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Influence of the OMCs pore structures on the capacitive performances of supercapacitor.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2009; 4: 654–659
Published online 26 June 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.311
Research Article
Influence of the OMCs pore structures on the capacitive
performances of supercapacitor
Gu-Zhen Nong, Hua Wang and Wen-Cui Li*
School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China
Received 29 October 2008; Revised 26 February 2009; Accepted 26 February 2009
ABSTRACT: In the present study, two mesoporous carbons OMC-KIT-6 and OMC-SBA-16 were nanocasted using
mesoporous silica of KIT-6 and SBA-16 as templates and furfuryl alcohol as carbon precursor. Transmission electron
microscopy (TEM) and X-ray diffraction (XRD) characterizations confirmed that the resultant samples are mesoporous
carbons, and the as-prepared OMC-KIT-6 has an Ia3d ordered structure, whereas OMC-SBA-16 belongs to Im3m space
group. The surface area and the average pore size are (1658 m2 g−1 and 3.4 nm) for OMC-KIT-6 and (1638 m2 g−1
and 2.9 nm) for OMC-SBA-16, respectively. The results of cyclic voltammograms and galvanostatic charge-discharge
tests show that these two mesoporous carbons have excellent capacitive performances. But the difference of capacitive
behavior between OMC-KIT-6 and OMC-SBA-16 may be a result of the difference of pore geometries of these two
carbons. In order to find out the function of mesopore in a supercapacitor, we compared the capacitive properties
of mesoporous and microporous carbons; the experiment results indicated that these two kinds of carbon exhibit
nearly ideal capacitive behavior at low scan rate. When the scan rate is enhanced up to 50 mV s−1 the performance
of mesoporous carbon is more stable than microporous carbon. This outcome demonstrated that mesopore plays an
important role in forming double layers in the electrode materials.  2009 Curtin University of Technology and John
Wiley & Sons, Ltd.
KEYWORDS: supercapacitors; ordered mesoporous carbons; pore structure
INTRODUCTION
Because of the climate change and the decreasing availability of fossil fuels, the society needs to move toward
sustainable and renewable energy. As a result, we are
observing an increase in renewable energy production
from wind and sun, as well as the development of electric vehicles or hybrid electric vehicles with low CO2
emissions. However the wind does not blow on demand,
the sun does not shine during the night and we all expect
to drive our car with at least a few hours of autonomy; energy-storage systems are also starting to play
an important role in our lives. At the forefront of these
are electrical energy-storage systems, such as batteries
and electrochemical capacitors (ECs).[1]
Supercapacitors, ultracapacitors and electrochemical
double-layer capacitors (EDLCs) are the names of electrochemical energy-storage devices that are fit for rapid
charge storage and release of energy.[2] Owing to their
highly reversible charge storage process, supercapacitors have longer cycle lives and can be both rapidly
*Correspondence to: Wen-Cui Li, School of Chemical Engineering,
Dalian University of Technology, Dalian 116012, China.
E-mail: wencuili@dlut.edu.cn
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
charged and discharged at power densities exceeding
1 kW kg−1 .[3] These features have generated great interest in the application of supercapacitors for a wide, and
growing, range of applications that include consumer
electronics, hybrid electric vehicles and industrial power
management.[4]
Because of the difference of power storage mechanism, supercapacitor is classified into two kinds: EDLC
and faraday psuedocapacitors. In this article we emphasize on EDLC. At present there are three kinds of materials that are used as supercapacitor electrode materials:
carbon materials, metal dioxides and conductive polymer materials. Carbon material is widely used in the
supercapacitor field as it is cheap, easy to obtain and
comes in many shapes of powder, fiber, felt and cloth.
At the same time because of its high conductivity, high
surface-area range and good corrosion resistance etc.,
and its chemical and physical properties, carbon-based
electric double-layer capacitors (EDLCs) are showing
a long cycle life and a high power density.[5] Based
on earlier research scientists found that there are many
factors that influence the value and performance of a
capacitor, such as specific surface area, pore sizes distribution, surface functional group and so on. The research
results show that the higher the specific area, the higher
Asia-Pacific Journal of Chemical Engineering
INFLUENCE OF OMCS ON THE PERFORMANCE OF A SUPERCAPACITOR
the capacitor value; but in fact capacitor value is not
always proportional to the specific surface areas. One
reason for this is that the effective surface area, which
is the available surface area of micropores to form the
electric double layer, is low because of low permeability
of the electrolyte into micropores.[6]
In carbon-based capacitors, different electrolytes
demand different pore sizes of the electrode materials. Until now, there is no accordant conclusion of the
lower limit of pore size to form double layers in electrolyte solutions. Shiraishi pointed out that the micropore size smaller than 0.7 nm has no contribution to the
EDLC, and the appropriate pore size in H2 SO4 solution
is 0.8–2 nm.[6] Chmiola discovered that when a pore
size close to the ion size will have a good effect on
capacitor, a micropore size smaller than 1 nm will produce high capacitance in organic or aqueous electrolyte
solutions.[7] These results challenge the long-held presumption that pores smaller than the size of solvated
electrolyte ions do not contribute to energy storage.[8]
In recent years, highly ordered mesoporous carbons
(OMCs) with large surface area, high and tunable mesoporosity, narrow pore-size distribution, high electrical conductivity and hydrophobic surface are particularly promising as EDLC electrodes for fundamental research and for practical applications.[9] OMCs
have attracted much attention for wide application in
methane and hydrogen storage,[10] as absorbents,[11] catalyst supports,[12,13] and in EDLC applications.[14] It
has been demonstrated by many research groups that
OMCs have much better electrochemical performance
than conventional high-surface activated carbon (AC)
at high current densities both in aqueous[15] and nonaqueous electrolytes.[16] Compared with a wide pore
distribution ranging from micropores to macropores and
a random pore connection of AC, the OMCs possess a
narrow distribution in the mesopore range and a uniform
pore connection, but the obtained capacitance, about
8–10 µF cm−2 , is still below the theoretical value estimated to be 20 µF cm−2 for carbon materials.[16]
That indicated, a great part of the pore surface in
OMCs cannot be effectively utilized in EDLCs. Till now
it has been principally agreed that the surface area, pore
size[17] and functional groups[1] of OMCs highly affect
the capacitive performances of supercapacitors. The rate
capability is critically dependent on the ion dispersion,
which can be estimated by the ion penetrability of the
electrode;[18] however, no clear picture has emerged
owing to the complexity of porous carbon. The key
point to promote the capacitive behaviors of OMCs is
to develop a pore structure that favors facile electrolyte
accessibility and rapid ion diffusion.
Recently, large numbers of studies have focused
on the influence of pore structures including surface
area, pore size, etc. of mesopores on the electrochemical performances.[19] Many groups have synthesized
ordered mesoporous silica, which can then be used as a
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
template for the preparation of carbons with controlled
pore sizes,[20,21] meeting the above mentioned capacitor
requirements. The templates are mesostructured silica
such as MCM-48, MCM-41 and SBA-15, whereas the
carbon source is a liquid or gas phase, e.g. a sucrose
solution, polyfurfuryl alcohol, propylene or pitch.[22,23]
After its deposition inside the pores of the template,
the carbon precursor is carbonized at temperatures close
to 800 ◦ C. The last step is the removal of the silica
template by dissolution in hydrofluoric acid. In our
present study, in order to investigate the influence of
pore arrangement excluding other factors, two OMCs
with different pore arrangement and nearly same surface
area and pore size were prepared by the template
method.
EXPERIMENTAL
Two mesoporous silica, KIT-6 and SBA-16, were
employed as templates and their properties have been
listed in Table 1. Furfuryl alcohol was used as carbon
precursor, and oxalic acid as catalysis. Figure 1 shows
the pore structure model of mesoporous silica KIT-6 and
SBA-16. KIT-6 has three-dimensional cubic structure
with an Ia3d space group; SBA-16 is composed of
spherical cavities connected by some nanosize windows
and the space group is determined as Im3m.
Briefly, mesoporous silica was impregnated with
furfuryl alcohol and oxalic acid (the molar ratio between
furfuryl alcohol and oxalic acid is of 200 : 1) and
then cured in air under 90 ◦ C for 2 days to form
polyfurfuryl alcohol, which was further carbonized at
850 ◦ C for 2 h under a nitrogen atmosphere. The OMC
was obtained after dissolution of the silica framework
with NaOH solution at 50 ◦ C for 18 h. The resultant
samples were denoted as OMC-KIT-6 and OMC-SBA16, respectively.
Concerning the electrode preparation, the obtained
OMC was mixed with polytetrafluoroethylene (PTFE)
in alcohol solution with the ratio of 19 : 1. The mixture
was dried at 50 ◦ C for a few minutes before being
pressed on the current collector of nickel foam. The
surface area of the electrode was 1 cm2 and the mass
was about 10 ± 2 mg.
Nitrogen adsorption measurement was carried out at
77 K with an ASAP 2020 system. The carbon powders
were dried for 4 h at 200 ◦ C before testing. The total
Table 1. Structure parameters of silica hard templates.
Sample
Space
group
SBET
(m2 g−1 )
Vtot
(cm3 g−1 )
Dmax
(nm)
KIT-6
SBA-16
Ia3d
Im3m
765
930
1.13
0.81
6.2
7.3
Asia-Pac. J. Chem. Eng. 2009; 4: 654–659
DOI: 10.1002/apj
655
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G.-Z. NONG, H. WANG, W.-C. LI
Asia-Pacific Journal of Chemical Engineering
electrolyte for 24 h to eliminate air in the electrodes
and let the electrolyte ions get into the pore structure of
electrode materials.
RESULTS AND DISCUSSION
In order to investigate the pore structure of the resultant
sample, TEM and XRD characterizations were carried
out. TEM images and XRD patterns of the obtained
mesoporous carbon OMC-KIT-6 and OMC-SBA-16 are
displayed in Fig. 2. The XRD pattern of OMC-KIT-6
shows two well-resolved sharp XRD diffraction peaks
in the region of 2θ = 0.5–1.5◦ , which correspond to
211 and 220 reflections, based on the cubic system,
and can be determined as Ia3d.[24] The XRD pattern of
OMC-SBA-16 shows three well-resolved Bragg diffraction peaks at angles below 5◦ , which correspond to the
cubic Im3m space group. Also, Fig. 2(b) shows TEM
images of these two carbon samples demonstrating the
highly ordered mesostructure.
N2 sorption isotherms and corresponding pore-size
distributions of the resultant samples are shown in
Fig. 3. As can be seen in Fig. 3(a), nitrogen adsorption–desorption isotherms for the two carbons featured
narrow capillary condensation steps, which indicate
Pore structure of mesoporous silica: (a) KIT-6 (b) SBA-16. This
figure is available in colour online at
www.apjChemEng.com.
Figure 1.
specific surface area (SBET ) and pore-size distribution in
the mesopore region of the carbon powders were evaluated from the analysis of the N2 adsorption isotherms
using the BET and BJH theories. X-ray diffraction
(XRD) was performed on a Rigaku D/Max-2400 X-ray
diffractometer (Cu Kα radiation, λ = 1.5432 Å). Transmission electron microscopy (TEM) and high-resolution
TEM (HRTEM) analyses were carried out with Tecnai
G2 20S-Twin equipment operating at 200 kV. Electrochemical characterization was carried out using a conventional three-electrode system with a platinum plate
and saturated calomel electrode (SCE) as the counter
and reference electrodes, respectively. Cyclic voltammograms (CVs) were recorded by polarizing the working electrode between 0 and 0.8 V vs SCE in a 30 wt%
KOH aqueous solution on a CHI606C electrochemical
workstation and the scan rate is 5 mV s−1 . Before electrochemical tests, the electrodes were impregnated with
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 2. XRD (a) and TEM images (b) of sample MC-KIT-6
and MC-SBA-16.
Asia-Pac. J. Chem. Eng. 2009; 4: 654–659
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
INFLUENCE OF OMCS ON THE PERFORMANCE OF A SUPERCAPACITOR
pores of 3.7 nm inner diameter, and another pore system of 2.5 nm diameter that is formed between the
tubes, was obtained by using cubic Im3m mesostructured mesoporous silica with a pore diameter of 7.3 nm
as the template.
All the samples given are type IV isotherms that have
the characteristics of mesoporous carbon. The poresize distributions of two samples are almost identical.
The textural parameters including specific surface areas,
pore volume and pore size are listed in Table 2.
CVs of two OMCs are presented in Fig. 4(a). The CV
curves of these two mesoporous carbons were close to
square and nearly the same, which demonstrated that
OMC-KIT-6 and OMC-SBA-16 exhibited nearly ideal
capacitance behavior. We can see a taper angle of the
curve that might have been caused by the essential
resistance of the carbon electrode materials or the
contact resistance between the electrolyte ions and the
carbon materials. The galvanostatic charge–discharge
Figure 3. N2 adsorption–desorption isotherms
(a) and pore-size distribution (b) of MC-KIT-6(solid
circle) and MC-SBA-16(solid star).
the high degree of pore-size uniformity. The nitrogen adsorption isotherms and the pore-size distribution of the OMC-SBA-16 [Fig. 3(a) solid star] show
that the inner diameter of the mesoporous carbon tube
was 3.7 nm. Two capillary condensation steps were
observed on the adsorption and desorption branches
of the nitrogen isotherms in the pressure ranges of
0–0.03 and 0.35–0.45, which correspond to pore sizes
of 2.5 nm and 3.7 nm [see Fig. 3(b) solid star], respectively. The pore size of the OMC-SBA-16 is in agreement with the larger one of the carbon material, thus
indicating that the mesoporous carbon replica, with
CVs (a) and galvanostatic
charge–discharge profiles (b) of OMC-KIT6 and OMC-SBA-16 at a potential scan
rate of 5 mV s−1 in a three-electrode cell.
This figure is available in colour online at
www.apjChemEng.com.
Figure 4.
Table 2. Properties of the resultant mesoporous carbon.
Sample
OMC-KIT-6
OMC-SBA-16
SBET
(m2 g−1 )
Vt
(cm3 g−1 )
Dpeak
(nm)
Average Pore size
(nm)
C
(F g−1 )
C
(µF cm−2 )
1657
1638
1.42
1.18
3.1
2.5/3.7
3.4
2.9
144
176
8.7
10.8
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2009; 4: 654–659
DOI: 10.1002/apj
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G.-Z. NONG, H. WANG, W.-C. LI
profiles of these two carbons [see Fig. 4(b)] under
the scan rate of 5 mA s−1 are shaped like triangles,
which demonstrates that they give good capacitive
performances. However, there is a little difference
of the discharge line of these two carbons, the line
of OMC-SBA-16 happens to have a light deviated
from the straight line. According to the difference of
mesopore structure of the two carbon samples, we found
OMC-KIT-6 had a three dimensional pore structure,
whereas OMC-SBA-16 exhibited body-center cubic
pore structure. The calculated capacitances based on the
galvanostatic charge–discharge profiles with textural
parameters of the OMC-KIT-6 and OMC-SBA-16 is
compiled in Table 2. The specific capacitance values
of OMC-KIT-6 and OMC-SBA-16 were 144 F g−1
and 176 F g−1 , respectively. According to the working
mechanism of EDLCs, the higher surface area of an
electrode will lead to a higher capacitance value. From
Table 1 we can see that for OMC-KIT-6, the pore
size concentrated on 3.1 nm, and for OMC-SBA-16,
there are two peaks: one is 3.7 nm, and another one
is 2.5 nm, corresponding to the outer- and inner-cage
sizes of the silica template, respectively. Comparing the
textural parameters, we found that these two carbons
have nearly same surface area and close pore size.
The only difference in pore structure is the pore
arrangement in space. That gives us an opportunity to
precisely investigate the relation of pore arrangement
and capacitive behavior, and exclude the influence of
surface area and pore size.
Therefore, the difference of capacitance and galvanostatic charge–discharge profiles between OMC-KIT6 and OMC-SBA-16 should arise from other factors.
Upon XRD analysis, pore arrangement of OMC-KIT6 belongs to Ia3d space group, whereas OMC-SBA16 belongs to Im3m ordered structure. That may be
responsible for the difference of electrochemical behaviors between the two OMCs. OMC-KIT-6 has threedimensional cubic periodic structures, which block the
ion transport in the pore. However, OMC-SBA-16
clearly shows the cage-like three-dimensional cubic
mesostructure, and the pore that provided a connective pore structure for electrolyte ions to transport is
straight, and then decreased the transported resistance.
The obvious differences in capacitances (µF cm−2 ) are
most likely arising from the intrinsic pore structure of
OMC-KIT-6 and OMC-SBA-16. In terms of easy accessibility of inner pore surface area, the pore structure of
OMC-SBA-16 is more advantageous than that of OMCKIT-6. Thus, at a fixed current load, the straighter the
open porous channels are, the higher the capacitance
(µF cm−2 ) will be.
In order to investigate the capacitor performances of
the resultant mesoporous carbons, we further enhanced
the scan rate to test the capacitive behavior of the
OMCs. Meanwhile, one microporous activated carbon
was also tested as reference, the BET surface area
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
Figure 5. CVs of MC-KIT-6 (a) and activated
carbon (b) in aqueous electrolyte (30 wt%
KOH solution). The scan rate is 2 mV s−1 ,
5 mV s−1 , 10 mV s−1 , 20 mV s−1 , 50 mV s−1
along the arrow. This figure is available in
colour online at www.apjChemEng.com.
and average pore size of which are 3142 m2 g−1 and
0.3 nm, respectively. The CV curves of these two
carbons can be seen in Fig. 5; we can see, at low scan
rate, both mesoporous and microporous carbons have
similar rectangular working window, indicating a close
capacitive behavior. However, when increasing the
scan rate high up to 50 mV s−1 , the working window
is seriously deviated from ideal capacitive behavior
for microporous carbon; in contrast, the deviation of
mesoporous carbon is not too obvious. As we know
the ion radius of hydrated K+ is roughly 0.133 nm, the
pore size of these two carbons is big enough to let the
electrolyte ions transport in the pore. So the difference
in the capacitive performances is owing to the different
of pore structures. When the scan rate is enhanced, ions
transport too fast in the pore; as a result, the surface
areas of the electrode materials cannot be fully used.
That means the effective surface area is low. With small
micropores, the adsorption–desorption of large cations
to form an electric double layer is not easy with a low
charge–discharge rate of 20 mA, so the contribution of
Smicro is very small, but it becomes even more difficult
at a high charge–discharge rate such as 200 mA.
The adsorption–desorption of large cations into large
Asia-Pac. J. Chem. Eng. 2009; 4: 654–659
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
INFLUENCE OF OMCS ON THE PERFORMANCE OF A SUPERCAPACITOR
mesopores of 3.7 nm in OMC-SBA-16 is much more
easy, as the contribution of Sext is about 10 times larger
than that of Smicro , and this process is not influenced as
much by the charge–discharge rate. This is the reason
the rate performance is determined by the mesoporous
surface area of the negative electrode carbon. The
negative electrode carbon should contain a large amount
of mesopores. Several studies have reported that high
surface area carbons with porosity formed essentially by
mesopores could be potentially more advantageous than
microporous carbons to provide high power density.
That indicates the presence of mesopores is beneficial to
enhance the capacitive behavior of electrode materials
and is crucial if a high propagation of charge is
demanded. For example, during the acceleration of
transportation system, power peak is required.
This result demonstrated that it is very important
to connect micropores and mesopores. The pores in
the porous carbon are interconnected to each other.
Macropores have many branches to connect with the
mesopores, and mesopores also have many branches
to link the micropores and supermicropores. Through
macropore and mesopore, electrolyte ions can reach
the surface of the micropore to form double layer.
Mesopores not only work as channels but can also form
double layer on its surface.
CONCLUSIONS
We have synthesized two OMCs with various ordered
structures and nearly identical surface area and pore size
via a nanocasting pathway using silica as template. The
characterization of textural properties and the capacitive behaviors of these nanocast mesoporous carbons
allow a better understanding of the relation between
structure and performance. The difference of electrochemical behaviors between two mesoporous carbons
is mainly because of the various pore arrangements in
the OMCs. And the presence of mesopores is important
for the rapid propagation of charge. As a result carbon
materials with mesopores are good for the ions to get
to the surface of the micropore and form double layer,
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
and improve the effective surface area. It is crucial to
find a balance between micropore and mesopore for the
development of electrode materials for supercapacitors.
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Asia-Pac. J. Chem. Eng. 2009; 4: 654–659
DOI: 10.1002/apj
659
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