)
DVm
100
Pore Diameter (Angstrom)
where Cm is the specific capacitance; I is the discharge current;
Dt is the space-time when the alteration of voltage was DV, and
m is the mass of carbon.
600
400
200
In an inorganic system, cyclic voltammetry (CV) method
with three electrodes would be used to evaluate the
performance in 1 M H2SO4 aqueous solution electrolyte.
From the CV the specific capacitance could be deduced from
the definition of capacitance
0
-200
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (p/p )
0
Fig. 1. N2 adsorption isotherms and BJH pore size distribution of the carbon
samples.
dQ
dU
dQ=dt
dU=dt
I
I
C Z
Z
Z 0Cm Z
(2)
n
nm
Table 1
Specific surface areas and pore volumes of porous carbons
where Cm is the specific capacitance; I is the instant current; n is
the scanning rate and m is the mass of carbon. For convenience,
in this work, the specific capacitance was calculated from half
of the difference between charge and discharge current at
650 mV on the CV curves observed.
Samples
SBET
(m2/g)a
Smic
(m2/g)b
Vtotal
(cm3/g)c
Vmeso
(cm3/g)d
Mesoporosity
(%)e
SHC
AC1
AC2
1142
1667
2322
179
1152
723
1.715
0.825
1.181
1.287
0.106
0.281
75.1
12.9
23.8
a
3. Results and discussion
BET-specific surface area.
Microsurface area.
b
c
d
e
Total pore volume.
3.1. Porosity and pore structure
Mesopore volume.
Determined by Vmeso/Vtotal
.
Fig. 1 shows the nitrogen adsorption isotherms of the
templated carbon (SHC) and two commercial activated carbons
(denoted as AC1, AC2, respectively).
2.2. Characterization
Nitrogen adsorption at 77 K gave the BET-specific surface
area and pore structure. BET-specific surface area was obtained
by BET method [8]; the mesopore distribution was gained from
BJH method [9]; pore volume was determined by using the
data of N2 adsorbed [10]; and micropores surface area was
obtained from t-plot method [11].
The type-I isotherms (in terms of IUPAC) of AC1 and AC2
indicated that almost all the pores inside them were
micropores; the type-II isotherm of SHC showed that SHC
possessed micropores and mesopores [10]. BJH mesopore size
distribution displayed that AC1 and AC2 had few mesopores,
and SHC had very rich mesopores mainly at the diameter of
4 nm. Table 1 showed that the BET-specific surface area of
SHC was much smaller than AC1’s and AC2’s, but its total
pore volume and mesopore volume, especially the mesopor-
osity were larger than others’. Above results indicated that
SHC was a mesopore carbon with large pore volume and high
mesoporosity.
2.3. Electrochemical study
In an organic electrolyte system, the 2 cm2 pressed plates of
the mixture of the templated carbon, PTFE (polytetrafluor-
oethylene) and carbon black with the weight ratio of 80:10:10
(a)
(b)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2
2
SHC
AC1
AC2
SHC
AC1
AC2
Discharge-0.5mA/cm
Discharge-0.05mA/cm
2.0
1.5
1.0
0.5
0.0
0
200 400 600 800 1000 1200 1400 1600 1800
Time(s)
0
5000
10000
Time(s)
15000
20000
Fig. 2. Constant-current discharge curves of the carbons in EDLC with LiPF6/PECCE electrolyte (a, at 0.05 mA/cm2; b, at 0.5 mA/cm2).

1188
G. Liu et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1186–1189
Table 2
It was much interesting that the specific surface area of
templated carbon is not larger, but its specific capacitance is
´
higher. In Lozano-Castello et al.’s report [13] at the similar
Specific capacitances of the carbon samples
Samples
Ash content
(%)
Specific capacitance (F/g)
conditions, if the specific capacitance of activated carbons
closed to SHC’s its BET-specific surface area might be nearly
3000 m2/g. Comparing the differences of the pore information
between SHC and AC1, AC2, we reasoned that the larger pore
size of SHC could easily form the electric double layer but
AC1’s and AC2’s could not form it because of their small pore
size and the large ion diameter of the electrolyte.
Organic system
(mA/cm2)
Inorganic system
(mV/s)
0.05
0.5
5
100
SHC
AC1
AC2
10.7
0.2
232
173
195
180
154
177
261
308
210
154
3.0
The charge/discharge curves (Fig. 3) showed that the
charge/discharge efficiency of carbons was in the sequence:
SHC!AC1!AC2; it might be the effect of impurities (Table 2
showed the ash contents of the samples). The impurities might
react with the electrolyte and caused dilution of the electrolyte.
The nonlinear character indicated that SHC possessed pseudo
capacitance and a large drop of the voltage, which was due to
the IR contributions from charging to discharging which
showed that SHC had larger resistance.
3.0
2.5
2.0
1.5
1.0
0.5
0.0
SHC
AC1
AC2
Cyclic voltammograms of SHC in 1 M H2SO4 at different
scanning rate (Fig. 4a) showed almost rectangular curves.
When the scanning rate increased from 20 to 200 mV/s the
similar shape of the CV curves indicated that SHC possessed a
good electrochemical stability. The specific capacitance of
SHC was calculated to be 261 F/g at 5 mV/s, smaller than the
AC2’s 308 F/g. When the rate was at 100 mV/s, the specific
capacitance of SHC decreased to 210 F/g, but AC2’s decreased
to 154 F/g, around half of the value at 5 mV/s (Table 2).
Fig. 4b indicated that the specific capacitances of all the
carbon samples reduced when the scanning rate increased. As
the scanning rate increases, the specific capacitance of SHC
was higher than AC2’s, but it was much lower than AC2’s at
the slow scanning rate. It indicated that the large-current
performance of SHC was excellent in H2SO4 system.
0
500
1000
1500
2000
2500
Time(s)
Fig. 3. Charge/discharge curves of the carbons in EDLC with LiPF6/PECCE
electrolyte (at 1 mA/cm2).
3.2. Electrochemical performance
When the current density was 0.05 mA/cm2, the discharge
curve of the three kind of carbons (Fig. 2a) indicated that the
specific capacitance of SHC was the highest; when the current
density increased to 0.5 mA/cm2 (Fig. 2b) the trend was the
same. According to Eq. (1), the specific capacitance of SHC
was calculated to be 232 F/g (see Table 2) at the current density
of 0.05 mA/cm2, much more than the AC1’s 173 F/g its
specific surface area was 1667 m2/g even comparing with the
AC2 with specific surface area of 2332 m2/g its specific
capacitance was 195 F/g. When the current density increased to
0.5 mA/cm2, the specific capacitance of SHC also kept the
highest value.
The specific capacitance depended on the formation of
electric double layer. The size of hydrogen ions was small
enough for diffusing into micopores of carbons in 1 M H2SO4,
thus for AC2 due to its large specific surface area and nearly all
the micopores, the specific capacitance was higher at the low
scanning rate; as the scanning rate increases so many ions
needed a large channel to transfer thus the decrement of the
400
(b)
300
200
100
0
(a)
SHC
AC2
In 1MH2SO4
350
300
250
200
150
100
50
SHC-200mV
SHC-100mV
SHC-50mV
SHC-20mV
-100
-200
-300
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
Potential Vs SCE (V)
1
10
100
1000
Scanning Rate (mV/s)
Fig. 4. Cyclic voltammetry (CV) analysis in 1 M H2SO4 aqueous solution (a, cyclic voltammograms of SHC; b, specific capacitance at different scanning rates).

G. Liu et al. / Journal of Physics and Chemistry of Solids 67 (2006) 1186–1189
1189
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electrolyte, the specific capacitance of SHC was larger than the
commercially activated carbons AC1s and AC2s; but in
inorganic electrolyte, it was smaller at low scanning rate but
larger at high scanning rate compared to AC2s. The templated
carbon SHC was a mesoporous carbon with large specific
capacitance in EDLC with organic electrolyte and with good
performance in H2SO4 system at large current.
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Acknowledgements
The authors truly appreciate the financial support of the
Natural Science Foundation of China (No.50272033).
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