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Nano iron oxide (Fe2O3)/carbon black electrodes for

electrochemical capacitors

Mahdi Nasibi a,n, Mohammad Ali Golozar b, Gholamreza Rashed a

a Technical Inspection Engineering Department, Petroleum University of Technology, Abadan, Iranb Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

a r t i c l e i n f o

Article history:

Received 8 May 2012Accepted 28 June 2012Available online 6 July 2012

Keywords:

Nano iron oxide

Microstructure

Electrochemical capacitors

Energy storage and conversion

a b s t r a c t

In this research, nano iron oxide (Fe2O3)/carbon black electrodes are prepared by mechanical pressing

method and evaluated as possible electrodes for electrochemical capacitors. Electrochemical propertiesof the produced electrodes are studied using cyclic voltammetry (CV) and electrochemical impedance

spectroscopy (EIS) tests in 2 M KCl electrolyte at different scan rates. Scanning electron microscopy

(SEM) is also used to characterize the microstructure and nature of the produced electrodes.

Electrochemical stability of the electrodes is investigated by switching the electrode back and forth

for 500 cycles at 20 mV s1.The results obtained show a specific capacitance of as high as 40.07 F g1

for 30:60:10 (carbon black:Fe2O3:polytetrafluoroethylene) electrode in 2 M KCl at 10 mV s1.

The proposed electrode exhibits good cyclic stability and maintains 80% of the capacitance after 500

cycles. SEM images confirm the porous structure of Fe2O3/carbon black electrodes.

Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction

Although the need for energy and energy storage will be far

greater in the future, the problem of ensuring power quality is

already upon us. Energy storage is being widely regarded as one

of the potential solutions to deal with the variations of variable

renewable electricity sources and it is the key to unlocking the

door renewable energy. Among different energy storage systems,

electrochemical capacitors store the electrical energy in interfaces

formed at a solid/electrolyte interface. Positive and negative ionic

charges within the electrolyte accumulate at the surface of the

solid electrode and compensate the electronic charge at the

electrode surface [1]. High cycle life, high life time, high energy

efficiency ranging from 85% up to 98% and high self-discharge

rate are some of the characteristics of supercapacitors [2,3].

Today, many laboratories are actively engaged in developmentof well-known type of supercapacitors, viz., electrochemical

double-layer, pseudo and hybrid supercapacitors, and most

researches have been focused on the development of different

electrode materials[4,5].

The aim of this work is to fabricate nano Fe2O3/carbon black

electrodes using mechanical pressing as a fast method. Then the

products were evaluated as possible candidate electrode for

electrochemical capacitors using cyclic voltammetry, electroche-

mical impedance spectroscopy and scanning electron microscopy.

2. Experimental

Materials: High purity (499%) nano iron oxide (Fe2O3)

(o50 nm), nickel foil (99.99% with 0.125 mm thickness) and

polytetrafluoroethylene (o2 mm) were purchased from Aldrich,USA. All other chemicals used in this study were purchased from

Merck, Germany. In order to prepare the electrodes, the mixture

containing different wt% Fe2O3and carbon black (CB) and 10 wt%

polytetrafluoroethylene (PTFE) was mixed well in ethanol to form

a paste and then was pressed onto the nickel foil (25 MPa), which

served as a current collector (surface was 0.785 cm2). The typical

mass weight of electrode material was 30 mg. The used electrolytewas 2 M KCl.

Characterization: Electrochemical behavior of Fe2O3/carbon

black electrodes was characterized using cyclic voltammetry

(CV) and electrochemical impedance spectroscopy (EIS) tests.

The electrochemical measurements were performed using an

Autolab (Netherlands) potentiostat Model PGSTAT 302 N. CV tests

were conducted at various scan rates (s) with recording of

potential response currents, I, which is related by CI/s where C

is the capacitance of the electrode interface. EIS measurements

were also carried out in the frequency range from 100 kHz to

0.005 Hz at open circuit potential with ac amplitude of 10 mV.

The specific capacitance C (F g1) of the active material was

Contents lists available at SciVerse ScienceDirect

journal homepage: www.elsevier.com/locate/matlet

Materials Letters

0167-577X/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.matlet.2012.06.109

n Corresponding author. Tel.: 98 911 3708480; fax: 98 631 4423520.

E-mail address: [email protected] (M. Nasibi).

Materials Letters 85 (2012) 4043
http://www.elsevier.com/locate/matlethttp://www.elsevier.com/locate/matlethttp://localhost/var/www/apps/conversion/tmp/scratch_2/dx.doi.org/10.1016/j.matlet.2012.06.109mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_2/dx.doi.org/10.1016/j.matlet.2012.06.109http://localhost/var/www/apps/conversion/tmp/scratch_2/dx.doi.org/10.1016/j.matlet.2012.06.109mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_2/dx.doi.org/10.1016/j.matlet.2012.06.109http://localhost/var/www/apps/conversion/tmp/scratch_2/dx.doi.org/10.1016/j.matlet.2012.06.109http://localhost/var/www/apps/conversion/tmp/scratch_2/dx.doi.org/10.1016/j.matlet.2012.06.109http://www.elsevier.com/locate/matlethttp://www.elsevier.com/locate/matlet


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determined by integrating either the oxidative or the reductive

parts of the cyclic voltammogram curve to obtain the voltam-

metric chargeQ(C). This charge was divided by the mass of active

material m (g) in the electrode and the width of the potential

window of the cyclic voltammogramDE(V), i.e., CQ/(DEm)[6].

The morphology and the nature of porous electrodes were studied

using scanning electron microscopy (TESCAN, USA).

3. Results and discussion

Specific surface area and porosity are two important para-

meters to prepare high efficient electrodes for electrochemical

capacitors. The morphology and the nature of the as-prepared

electrodes were analyzed using SEM. As shown in Fig. 1, images

show good dispersion of the nanoparticles on the surface of the

electrodes. This increases the specific surface area and makes the

surface of the electrodes porous. This porous surface greatly

improves the charge storage and charge delivering capability of

the electrodes.

Fig. 2(a) shows the CVs of various Fe2O3-containing electrodes

at the scan rate of 10 mV s1 in 2 M KCl electrolyte. CVs exhibit a

rectangular shaped profile (Fig. 2(a)), which is a characteristic of

ideal capacitive behavior [7,8]. The electrodes exhibited almost

potential-independent capacitance for which current leakage may

be the reason[7]. As the Fe2O3 nanoparticles were added to the

electrode material, potential window was shifted to the negative

direction of potentials to overcome the overcharging and/or

overdischarging of the electrodes. This could result in a decrease

in the active materials available for further cycling. As the nano

material content increased (up to 80%) current peak decreased

and caused to lower the calculated capacitance. This current peak

reduction may be due to increase in the electrode resistance

(Fig. 2(b)) in the presence of iron oxide nanoparticles. The point of

intersecting with the real axis of Nyquist curves in the range of

high frequency (Fig. 2(b)) is the equivalent series resistance (ESR).

It indicates the total resistance of the electrode, the bulk electro-

lyte resistance and the resistance at the electrolyte/electrodeinterface [9]. As shown in Fig. 2(b), an obvious difference

observed among these three Nyquist spectra is the pseudo-charge

transfer resistance (Rct) in high Fe2O3-containning electrodes. This

could be attributed to the Faradic reactions of active materials on

the electrode surface. Generally, by addition of Fe2O3 nanoparti-

cles into the electrode material, the following three parameters

will act simultaneously: Faradic reactions on the metal oxide

particles, increase in the specific surface area in the presence of

nanoparticles and increase in the electrical resistance of the

electrode. Finally, it is concluded that the 30:60:10 electrode

shows as high as 40.07 F g1 capacitance in 2 M KCl electrolyte at

a scan rate of 10 mV s1. This electrode was selected for further

investigation, i.e. scan rate and chargedischarge cycles.

The CV curves at various scan rates between 10 and

100 mV s1 for 30:60:10 electrodes are shown in Fig. 3(a). As

the potential scan rate is increased no obvious distortion in the CV

Fig. 1. SEM images obtained from 30:60:10 electrode showing porous surface.

Fig. 2. (a) CV curves and (b) Nyquist diagrams for various Fe2O3-content electro-

des in 2 M KCl electrolyte.

M. Nasibi et al. / Materials Letters 85 (2012) 4043 41


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curves is observed. Even at a high scan rate of 100 mV s 1, the CV

curves still retain their rectangular shape. However, CV curves at

such a high scan rate reveal a low capacitance (due to energy

losses) and a low current response on voltage reversal at each end

potential. It has been proposed that the low current response on

the voltage reversal represents a high ESR of the electrode as well

as a low diffusion of electrolyte ions within the pores of the

electrode[10]. In the case of Fe2O3/CB electrodes, Fe2O3 particles

(o50 nm) on the surface of the carbon black particles can greatly

reduce the diffusion length of K during the chargedischarge

processes.

In order to gain the quantitative information on the utilization

of 30:60:10 Fe2O3/CB electrodes, the voltammograms were ana-

lyzed as a function of scan rate, according to the procedure

reported by Ardizzone et al. [11]. The scan rate dependence of

the capacitance can be related to the less accessible surface area

(pores, cracks, etc.) which become excluded as the rate reaction is

enhanced. In charge and discharge cycles, the total charge can be

written as the sum of an inner charge from the less accessible

reaction sites and an outer charge from the more accessiblereaction sites, i.e., qnTq

n

I qn

O, where qn

T, qn

I and qn

O are the total

charge, and charges related to the inner and the outer surface,

respectively. The extrapolation ofqn tov0 from 1/qn vs. v1/2 plot

gives the total charge qnT of 33.3 C g1 cm2 which is the charge

related to the entire active surface of the electrode. In addition,

extrapolation of qn to vN (v1/20) from the qn vs. v1/2 plot

gives the outer charge qnO, 9.80 C g1 cm2, which is the charge

due to the redox process on the most accessible active surface

[12]. Therefore, this nanocomposite electrode shows low ratio of

the outer charge to total charge (qnO/qn

T) of 0.3, which confirms the

low current response of the prepared electrodes at high scan rate.

Regarding the practical applications, the cycle stability of

supercapacitors is a crucial parameter. The cycle lives of both

conducting polymers and metal oxides, as candidates for pseudo-

capacitive materials, are much shorter than those of the carbon

base materials because of the loss of active materials [4]. Incor-

poration of carbon black into the pseudo-capacitive materials is

an effective method to improve their cycle performance. In the

case of nano Fe2O3/CB electrodes the cycle stability was evaluated

by repeating the CV at a scan rate of 20 mV s1 for 500 cycles

(Fig. 3(b)). Simultaneously, EIS tests were used to evaluate the

electrode changes (Fig. 3(c)). These electrodes were found to

exhibit excellent stability over the entire cycle numbers

(Fig. 3(b)). In these electrodes, carbon black can provide con-

ductive channels due to the excellent conductivity; but, during

the 500 chargedischarge cycles the equivalent series resistance

increases (Fig. 3(c)) and the Nyquist plots shift to some higher

values which can be attributed to the electrolyte decomposition

on the electrode surface. Increased electrode resistance with

increasing cycle number can decrease the capacitance (20% after

500 cycles). Electrolyte deposition and active material dissolution

during cycling decrease the specific surface area. So fewer

electrochemically active sites were available for energy storage.

4. Conclusions

In summary, nano Fe2O3/carbon black is a good candidate

electrode material for electrochemical capacitors at low scan

rates. This is based on the good electrochemical performance

observed in the potential range from 0.3 to 0.25 V (vs. SCE) in

2 M KCl electrolyte. The surface showed a porous structure that

greatly improves charge storage and charges delivery of electrode

capability. It shows a pseudo-capacitance characteristic and low

ratio of the outer charge to total charge (qnO/qn

T) of 0.3. The

30:60:10 (CB:Fe2O3:PTFE) electrode has as high as 40.07 F g1

specific capacitance in 2 M KCl electrolyte at a scan rate of

10 mV s1 and showed a good cycling performance.

References

[1] Kotz R, Carlen M. Electrochim Acta 2000;45:248398.[2] Hadjipaschalis I, Poullikkas A, Efthimiou V. Renew Sustain Energy Rev

2009;13:151322.[3] Sharma P, Bhatti TS. Energy Convers Manage 2010;51:290112.[4] Zhang Y, Feng H, Wu X, Wang L, Zhang A, Xia T, et al. Int J Hydrogen Energy

2009;34:488999.

Fig. 3. (a) CV curves obtained at various scan rates, (b) representative cyclicvoltammograms obtained at 20 mV s1 and (c) Nyquist plots before and after 500

CV cycles for 30:60:10.

M. Nasibi et al. / Materials Letters 85 (2012) 404342


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