Accepted Manuscript
Fe3O4/Carbon nanocomposite: Investigation of capacitive & magnetic properties for
supercapacitor applications
Neriman Sinan, Ece Unur
PII:
S0254-0584(16)30679-4
DOI:
10.1016/j.matchemphys.2016.09.016
Reference:
MAC 19162
To appear in:
Materials Chemistry and Physics
Received Date: 13 July 2016
Revised Date:
5 September 2016
Accepted Date: 9 September 2016
Please cite this article as: N. Sinan, E. Unur, Fe3O4/Carbon nanocomposite: Investigation of capacitive
& magnetic properties for supercapacitor applications, Materials Chemistry and Physics (2016), doi:
10.1016/j.matchemphys.2016.09.016.
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Fe3O4/Carbon nanocomposite: Investigation of capacitive & magnetic properties for
supercapacitor applications
Neriman Sinan a, Ece Unur b*
Department of Advanced Technologies, Materials Science and Engineering Program, Bursa Technical University,
Yildirim 16310, Bursa, Turkey (sinanneriman@gmail.com)
b
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a
Department of Energy Systems Engineering, Bursa Technical University, Yildirim 16310, Bursa, Turkey
(eceunur@yahoo.com)
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*Corresponding author
ABSTRACT: Fe3O4 nanoparticles with ~10 nm diameters were synthesized by an extremely low-
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cost, scalable and relatively biocompatible chemical co-precipitation method. Magnetic
measurements revealed that Fe3O4 nanoparticles have bifunctional superparamagnetic and
ferromagnetic character with saturation magnetization (Ms) values of 64 and 71 emu g-1 at 298 K and
10 K, respectively. Pseudocapacitive Fe3O4 nanoparticles were then integrated into hazelnut shells -
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an abundant agricultural biomass - by an energy efficient hydrothermal carbonization method.
Presence of magnesium oxide (MgO) ceramic template or its precursor in the hydrothermal reactor
allowed simultaneous introduction of pores into the composite structure. Hierarchically micro-
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mesoporous Fe3O4/C nanocomposite possesses a high specific surface area of 344 m2 g-1.
Electrochemical properties of Fe3O4/C nanocomposite were investigated by cyclic voltammetry and
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galvanostatic charge-discharge measurements in a conventional three-electrode cell. The Fe3O4/C
nanocomposite is able to operate in a large negative potential window in 1M Na2SO4 aqueous
electrolyte (-1.2 - 0 V vs. Ag/AgCl). Synergistic effect of the Fe3O4 and carbon leads to enhanced
specific capacitance, rate capability and cyclability making Fe3O4/C nanocomposite a very promising
negative electrode material for asymmetric supercapacitors.
Keywords: magnetic nanocomposite, biomass, hydrothermal treatment, MgO templating,
supercapacitor
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1. Introduction
Utilizing abundant renewable energy sources, e.g., sunlight, tides, wind, to generate clean and
sustainable electricity on a large scale is a key solution to halt climate change and ensure energy
sustainability. Supercapacitors present the most versatile solution for integration of intermittent
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renewable energy to the power grid as they are more suited to overcome sudden power fluctuations
than batteries with their higher rate capabilities [1–3]. High power and energy density
supercapacitors are prone to offer solutions for a number of applications, such as electric vehicles,
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portable devices, wearable and flexible electronics [4,5].
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Carbonaceous materials possess high specific surface areas and electrical conductivities,
providing high power density along with a long cycle life owing to non-faradaic/electrostatic charge
storage mechanism within the electric double layer (EDL). But, their low energy density restricts
their widespread use in energy storage applications. Transition metal oxides with their pseudocapacitive charge storage mechanism, that relies on reversible surface redox reactions, enable
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capacitances and energy densities superior to pure EDL capacitors’ [6]. However, poor conductivity
(aggravating power performance and cycle life) and high manufacturing costs are the major
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weaknesses of metal oxides when compared to carbonaceous materials.
Nano-sized transition metal oxides (MO, M = Mn, Ti, Cu, Ni, Fe) constitute a broad family of
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functional materials which offer exceptional physical properties. Among them, magnetite (Fe3O4)
has been widely studied because of its eco-friendliness, natural abundance, higher electrical
conductivity and low cost. Electrons hopping between ferric and ferrous ions at neighboring
octahedral sites induce unique electrical and magnetic properties to cubic inverse spinel magnetite
[7,8]. Superparamagnetic Fe3O4 nanoparticles (< 20 nm) [9] which are biocompatible and noncytotoxic to humans have been extensively studied for application in rapidly emerging fields of
biomedical research including targeted tumor/cancer diagnosis and treatment, drug delivery and
magnetic resonance imaging (MRI) [10–13]. In recent years, nanoscale magnetite particles have also
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attracted increasing attention in energy storage applications. High theoretical specific capacitance
(estimated as 347 F g-1 at 1.2 V potential window from [14]) and conductivity (102-103 Ω-1 cm-1) [7]
of Fe3O4 suggest a high energy density storage in supercapacitors through redox reactions. Fe3O4
nanoparticles can be synthesized using numerous techniques, such as co-precipitation [15–17],
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hydrothermal [18,19], sol-gel [20], sonochemical [21], microemulsion, thermal decomposition [12]
and etc. However, many of these methods result in low product yields and inadequate crystallization,
and they are not suitable for industrial-scale production due to high-cost and high energy
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consumption. Among them the co-precipitation of Fe2+ and Fe3+ in an alkaline environment, reported
by Massart [22], has been the most widely used and commercialized chemical synthesis method for
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preparing spherical ‘‘superparamagnetic’’ Fe3O4 nanoparticles with uniform sizes [11]. In spite of
some difficulties in control of agglomeration and particle size distribution, the method is still more
suitable for large-scale production due to its extremely low-cost, high product yield, mild and simple
synthesis procedure which use relatively non-toxic starting materials [12].
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Coupling different conductive carbon phases with metal oxides has been a frequently visited
method in pursuit of higher energy and power density electrode materials for energy storage
applications [23,24]. Compared to more sophisticated conductive phases (CNTs, graphene, and etc.),
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porous carbon particles can be easily obtained on a larger scale without defects [24,25]. Fe3O4 serves
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as a high energy negative electrode for supercapacitors [2,26] owing to its high hydrogen evolution
potential in aqueous electrolytes [27]. Fe3O4/C nanocomposites are known to provide a dramatically
improved electrochemical performance when compared to bare Fe3O4 and C. In the Fe3O4/C
composite structure, active carbon prevents Fe3O4 aggregation, buffers the volume changes caused
by charge/discharge processes and improves the rate capability and cycling stability (reversibility) by
providing ion transport highways [28]. Moreover, high surface area of carbon increases the
electrolyte exposure of active materials and thus improves the EDL- and pseudo-capacitance.
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In this work, porous Fe3O4/C nanocomposite material was prepared by an undemanding
methodology in terms of its complexity, environmental impact and cost. First, high-purity magnetite
nanoparticles with uniform sizes were produced by chemical co-precipitation method. The obtained
Fe3O4 nanoparticles were further modified with carbon and subjected to MgO templating
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simultaneously in one pot through a commonly used hydrothermal route by introducing hazelnut
shells as a cheap and sustainable carbon source and MgAc as a non-toxic template precursor. First,
structural and magnetic properties of both Fe3O4 and Fe3O4/C were discussed in detail, and then by
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using a conventional three-electrode cell (vs. Ag/AgCl), the electrochemical characteristics of the
Fe3O4/C electrode were investigated in 1M Na2SO4 aqueous solution. High energy density and
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specific capacitance along with a good cycling stability were induced by the co-existence of
magnetite and carbon in the composite electrode. When compared to previous reports on Fe3O4
based composites in aqueous electrolytes, a relatively large operational potential range (-1.2 - 0 V vs.
Ag/AgCl) of the Fe3O4/C nanocomposite prepared in this work enabled the electrode to reach a
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superior energy density (27.2 Wh kg-1 at 1 A g-1). Thus, the Fe3O4/C nanocomposite prepared in this
work promises as a very suitable negative electrode material for asymmetric supercapacitors
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delivering high energy/power density.
2. Materials and Methods
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2.1 Synthesis of Fe3O4 nanoparticles
Simple and cost-effective chemical co-precipitation method was used to prepare nano-sized
magnetite (Fe3O4) particles [15,16]. A solution of 0.12 mol ferric chloride (FeCl3, anhydrous,
Panreac) was mixed with 0.06 mol ferrous chloride tetrahydrate (FeCl2.4H2O, Merck) in 600 mL
deionized water (molar ratio of Fe(III)/Fe(II) = 2). Then, the iron oxide particles were precipitated
from the above reaction mixture via dropwise addition of 170 ml of ammonia solution (NH4OH,
25%, Merck) under constant mechanical stirring at 80 °C for 30 min. The resulting black solution
was filtered and washed with deionized water until neutral pH is reached and the obtained magnetite
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precipitate was dried at 80 °C. Fe3O4 nanoparticles formed through the redox reactions between Fe2+
and Fe3+ species. The overall magnetite formation mechanism can be written as follows [15–17]:
+ 2 + 8
.
→ ↓ +8
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2.2 Preparation of Fe3O4/C nanocomposite
Similar to our previous report, hydrothermal carbonization (HTC) and MgO templating were
realized simultaneously in one pot to prepare porous Fe3O4/C nanocomposite [29]. The as-
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synthesized Fe3O4 nanoparticles were mixed with hazelnut shell powder (HS) and magnesium
acetate (MgAc, Mg(CH3COO)2.4H2O, Merck) porogen at a weight ratio of 1:4:1 in deionized water
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and placed in a hydrothermal reactor. The mixture in the closed reactor was subjected to 200 ºC for 8
hours in an electric oven. Sample obtained was etched with dilute acetic acid solution. Mesopores
form as the acid solution dissolves out the ceramic template (MgO) trapped within the composite
structure during carbonization. Samples washed with deionized water were filtered and dried at 80
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ºC. In order to improve its thermal and electrical stability, sample was subjected to thermal treatment
under argon flow at 700 ºC for 2h with a 10 ºC min-1 heating rate. The resulting magnetite-carbon
nanocomposite powder was named Fe3O4/C.
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2.3 Materials characterizations
The as-prepared samples were examined with Rigaku Ultima IV X-ray diffractometer to
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determine the crystalline properties. Thermal decomposition of the samples were investigated by
Perkin Elmer Pyris 1 thermogravimetric analyser (TGA) as they are heated from room temperature to
950 °C with a 10 °C min-1 rate under air. LECO Elemental Analyser (CHNS-932) was employed to
perform elemental studies. The microstructure and morphology of the samples were investigated via
JEOL JEM-2100F high-resolution transmission electron microscope (HR-TEM) with 200 kV
accelerating voltage and QUANTA 400F Field Emission scanning electron microscope (FE-SEM).
N2 adsorption/desorption isotherms were recorded at 77 K with Autosorb-6 Analyser (Quantachrome
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Corp.). Prior to analyses, samples were evacuated at 200 °C for 10 hours. To determine the specific
surface area the Brunauer, Emmett and Teller (BET) theory was used. Then the resulting pore size
distributions of samples were obtained by the Density Functional Theory (DFT). Magnetic
measurements were realized at both room temperature (298 K) and low temperature (10K) by
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vibrating sample magnetometer (VSM) instrument of Cryogenic Limited.
2.4 Electrochemical testing of Fe3O4/C nanocomposite
To fabricate the working electrode, Fe3O4/C nanocomposite powder, polyvinylidene fluoride
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(PVDF, Kynar® HSV 900) binder and conducting carbon black (Timcal Graphite & Carbon Super
P®) were mixed at an 80:10:10 weight ratio. To form a viscous paste, minimum amount of N-
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methylpyrrolidone (NMP, Merck) was dropped into the mixture. The paste was casted on a stainless
steel (0.1 mm, MTI) in the form of a 500 µm thick film and left to dry naturally and then dried under
vacuum at 120 °C for 4 hours.
Electrochemical
characterizations
were
performed
on
a
Gamry
Reference
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potentiostat/galvanostat. In a conventional three-electrode cell, the working, counter and reference
electrodes were the Fe3O4/C composite, a platinum wire and an Ag/AgCl (saturated KCl),
respectively. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements of
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Fe3O4/C nanocomposite were realized at various scan rates (2 - 100 mV s-1) and current rates (0.25 -
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5 A g-1) in aqueous solution of 1M sodium sulphate (Na2SO4, -1.2 - 0 V vs. Ag/AgCl). Data were
collected by applying 5 repetitive cycles to the working electrode at each scan rate and each current
rate. Electrode’s active material loading was ~3.5 mg cm-2.
Eqn. (1) can be used to calculate the specific capacitance ( ) of the electrode [30]:
( ) =
× ∆
×∆
(1)
where I (A) is the constant discharge current, ∆t (s) is the time of the discharge, m (g) is the active
material mass in the electrode and ∆V (V) is the potential window excluding IR drop.
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Corresponding specific energy and power densities are derived from Eqn. (2) and (3),
respectively [31,32]:
(2)
+(# % ) =
"
× 3600
(3)
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0.5 ( ( ∆ 3.6
"(#ℎ % ) =
where (s) is the time of discharge.
3. Results and discussions
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Primary compositional information on the Fe3O4/C nanocomposite was obtained by TGA
analysis (Fig. 1). The weight loss observed from 400 °C to 550 °C is ascribed to decomposition of
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organic components. The 43% weight retention quantifies the total amount of Fe3O4 within the
nanocomposite, as the pure Fe3O4 is not expected to exhibit any weight loss and degradation [33–
35]. According to elemental analysis results, Fe3O4/C nanocomposite is composed of 54% carbon,
2% hydrogen, 0.5% nitrogen, 0.5% oxygen and 43% inorganics (Fe3O4) by weight. These results
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confirm the TGA findings. A phase transformation, such as oxidation of magnetite into hematite
(Fe2O3) in air is characterized by a weight gain between 200 and 400 °C in TGA [36–38]. Absence
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of such a gain in Fig. 1 indicates the stable nature of as-synthesized Fe3O4.
Fig. 1 TGA curve of Fe3O4/C nanocomposite under dry air
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The crystalline structure of the samples were examined by XRD. The diffraction pattern of
Fe3O4 nanoparticles has matched well with the reference pattern of magnetite (Fig. 2b). The sharp
and intensive diffraction peaks indicate highly crystalline nature of Fe3O4. The peaks located at 2θ=
18.6°, 30.5°, 35.8°, 43.5°, 53.9°, 57.5°, and 63.1° in Fig. 2a are assigned to the (111), (220), (311),
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(400), (422), (511), and (440) reflections of the cubic Fe3O4 with Fd3m space group, respectively.
The porous Fe3O4/C nanocomposite exhibits high intensity XRD peaks similar to bare Fe3O4
particles’. The broad diffraction band observed for Fe3O4/C nanocomposite at 25º is ascribed to the
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(002) plane reflections of amorphous carbons [39]. Absence of characteristic peaks associated with
impurities/other crystalline forms of iron oxide, such as maghemite (γ-Fe2O3) and hematite (α-
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Fe2O3), notes the purity of the Fe3O4 particles and Fe3O4/C nanocomposite. It is known that the
magnetite oxidizes to maghemite slowly at room temperature, and then to hematite at higher
temperatures in air. Additionally, magnetite particles synthesized from aqueous solution could
transform to γ-Fe2O3 at ~200 °C [7]. Maghemite phase is not present in our samples, since its typical
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(110), (210) and (211) reflections at lower 2θ= ~15.0° , 24.0° and 26.0°, respectively, are absent in
the XRD patterns [40,41]. Non-existence of strong peaks at 2θ= ~24.1°, 33.2° and 40.9° belonging
to the (012), (104) and (110) reflections of hematite phase, respectively, also rules out the presence
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of this phase, in line with the TGA findings [42,43]. The formation of impurities or other iron oxide
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phases might have been avoided by use of NH4OH (instead of NaOH) to precipitate magnetite
particles [16].
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Fig. 2 (a) XRD results of bare Fe3O4 nanoparticles and Fe3O4/C nanocomposite (b) reference pattern
of magnetite
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Analysis of X-ray diffractograms with the Debye–Scherrer method gives the average
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crystallite size (,) of Fe3O4 [35,44]:
-.
/ 0123
where K is shape factor (0.89 for spherical particles), λ is the X-ray wavelength (0.15406 for 4-5 ),
θ is the diffraction angle and β in radians is the (full width at half maximum (FWHM). Using the
(311) reflection, Fe3O4’s crystallite size was obtained to be 12 nm.
HR-TEM images (Fig. 3) revealed that the Fe3O4 nanoparticles display spherical morphology
and good uniformity with ~10 nm diameter matching well with the crystallite size obtained from the
XRD results. Fig. 3b clearly shows the lattice fringes of the Fe3O4 sample indicating highly
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crystalline nature. Interparticle agglomeration caused by magnetic dipole-dipole interactions is also
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observed from the images [45].
Fig. 3 HR-TEM images of bare Fe3O4 nanoparticles
The morphological properties of bare Fe3O4 and composite Fe3O4/C particles were examined
by FESEM. Fig. 4a and b show that the aggregates of iron oxide nanoparticles are successfully
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synthesized with uniformly sized spherical morphology. Fig. 4c and d show that the Fe3O4
nanoparticles are homogeneously dispersed within porous carbon matrix and denser morphology is
obtained through nanostructuring and pyrolysis. Nanostructuring improves reversibility of Fe3O4
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redox reactions and allows high capacitance retention due to decrease in ion diffusion distances
[46,47]. The aggregation of Fe3O4 limits access to electroactive sites, resulting in lower charge
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storage capacity [19]. The carbon matrix increases electrical double layer capacitance and serves as a
spacer to minimize aggregation of Fe3O4 nanoparticles [27,38]. Particle sizes of Fe3O4/C
nanocomposite are observed to be in the range of 16-25 nm from the SEM images, implying 6-15
nm carbon coating on Fe3O4.
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Fig. 4 SEM images of the samples: (a-b) bare Fe3O4 nanoparticles and (c-d) Fe3O4/C nanocomposite
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The N2 adsorption/desorption isotherms of the bare Fe3O4 and Fe3O4/C composite are given
in Fig. 5a and Fig. 5b, respectively, with the insets showing the corresponding pore size
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distributions. A hysteresis loop (characterizing Type IV isotherm) observed in both samples indicates
capillary condensation in mesopores based on the IUPAC classification [48]. In the case of bare
Fe3O4, the Type IV isotherm accompanied by H3 type hysteresis loop denotes slit-shaped pores
formed by aggregates of loosely bound particles that do not show any limiting adsorption at high
P/P0. Unlike bare Fe3O4, the dramatic increase in N2 volume at low P/P0 (<0.1) appearing in the
Fe3O4/C isotherm -a characteristic of Type I isotherm- accompanied by H4 type hysteresis loop
indicates existence of micropores. In conclusion, the mixed Type I and Type IV isotherm character
observed for Fe3O4/C in combination with H4 type hysteresis loop denotes hierarchical (micro- and
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meso) pore structure. Fe3O4 nanoparticles exhibit very low specific surface area (82 m2 g-1) and pore
volume (0.07 cm3 g-1). After carbon modification, the specific surface area and the pore volume
increases to 344 m2 g-1 (~4 times higher than bare Fe3O4’s) and 0.16 cm3 g-1, respectively. This
significant enhancement can be ascribed to mesopore formation by MgO templating [49] and
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micropore openings from gas evolution during the pyrolysis [50] at 700 0C, confirmed by the pore
size distribution curves of bare Fe3O4 (1.3 nm) and Fe3O4/C composite (1.3 and 6.0 nm). The specific
surface area we obtained in this work (344 m2 g-1) is significantly higher than the previously reported
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carbonaceous Fe3O4 composites prepared by more sophisticated materials and methods (Fe3O4reduced graphene oxide: 147 m2 g-1 [2] and Fe3O4-carbon nanosheets: 229 m2 g-1 [51]).
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Interconnected micro- and mesopores of Fe3O4/C provides a large and a highly accessible surface
area enhancing EDL and pseudo- capacitance values. Mesopores facilitate fast ion diffusion via short
transport pathways during the charge/discharge process providing high rate capability and thus high
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power density [52].
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Fig. 5 N2 sorption isotherms of (a) bare Fe3O4 nanoparticles and (b) Fe3O4/C nanocomposite (insets
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show the pore size distributions)
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Magnetic properties were investigated via magnetic field dependent magnetization by applying a
maximum magnetic field of ±10 kOe (1 Oe = 79.59 A m-1). Typical magnetic hysteresis loops,
magnetization (emu g-1, M) vs. magnetizing field (Oe, H), at room temperature (298 K) and low
temperature (10 K) are depicted in Fig. 6. Magnetic properties of the samples i.e., saturation (Ms)
and remanent (Mr) magnetizations, and coercivity field (Hc) are summarized in Table 1. Fe3O4
nanoparticles that are less than 20 nm in size, generally show superparamagnetic character with
almost zero coercivity and remenance values [9]. Mitchell et al.[20] have synthesized cubic iron
oxide nanoparticles (8±2 nm ) using a sol-gel technique and obtained bifunctional magnetic behavior
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(superparamagnetic at 300K and ferromagnetic at 10K). Analogously, our bare Fe3O4 nanoparticles
showed bifunctional superparamagnetic and ferromagnetic behavior at 298 K and 10 K, respectively.
Saturation magnetization (Ms) of 64 emu g-1 obtained at 298 K is lower than the theoretical Ms of
bulk Fe3O4 (92 emu g-1) [53], but still higher than the recently reported values [8,20], confirming that
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the cheap chemical co-precipitation method is able to produce high quality magnetite nanoparticles.
Fig. 6 Magnetization curves of samples at 298 K and 10 K: (top) bare Fe3O4 nanoparticles and
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(bottom) Fe3O4/C nanocomposite
The narrow hysteresis loops observed for the Fe3O4/C nanocomposite at both temperatures
indicate soft ferromagnetic behaviour [18]. As seen from Table 1, the saturation magnetization of
Fe3O4 decreases when modified with carbon (Fe3O4/C), as the magnetic alignment of amorphous
carbon counteracts the alignment of Fe3O4 particles [18]. Therefore, the amount of carbon present in
the composite material should be optimized for applications that benefit from the magnetic properties
of Fe3O4.
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Table 1. Magnetic properties of bare Fe3O4 nanoparticles and Fe3O4/C nanocomposite at 298K and
10K
Sample
Saturation magnetization/
Ms (emu g-1)
Remanent magnetization/
Mr (emu g-1)
Coercivity field/
Hc (Oe)
10 K
298 K
10 K
298 K
Fe3O4
64
71
0
34
0
228
Fe3O4/C
34
36
8
14
66
322
10 K
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298 K
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Electrochemical performance of the Fe3O4/C nanocomposite electrode in 1M Na2SO4
Electrochemical characterization of the Fe3O4/C was realized in a conventional threeelectrode cell (vs. Ag/AgCl) by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD)
techniques to explore its applicability in supercapacitors. CV curves of the Fe3O4/C were recorded at
the scan rate of 50 mV s-1 for three different potential windows using 1M Na2SO4 solution as an
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electrolyte (Fig. 7). Similar to previous studies on iron oxide based electrodes, the CV curves exhibit
higher cathodic currents than anodic ones as the negative potential limit is shifted towards more
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negative values [27]. Larger CV area observed at more negative potential window (-1.2 - 0 V vs.
Ag/AgCl) suggests a higher capacitance value. This result may be attributed to the space-charge-
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limited capacitance for a semiconductor-electrolyte interface [27,54].
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Fig. 7 Cyclic voltammograms of Fe3O4/C nanocomposite in 1M Na2SO4 at various potential limits
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CV studies at various scan rates (2-100 mV s-1) were further carried out in the -1.2 to 0 V
range. The curves yield a nearly-rectangular shape (Fig. 8a) because of the simultaneous occurrence
of faradaic and non-faradaic processes to store energy. Obvious distortion of voltammograms
observed at higher scan rates is related to the slow charge transfer and cation diffusion processes
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leaving the interior surfaces unutilized. Fig. 8b shows CV curves of a composite electrode and
blank/uncoated stainless steel (SS) at 2 mV s-1. The SS voltammogram exhibits a linear shape as it
presents negligible current during CV test, proving its inactivity on the charge storage. The
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pseudocapacitive reaction mechanism of Fe3O4 is based on the reversible surface reactions of Fe2+/
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Fe3+ redox couple accompanied by the intercalation of 7 ions in 1M Na2SO4 solution [31]:
2 88 + 7 ↔ ( 888 ) + 7 ( 888 ) + 2 High rate capability (high capacitance retention at high current rates) is essential to achieve
high power density electrode materials. Conductive carbon matrix is advantageous in achieving high
rate capability as it provides a highway for rapid electron/ion transport throughout the composite. It
is important to note that the composite electrode was conditioned (continuously charged-discharged
at 1 A g-1 for 150 cycles) prior to further experiments to achieve material’s full potential (Fig. 8c).
The conditioning process ensures complete wetting of the composite electrode surface and redeems
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ions that are trapped in the preliminary cycles [27,55]. The specific capacitance of the composite
electrode (~115 F g-1) has improved by 20% to ~137 F g-1 after 150 cycles of conditioning (Fig. 8c).
Experiments repeated for new samples confirmed the necessity of conditioning. Fig. 8d shows the
galvanostatic charge-discharge (GCD) curves of the Fe3O4/C nanocomposite electrode that were
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recorded at various current rates (0.25-2 A g-1) after conditioning. The curves exhibit nearly
symmetric shape with slight distortions suggesting the accompaniment of the EDL capacitance by
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the pseudocapacitive redox reactions.
Fig. 8 Cyclic voltammetry curves of (a) Fe3O4/C nanocomposite at various scan rates (b) stainless
steel (SS) blank and Fe3O4/C nanocomposite at 2 mV s-1; (c) 150 cycle conditioning of Fe3O4/C
nanocomposite at 1 A g-1 (d) charge-discharge curves of conditioned Fe3O4/C nanocomposite
electrode at various current rates in 1M Na2SO4 (-1.2 - 0 V vs. Ag/AgCl)
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Table 2 summarizes the specific capacitances of the Fe3O4/C nanocomposite obtained at
various current rates (0.25 to 5 A g-1). The gradual decrease in capacitance with increasing current
rate is due to the insufficient time for electrolyte ions to intercalate into the inner surfaces, offering
less electroactive surface area for charge storage and thus a lower specific capacitance [20,31]. At
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higher current rates, only the outer surface of the active material can participate in energy storage
mechanisms due to the limited diffusion time [18,20,31,37]. Small voltage (IR) drops observed at the
start of the discharge curves are related to the equivalent series resistance (ESR) phenomenon [56].
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Table 2. Specific capacitances of the Fe3O4/C nanocomposite electrode measured at different current
Sample
0.25A g-1 0.50A g-1 0.75A g-1
1.0A g-1 1.5A g-1 2A g-1 3A g-1 4A g-1 5A g-1
280.8
136.2
178.8
149.8
115.9
97.2
75.2
60.9
50.2
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Fe3O4/C
Specific Capacitance/ F g-1
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rates in 1M Na2SO4
The energy and power densities of the Fe3O4/C nanocomposite electrode at 1 A g-1 are
calculated to be 27.2 Wh kg-1 and 705.5 W kg-1, respectively (discharge time= 139 s). At 5 A g-1
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current rate, an increased power density of 6255.5 W kg-1 was obtained while ~40% of the original
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energy density (10.0 Wh kg-1) was preserved (discharge time= 6 s). Comparison of our results to
previously reported iron oxide based electrode results is given in Table 3. The Fe3O4/C
nanocomposite electrode we have prepared in this work shows higher energy densities than recently
reported values [31,32,57] even at high current loadings due to its extended operational potential
window of 1.2 V.
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Potential
range (V)/
Electrolyte
Reference
electrode
Current
density
(A g-1)
Specific
capacitance
(F g-1)
Fe3O4 nanosheets
modified with carbon
nanofibers
-0.9 - 0.1/
1M Na2SO3
SCE
0.42
Reduced graphene oxideFe3O4 composite
0 - 0.8/
0.5M Na2SO4
Ag/AgCl
1
Fe3O4/carbon
nanocomposite obtained
from metal-organic
framework
-1 - 0/
1M KOH
Ag/AgCl
Fe3O4@C core-shell
microspheres
-1 - 0.5/
1M KOH
Fe3O4/C nanocomposite
-1.2 - 0/
1M Na2SO4
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Sample
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Table 3. Comparison of our results to previously reported Fe3O4-based composites.
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1
Ref.
135
[58]
154
[31]
139
[59]
0.5
110.8
[36]
Ag/AgCl
1
136.2
This
work
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SCE
Decent cycling stability is essential for practical applications. Due to the volume changes
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arising from the redox reactions, metal oxides generally exhibit much shorter cycle lives than
carbonaceous materials. Incorporation of pseudo-capacitive metal oxides into carbonaceous materials
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C
improves the degree of reversibility and cycling stability [56]. Cycling performance of Fe3O4/C
nanocomposite was investigated at 2 A g-1 by continuous charge-discharge between -1.2 and 0 V vs.
Ag/AgCl for 1000 cycles. The capacitance of Fe3O4/C nanocomposite (~115 F g-1) remains stable
for 1000 cycles without an obvious decay (Fig. 9) as the conductive carbon matrix buffers the
probable volume changes and prevents aggregation of Fe3O4 nanoparticles upon continuous chargedischarge process [37].
19
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Fig. 9 Cycling stability of Fe3O4/C nanocomposite at 2 A g-1 rate (-1.2 - 0 V vs. Ag/AgCl in 1M
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Na2SO4, 1000 cycles)
4. Conclusions
Spherical Fe3O4 nanoparticles (ca. 10 nm in diameter) were obtained via practical coprecipitation technique. Highly crystalline, uniform and bifunctional Fe3O4 nanoparticles exhibiting a
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superparamagnetic character with an Ms (saturation magnetization) of 64 emu g-1 at 298 K and a soft
ferromagnetic character with an Ms of 71 emu g-1 at 10 K promise for biomedical applications.
Hierarchically porous Fe3O4/C nanocomposite presenting a high specific surface area of 344 m2 g-1
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was prepared via eco-friendly and cost-effective route composed of hydrothermal carbonization and
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MgO templating methods. The resulting composite material, synergistically coupling the EDL
capacitance of carbon with the faradaic pseudocapacitance of Fe3O4, showed a specific capacitance
of 136 F g-1 at 1 A g-1 in 1M Na2SO4 (-1.2 - 0 V vs. Ag/AgCl). The corresponding energy and power
densities were calculated as 27.2 Wh kg-1 and 705.5 W kg-1, respectively. Furthermore, Fe3O4/C
nanocomposite showed stable cycling performance with no capacitance decay for 1000 cycles at 2 A
g-1. Compared to previous reports on Fe3O4 composites with more sophisticated carbons, such as
graphene, nanotubes and etc., our Fe3O4/C sample exhibits higher energy density. As Fe3O4/C is able
to operate within a large negative potential range in a neutral medium, it can be coupled with a
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suitable positive electrode (e.g. carbon, MnO2) in an asymmetric supercapacitor to extend the
potential window and thus the energy density of the device.
Acknowledgements
Authors are grateful to TUBITAK (the Scientific and Technological Research Council of Turkey) for
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funding this work (Project No: 112T570) and the Middle East Technical University Central
Laboratory for their support in material characterizations.
T. Ma, H. Yang, L. Lu, Development of hybrid battery-supercapacitor energy storage for
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[1]
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•
Fe3O4 (magnetite) particles with ~10nm dia. were prepared by a facile chemical coprecipitation.
•
Fe3O4 nanospheres are superparamagnetic at 298K with high saturation magnetization
of 64 emu g-1.
Porous Fe3O4/C nanocomposite was also prepared by a green HTC method combined
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•
with MgO templating.
•
Electrochemical properties of Fe3O4/C were studied in 1M Na2SO4 (between -1.2 and
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Nanocomposite electrode showed high energy density of 27.2 Wh kg-1 at 1 A g-1.
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•
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0 V vs. Ag/AgCl).