CHEM. RES. CHINESE UNIVERSITIES 2012, 28(5), 780—783
Preparation and Supercapacitive Properties of Fe2O3/Active
Carbon Nanocomposites
LUO Pei-wen1, YU Jian-guo1, SHI Zhi-qiang1, HUANG Hua1, LIU Lang1, ZHAO Yong-nan1*,
LI Guo-dong2 and ZOU Yong-cun2
1. Institute of Nanostructured Materials & Tianjin Key Laboratory of Fiber Modification and Functional Fiber,
School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300160, P. R. China;
2. State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,
Jilin University, Changchun 130012, P. R. China
Abstract Fe2O3/active carbon(Fe2O3/AC) nanocomposites were readily fabricated by pyrolyzing Fe3+ impregnated
active carbon in a nitrogen atmosphere. The as-prepared composites were studied by X-ray powder diffraction(XRD),
X-ray photoelectron spectroscopy(XPS) and transmission electron microscopy(TEM). The capacitive property of the
composites was investigated by cyclic voltammetry(CV) and galvanostatic charge-discharge test. Physical characterizations show that the γ-Fe2O3 fine grains dispersed in the AC well, with a mean size of 21.24 nm. Electrochemical
tests in 6 mol/L KOH solutions indicate that the as-prepared nanocomposites exhibited improved capacitive properties. The specific capacitance(SC) of Fe2O3/AC nanocomposites was up to 188.4 F/g that was derived from both electrochemical double-layer capacitance and pseudo-capacitance, which was 78% larger than that of pristine AC. A
symmetric capacitor with Fe2O3/AC nanocomposites as electrode showed an excellent cycling stability. The SC was
only reduced by a factor of 9.2% after 2000 cycles at a current density of 1 A/g.
Keywords Nanocomposite; Fe2O3; Active carbon; Supercapacitor
Article ID 1005-9040(2012)-05-780-04
1
Introduction
In recent years, electrochemical capacitors(ECs) or supercapacitors that fill the gap between battery and conventional
capacitor have attracted much attentions due to their long cycle
life, high power density and environmentally friendly characters[1,2]. Designing and fabricating novel structured active materials with enhanced energy density, long cycling life and high
power operation is one of the key factors to render ECs a wide
application in portable appliances and electronic vehicles.
Many researchers have investigated into carbon based composite materials[such as MnO2/graphene[3], carbon nanotube
(CNT)/polyaniline[4]] as the electrode active materials for supercapacitors due to their excellent supercapacitive performance. They combine the advantages of carbon materials
(e.g., fine conductivity and large surface area) with those of
metal oxides or polymers[e.g., high specific capacitance(SC)][5].
Iron oxides have been considered as one of the most
promising electrode materials for ECs due to their suitable
electrochemical performance, natural abundance and environmental compatibility[6,7]. Although iron oxides were extensively
studied as anode material for Li-ion batteries[8,9], few focused
on their application in ECs. Actually, the practical performance
of iron oxide electrodes for ECs exhibits a small SC, a low
energy density and poor cyclic performance[7,10,11]. FeOx combined with carbonaceous materials(e.g., graphene or CNT) has
shown enhanced capacitive properties[8]. High SC of 480 F/g as
well as an improved cycling performance was achieved in
Fe3O4/reduced graphene oxide nanocomposites[12]. However,
the high cost of graphene limits its wide application.
Active carbon(AC) has the advantages over other carbon
materials due to its accessibility, easy processability and relatively low cost[13]. Considerable endeavor has exerted on
asymmetric capacitor via iron oxide and AC to achieve high
operating voltage and large energy density[14]. However, the
iron oxide/AC nanocomposites have been seldom investigated.
Herein lie Fe2O3/AC nanocomposites fabricated to acquire
improved supercapacitive properties.
2
2.1
Experimental
Synthesis of Fe2O3/AC Nanocomposites
Fe2O3/AC nanocomposites were fabricated by heating the
Fe3+ impregnated AC in a N2 flow. Typically, commercial
AC(Vulcan-32) was modified by HNO3(63%, mass fraction)
for 3 h at 80 °C to make it hydrophilic. Then 0.3 g of modified
AC and 2.4 g of Fe(NO3)3·9H2O were dispersed in 40 mL of
H2O. After vigorously stirring for 30 min and ultrasonicating
for 1 h, the mixture was filtered and dried. The dried filter
———————————
*Corresponding author. E-mail: [email protected]
Received October 10, 2011; accepted November 29, 2011.
Supported by the National Natural Science Foundation of China(No.21271138) and the Natural Science Foundation of Tianjin City, China(No.10JCZDJC21500).
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LUO Pei-wen et al.
residue was then heated at 750 °C for 3 h in a N2 flow.
2.2
Characterizations and Electrochemical Test
The physical characterizations were performed by X-ray
diffraction(XRD, Bruker D8 Discover, λ=0.15406 nm), X-ray
photoelectron spectroscopy(XPS, Thermo Fisher PHI5300) and
transmission electron microscopy(TEM, Hitachi H-7650). The
content of iron oxides in the nanocomposite was measured by
thermogravimetric analysis(TGA), which was performed on a
Netzsch STA 409 PG/PC in an oxygen atmosphere at a heating
rate of 10 °C/min. Cyclic voltammetry(CV) was run on a LK
3200 electrochemical workstation in a three-electrode cell. A Pt
foil and a saturated calomel electrode(SCE) were used as the
counter and reference electrodes, respectively. Nickel foam
loaded with a mixture of Fe2O3/AC(85%, mass fraction), acetylene black(10%, mass fraction) and polytetrafluoroethylene(5%, mass fraction) as the working electrode. Galvanostatic
charge-discharge tests were run on a BTS-5 V-100 mA instrument in a two-electrode cell with equal mass of nanocomposites loading on each electrode. All the electrochemical tests
were conducted in 6 mol/L KOH solution.
3
3.1
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determine the valence state of iron. Two intensive peaks at
710.8 and 724.6 eV in the XPS spectrum(Fig.4) correspond to
Fe2p3/2 and Fe2p1/2 binding energies, respectively, which are in a
good agreement with documented values of Fe3+ species[16]. So
the embedded nanoparticles are confirmed as γ-Fe2O3. The
TGA curve gives a content of 9.41%(mass fraction) of Fe2O3
(Fig.5).
Fig.2
Size distribution histogram of Fe2O3/AC
nanoparticles
Results and Discussion
Characterizations
As shown in Fig.1(A), compared with AC[Fig.1(B)], the
Fe2O3/AC nanocomposite contains lots of well-dispersed nanoparticles with a mean size of 21.24 nm(Fig.2). The XRD
pattern(Fig.3) confirms the presence of γ-Fe 2 O 3 (JCPDS
No.39-1346) in the products. The carbon matrix is still
amorphous after heat treatment. Amorphous carbon is beneficial to the supercapacitor due to the feasibility of ion migration[15]. Owing to the diversity of iron oxides and the structural
similarity between γ-Fe2O3 and Fe3O4, XPS was used to
Fig.3 XRD pattern of Fe2O3/AC nanocomposite
Fig.4 XPS pattern of Fe2O3/AC nanocomposite
Fig.5
3.2
Fig.1
TEM images of Fe2O3/AC nanocomposite(A)
and AC(B)
TGA curve of Fe2O3/AC nanocomposite
Electrochemical Testing
CV is considered to be an effective and suitable tool to
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CHEM. RES. CHINESE UNIVERSITIES
investigate electrochemical performances of electrode materials.
So, CV tests were carried out in a three-electrode cell in 6
mol/L KOH solution. The CV curves for Fe2O3/AC nanocomposites and pristine AC vary greatly[Fig.6(A)]. The roughly
rectangular shape of the CV curve of AC indicates a typical
capacitive behavior. However, for the CV curves of Fe2O3/AC,
current increases are discovered at around –0.65 V and followed by a pair of redox peaks, which are ascribed to the reaction of Fe(III)/Fe(II) on the surface of active carbon[17]. That
implies the presence of Faradic pseudo-capacitance for the
Fe2O3/AC nanocomposite. Furthermore, the intensively increased response current of Fe2O3/AC nanocomposite demonstrates an augmented capacity. Beyond all doubt, the enhanced
capacitive performances are ascribed to the large capacitance
contributed by iron oxide. Notably, Fe2O3/AC nanocomposites
also exhibit an enlarged potential window(–1.3―0.3 V vs.
SCE), which possibly affords a high working voltage for
supercapacitors, and may greatly enhance the energy density.
These results imply that the Fe2O3/AC nanocomposites are
promising electrode candidates for supercapacitors.
Fig.6
CV curves of AC(a) and Fe2O3/AC nanocomposite(b) at a scan rate of 50 mV/s(A) and CV
curves of Fe2O3/AC nanocomposite at different
scan rates(B)
(B) Scan rate/(mV·s–1): a. 5; b.10; c. 20; d. 50.
The CV curves of Fe2O3/AC nanocomposite at different
scan rates varying from 5 mV/s to 50 mV/s are presented in
Fig.6(B). All the CV curves exhibit obvious redox peaks. Apparently, the peak current density positively relates to the scan
rate without obvious shape change of the CV curve, indicating
a good rate for Fe2O3/AC[18]. However, the increased scan rate
caused a positive shift of the anodic peaks and a negative
movement of the cathodic peaks. This phenomenon indicates
that the polarization of Fe(III)/Fe(II) reaction increases at a
high charging/discharging rate.
The galvanostatical charge-discharge curves of symmetric
Vol.28
two-electrode cells are shown in Fig.7. The AC capacitor shows
a linear curve[Fig.7(A)], which demonstrates single
double-layer capacitance that is consistent with CV analysis for
AC. The SC could be evaluated from the charge-discharge
curves based on the following equation:
I Δt
(1)
SC = 4
mΔV
where SC(F/g) represents the specific capacitance of a single
electrode, I denotes the charge-discharge current, ∆t is the discharging time in second, ∆V is the potential change in volt, m
symbolizes the total mass of the loaded active material on both
the electrodes of symmetric capacitors.
Fig.7
Galvanostatic charge-discharge curves of the
symmetric cells of various samples
(A) γ-Fe2O3(20―30 nm)(a), AC(b) and Fe2O3/AC(c) tested at 0.25 A/g;
(B) mixtures of 90.59% AC and 9.41% γ-Fe2O3 tested at 0.25 A/g;
(C) Fe2O3/AC nanocomposite tested at current densities of 0.25(a), 0.5(b),
0.75(c) and 1 A/g(d). Inset in (B) shows the TEM image of Fe2O3 nanoparticles. Inset in (C) shows the SC values at different current densities.
With the help of equation (1), the SC of γ-Fe2O3(20―30
nm), pristine AC or Fe2O3/AC is 14.1, 105.8 or 188.4 F/g,
respectively. The capacitance of γ-Fe2O3 is very low, which
is attributed to its poor conductivity. For comparison, the
physical mixed materials of 90.59%(mass fraction) AC and
9.41%(mass fraction) γ-Fe2O3 nano-powders(purchased from
Beijing DK Nano S&T Ltd., 20―30 nm) were tested under the
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LUO Pei-wen et al.
same conditions. This material gave a SC value of 122.4
F/g[Fig.7(B)]. The capacitance of Fe2O3/AC is 78% larger than
that of pristine AC, which is contributed by both electrochemical double-layer capacitance and pseudo-capacitance. The increased capacitance of Fe2O3/AC composite is possibly due to
the fine and well dispersed Fe2O3 grains in the active carbon
which could provide more active sites for redox actions and
yield a high capacitance. In addition, the in-situ formed Fe2O3
nanoparticles closely contact with AC to accelerate the electron
transport and facilitate the redox reactions of iron oxides[5]. A
high capacitance value of about 877 F/g was calculated based
on the mass of Fe2O3 in the Fe2O3/AC nanocomposite. To our
knowledge, prior studies in aqueous electrolytes with
FeOx-based electrodes have given a varied FeOx-normalized
capacitance in a range of 5－350 F/g[7,19,20]. Although a high
capacitance of 510 F/g for Fe3O4 have been achieved by Wu et
al.[21] under a very limited circumstance[a low oxide mass
loading(3%)], the normalized capacitance is only 40 F/g.
Compared with the SC at a current density of 0.25 A/g,
91.2%, 86.2%, and 85.3% of the capacitance have been maintained at 0.5, 0.75, and 1.0 A/g, respectively, indicating high
capacitance retention under high power operations[Fig.7(C)].
This phenomenon is possibly ascribed to the porous structures
of AC that facilitate the ion diffusion and maintain fine performances at a large current density. Furthermore, AC which acts
as carbon matrix directly contacts with iron oxides. Such a
structure could make electron transport easyly and enhance the
electrochemical performances of the nanocomposites[5].
Cycling stability is another important parameter for supercapacitors. The charge-discharge cycling stability of the
Fe2O3/AC nanocomposite was also evaluated(Fig.8). The SC
slowly increases from 83 F/g to 190 F/g in the initial 1350
cycles at a current density of 0.25 A/g. The increased SC within
the starting 1350 cycles is possibly due to the wetting or activating process[22]. After the wetting process completed, no obvious SC fade occurred within 2000 cycles. When the current
density was elevated to 1 A/g, the nanocomposite also exhibited a good stability within 2000 cycles. Only 9.2% declination occurred in the SC. The excellent cycling stability at a
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large current density indicates a highly electrochemical reversibility of the nanocomposites. Therefore, the fabrication of
Fe2O3/AC composites with AC as a support not only accelerates the electron transport and facilitates the redox reactions of
iron oxides, but also improves the cycling stability of
FeOx[7,11,23].
4
Conclusions
Fe2O3/AC nanocomposites were readily fabricated by pyrolyzing Fe3+ impregnated active carbon. The nanocomposites
exhibited enhanced capacitive properties in 6 mol/L KOH solution with a high SC of 188.4 F/g. The symmetric capacitor with
Fe2O3/AC as electrodes showed an excellent cycling stability
within 2000 cycles at 1 A/g. These results indicate that the
Fe2O3/AC nanocomposites are promising electrode materials
for supercapacitors.
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Fig.8
Charge-discharge cycling stability tests of
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