Electrical double-layer capacitance of zeolite-templated carbon in organic electrolyte

Article abstract of DOI:10.1149/1.3002375

Porous carbon materials have been prepared using chemical vapor deposition of ethylene and acetonitrile on zeolite Y templates with the subsequent removal of the template. The use of zeolite templates enabled tuning of the carbon chemistry, microstructure, and specific surface area while maintaining ordered pores and similar pore-size distribution. Evaluation of the performance of the templated carbons in electrochemical capacitors with tetraethylammonium tetrafluoroborate salt solution in acetonitrile showed a high gravimetric capacitance of up to 146 Fg and a short time constant of up to 3 s, indicating fast charge/discharge capability. Nitrogen doping limited the capacitor performance at high current densities without enhancing the energy-storage characteristics at low current densities. Specific capacitance at a constant pore size was found to linearly depend on the surface area, suggesting a potential for further improvements. Carbons produced by zeolite-templating method are promising for use in high-energy, high-power electrochemical capacitors employing organic electrolytes.

Full text of DOI:10.1149/1.3002375

Journal of The Electrochemical Society, 156 ͑1͒ A1-A6 ͑2009͒
A1
0
013-4651/2008/156͑1͒/A1/6/$23.00 © The Electrochemical Society
Electrical Double-Layer Capacitance of Zeolite-Templated
Carbon in Organic Electrolyte
a,
b
c
a,
b
C. Portet, * Z. Yang, Y. Korenblit, Y. Gogotsi, ** R. Mokaya, and
c,z
G. Yushin
a
A. J. Drexel Nanotechnology Institute and Department of Materials Science and Engineering, Drexel
University, Philadelphia, Pennsylvania 19104, USA
b
School of Chemistry, University of Nottingham, Nottingham NG7 2RD, United Kingdom
c
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta,
Georgia 30332, USA
Porous carbon materials have been prepared using chemical vapor deposition of ethylene and acetonitrile on zeolite Y templates
with the subsequent removal of the template. The use of zeolite templates enabled tuning of the carbon chemistry, microstructure,
and specific surface area while maintaining ordered pores and similar pore-size distribution. Evaluation of the performance of the
templated carbons in electrochemical capacitors with tetraethylammonium tetrafluoroborate salt solution in acetonitrile showed a
high gravimetric capacitance of up to 146 F/g and a short time constant of up to 3 s, indicating fast charge/discharge capability.
Nitrogen doping limited the capacitor performance at high current densities without enhancing the energy-storage characteristics
at low current densities. Specific capacitance at a constant pore size was found to linearly depend on the surface area, suggesting
a potential for further improvements. Carbons produced by zeolite-templating method are promising for use in high-energy,
high-power electrochemical capacitors employing organic electrolytes.
©
2008 The Electrochemical Society. ͓DOI: 10.1149/1.3002375͔ All rights reserved.
Manuscript submitted July 29, 2008; revised manuscript received September 22, 2008. Published October 30, 2008.
Electric double-layer capacitors ͑EDLCs͒, also called ultra- or
thesis is to use inorganic templates for carbon deposition, followed
16-21
supercapacitors, are rechargeable electrochemical energy-storage de-
by the subsequent template removal.
This approach allows
1
vices like batteries but with different storage mechanisms and per-
keeping the pore-size constant and understanding effects of other
structural parameters, such as nitrogen doping, available surface
area, or carbon ordering, on the electrochemical performance of po-
formance characteristics. The charge-storage mechanism in EDLCs
is purely electrostatic in nature. During charging and discharging of
EDLCs, no electron transfer takes place at the carbon electrode/
electrolyte interface. Due to the nature of the storage, one of the
most interesting characteristics of EDLCs is their practically unlim-
ited cycling life. The energy storage of an EDLC is determined by
the maximum charge voltage and the capacitance of its electrodes or
the amount of charge accumulated on the electrode per unit voltage.
The power characteristic of an EDLC is mostly determined by the
electrode capacitance and the ability of the electrolyte ions to rap-
idly move through the porous electrodes.
2
2
rous carbons. Pioneering work by Kyotani et al. suggested the use
of zeolites with strictly defined pores as templates for the formation
of carbons with uniform structure, high surface area, and pores in
the 1–3 nm range. Subsequent work by Paredes and Kyotani et
2
3,24
25-28
29
al.,
al.
Pacula and Yang et al.,
Gaslain et al., and Garsuch et
reproduced and confirmed the preservation of the structural
regularity of zeolites in carbons. In addition, zeolite-templated car-
3
0-32
2
5,26
bons demonstrated excellent gas-sorption properties.
Recent
3
3
work by Ania et al. demonstrated that zeolite-templated carbons
doped with nitrogen have excellent capacitance characteristics when
used in aqueous electrolytes due to the pseudocapacitance effects of
nitrogen-containing functional groups. However, most commercial
EDLCs use organic electrolytes capable of withstanding higher op-
erating voltages and thus offering a higher energy density. In addi-
tion, organic electrolytes offer increased cycling life, which is lim-
ited in the case of aqueous electrolytes by redox reactions. It is
therefore important to investigate zeolite-templated carbons in or-
ganic electrolytes, which are of practical interest for EDLC applica-
tions. In this work we explore the potential of zeolite-templated
carbons for EDLC in organic electrolytes and discuss the correlation
between their performances and the carbon structure and properties,
such as specific surface area, ͑SSA͒, pore volume, and nitrogen
doping.
EDLCs possess much higher energy density than conventional
capacitors due to the large surface area of the porous carbon elec-
1
trode film. EDLCs can often store more power in a smaller volume
1,2
and at a lower cost than batteries and fuel cells. Their unique
characteristics allow them to complement or replace batteries and
fuel cells in applications where high power and low weight are
essential, such as mobile electronic and communication devices,
back-up power storage, power-quality devices, and peak power
sources for hybrid 3-e5lectric vehicles. Power systems with
6
battery-supercapacitor,
engine-supercapacitor,
and
fuel
7
cell-supercapacitor combinations are attractive due to their unprec-
edented efficiency, power, and energy characteristics.
State-of-the-art activated carbons ͑ACs͒ used in commercial
EDLCs offer 80–120 F/g when used in organic electrolytes. The
intrinsic properties of the ACs and the challenges associated with
precise and consistent control over porosity, surface chemistry, and
microstructure during activation limit the performance characteris-
Experimental
Carbon synthesis.— The zeolite Y powder purchased from Fluka
8
͑
Buchs, Switzerland͒ was used as the template for carbon deposi-
tics of EDLCs. Carbon synthesis methods provide better control
27
tion. The detailed preparation procedure is described elsewhere.
over the structure and properties most vital for EDLC performance
improvements. For example, the use of metal carbides as inorganic
precursors for porous carbon synthesis allows a tight control over
Briefly, zeolite samples were heated to the deposition temperature
under a nitrogen flow. Once the desired temperature was reached,
pure ethylene ͑series E samples͒ or nitrogen saturated with acetoni-
9-11
pore-size distribution in the 0.6–1 nm range.
The microporosity
trile vapors ͑series Y samples͒ were introduced into the system at
developed in those carbons resulted in high capacitance values in
3
12-15
1
00 cm /min for 3 h to deposit carbon onto the zeolite. Synthesis
organic electrolytes
but rather limited frequency response. An-
temperatures were selected within the range of pyrolysis of precur-
other attractive route to control pore size and structure during syn-
sors used and were chosen to be 550, 650, and 750°C for E samples
͑
C H ͒, and 700, 800, 850, and 900°C for Y samples ͑acetonitile͒.
2 4
Deposition temperatures above 750°C for E samples and below
00°C for Y samples resulted in substantially smaller SSA, and
these samples were not considered in the present study. After carbon
*
Electrochemical Society Student Member.
7
*
* Electrochemical Society Active Member.
z
E-mail: yushin@gatech.edu
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A2
Journal of The Electrochemical Society, 156 ͑1͒ A1-A6 ͑2009͒
Figure 1. Electron microscopy of the
templated carbon samples: ͑A͒ low- and
͑
B͒ high-magnification SEM micrographs
demonstrating conservation of zeolite
shape during carbon synthesis and ͑C͒
low- and ͑D͒ high-resolution TEM micro-
graphs demonstrating formation of coat-
ings on the surface of the porous carbon
particles.
deposition, the samples were cooled down to room temperature un-
der a nitrogen flow. Zeolite templates were dissolved using 10% HF
solution in order to produce zeolite-free porous carbon with less
than 5% oxide residues, according to the thermogravimetric
50 ␮m thick polymeric separator ͑W. L. Gore & Associates, Inc.,
Newark, DE, USA͒ was used. PTFE plates and stainless steel
clamps were used to maintain current collector/electrode/separator/
electrode/current collector assembly under a pressure of approxi-
mately 0.2 MPa. An acetonitrile ͑Acros Organic, Geel, Belgium͒
solution of a vacuum-dried NEt BF salt ͑Alfa Aesar, Ward Hill,
MA, USA͒ was used as the electrolyte. The electrochemical capaci-
tor was assembled in a glove box, and the moisture and oxygen
content were kept below 10 ppm.
2
7
analysis.
4
4
Material characterization.— Scanning electron microscopy
SEM͒ and transmission electron microscopy ͑TEM͒ studies were
͑
undertaken in order to analyze the structure and morphology of the
synthesized samples. SEM analysis was done at a working distance
of 2–4 mm and an accelerating voltage of 2–5 kV ͑Supra 50VP
field-emission scanning electron microscope, Carl Zeiss,
Oberkochen, Germany͒. TEM samples were prepared by sonicating
powders dispersed in ethanol. A droplet was deposited on a lacey-
carbon-coated copper grid ͑200 mesh͒. Observations were per-
formed using an accelerating voltage of 200 kV ͑JEOL 2010F, To-
kyo, Japan͒. Nitrogen sorption measurements were performed at
Galvanostatic cycling measurements were performed using an SI
286 galvanostat/potentiostat ͑Solartron Analytical, Farnborough,
1
Hampshire, UK͒ between 0 and 2.3 V at various currents ranging
from 10 to 200 mA. The gravimetric capacitance, C ͑F/g͒, was cal-
culated according to Eq. 1
2I
C =
͓1͔
͑
dV/dt͒ ϫ mAM
where I is the current ͑A͒, dV/dt is the slope of the discharge curve
V/s͒, and m is the mass of the active material in a single elec-
7
7 K using an ASAP 2020 ͑Micromeritics, Norcross, GA, USA͒ or
Quadrasorb ͑Quantachrome Instruments, Boynton Beach, FL, USA͒
͑
pore size and surface area analyzer. The SSA was calculated using
AM
trode ͑g͒.
the Brunauer–Emmett–Teller ͑BET͒ method from N adsorption iso-
2
Cyclic voltammetry ͑CV͒ and electrochemical impedance spec-
troscopy ͑EIS͒ were performed using a PCI4/300 potentiostat
therms in the range of relative pressures from 0.05 to 0.2. The pore-
size distribution was obtained from the adsorption isotherms using
the nonlocal density functional theory ͑DFT͒. The pore-size distri-
͑
Gamry Instruments, Warminster, PA, USA͒. Cyclic voltammograms
3
4
were recorded between −2 and 2 V at a scan rate of 10 mV/s. The
capacitance was calculated from the voltammograms according to
Eq. 2
butions were determined assuming a cylindrical pore shape. The
range of relative pressures from 0.05 to 0.2 was used for BET SSA
calculations. X-ray photoelectron spectroscopy ͑XPS͒ analysis was
conducted using a model 1600 system ͑Physical Electronics-PHI,
Chanhassen, MN, USA͒ to determine the elemental composition of
the synthesized samples. Al foil was used as a sample holder. Charg-
ing was reduced by covering the sample with a conductive mesh and
using a discharging gun.
Ia − Ic
C =
͓2͔
SR ϫ mAM
where Ia and Ic are the anodic and cathodic currents ͑A͒, respec-
tively, and SR is the scan rate ͑0.01 V/s͒. EIS was performed on a
fully discharged cell ͑0 V͒ in a frequency range between 7 mHz and
Electrochemical characterization.— Electrochemical tests were
performed in a symmetrical two-electrode configuration with 2
ϫ 2 cm electrodes. Aluminum foil 300 ␮m in thickness was rough-
ened using SiC 600 grit sandpaper and coated with a conductive
polyurethane paint containing acetylene black particles to serve as a
current collector. The employed procedure was found to provide low
interface resistance between the Al current collectors and the
5
0 kHz with a signal amplitude of 5 mV.
Results and Discussion
All samples retained the size and shape of the zeolite template
2
5-28
particles ͑Fig. 1A and B͒, as previously observed.
Because the
particle size is known to affect the electrochemical performance of
3
5,36
37
electrodes.
The active material consisted of 95 wt % carbon and
porous carbons, conservation of size and shape was an important
5
wt % poly͑tetrafluoroethylene͒ ͑PTFE͒ ͑60 wt % suspension in
parameter for the performed parametric studies. The microstructure
of the synthesized carbons slightly varied within the produced
samples. Each particle consisted of two regions, an internal highly
water, Sigma-Aldrich, St. Louis, MO, USA͒. The mass of the active
material was kept constant at 40 mg for all the tested samples. A
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Journal of The Electrochemical Society, 156 ͑1͒ A1-A6 ͑2009͒
A3
Table I. Composition of the templated carbons derived from XPS
analysis.
Table II. Porosity of the synthesized carbons.
a
2
2
3
Sample BET SSA ͑m /g͒ DFT SSA ͑m /g͒ Pore volume ͑cm /g͒
Sample
C ͑atom %͒
N ͑atom %͒
F ͑atom %͒
Y 700
Y 800
Y 850
Y 900
E 550
E 650
E 750
1122
1941
1987
1403
853
1179
2196
2205
1606
1120
1625
1075
0.81
0.97
1.08
0.81
0.46
0.74
0.60
Y 700
Y 800
Y 850
Y 900
E 550
E 650
E 750
89.9
92.8
90.7
95.2
96.7
98.4
99
6.5
4.6
7.2
4.4
0
3.6
2.6
2.1
0.4
3.3
1.6
1
1370
1032
0
0
a
Oxygen detected during the analysis is not considered in the present
table.
in the micropores after exposure of the samples to the ambient at-
mosphere. The presence of oxygen-containing functional groups
was neglected. As expected, Y samples synthesized by decomposi-
tion of acetonitrile contained a substantial amount of nitrogen ͑up to
ϳ7 atom %͒, which was not present in the E series prepared using
ethylene precursor. Detected fluorine is believed to come from the
HF treatment. Samples in each series synthesized at higher tempera-
porous and disordered carbon core and a graphitic surface coating
Fig. 1C and D͒. The thickness of the coatings varied from particle
͑
to particle ͑2–20 nm͒ but, in general, was larger for samples pro-
duced at higher temperatures and for E series of samples ͑not
shown͒. It may result from carbon deposition on the surface of the
particles during the precursor pyrolysis after all zeolite pores have
been filled with carbon. The coating is porous and, as is discussed in
the following sections, allows diffusion of gas molecules and ions
into the high-surface-area core of the particles. Existence of such a
coating may contribute to lowering the pore volume and surface area
but, at the same time, is expected to improve the electrical conduc-
tivity of the carbon electrodes.
tures trapped less fluorine because of
graphitization and fewer dangling bonds.
a higher level of
2
7
The shape of the N2 sorption isotherms of the synthesized car-
bons ͑not shown͒ indicates the presence of both micropores and
mesopores. Pore-size distribution determined from the isotherms of
all the samples ͑Fig. 2͒ is similar. The most pronounced peaks are at
2
4,33
around 1.5 and 2 nm, similar to previous reports.
The
0.7–1.5 nm pores are likely to be directly templated by the zeolite
framework, while the larger pores arise from incomplete filling of
A prior XPS study indicated that the chemical composition of the
nitrogen-doped ͑i.e., acetonitrile-derived͒ zeolite-templated carbons
3
3
the zeolite pores during the templating process. The pore volume
and SSA showed substantial variations, depending on the synthesis
conditions ͑Fig. 3 and Table II͒. The peak of the pore volume and
SSA corresponded to 650 and 850°C for E and Y samples, respec-
tively. Y samples demonstrated a noticeably better developed poros-
ity with nearly double the pore volume ͑Fig. 3 and Table II͒, as
2
5
is the same on the surface and in the bulk of the samples. As such,
we expect that nitrogen is homogeneously distributed in the carbon
structure. Our XPS studies identified four distinguished peaks cor-
responding to C , O , N , and F core-level transitions ͑not
1
S
1S
1S
1S
shown͒. Chemical composition of the samples presented in Table I
was obtained from analyzing the XPS spectra. In these calculations
we assumed that oxygen content ͑commonly in the range of 3–5
atom % for our samples͒ mostly comes from the moisture adsorbed
2
3
expected from the prior studies. Differences in the micropore vol-
ume were slightly less pronounced ͑Fig. 3B͒. Figure 3B shows that
the micropore volume does not follow the same trend as the SSA
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.7
0.6
Y 800
Y 850
Y 900
E 550
E 650
E 750
0.5
0.4
0.3
0.2
0.1
Figure 2. ͑Color online͒ Pore-size distri-
bution calculated from N2 isotherms col-
lected at 77 K. The shape of the pores was
assumed to be cylindrical.
0.0
1
2
3
4
5
1
2
3
4
5
Pore diameter (nm)
Pore diameter (nm)
A
B
1
1
0
0
0
0
0
.1
0.40
E series
Y series
0
0
.36
.32
E series
Y series
.0
.9
.8
.7
.6
.5
0.28
0
0
.24
.20
Figure 3. ͑Color online͒ Porosity varia-
tion with the precursor and the synthesis
temperature: ͑A͒ total pore volume and
0.16
͑
B͒ volume of pores Ͻ2 nm ͑micropores͒.
0
0
.12
.08
0.04
.00
0
6
00
700
800
900
600
700
800
900
Synthesis temperature (°C)
Synthesis temperature (°C)
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A4
Journal of The Electrochemical Society, 156 ͑1͒ A1-A6 ͑2009͒
Figure 4. ͑Color online͒ Specific capaci-
tance dependence on ͑A͒ BET SSA and
͑
B͒ pore volume.
and the total pore volume for the Y samples; it continuously de-
creases with the synthesis temperature. On average, a slightly higher
pore volume is obtained for the Y compared to the E sample series.
Figure 4 represents the specific capacitance of the E and Y series
measured by galvanostatic cycling measurements at low current
Several authors have suggested that nitrogen enrichment of car-
bon may increase the capacitance in organic electrolytes due to the
4
0
pseudocapacitance effect. While in our case the highest capaci-
tance is achieved in the sample containing nitrogen, we attribute the
high capacitance solely to the highest SSA. Experimental data for
the Y series do not show any signs of pseudocapacitance; cyclic
voltammograms obtained for the Y series presented no additional
peaks ͑not shown͒. Capacitance measurements obtained from both
the discharge curves and the cyclic voltammograms show a good
consistency with the capacitance variation of about 5%, which is
traditionally observed in such measurements.
͑
10 mA͒. A linear charge–discharge was observed during cycling
with the same charge and discharge time, suggesting that no faradaic
reactions occurred between the carbon and the electrolyte. Both the
1
2
37
pore size and the particle size of porous carbons are among the
parameters affecting the specific capacitance. However, those are
similar in all our samples. As a result, we observed a linear relation
between the gravimetric capacitance with the BET SSA and total
pore volume ͑Fig. 4͒. The average value for specific capacitance per
BET SSA was ϳ8 ␮F/cm . The maximal capacitance of 146 F/g
was obtained for the carbon sample having the largest SSA and pore
4
1
As compared to carbon nanotubes and onions, commercial ac-
4
2
37
tivated carbons, and carbide-derived carbons, a larger capaci-
tance decrease was observed in our samples at high discharge rates
͑Fig. 5͒. It is generally assumed that the capacitance loss under large
currents is due to the presence of micropores. In our case, the vol-
ume of micropores is rather small, and no clear correlation between
the micropore volume and capacitance fading was observed. For
example, Y 800 possesses the worst overall micropore volume ͑Fig.
3B͒, while Y 850 demonstrates the highest capacitance fading. We
hypothesize that two main factors contributed to the impeded trans-
port of ions at high current density: ͑i͒ dangling bonds and surface
functionalities, particularly the nitrogen-containing ones ͑Table I͒,
and ͑ii͒ graphitic coating of the particles, which is likely to serve as
a bottleneck and restrict the ion transport toward the internal volume
of the particles ͑Fig. 1D͒. The first factor clearly has the largest
impact ͑Fig. 5͒. The Y series demonstrated worse retention of the
capacity than the E series. Y 850 with the highest nitrogen content
showed the largest capacitance fading. If our hypothesis is correct,
future work directed toward synthesis of high-surface-area tem-
plated carbons with the smallest content of impurities ͑fluorine, ni-
trogen, and oxygen͒ may produce excellent materials for EDLCs
operating in organic electrolytes. This is in contrast to the use of
2
volume ͑Fig. 3 and 4͒. In contrast to capacitance saturation observed
2
38
in activated carbons at ϳ1500 m /g by Barbieri et al., capacitance
of the zeolite-templated carbon samples with increased order and
uniformity in the pore structure continues to grow with more devel-
oped porosity. This result shows the significant potential advantage
of the periodic structure obtained in the zeolite-templated carbon.
The micropore volume contribution generally observed in high-
12,39
capacitive carbons
does not significantly affect the variation in
capacitance of our samples, presumably due to the small relative
volume of micropores. As a result, we did not observe any clear
correlation between the capacitance and the micropore volume.
While the specific capacitance per unit of SSA may be lower for
12
mesopores compared to subnanometer pores, high capacitance val-
ues can be achieved for mesoporous carbons, if the available SSA
and pore volume are large enough. Small mesopores or meso- and
2
micropores together can provide SSA in excess of 2000 m /g and
3
pore volume above 1 cm /g, which have not been achieved in sub-
nanometer pores so far.
1
00
95
90
85
A 1
00
Y 800 C =130 F/g
0
E 550 C =88 F/g
0
Y 850 C =146 F/g
0
E 650 C =100 F/g
0
Y 900 C =115 F/g
9
9
8
8
7
7
5
0
5
0
5
0
E 750 C =88 F/g
0
8
7
7
6
6
5
5
4
4
0
5
0
5
0
5
0
5
Figure 5. ͑Color online͒ Variation of ca-
pacitance with current density. C0 repre-
sents the maximum capacitance at 10 mA.
0
0
0
0
.00
0.05
0.10
0.15
0.20
.00
0.05
0.10
0.15
0.20
Current (A)
Current (A)
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Journal of The Electrochemical Society, 156 ͑1͒ A1-A6 ͑2009͒
A5
A
B
7
mHz
8
8
6
4
2
0
0
0
0
0
E series
Y series
E 650
E 750
Y 800
Y 850
Y 900
7
6
5
4
3
2
1
Figure 6. ͑Color online͒ Frequency re-
sponse of the synthesized carbons: ͑A͒
Nyquist plots and ͑B͒ variation of ESR
with synthesis temperature.
Z"=0, f=1 kHz
10 20 30
Z' (Ω.cm )
0
40
600
700
800
900
2
Synthesis temperature (°C)
3
3
templated carbons in aqueous solutions, where surface functional
groups substantially contribute to the overall capacitance and do not
impede the transport of smaller hydrated ions to any significant de-
gree.
ionic resistance of the electrolyte, among others. The resistance de-
crease with increasing synthesis temperature could be due to an
increase in the electrical conductivity of the carbon. However,
sample E 750 breaks the observed trend. Some inconsistency in the
material or the electrode preparation might be responsible for the
observed deviation.
EIS provides complementary information about the frequency
response of carbons in EDLCs. The Nyquist plots of cells assembled
with templated carbon electrodes ͑Fig. 6A͒ demonstrate no high-
frequency loop, indicating a good contact between the current col-
lector and the active material. The frequency behavior varies from
one cell to another. The higher ZЈ and ZЉ at low frequency observed
in the E sample series indicates their lower capacitance values, in
agreement with CV and discharge experiments ͑Fig. 4 and 5͒. From
these plots, the equivalent internal series resistance ͑ESR͒ of the
cells measured at 1 kHz and corresponding to ZЉ = 0 was plotted as
a function of the synthesis temperature ͑Fig. 6B͒. There are several
contributions to ESR, including the electrical resistance of the car-
bon electrodes and the current collector/electrode interfaces and
The variation of the real and imaginary capacitance with fre-
4
0
quency ͑Fig. 7͒ was calculated according to Eq. 3
1
1
CЈ = −
;
CЉ =
͓3͔
2␲fZ
Љ
2␲fZ
Ј
where f is the frequency. The plots of the real part of the complex
capacitance CЈ as a function of the frequency ͑Fig. 7A and B͒ dem-
onstrated variations of the frequency response among the samples.
Furthermore, even at the lowest frequency ͑7 mHz͒ the graphs do
not show saturation, and the full capacitance is not reached. Among
A
B
1
.0
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
(
(
(
Y 800)
Y 850)
Y 900)
E 550
E 650
E 750
0.8
0
0
0
0
.6
.4
.2
.0
0.01
0.1
1
10
100
0
.01
0.1
1
10
100
Frequency (Hz)
Figure 7. ͑Color online͒ Variation of the
A and B͒ real part CЈ and ͑C and D͒
Frequency (Hz)
͑
imaginary part CЉ of the capacitance for
the ͑A and C͒ E and ͑B and D͒ Y sample
series.
C
0.5
E 550
E 650
E 750
D
0.9
Y 800
Y 850
Y 900
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
0
0
0
0
.4
.3
.2
.1
.0
0.01
0.1
1
10
100
0
.01
0.1
1
10
100
Frequency (Hz)
Frequency (Hz)
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A6
Journal of The Electrochemical Society, 156 ͑1͒ A1-A6 ͑2009͒
show saturation at high surface areas, opening avenues for further
1
2
0
8
6
4
2
0
capacitance enhancement. XPS studies revealed up to ϳ7 atom % of
nitrogen in samples synthesized using an acetonitrile precursor. In
contrast to expectations, nitrogen doping was not found to increase
the total capacitance values but negatively affected the capacitance
retention at high current densities. This suggests that pure carbon
electrodes might be better for EDLCs using organic electrolytes.
The frequency response of the synthesized materials was better than
that of the high-capacitance microporous carbons. The combination
of high capacitance with a short time constant makes zeolite-
templated carbon a promising material for high-energy, high-power
EDLCs using organic electrolytes.
1
τ0 E series
τ
Y series
0
6
50
700
750
800
850
900
Synthesis temperature (°C)
Georgia Institute of Technology assisted in meeting the publication costs
of this article.
Figure 8. ͑Color online͒ Variation of the time constant with synthesis tem-
perature.
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4
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