High capacitance carbon-based xerogel film produced without critical drying

Article abstract of DOI:10.1063/1.2976684

We report the production of carbon-based xerogel film without the need for supercritical drying. Xerogel samples were characterized with field emission scanning electron microscopy, nitrogen adsorption/desorption at 77 K, Raman spectroscopy, thermogravime

Full text of DOI:10.1063/1.2976684

High capacitance carbon-based xerogel film produced without critical drying
Yousheng Tao, Morinobu Endo, Risa Ohsawa, Hirofumi Kanoh, and Katsumi Kaneko
Citation: Applied Physics Letters 93, 193112 (2008); doi: 10.1063/1.2976684
View online: http://dx.doi.org/10.1063/1.2976684
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APPLIED PHYSICS LETTERS 93, 193112 ͑2008͒
High capacitance carbon-based xerogel film produced without
critical drying
Yousheng Tao,1,a͒ Morinobu Endo,1 Risa Ohsawa,2 Hirofumi Kanoh,2 and
Katsumi Kaneko2,b͒
1Institute of Carbon Science and Technology, Shinshu University, Nagano 380-8553, Japan
2Department of Chemistry, Graduate School of Science, Chiba University, Chiba 263-8522, Japan
͑Received 8 May 2008; accepted 10 August 2008; published online 13 November 2008͒
We report the production of carbon-based xerogel film without the need for supercritical drying.
Xerogel samples were characterized with field emission scanning electron microscopy, nitrogen
adsorption/desorption at 77 K, Raman spectroscopy, thermogravimetric analysis, and electrical
conductivity and cyclic voltammetry measurements. Experimental results reveal that the film is
largely crack free and homogeneous in thickness, and, importantly, has high surface area, large
nanopore volume, and an excellent performance for electrical charge storage—both per unit mass
and unit volume. These results indicate that the film has potential applications for electrical energy
storage devices. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2976684͔
Nanoporous carbon films have been of great interest for
many years because of their potential use in separation and
purification, catalysis, and chromatographic separation and
for such applications as electric double-layer capacitors, bio-
catalytic sensors, and microscopes.1–7 Although numerous
methods such as chemical vapor deposition, hydrothermal
decomposition of carbide compounds, and polymer coating
and pyrolysis have been developed for the fabrication of car-
bon film,8–10 easily scalable self-supported nanoporous car-
bon films have not yet been obtained with any of them. The
following promising methods for fabricating the pore struc-
ture of carbon film have recently been reported: colloidal
silica imprinting, microbead patterning, and presynthesized
mesoporous silica scaffolding.11–14 However, these methods
are tedious and prone to surfactant cost, incomplete infiltra-
tion of carbon precursors, formation of nonporous carbon on
the external surface of the mesoporous powders, massive
loss of carbon in the form of volatile carbon-containing spe-
cies during pyrolysis, and difficulty in controlling the mac-
roscopic morphology.15,16 Moreover, these methods are ex-
tremely difficult to apply to the fabrication of large-scale
uniform nanoporous films.15 Although efforts have been
made recently to synthesize nanoporous carbon by direct
pathways of nanocasting17 and to prepare ordered porous
and for use in supercapacitors, gas diffusion electrodes in
proton exchange membrane fuel cells, and anodes in re-
chargeable lithium ion batteries.21–23 Unfortunately, the su-
percritical drying required to prevent collapse of the highly
nanoporous aerogel is expensive although supercritical pro-
cesses can be used on a large commercial scale. In this letter
we report a supercritical drying-free method for preparing
uniformly thin carbon xerogel film ͑CXF͒. The prepared
CXF exhibited excellent electrical charge storage.
The CXF was synthesized using the slit-space film
model and resorcinol ͓C6H4͑OH͔͒ ͑min. 99.0%͒, formalde-
2
hyde solution ͑HCHO͒ ͑36.0%–38.0%͒, sodium carbonate
͑Na2CO3͒ ͑min. 99.5%͒, and ion-exchanged water as
sources, of which the molar ratio was 1:2:0.025:0.4. All
agents were used as received from Wako Pure Chemical In-
dustries, Ltd. without further purification. The polyconden-
sation and polymerization of resorcinol with formaldehyde
was performed for 1 day at room temperature, afterward for
1 day at 323 K, and successively for 3 days at 363 K. After
the reaction cell was opened, the resorcinol-formaldehyde
polymer xerogel film was dried in air at 298 K and 101.3
ϫ103 Pa. The dried film was pyrolyzed at 1323 K in a ni-
trogen atmosphere. A shiny black and slightly flexible CXF
was obtained. Field emission scanning electron micrographs
͑FE-SEMs͒ of the samples were obtained using a scanning
electron microscope ͑JEOL, JSM-633F͒ operated at an accel-
erating voltage of 5.0 kV. The nitrogen adsorption/desorption
isotherms were measured at 77 K using a gas adsorption
analyzer ͑Quantachrome, Autosorb-1͒. The samples were
evacuated at 10−4 Pa and 383 K for 2 h prior to the adsorp-
tion measurement. The Raman spectra were obtained with a
laser Raman spectrometer ͑JASCO, NRS-1000͒ controlled
by computer. Sample excitation was done with the 532 nm
line of a Spectra-Physics LD laser. The scattered light was
detected with a LN/CCD-576E GaAs photomultiplier tube.
Thermogravimetric analysis ͑TGA͒ of the samples was per-
formed using a thermal analyzer ͑Seiko Instruments, TG/
DTA 6200͒ up to 1273 K at a heating rate of 1 K min−1 in a
mixed flow of oxygen and nitrogen at 200 ml min−1. The
cyclic voltammetry measurement was performed within the
potential range from 0 to 0.8 V using a 2.0M H2SO4 aqueous
carbon film through self-assembly of block copolymers,15
a
scalable method for preparing nanoporous carbon films has
not yet been established.
Since the pioneering demonstration of Kistler18 of the
pore continuity of aerogels, much attention has been directed
at the sol-gel process followed by supercritical drying for
fabricating porous materials.18 Pekala and co-workers19,20
succeeded in preparing carbon aerogel, which is carbonized
resorcinol-formaldehyde aerogel pyrolyzed in an inert atmo-
sphere such as nitrogen. Due to their large surface area and
easily tunable nanoporosities, such materials have received
considerable attention in the study of the fundamentals of
materials science and for commercial application to hydro-
gen fuel storage, catalysis, and chromatographic separation
a͒
Electronic mail: tao@endomoribu.shinshu-u.ac.jp.
Electronic mail: kaneko@pchem2.s.chiba-u.ac.jp.
b͒
0003-6951/2008/93͑19͒/193112/3/$23.00
93, 193112-1
© 2008 American Institute of Physics
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202.28.191.34 On: Fri, 19 Dec 2014 16:05:21

193112-2
Tao et al.
Appl. Phys. Lett. 93, 193112 ͑2008͒
FIG. 3. Cyclic voltammetry graphs of CXF in 2.0M H2SO4 solution at scan
FIG. 1. High- and low-magnification ͑inset͒ FE-SEM of CXF.
rate of 10 mV s−1
.
solution as electrolyte and Ag/AgCl as a reference electrode.
The potential scan rate was fixed at 10 mV s−1. The direct-
current volume electrical conductivity was measured using a
potentiostat/galvanostat ͑Hokuto Denko, HA-501G͒ at room
temperature.
The high- and low-magnification FE-SEM images of the
sample in Fig. 1 show that the sample had relatively uniform
pore sizes with a highly disordered pore structure. Mesopo-
res ϳ10 nm in width are evident. The CXF was homoge-
neous with a thickness of several micrometers. It can be
fabricated in sizes up to several square centimeters without
cracks, as shown by the low-magnification image.
Low-temperature nitrogen adsorption measurement is a
common means for evaluating the pore structure of a solid.
The adsorption and desorption isotherms of nitrogen on the
CXF at 77 K, plotted in Fig. 2, rose steeply below P/P0
=0.02, and there was a hysteresis loop extending from
P/P0=0.70 to 0.90, suggesting the coexistence of mi-
cropores and mesopores in the film. There were type IV iso-
therms and a clear H1 hysteresis loop, suggesting that the
mesopores were quite uniform. The micropore and mesopore
size distributions ͑Fig. 2, inset͒, respectively, determined by
the Horváth and Kawazoe method and the Barrett–Joyner–
Halenda method, were narrow.. The average pore sizes were
0.6 and 9 nm, respectively. This mesopore size is consistent
with previously estimated ones from FE-SEM observations.
Subtracting pore effect analysis24 of the N2 adsorption iso-
therm showed that the surface area was 915 m2 g−1, the mi-
cropore volume was 0.37 cm3 g−1, and the mesopore volume
was 0.73 cm3 g−1. This high porosity and the bimodal pore
size distribution are distinct advantages for carbon film in
terms of capacitance storage because the nanopores ͑we refer
nanopores to micropores and mesopores for convenience25͒
ensure high-capacity storage, and the interconnected meso-
pore channels provide fast mass transfer.
The electrochemical behavior of the CXF was character-
ized by means of the electrical conductivity and cyclic vol-
tammetry measurements. The CXF had a direct-current vol-
ume electrical conductivity of 3.5ϫ103 S m−1 at room
temperature. The cyclic voltammogram of CXF in 2M
H2SO4͑aq͒ electrolyte solution at a scan rate of 10 mV s−1
was very close to an ideal rectangular shape ͑Fig. 3͒, sug-
gesting that energy is retrievable in the discharge over the
same potential range as that required to store the energy on
charging. The CXF showed a rapid current response on volt-
age reversal at each end potential. Its appearance density was
0.91 g cm−3, much higher than that of ordinary activated
carbons. It is noteworthy that the CXF had both a high mass-
specific capacitance and a volume-specific capacitance—
both of which are required for practical application. Carbon
xerogels can be activated with gases such as air, steam, and
CO2 to greatly increase the surface area and to modify their
surface.23,24,26 This means that the capacitance characteristics
of CXF can be further improved, which is a future research
topic.
The CXF was further characterized by Raman spectros-
copy and TGA to investigate its chemical structure. The Ra-
man spectrum ͓Fig. 4͑a͔͒ had two distinct peaks, at 1350 and
1580 cm−1 shifts, which were, respectively, assigned to dis-
ordered and ordered carbon phases. From the Raman spec-
trum, the in-plane microcrystallite size was determined using
a method reported elsewhere19 to be 2.4 nm. The TG/
derivative thermalgravimetry ͑DTG͒ data showed that the
CXF decomposed at 773 K in a mixed flow of oxygen and
nitrogen ͓Fig. 4͑b͔͒. The Raman and TG analysis revealed
that the CXF had a chemical structure remarkably similar to
that of carbon aerogels synthesized using the standard CO2
supercritical drying process.19,20,26 In short, the film was
chemically, thermally, and mechanically stable.
FIG. 2. Nitrogen adsorption ͑k͒ / desorption ͑b͒ isotherms at 77 K on
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FIG. 4. ͑a͒ Raman spectrum of CXF, and ͑b͒ TG ͑solid line͒, and DTG
CXF. Insets show the micropore and mesopore size distributions.
͑dotted line͒ curves of CXF in a mixed flow of oxygen and nitrogen.

193112-3
Tao et al.
Appl. Phys. Lett. 93, 193112 ͑2008͒
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based xerogel film with bimodal micropores and mesopores
can be directly fabricated under ambient conditions using a
thin reaction space without supercritical drying. The carbon
film has a large surface area and well-developed nanoporos-
ity and pore interconnectivity, making it well suited for high-
capacitance storage. Its simple and economic preparation
should lead to the use of conductive CXF in many electro-
chemical applications such as gas sensors, biological sensors,
and supercapacitors.
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