Morphological Effects on the Electrical and Electrochemical Properties of Carbon Aerogels

Article abstract of DOI:10.1149/1.1368104

Carbon aerogels are prepared by a sol-gel process. By controlling the mass ratio of reactants and the molar ratio of resorcinol to catalyst (R/C), carbon aerogels with different microstructures can be developed. The electrical conductivity is measured by the van der Pauw method. The results show that the electrical conductivity increases with increase of both temperature and density of carbon aerogel. Cyclic voltammetry is a useful technique to investigate the electrochemical properties of carbon aerogel electrodes. The maximum specific capacitance of carbon aerogel electrode in H₂SO₄ electrolyte is -86 F/g.

Full text of DOI:10.1149/1.1368104

Journal of The Electrochemical Society, 148 ͑6͒ D75-D77 ͑2001͒
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0013-4651/2001/148͑6͒/D75/3/$7.00 © The Electrochemical Society, Inc.
Morphological Effects on the Electrical and Electrochemical
Properties of Carbon Aerogels
J. Wang,a S. Q. Zhang,a Y. Z. Guo,a J. Shen,a S. M. Attia,a B. Zhou,a G. Z. Zheng,b
and Y. S. Guib
aPohl Institute of Solid State Physics, Tongji University, Shanghai 200092, China
bShanghai Institute of Technical Physics, Shanghai 200092, China
Carbon aerogels are prepared by a sol-gel process. By controlling the mass ratio of reactants and the molar ratio of resorcinol to
catalyst ͑R/C͒, carbon aerogels with different microstructures can be developed. The electrical conductivity is measured by the van
der Pauw method. The results show that the electrical conductivity increases with increase of both temperature and density of
carbon aerogel. Cyclic voltammetry is a useful technique to investigate the electrochemical properties of carbon aerogel elec-
trodes. The maximum specific capacitance of carbon aerogel electrode in H2SO4 electrolyte is ϳ86 F/g.
© 2001 The Electrochemical Society. ͓DOI: 10.1149/1.1368104͔ All rights reserved.
Manuscript submitted July 26, 1999; revised manuscript received November 30, 2000.
Carbon aerogel, derived from resorcinol-formaldehyde ͑RF͒
aerogel, can be prepared by the sol-gel process and supercritical
drying. The molar ratio of resorcinol to catalyst ͑R/C͒ controls the
particle size, while the mass ratio of reactants determines the density
of the material.1 Because of its monolithic structure, high specific
surface area ͑400-1100 m2/g͒, and high electrical conductivity, car-
bon aerogel can be used as an electrode for electrochemical double-
layer capacitors ͑EDLCs͒.2 The basic principle of EDLCs is that
energy can be stored via separation of charges across a polarized
electrode/electrolyte interface. The stability of charge of the
electrode/electrolyte interface will influence the characteristics of
EDLCs, such as self-discharge property. It is required that elec-
trodes should be good conducting materials, and that there is no
participation in faradaic reactions with the electrolyte at the applied
voltage.
Results and Discussion
Electrical conductivity.—The temperature dependence of the
electrical conductivity ͑␴͒ for all prepared samples is shown in Fig.
1. Figure 1a shows the behavior of the electrical conductivity for
samples having R/C ratio of 200 and mass ratios of reactants 30, 40,
and 50%, while Fig. 1b shows the behavior of the electrical conduc-
tivity for samples having the mass ratio of reactants 30% but differ-
ent R/C ratios of 50, 100, and 200. It is shown that the electrical
conductivity increases with increase of both temperature and density
of the samples.
Figure 2 shows the scanning electron micrographs ͑SEMs͒ of
carbon aerogels. The microstructure of carbon aerogels consists pri-
marily of interconnected carbon grains. Raman scattering and X-ray
diffraction ͑XRD͒ experiments4 showed that there are graphitic car-
bon ribbons present within the single particles of carbon aerogels.
These ribbons are cross-linked with each other within the particles,
so the particle can be viewed as conducting, while the structural
links between neighboring grains are not necessarily electrically
conducting.
In this paper, carbon aerogels are developed under different con-
ditions. The relation between the electrical conductivity and the mi-
crostructure of carbon aerogels is investigated in the temperature
range from 20 to 300 K, while the electrochemical properties of
carbon aerogel are investigated by cyclic voltammetry.
By changing the mass ratio of reactants during the sol-gel pro-
cess, carbon aerogels with various densities can be developed. With
increasing mass density, the particle size remains constant, but the
packing ratio of these particles increases, and the space due to the
mesopores that span the distance between chains of interconnected
particles decreases. Therefore, the more closely packed the conduct-
ing grains, the higher the electrical conductivity of carbon aerogels.
As the mass ratio of reactants determines the packing ratio, the
R/C ratio determines grains size. Under high catalyst concentration
͑e.g., R/C ϭ 50͒, many clusters are generated during the sol-gel
process but they cannot grow very large in diameter. The resultant
carbon aerogel has a polymeric microstructure, in which no distinct
single grains present, if some, with sizes 7-9 nm. Under low catalyst
concentration (R/C ϭ 200, 300), fewer clusters are generated and
they can grow larger in diameter than their high catalyst counter-
parts. The resultant carbon aerogel has a colloidal microstructure, in
which some distinct grains are present, with sizes 11-14 nm. There-
fore, it is expected that more nonconducting regions ͑defects͒ are
present in polymeric carbon aerogels, leading to lower electrical
conductivity at a given density as compared to colloidal carbon
aerogels.
Experimental
Two sets of carbon aerogel samples were prepared; the first set
had the same R/C ratio of 200 with different mass ratios, 30, 40, and
50%, respectively; while the second set had the same mass ratio of
30% with different R/C ratios of 50, 100, and 200, respectively. The
preparation of RF aerogels and carbon aerogels have been described
in Ref. 3 in detail. Briefly, resorcinol and formaldehyde were mixed
in a 1:2 molar ratio. Deionized water was used as a solvent to
control the final gel concentration. Sodium carbonate was added as a
base catalyst. Each mixture was poured into individual glass con-
tainers and kept at ϳ85 Ϯ 5°C. Next, the cross-linked gels were
exchanged with acetone and subsequently dried supercritically using
liquid carbon dioxide (Tc ϭ 31°C, Pc ϭ 7.4 MPa͒. RF aerogels
were pyrolyzed in a flow of N2 for 4 h at 1050°C in a tube furnace.
Carbon aerogel was cut into 1.0 mm thick wafers. The electrical
conductivity was measured using van der Pauw method, and four
contacts were made. The measurement was carried out in the tem-
perature range 20-300 K.
From Fig. 1b, we cannot confirm the role played by the particle
size on the electrical conductivity of carbon aerogels. The R/C
ϭ 50 carbon aerogel is more microscopically disordered ͑more de-
fects͒ than R/C ϭ 200, 100 samples, which depresses the increment
of the electrical conductivity of R/C ϭ 50 carbon aerogels. This is
effecting the favorable effect due to the high packing ratios of grains
for R/C ϭ 50 sample which can counteract the negative effect of
defects.
Cyclic voltammetry was used for the investigation of the electro-
chemical properties of carbon aerogel electrode/H2SO4 electrolyte
system. A Pb wafer was used as the counter electrode, and a satu-
rated calomel electrode ͑SCE͒ was used as a reference electrode.
The voltage sweep of the cyclic voltammetry was from Ϫ600 to 600
mV. The measurement was carried out by using a potentiostat
͑ZF-3͒ and a voltage scanner ͑ZF-4͒, and the sweep rates were 1.67,
5, and 10 mv/s, respectively.
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Journal of The Electrochemical Society, 148 ͑6͒ D75-D77 ͑2001͒
Figure 3. Cyclic voltammetry of carbon aerogel electrode/H2SO4
electrolyte.
loops. It is shown that, for the first loop, where the sweep rate is
1.67 mV/S, the current is constant for a wider range ͑from Ϫ200 to
ϩ400 mV͒ than those currents for other loops with higher sweep
rates. We have to mention that in order to get any one of the loops,
we had to repeat the measurements at the same condition 20 times to
get stable results. The relation between the current ͑at 100 mV vs.
SCE, the midpoint of the current plateau͒ and the sweep rate is
linear as shown in Fig. 4. As the sweep rate increases, the electric
current increases. The specific capacitance of carbon aerogel elec-
trode in H2SO4 electrolyte is calculated using the relation5
C ϭ I/ V M͒
͑
s
s
Here I is the current ͑mA͒ at 100 mV vs. SCE, Vs is the sweep
rate ͑mV/S͒, and M is the weight of the electrode ͑g͒. The maximum
value of Cs is found to be ϳ86 F/g for the carbon aerogels with the
R/C ratio of 200 and the mass ratio of 40%. Previous work has
shown that the capacitance density appears to peak or plateau near
aerogel density of ϳ0.8 and ϳ1.0 g/cm3 for the organic electrolyte
and 5 M KOH. The specific capacitance values of carbon aerogels in
5 M KOH is 45 F/g, while in 1 M H2SO4 is 95 F/g.6
Figure 1. Effect of temperature on electrical conductivity of carbon
aerogels.
It is found that every cyclic loop begins to overlap at a constant
sweep rate after the measurement is repeated 20 times. When the
voltage signal begins to scan for the first time ͑e.g., from 0 to 600
mV͒, ions can occupy some pores within the electrode to participate
the formation of the electrochemical double-layer. While the voltage
signal begins to scan reversely ͑e.g., from 600 mV to 0͒, the voltage
imposed on the electrode is decreased gradually, and the ions col-
lected on the electrode begin to leave the electrode, but some ions
Cyclic voltammetry.—The electrochemical behavior of carbon
aerogels is characterized by cyclic voltammetry ͑CV͒ shown in Fig.
3. The measurement is carried out over potential ranges from Ϫ600
to 600 mV vs. saturated calomel electrode ͑SCE͒. The measure-
ments are carried out with different sweep rates and we get three
Figure 2. SEM of the carbon aerogel.
Figure 4. The relation between the current and sweep rate.
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Journal of The Electrochemical Society, 148 ͑6͒ D75-D77 ͑2001͒
plunged into the pores cannot come out immediately, resulting in the Acknowledgment
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next loop not overlapping with the former one. With increasing scan
cycles, some pores become larger because of the effect of the
‘‘plunging’’ and ‘‘squeezing’’ of the ions, so such pores become
unblocked for ions, which results in the overlapping of the later
loops at a constant sweep rate. To increase the capacitance, some
international study groups usually take the method of activating the
electrode.7,8 The purpose is to improve pore structure, which is use-
ful to the formation of the electrochemical double layer.
This work was performed under the auspices of the Trans-
century Talents Program by Chinese Educational Department for
youths who have great achievement in scientific fields.
The Pohl Institute of Solid State Physics assisted in meeting the publica-
tion costs of this article.
References
1. R. W. Pekala and F. M. Kong, J. Phys. Colloq., C4, 33 ͑1989͒.
2. J. C. Farmer, D. Fix, G. V. Mack, R. W. Pekala, and J. F. Poco, J. Electrochem.
Soc., 143, 159 ͑1996͒.
3. S. Q. Zhang, J. Wang, J. Shen, B. Zhou, Z. Q. Lai, and L. Y. Chen, in Symposia
Proceedings of C-MRS, 105 ͑1997͒.
4. Y. Maeda, Y. Okemoto, and M. Inagaki, J. Electrochem. Soc., 132, 2363 ͑1985͒.
5. I. Tanahashi, A. Yoshida, and A. Nishino, Bull. Chem. Soc. Jpn., 63, 3611 ͑1990͒.
6. R. W. Pekala, J. C. Farmer, C. T. Alviso, T. D. Tran, S. T. Mayer, J. M. Miller, and
B. Dunn, J. Non-Cryst. Solids, 225, 74-80 ͑1998͒.
Conclusion
The electrical conductivity of carbon aerogel increases with in-
creasing both the temperature and density of the sample. Cyclic
voltammetry is a useful technique for investigating the electro-
chemical properties of carbon aerogel electrodes. The maximum
special capacitance of carbon aerogel electrode in H2SO4 electrolyte
is ϳ86 F/g.
7. C. Jones and E. Sammann, Carbon, 28, 509 ͑1990͒.
8. R. Saliger, U. Fischer, C. Herta, and J. Fricke, J. Non-Cryst. Solids, 225, 81 ͑1998͒.
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