Carbon 44 (2006) 2122–2129
www.elsevier.com/locate/carbon
Electrical double-layer characteristics of novel
carbide-derived carbon materials
J. Leis
a,b,*
, M. Arulepp a, A. Kuura a, M. Lätt
a
a,b
, E. Lust
b
Tartu Technologies Ltd., 185 Riia Str., 51014 Tartu, Estonia
b
University of Tartu, 2 Jakobi Str., 51014 Tartu, Estonia
Received 15 November 2005; accepted 7 April 2006
Available online 24 May 2006
Abstract
A variety of nanoporous carbide-derived carbon materials possessing improved pore size distributions were synthesised from a mixture of titanium carbide and titanium dioxide. It was observed that TiO2 caused partial oxidation of the carbon particles created during
high-temperature chlorination of the TiC/TiO2 mixture. The resulting carbon powder is characterised by narrow pore size distribution
with a peak pore size of around 8 Å and a noticeably smaller amount of pores below 6–7 Å compared to the carbon derived from pure
TiC. Electrochemical and electrical double-layer characteristics of novel carbon materials in the acetonitrile solution of triethylmethylammonium tetrafluoroborate were obtained by using cyclic voltammetry and constant current methods. Carbon electrode materials of
this study were tested over the temperature range from 10 °C to +60 °C. Results of this study affirmed a great potential of the
synthesised advanced carbide-derived carbon, whose specific double-layer capacitance reaches approximately 90 F cm3 and 125 F g1.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Porous carbon; Electrodes; Oxidation; Adsorption; Electrochemical properties
1. Introduction
The excellent adsorptivity of carbide-derived carbon
(CDC) were known already in the middle of last century
[1], however, the unique nanoporous structure, narrow
pore size distribution and the possibility of fine-tuning
the pore size were confirmed only lately [2–5]. Wide variability of the nanostructure and the pore size distribution
of CDC materials are practically achieved by varying the
precursor carbide and thermal conditions of extracting
the carbide-forming element from the crystal lattice of
the carbide used [6]. CDC is suggested as a suitable material in fuel cells [7] and for selective adsorption processes,
e.g., for the extraction/purification of noble gases [8] and
for hydrogen storage [9], etc. Another rapidly developing
*
Corresponding author. Address: Institute of Chemical Physics, University of Tartu, 2 Jakobi Str., 51014 Tartu, Estonia. Tel.: +372 738 3057;
fax: +372 742 8467.
E-mail address: [email protected] (J. Leis).
0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2006.04.022
application field based on the adsorption behaviour of
nanoporous carbon is super- or ultracapacitors, which
are electrochemical energy-storing devices [10,11]. Many
types of CDC have been tested in supercapacitors, the most
promising among of them being the carbon derived from
titanium carbide [12,13].
The mass balance of titanium carbide chlorination reaction is expressed by
TiC + 2Cl2 ! C + TiCl4 "
ð1Þ
It is known that the carbon synthesised from TiC below
900 °C is predominantly amorphous and does not contain
any long-range structures according to the X-ray diffraction and high-resolution transmission electron microscopy
studies [14]. Such carbon is highly disordered, consisting
mainly of randomly curved graphene layers. The atomic
structure of carbide-derived nanoporous carbons prepared
by the chlorination of TiC at different temperatures has
been studied by means of the reverse Monte Carlo modelling of neutron diffraction data [15]. It is generally observed
J. Leis et al. / Carbon 44 (2006) 2122–2129
that the TiC-derived amorphous carbon possesses a BET
surface area up to 1400 m2 g1 and is almost completely
microporous with a peak pore size of 7 Å. The homogeneity and highly nanoporous structure cause a noticeable
sterical hindrance inside carbon particles. It inevitably
reduces the velocity of mass transport, which is undesirable
for most of the adsorption-based applications. In fact, the
very small nanopores, below 6–7 Å, are inaccessible to electrolyte ions [12,16]. The impact of pores below 6 Å can be
reduced by using different oxidative post-modification
treatment methods previously discussed in detail [17].
Fairly recently a new method was discovered that
enabled the minimization of such inaccessible nanopores
during the chlorination of carbide [18]. The method implements the known carbothermal reduction of TiO2 in chlorine atmosphere:
C + TiO2 + 2Cl2 ! TiCl4 + CO2 :
ð2Þ
When two processes expressed by Eqs. (1) and (2) are running simultaneously in the reaction medium, the overall
mass-balance of the reagents and products is consequently
presented by
TiC + xTiO2 + 2(1 + x)Cl2
! (1 x)C + (1 + x)TiCl4 + xCO2 ;
ð3Þ
where x is a stoichiometrical constant corresponding to the
mole-amount of TiO2 participating in the reaction. The essence of this method is that in chlorine atmosphere TiO2
oxidizes a predetermined small part of the carbon created
during the etching of carbide with chlorine. The process,
thereby, is well controlled by the amount of TiO2 in the
reaction medium. The reaction temperature, which particularly regulates the equilibrium between CO2 and CO, provides additional control over the quantity of the carbon
oxidised during the reaction. The desired oxide/carbide
mixture can be made by mechanical mixing of oxide and
carbide powders or made during carbide synthesis by using
incomplete conversion of metal oxide into carbide.
The aim of the present paper is to analyse the suitability
of TiO2-assisted carbide-derived carbon for the supercapacitor and to compare its performance with the other
TiC-derived carbon materials. Cyclic voltammetry (CV)
and constant current method (CC) have been used for
the detailed analysis of the materials synthesised.
2123
(AGA, 99.999%) for 1 h at the desired temperature (700–
900 °C). The flow rate of chlorine was 1.5 l min1. The
by-product, TiCl4, was led away by the stream of excess
chlorine and passed through a water-cooled condenser into
a collector. After that the reactor was flushed with a slow
stream of argon (0.5 l min1) at 1000 °C for 1 h to remove
the excess of chlorine and residues of gaseous by-products
from carbon. During heating and cooling, the reactor was
flushed with argon (0.5 l min1). The resulting carbon powder was thereafter treated with hydrogen at 800 °C for 2.5 h
to dechlorinate thoroughly the carbon as well as remove
oxygen-containing functional groups from the surface of
carbon.
The conditions of synthesis and final yields of carbon
are presented in Table 1. Somewhat lower yields of samples
1–3, especially at higher synthesis temperatures, from those
expected according to Eq. (3) are probably due to the etching of carbon by chlorine catalysed particularly with TiCl4.
Comparative samples 4 and 5 were made by chlorinating
titanium carbide without additives at 800 °C. Sample 5
was additionally oxidised by using the method of high-temperature treatment of carbon saturated with trapped water
in nanopores [17]. Sample 6 is an activated high-area carbon of coconut shell origin.
Purity of TiC-derived carbon powders was roughly confirmed by recording the X-ray diffraction spectra for the
solid products of carbide chlorination as earlier described
Table 1
Chlorination temperature and the yield of TiC-derived carbon materials
investigated
Carbon
Chlorination
temperature [°C]
Yield of carbon
from theoretical [%]
1
2
3
4
5
700
800
900
800
800
78
78
50
95
88
2. Experimental
2.1. Nanoporous carbon materials
The carbide-derived carbon was made from fine-grain
TiC in a stationary bed reactor. A detailed description of
making samples 1–3 is given below.
Titanium carbide (H.C. Starck, grade C.A., 5 g) with an
average particle size of 1.3-3 lm was thoroughly mixed
with 10 wt% of titanium oxide powder (Kemira, 1 lm,
0.5 g). This mixture was thereafter loaded into a horizontal
quartz bed reactor and reacted with a flow of chlorine
Fig. 1. Characteristic XRD patterns for carbon sample 2 and precursor
materials TiC and TiO2.
2124
J. Leis et al. / Carbon 44 (2006) 2122–2129
in Ref. [3]. The example of XRD spectrum of carbon 2
together with the spectra of precursor materials, TiC and
TiO2, is shown in Fig. 1. It shows a typical powder XRD
spectrum of the amorphous carbide-derived carbon characterized by broad d100 reflection and completely missing 0 0 l
peaks.
3. Adsorption measurements
The porous structures of carbon materials were characterised by using different nitrogen and benzene sorption
methods. Low-temperature nitrogen sorption experiments
were performed by using a Gemini Sorptometer 2375
(Micromeritics). The specific surface areas of carbon materials, SBET and SL, were calculated according to BET and
Langmuir theories, respectively, up to the nitrogen relative
pressure (P/P0) of 0.2. The total volume of pores, VP, was
calculated from nitrogen adsorption at a relative pressure
(P/P0) of 0.95. The volume of micropores, VMP, was calculated from the t-plot of the adsorption isotherm and the
volumes of pore size fractions, V(P<11Å) and V(P>11Å), were
derived from the data of BJH pore size distribution. The
size distribution of micropores was calculated from the
nitrogen sorption data measured by ASAP2000 (Micromeritics) by using the method based on the density functional theory (DFT).
Adsorption dynamics of benzene vapour was studied at
room temperature by using the computer-controlled weighing method of carbon samples in benzene vapour at normal
pressure and room temperature in time. The volume of
pores that adsorbed benzene under the above-described
conditions was calculated according to
W s ¼ ðm2 m1 Þ=m1 d C6 H6
½cm3 g1 ;
ð4Þ
where m1 and m2 are the initial and final weights of the testsample, respectively, and d C6 H6 is the density of benzene at
room temperature.
3.1. Evaluation of the EDLC parameters
The general method of making carbonaceous polarizable electrodes from carbon powders was the following.
A mixture of 92 wt% nanoporous carbon and 8 wt% polytetrafluoroethylene (PTFE, Aldrich, 60% suspension in
water) was thoroughly mixed by using a small amount of
ethanol as a mixing aid and after that gently pressed until
the formation of a flat cake. Thereupon ethanol was evaporated. The cake was then impregnated with heptane,
shaped into a cylinder and extruded by rolling the body
in the axial direction of the cylinder. This procedure was
repeated until the appearance of elastic properties. Finally,
heptane was removed at 75 °C and the extruded cake was
rolled stepwise down to the thickness of 90 ± 5 lm. After
drying in a vacuum at 170 °C, the raw electrode sheets were
plated on one side with a thin aluminium layer (3 ± 1 lm)
by using the plasma-activated physical vapour deposition
method.
The electric double-layer capacitors (SC 1–6) were
assembled from a pair of carbon electrode discs separated
with ion-permeable separator paper from Kodoshi Nippon. The geometric surface area of the electrode was
2.27 cm2. The test-cells were vacuumed at 90 °C for 24 h
prior to impregnation with the electrolyte. The electrolyte
used in this study was 1.2 M triethylmethylammonium
tetrafluoroborate (TEMA, Stella) in anhydrous acetonitrile
(AN, Riedel-de Haën, H2O < 0.003%). The TEMA salt was
selected because of the use of highly nanoporous carbon
materials which, according to former studies, prefer smaller
Et3MeN+ cations, but not Et4N+ ions commonly used in
non-aqueous supercapacitors [12,13,16,19,20].
Before evaluation, the electric double-layer capacitor
(EDLC) cells were preconditioned at +60 °C during 48 h
needed to saturate the dense nanoporous electrode body
by the electrolyte. Thereafter continuous galvanostatic
cycling between 2.5 V and 1.25 V with the current
I = 100 mA was carried out prior to performing further
electrochemical studies. The purpose of preliminary galvanostatic cycling is to distribute the electrolyte ions and to
decompose and inactivate the traces of H2O or other possible impurities. Usually 5 cycles were needed to establish
a stable double-layer performance of the electrochemical
system.
Measurements of EDLCs in constant current charge/
discharge regimes by using the voltage range from 1.25 V
to 2.5 V were performed in order to evaluate the capacitance and resistance characteristics of carbon materials
corresponding to the current varied from 10 mA to
500 mA. The discharge capacitance was calculated from
the data of the tenth cycle according to
C¼I
Dt
:
DU
ð5Þ
Measurements were repeated at various fixed temperatures
in the region from 10 °C to +60 °C to establish the influence of temperature on the stability of EDLC parameters.
The internal resistance values were calculated from the initial IR-drop value (at fixed Dt = 10 ms) according to
R¼
dU 1
:
2I
ð6Þ
4. Results and discussion
Carbon materials chosen for this study exhibit moderately high porosity. The specific surface area varies from
1400 m2 g1 to 1900 m2 g1. All carbons possess a nongraphitic disordered carbon network according to the XRD
analysis and, therefore, a different surface area must be
caused by a different pore size distribution in carbon particles. On the other hand, the larger are the pores, the lower
should be the apparent density of the respective carbon.
The findings of this study particularly agree with this
assumption as it appears from the comparison of the densities and pore volume fractions (V(P<11Å) and V(P>11Å)) in
J. Leis et al. / Carbon 44 (2006) 2122–2129
2125
Table 2
Pore structure characteristics of carbon materials as defined in the text, calculated from nitrogen and benzene sorption data
Carbon
d a [g cm3]
SBET [m2 g1]
SL [m2 g1]
VP [cm3 g1]
VMP [cm3 g1]
V(P<11Å)b [cm3 g1]
V(P>11Å)b [cm3 g1]
Ws [cm3 g1]
1
2
3
0.73
0.71
0.64
1453
1588
1841
1847
2019
2367
0.69
0.76
0.93
0.61
0.65
0.72
0.35
0.37
0.20
0.34
0.39
0.73
0.72
0.71
0.92
4
5
6
0.71
0.70
0.62
1396
1451
1898
1751
1822
2473
0.66
0.69
0.98
0.57
0.60
0.76
0.40
0.40
0.07
0.26
0.29
0.91
0.62
0.64
0.90
a
b
d is an apparent density of the carbon sheet including 8 wt% of PTFE (6 wt% for 4 and 5).
Calculated from BJH pore size distribution data.
Fig. 2. HRTEM images of the nanoporous carbon derived through the chlorination at 800 °C from TiC without the influence of TiO2 (a) and from the
mixture of TiC and TiO2 in 10:1 weight ratio (b). The pictures (a) and (b) correspond to samples 4 and 2 of this study.
network and nanopores smaller than 6–7 Å are hardly penetrated by electrolyte ions. An example of such carbon is
shown on the TEM picture in Fig. 2a. Contrariwise, commonly used high-surface activated carbon materials, as represented by carbon 6 in this study, possess a significantly
larger average pore size and consequently very good penetrability. The pore size distribution of different carbon
materials studied by means of DFT is compared in
Fig. 3. Nevertheless, the density of activated carbon (6) is
rather low and, consequently, the volumetric EDLC characteristics are not good as further discussed in this paper.
Studies on the partly post-oxidised TiC-derived carbon
materials (carbon 5 of this study) have shown that only a
Incremental pore area / m2g-1
Table 2. In the present work, the densities of compacted
carbon sheets were used for the comparison of the volumetric characteristics. As far as the particle size of different carbon samples is similar and the thickness values of carbon
sheets are equal, it is assumed that the compaction and
consequently the unoccupied space between carbon particles is equal, too. The statement is in accordance with the
almost linear relationship between the density of the carbon sheet and the total pore volume (VP) of carbon powder
according to N2 adsorption. Small deviation with respect
to carbons 4 and 5 is caused by a slightly smaller amount
(6 wt%) of the binder (PTFE) used in the carbon sheets
compared to the 8 wt% PTFE in the rest of the samples.
According to the sorption data in Table 2, the SBET
value is reciprocally proportional to the density of the carbon studied. From the point of view of practical EDLC
applications, a high surface area is needed to reach a high
double-layer capacitance, whereas low density undesirably
reduces the volumetric energy and power output of the
capacitor. Therefore, it is important to find the optimum
balance between these two parameters. The most reasonable way to do that is to find out the smallest possible pore
size that can still accommodate electrolyte ions. The excellent high content of nanopores and the narrow pore size
distribution is observed in the TiC-derived carbon materials made at temperatures below 900 °C. However, experimental studies have revealed that too dense a carbon
400
Carbon 2
Carbon 4
Carbon 5
Carbon 6
300
200
100
0
5
10
15
20
25
30
Pore width / Angstrom
Fig. 3. DFT pore size distribution of activated carbon (6) and carbon
powders from TiC (2, 4 and 5) chlorinated at 800 °C.
2126
J. Leis et al. / Carbon 44 (2006) 2122–2129
slight oxidative treatment of originally dense nanoporous
carbide-derived carbons is sufficient to reach a desired balance between a high double-layer capacitance and a reasonably high density of carbon electrodes. Further, the
adsorption analysis of the novel carbide-derived carbon
made from TiC/TiO2 mechanically mixed composites,
wherein the oxidation of CDC is executed simultaneously
during the chlorination of carbide, confirms a similar pore
size distribution to that of post-oxidized CDC material.
The dominating pore size is shifted from 7 Å to 8 Å
according to DFT (cf. samples 2 and 5 in Fig. 3). Therefore, it was suggested that the TiC/TiO2-derived material
could also give a more promising EDLC performance.
Improved mass transport behaviour of the novel material
compared to the conventional nanoporous TiC-derived
carbon is also expected according to comparative TEM
images in Fig. 2. Carbon from the TiC/TiO2 mixture is
noticeably coarser than the one from pure TiC.
To establish the suitability of TiC/TiO2-derived materials as supercapacitors and compare the performance of
these materials with other CDC materials, a comparative
study of EDLC characteristics was performed by using
an acetonitrile-based electrolyte. The cyclic voltammograms were recorded in the voltage range DU from 0 V to
2.5 V by using a potential sweep-rate 5 mV s1 < v <
50 mV s1 according to v = ±dU/dt. The results are presented in Figs. 4 and 5. It was established that EDLC cells
were ideally polarizable in the applied voltage range. The
experimental data revealed that for all capacitors the
0.08
50mV s-1
I/A
0.04
10mV s-1
5mV s-1
0
-0.04
-0.08
-3
-2
-2.5
-1.5
-1
0
-0.5
0.5
U/V
30
30
20
20
10
10
0
SC 1
SC 2
C / F cm-3
C / F cm-3
Fig. 4. Cyclic voltammograms of SC 5 at different potential scan rates,
noted in figure.
SC 3
-10
-20
SC 4
0
SC 5
SC 6
-10
-20
-30
-30
-3
(a)
C, U-curves were independent of the number of current
cycles, n, if n P 5.
It was observed that all capacitors studied have an excellent, nearly rectangular shape of C, U-curves, although SC
4 and SC 6 possess lower capacitance caused by a smaller
content of working nanopores in respective electrode materials. In particular, the carbon used in SC 4 contains a significant fraction of nanopores below 7 Å, which cannot be
accessed by the cations of the electrolyte [21,22]. Conversely, the pores of the carbon in SC 6 are bigger than
needed to absorb the electrolyte ions whose diameters
range from 5 Å to 7 Å [23]. Bigger pores lead to a smaller
volumetric surface and consequently to a lower volumetric
double-layer capacitance. Recent studies show that the
highest volumetric capacitance was achieved with the carbon materials having the pore size of 8–9 Å [19].
Further, the positive and negative electrodes in each cell
were of the same thickness. It is known that the capacitance
of positively and negatively charged nanoporous carbidederived carbon differs by 10%. This phenomenon makes
cells unbalanced and the achieved overall capacitance as
well as the EDLC energy are somewhat lower than for
the balanced electrode pairs. The tendency of nanoporous
carbon electrodes for such disbalance is discussed in Ref.
[19].
It was observed that C, U-curves of SC 6 are not symmetrical to the y-axis, which is interesting due to the fact
that SC 6 was the only cell where activated carbon other
than carbide-derived carbon was used. Usually, the voltammograms become distorted from the ideal rectangular
shape due to surface functional groups [10].
The increased capacitance at higher voltage is caused by
the reduced effective size of electrolyte ions. This effect was
recently observed and discussed for carbide-derived carbon
electrodes [13,16,19,20]. Some similarity in galvanostatic
cycling curves of this study is particularly influenced by a
similar apparent density of respective electrodes. It is
important that after the fifth cycle there was no charge–
discharge cycle degradation during galvanostatic cycling.
The cycling stability of CDC materials was previously
reported in [13]. It is obvious that the durability of the discharge cycle (Fig. 6) decreases with the decreasing of the
density of electrodes (see d values in Table 2). Therefore,
-2.5
-2
-1.5
U/V
-1
-0.5
0
(b)
-3
-2.5
-2
-1.5
-1
-0.5
0
U/V
Fig. 5. Cyclic voltammograms expressed as capacitance vs. voltage of SC 1–3 (a) and SC 4–6 (b) at potential scan rate 50 mV s1. Capacitance is
calculated per electrode pair volume.
J. Leis et al. / Carbon 44 (2006) 2122–2129
100
3
U/V
2
1.5
1
SC 6
0.5
SC 3
SC 1
SC 2
0
0
100
200
300
400
1
2
3
4
5
6
1-sec
0.1
Resistance
Capacitancea
[X cm2]
[F cm3]
[F g1]
0.57
0.53
0.49
0.65
0.58
0.51
87
89
74
84
90
50
120
126
115
118
129
81
a
Capacitance is calculated per weight and volume of a single electrode
at 10 mA (CC regime).
the shortest discharge cycle was observed for capacitor SC
6, which also possesses the lowest capacitance among all
EDLC cells studied.
The chronopotentiometric (voltage/time) curves under
the constant current charge/discharge regime have a nearly
linear shape for all the cells studied. At higher current values, the IR-drop is well expressed. The calculated gravimetric capacitance values at I = 10 mA, presented in Table 3,
are rather similar for most of the carbide-derived materials
and significantly exceed the activated carbon characteristics. The difference established is expectedly caused by a
smaller volumetric content of micropores and a lower
apparent density of the carbon in SC 6. It should also be
noted that the capacitance of nanoporous carbide-derived
carbon decreases almost proportionally with the increasing
of the current [13].
The specific resistance (X cm2) of carbon electrodes,
derived from the inner resistance of the EDLC, is influenced by the choice of materials (i.e., electrolyte, carbon,
separator, current outlet, etc.), contact resistance between
carbon particles and the resistance of carbon/metal interface, the length and design of electrodes and current collectors, etc. [10] Systematic analysis of experimental results
also demonstrates a significant impact of electrode thickness [20]. Nevertheless, all the cells tested in the present
work have the same design; almost the only variable is
the carbon in the electrodes. Therefore, the difference in
the resistance values of EDLC cells should mainly be
caused by different conductivity of carbon materials, that
SC 1
SC 2
SC 3
SC 4
SC 5
SC6
1
Power density / W
Fig. 6. Constant current charge/discharge profile of selected SC cells
(noted in figure) at current 10 mA.
Table 3
Specific resistance and capacitance of carbon electrodes measured at RT
10-sec
10
Time / Second
SC
SC
SC
SC
SC
SC
100-sec
Energy density/ J cm-3
2.5
Capacitor
2127
10
100
cm-3
Fig. 7. Ragone plots for SC cells (noted in figure) by discharging of cell
from 2.5 V to 1.25 V at RT. Energy and power densities are calculated per
volume of the electrode pair.
is a combination of solid phase resistance, liquid phase
resistance for the electrolyte in pores or cavities and the
resistance of solid/liquid interface as discussed in detail
elsewhere [24].
The energy and power relationship of the capacitors was
derived from the constant current curves recorded at room
temperature in the voltage interval (DU) from 2.5 V to
1.25 V. The energy density (E) at constant current (I) was
calculated according to
Z t2
I
E¼
U dt;
ð7Þ
V t1
where t is the discharge time and V is the volume of the pair
of electrodes. The power density (P) corresponding to each
current value was calculated according to
P¼
E
:
Dt
ð8Þ
The results established are summarised as the Ragone plot
in Fig. 7. The time frames 100 s, 10 s and 1 s noted in the
figure indicate the application times of capacitors.
The volumetric energy densities at moderately low
power density are rather similar for all CDC-based capacitors tested. Capacitor SC 6 with conventional activated
carbon electrodes possesses the lowest energy density due
to a relatively low apparent density of carbon, caused particularly by larger pores. For the same reason, the SC 3
characteristics are slightly different from the other CDCbased capacitor parameters. In particular, the carbon in
SC 3 has a higher specific surface area because it was deeper oxidised during chlorination of the carbide as the effect
of higher temperature. Above 800 °C the Boudouard’s
equilibrium between C, CO and CO2 favours the formation
of CO instead of CO2 (cf. Eq. (3)) [25,26]. Significant
energy decrease at higher power values was observed for
SC 4, which is a clear indication of the restricted mobility
of ions in nanopores. Comparison of data for capacitors
SC 4 and SC 5 confirms a significant improvement of the
power performance, when the nanoporous carbide-derived
carbon is additionally oxidized in nanopores prior to
2128
J. Leis et al. / Carbon 44 (2006) 2122–2129
15
10
5
-20
(a)
10-Second power density / W cm-3
2-Second power density / W cm-3
5
0
SC 1
SC 2
SC 3
SC 4
SC 5
SC 6
20
40
3
2
1
-20
60
Temperature / ºC
4
SC 1
SC 2
SC 3
SC 4
SC 5
SC 6
0
(b)
20
40
60
Temperature / ºC
Fig. 8. Generated power density, temperature plots for the EDLC cells obtained at 2 s (a) and 10 s (b) discharge time.
making the electrodes. These observations are also in good
accordance with capacitance values and charge/discharge
profiles established in the constant current regime.
All the capacitors based on carbon prepared from the
TiC/TiO2 mixture, have moderately high energy density
and power performance upon a short application time
(i.e., below 10 s).
The power characteristics of SC cells at different application times and the temperature-dependence of power density are presented in Fig. 8. The conventional carbon
capacitor has the lowest volumetric power in almost the
whole scale of applications. The capacitors based on the
carbide-derived carbon show similar power characteristics
in applications exceeding the time limit of 10 s. The shorter
is the application time, the clearer is the advantage of TiC/
TiO2-based carbon materials.
According to Fig. 8 it appears that highly nanoporous
CDC materials are characterised by a noticeable temperature dependence of power densities. Contrariwise, the
power performance of activated carbon (SC 6), possessing
the lowest apparent density, is almost independent of temperature. Upon shorter applications the mobility of electrolyte ions becomes more important and therefore the
slope of power density vs. the temperature (P–T) plot
increases. During the 2-s discharge, the powers of SC 1,
SC 2 and SC 3 using TiO2-assisted carbon are rather similar and clearly exceed those of the rest of carbon materials of this study. This is directly related to an outstanding
energy density of respective carbon materials. The ‘‘10-s
power’’ in Fig. 8b is a characteristic that reflects the practical demands for supercapacitors in automotive applications. The highest ‘‘10-s power’’ at all application
temperatures tested is also observed for the supercapacitors using TiO2-assisted carbon. This confirms a good balance between the size of ion-adsorbing nanopores and the
apparent density of the carbon, thereby considering the
fact that the thickness of the electrodes and carbon particle size of the electrodes is the same in all capacitors
tested.
5. Conclusions
Double-layer capacitance of the carbon electrode material is proportional to its surface area. The smaller are the
pores (cavities) in carbon, the larger could be the quantity
of the pores and consequently the surface area in the volume of the electrode material. It is known that the carbon,
derived from certain mineral carbides, may possess very
high nanoporosity whereby characterised by pore size distribution as narrow as usually seen in ceramic molecular
sieves. However, the problem is that the carbide-derived
carbon tends to be too dense, which therefore creates
restricted access to the nanopore surface, high pore resistance and consequently relatively low power characteristics. According to our study, the pore size distribution of
the nanoporous carbide-derived carbon is significantly
improved if the sample is slightly oxidised in situ during
the etching of the carbide/oxide mixture with chlorine
gas. Electrochemical and electrical double-layer characteristics of these novel carbon materials studied in 1.2 M
triethylmethylammonium tetrafluoroborate (TEMA) acetonitrile solution affirm their greatly improved power
characteristics over a wide temperature range. Specific
double-layer capacitance of the advanced carbide-derived
carbon reaches 90 F cm3 and 125 F g1 in 1.2 M
TEMA/AN electrolyte.
Acknowledgements
Colleagues from Tartu Tehnoloogiad are gratefully
acknowledged for the assistance in preparation of the manuscript. Dr. Gunnar Svensson and Ms. Sigita Urbonaite,
Stockholm University, are thanked for the HRTEM study
of carbon materials.
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