全 文 ：[Article] www.whxb.pku.edu.cn
物理化学学报(Wuli Huaxue Xuebao)
Acta Phys. ⁃Chim. Sin. 2011, 27 (12), 2836-2840 December
Received: July 18, 2011; Revised: September 26, 2011; Published on Web: October 13, 2011.
ⒸCorresponding authors. GUO Pei-Zhi, Email: pzguo@qdu.edu.cn; Tel: +86-532-83780378. ZHAO Xiu-Song, Email:chezxs@qdu.edu.cn.
The project was supported by the National Natural Science Foundation of China (20803037, 21143006), Foundation of Qingdao Municipal Science
and Technology Commission, China (11-2-4-2-(8)-jch) and“Taishan Scholar”Program of Shandong Province, China.
国家自然科学基金(20803037, 21143006),青岛市应用基础研究项目(11-2-4-2-(8)-jch)和“泰山学者”计划资助
Ⓒ Editorial office of Acta Physico⁃Chimica Sinica
花生壳制备微孔炭及其在电化学超级电容器中的应用
郭培志 1,* 季倩倩 1 张丽莉 2 赵善玉 2 赵修松 1,2,*
(1青岛大学化学化工与环境学院,纤维新材料与现代纺织国家重点实验室培育基地,山东青岛 266071; 2Department of
Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576)
摘要: 以未使用和使用氢氧化钠溶液处理的花生壳为碳源分别制备出微孔炭PSC-1和PSC-2. PSC-1和
PSC-2的比表面积分别为552和 726 m2·g-1,其主要孔径都约为0.8 nm.用PSC-1和PSC-2制备的电极和对
称型超级电容器的循环伏安曲线均接近矩形,表明其具有良好的电容特性.在以微孔炭电极为工作电极、铂电
极为对电极和银/氯化银电极为参比电极组成的三电极体系测量表明,在 0.1 A·g-1的电流密度下, PSC-1和
PSC-2的比电容达到233和378 F·g-1.经过1000次恒电流充放电循环后,在三电极体系和超级电容器中电极
均表现出良好的稳定性和电容保持率.基于实验结果探讨了微孔炭的形成机理及其结构与电化学性质之间的
联系.
关键词: 超级电容器; 电极; 微孔炭; 花生壳; 电容
中图分类号: O646
Preparation and Characterization of Peanut Shell-Based Microporous
Carbons as Electrode Materials for Supercapacitors
GUO Pei-Zhi1,* JI Qian-Qian1 ZHANG Li-Li2 ZHAO Shan-Yu2 ZHAO Xiu-Song1,2,*
(1Laboratory of New Fiber Materials and Modern Textile, the Growing Base for State Key Laboratory, School of Chemistry,
Chemical Engineering and Environmental Sciences, Qingdao University, Qingdao 266071, Shandong Province,
P. R. China; 2Department of Chemical and Biomolecular Engineering, National University of Singapore,
4 Engineering Drive 4, Singapore 117576)
Abstract: Microporous carbons (PSC-1 and PSC-2) were obtained directly by the carbonization of
peanut shells without and with NaOH solution pretreatment, respectively. Both samples have a main pore
size of ~0.8 nm. The surface area increases from 552 m2·g-1 for PSC-1 to 726 m2·g-1 for PSC-2. Cyclic
voltammograms (CVs) of the PSC-1 and PSC-2 electrodes and the symmetrical supercapacitors show
almost rectangular shape indicating excellent capacitance features. The specific capacitances of PSC-1
and PSC-2 can reach 233 and 378 F·g-1, respectively, at a current density of 0.1 A·g-1 in a three-electrode
system using porous carbon as the working electrode, a platinum electrode as the counter electrode and a
Ag/AgCl electrode as the reference electrode. Furthermore, the electrodes in both three-electrode systems
and supercapacitors show high stability and capacitance retainability after 1000 cycles. The formation
mechanisms for the two microporous carbons and the relationship between the carbon materials and their
electrochemical properties are discussed based on the experimental results.
Key Words: Supercapacitor; Electrode; Microporous carbon; Peanut shell; Capacitance
doi: 10.3866/PKU.WHXB20112836
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GUO Pei-Zhi et al.: Preparation and Characterization of Peanut Shell-Based Microporous CarbonsNo.12
1 Introduction
Electrochemical double-layer capacitors (EDLCs) are prom-
ising power sources because the demand for energy storage de-
vices is increasing as a result of the fast-growing market for
portable electronic devices and hybrid electric vehicles.1-3 The
capacitance of EDLCs comes from charge accumulation at the
electrode/electrolyte interface, and is therefore strongly depen-
dent on the pore size and the surface area of the electrode ac-
cessible to the electrolyte.4-6 Among the various electrode mate-
rials for EDLCs, porous carbons are the most popular because
of their high surface area, low cost, good electrical conductivi-
ty, and excellent chemical stability.6-9
There are several approaches to prepare porous carbon. One
is the direct high-temperature carbonization of appropriate pre-
cursors.10,11 The other uses a template to prepare composites of
the template and carbon precursor, and the mixed composites
are then carbonized.6,7 Many researchers also use alkali/salt ac-
tivation methods consisting of treating a mixture of the precur-
sors and an alkali/salt at high temperature.12,13 For example,
Béguin et al.10 reported that porous carbon derived directly by
carbonization of sodium alginate had a capacitance as high as
200 F·g-1. Zhang et al.12 reported that oxygen-rich activated
carbons prepared from bituminous coal by a high-temperature
activation method had a specific capacitance of 370 F∙g-1. Re-
cently, we have shown that the carbonization temperature has a
significant effect on the electrochemical properties of chitosan-
based porous carbons.11
Peanuts are widely planted in China, and the total amount
can reach 14 million tons per year. Some of the peanut shells
are used as animal feed and for cultivation of edible fungi, but
a large proportion of the shells are unused. It is necessary to ex-
plore possible applications of peanut shells as well as other
waste biomass.14-21 Recently, peanut shells have been used to
fabricate porous carbons, which show many potential applica-
tions,14-16 for example as sorbents for metal ions14 and as Li-ion
battery electrode materials.16 The activation method is usually
use to fabricated activated carbons from peanut shell based on
the solid mixture of the precursors and KOH or ZnCl2,10-13 how-
ever, peanut shells are pretreated using aqueous NaOH solu-
tion before carbonization in our work. It is found that the pre-
treatment process has a strong effect on the physical and elec-
trochemical properties of the peanut-shell-based microporous
carbons. The electrochemical properties of the samples are
characterized by cyclic voltammetry (CV), galvanic charge-dis-
charge, and cycling experiments.
2 Experimental
2.1 Materials
Peanut shells were obtained from peanuts purchased at Qing-
dao market. NaOH and KOH (AR grade) were purchased from
the Sinopharm Chemical Reagent Company. Acetylene carbon
black (99.99%) and polytetrafluoroethylene (PTFE, with mass
fraction of 60%) latex were purchased from Strem Chemicals
and Aldrich, respectively. All chemicals were used without fur-
ther purification.
2.2 Preparation of porous carbons
The peanut shells were rinsed with water, dried, and then cut
into small pieces (PS-1). Some peanut shells had been pretreat-
ed in an aqueous NaOH solution (1 mol·L-1) at 80 °C for 12 h,
and then the treated peanut shells were washed with water,
dried and cut into pieces (PS-2). Porous carbons were obtained
by carbonization of the peanut shells in a tube furnace at 800 °C
for 90 min under a nitrogen flow. The heating rate was 10 °C·
min-1. The obtained black solid was then immersed into HF
(20%, mass fraction) solutions for 48 h. Finally, the solid was
filtered with water and dried at 60 °C for 6 h. The samples re-
ferred to as PSC-1 and PSC-2 were derived from PS-1 and
PS-2, respectively.
2.3 Characterization of porous carbons
The pore structures of the samples were investigated by
physical adsorption of nitrogen at liquid nitrogen temperature
(77 K) on an automatic volumetric sorption analyzer (NOVA
1100, Quantachrome). The specific surface area was deter-
mined by the Brunauer-Emmett-Teller (BET) method. Pore
size distribution was evaluated by the Barrett-Joyner-Halenda
(BJH) method. Elemental analyses were performed on a
VARIO EL III elemental analyses system (Elementar Analysen-
systeme GmbH, Hanau, Germany). Fourier transform infrared
(FTIR) spectra were recorded on a Thermo Nicolet 5700 spec-
trophotometer. X-ray powder diffraction (XRD) measurements
were determined using a Bruker D8 advanced X-ray diffract-
meter equipped with Cu Kα radiation (λ=0.15418 nm).
2.4 Fabrication of porous carbon electrodes
The electrochemical measurements were performed on an
Autolab PGSTAT302N in an aqueous KOH solution (6 mol·
L-1) at room temperature using a three-electrode cell with po-
rous carbon as the working electrode, a platinum electrode as
the counter electrode, and an Ag/AgCl electrode as the refer-
ence electrode. The porous carbon electrodes were obtained by
pressing a well-mixed slurry (80% (mass fraction) carbon,
15% acetylene carbon black, and 5% PTFE) onto a nickel
foam grid (1 cm× 1 cm) at 1.25×107 Pa. The typical mass load
of each electrode material was about 5 mg. The electrodes
were vacuum dried at 110 °C. Symmetrical sandwich-type su-
percapacitors made of two carbon pallets separated by fibrous
paper and the electrode materials composed of 80% carbon (10
mg), 10% acetylene carbon black, and 10% PTFE.
3 Results and discussion
Nitrogen sorption isotherms were recorded to determine the
pore properties of the peanut-shell-based porous carbons
(Fig.1). It can be seen that the data for PSC-1 and PSC-2 are al-
most the same, except that the surface area of PSC-2 is larger
than that of PSC-1. At low pressure, the initial step region as-
cends abruptly and then follows a plateau (Fig.1A), indicating
that adsorption has virtually stopped because of the pore wall
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Acta Phys. ⁃Chim. Sin. 2011 Vol.27
multilayer. Both isotherms are type IV isotherm curves. It can
be seen from Fig.1B that for both porous carbons the major
pore size of particles is ~0.8 nm, with a minor part of ~1.3 nm.
However, the surface area increases from 552 m2·g-1 for
PSC-1 to 726 m2·g-1 for PSC-2. These changes are ascribed to
the pretreatment with NaOH solution, which may enable some
pectin and hemicelluloses (lignin, and wax like substances) to
separate from the peanut shell and contribute to the increase in
the specific surface area and pore volume.22 This will be further
confirmed by the FTIR results. The pore sizes of PSC-1 and
PSC-2 are suitable for EDLC;1,23 they allow ion migration of in-
organic electrolytes and electronic adsorption because the di-
ameters of K + and OH − in the KOH solution are smaller than
0.4 nm.24
FTIR spectroscopy can give direct structural information in
the peanut shells during various chemical treatments. It can be
seen from the FTIR spectra of PS-1 and PS-2 (Fig.2) that most
of the absorption peaks are not shifted. For example, the ab-
sorption peaks at 3420 cm-1, ascribed to the OH group, and the
peaks in the fingerprint regions at 1424, 1158, and 1054 cm-1,
attributed to the cellulose structure, are virtually unchanged in
both peanut shell samples.22 The absorption band at 1635 cm-1,
ascribed to asymmetric COO― stretching, and the bands at
1458 and 1378 cm-1, assigned to the CH2 and CH symmetric
bending modes, are also unchanged. However, the vibrational
peak at 1738 cm-1 in the PS-1 spectrum, which is ascribed to
C＝O stretching of the methyl ester and carboxylic acid groups
in pectin, or the acetyl group in the hemicelluloses, disap-
peared from the PS-2 spectrum. This indicates that pectin and
hemicelluloses can be successfully extracted by a simple alka-
line solution treatment.22 A new band at 877 cm-1 in the PS-2
spectrum, ascribed to an epoxy compound, gives further infor-
mation on the efficiency of the pretreatment. Furthermore, the
mass contents (%) of C, N, S, and H, based on the elemental
analyses data, are 92.13, 0.79, 0.36, and 1.01 for PSC-1, and
92.30, 0.48, 0.36, and 0.81 for PSC-2.
Fig.3 shows the CV curves of PSC-1 and PSC-2 based elec-
trodes in three-electrode systems or symmetrical supercapaci-
tors at different scan rates. Fig.3(A, C) displays that the CV
curves of the electrodes in three-electrode systems have an al-
most rectangular shape, even at high scan rates, indicating that
electrodes based on PSC-1 and PSC-2 have good capaci-
tance4-7 and fast charge-discharge switching.25 PSC-1 and
PSC-2 electrodes all have large specific currents. Furthermore,
the specific currents for the PSC-2 electrode are larger than
those of the PSC-1 electrode, indicating that the PSC-2 elec-
trode has a lower resistance and higher capacitance than that of
the PSC-1 electrode.3 For symmetrical supercapacitors (Fig.3
(B, D)), all the CVs of the composites at a scan rate of 50 mV·
s-1 are nearly rectangular. However, the shape of the CVs are
somewhat tilted at 80 mV·s-1. The shape evolution of the CVs
at different scan rates indicates that the ohmic resistance of the
carbon electrodes is large at high scan rates.26
Galvanostatic charge-discharge is a commonly used method
for studying electrochemical capacitors; much information,
such as the capacitance and the long cycle capability of elec-
trode materials, can be obtained from the experiments.27 The
charge-discharge curves of the PSC-2 based electrodes and su-
percapacitors are shown in Fig.4. The coulombic efficiency of
carbon electrodes is nearly 100% , although the charge/dis-
charge curves are not exactly linear.28 The specific capacitance
can be calculated from the equation C=Itd·(m·ΔV)-1,12,29 where
I is the current in the galvanostatic charge-discharge measure-
ment, m is the mass of the active materials, td is the variance
metric of charge or discharge time, and ΔV is the potential win-
dow during the discharge process after IR drop. As depicted in
Fig.4A, the PSC-2 electrode had a capacitance of 378 F·g-1 at
a current density of 0.1 A·g-1, and the capacitances decreased
Fig.1 Nitrogen adsorption-desorption isotherms (A) and pore
size distribution (B) of PSC-1 and PSC-2
A
B
Fig.2 FTIR spectra of PS-1 and PS-2
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GUO Pei-Zhi et al.: Preparation and Characterization of Peanut Shell-Based Microporous CarbonsNo.12
to 322 and 310 F·g-1 at 0.5 and 1 A·g-1, respectively. Howev-
er, the capacitance of the PSC-2 electrode dropped to 233 F·
g-1 at a current density of 0.1 A·g-1 and decreased to 208 and
205 F·g-1 at current densities of 0.5 and 1 A·g-1, respectively.
For symmetrical supercapacitors,29,30 the capacitances were
130, 109, and 75 F·g-1 for PSC-2 at current densities of 0.2,
0.5, and 1 A·g-1 (Fig.4B), respectively, while the capacitance
was 102 F·g-1 for PSC-1 at 0.2 A∙g-1 and decreased to 74 and
52 F·g-1 at current densities of 0.5 and 1 A·g-1, respectively.
These results indicate that the pore sizes of PSC-1 and PSC-2
are suitable for rapid diffusion of ions, similar to those of car-
bide-derived microporous carbons.1-3 The difference in the ca-
pacitances of PSC-1 and PSC-2 is mainly caused by variations
in the specific surface areas.
The electrochemical stabilities of the active materials and
their repeatability were also investigated by galvanostatic
charge-discharge measurement. Fig.5 shows the results of a cy-
cling performance test on the PSC-2-based electrodes and su-
percapacitors. For the PSC-2 based symmetric supercapacitor,
the capacitance retention was over 90.0% at a current density
of 0.5 A·g-1 after 1000 cycles (Fig.5A). After 1000 cycles, the
specific capacitance of the PSC-2 electrode at a current density
of 2 A·g-1 was maintained more than 94.5% of the original val-
ue (Fig.5B). These results indicate that the peanut shell-based
carbon electrodes and supercapacitors have good stability and
capacitance retainability.
The excellent electrochemical performances of PSC-1 and
PSC-2 can be attributed to their unique micropore structure
and appropriate surface areas which afford a large enough elec-
trode/electrolyte interface for charge accommodation. It should
be noted that no products were obtained when solid mixtures
of peanut shells and NaOH were undergone the same calcina-
Fig.3 CV curves of PSC-1 (A, B) and PSC-2 (C, D) based electrodes (A, C) and supercapacitors (B, D)
Fig.4 Galvanostatic charge-discharge curves of PSC-2-based electrodes (A) and supercapacitors (B)
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Acta Phys. ⁃Chim. Sin. 2011 Vol.27
tion procedure. The new procedure consisting of sample pre-
treatment with alkaline solutions increases the specific surface
areas while the pore structure can be maintained. FTIR spectra
showed that the peak at 1738 cm-1 in the PS-1 spectrum, as-
cribed to the C＝O stretching of the methyl ester and carboxyl-
ic acid groups in pectin, or the acetyl group in hemicelluloses,
disappeared from the PS-2 spectrum. This indicates that pectin
and hemicelluloses can be successfully extracted by a simple
alkaline solution treatment. And thus microporous carbons
with superior electrochemical properties from peanut shells are
produced.
4 Conclusions
The alkaline solution activation method has been used to fab-
ricate peanut-shell-based microporous carbons. It is found that
PSC-2 derived via the activation method displays a higher sur-
face area than that of PSC-1 produced by a non-activation
method. The pore size distribution is unchanged. The CV
curves of the samples in three-electrode systems and superca-
pacitors display rectangular shapes denoting the fast charge-dis-
charge switching. The PSC-2 electrode shows a specific capaci-
tance as high as 378 F·g-1 and good cycle stability. These re-
sults suggest that microporous carbon electrodes made from
peanut shells have a potentially broad application as electro-
chemical capacitor electrode materials.
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Fig.5 Cycling performance of the PSC-2-based supercapacitors
(A) and electrodes (B)
2840