Electrochemical characterization of the
redox couple of Fe(III)/Fe(II) mediated by
grafted polymer electrode
M. M. Radhi, Emad A. Jaffar Al-Mulla &
W. T. Tan
Research on Chemical Intermediates
ISSN 0922-6168
Res Chem Intermed
DOI 10.1007/s11164-012-0954-6
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Res Chem Intermed
DOI 10.1007/s11164-012-0954-6
Electrochemical characterization of the redox couple
of Fe(III)/Fe(II) mediated by grafted polymer electrode
M. M. Radhi • Emad A. Jaffar Al-Mulla • W. T. Tan
Received: 5 October 2012 / Accepted: 23 November 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract A new grafted polymer electrode (GPE) (polystyrene as polymer) was
grafted with acrylonitrile as a monomer using gamma irradiation to produce a new
grafted polymer. The redox process of K3Fe(CN)6 during cyclic voltammetry was
studied by the new GPE. The ratio of Ipc/Ipa [1 of GPE to GCE Ipc/Ipa = 1.7,
indicating that this electrode is a reversible electrode and can be used in conductivity studies by voltammetric analysis. The physical properties of the new electrode
GP have good hardness, insolubility, and stability at different high temperatures and
at different pH. Also, the sensitivity under conditions of cyclic voltammetry is
significantly dependent on pH, electrolyte, and scan rate. At different scan rates, two
oxidation peaks and two reduction peaks of Fe(III) were observed in a reversible
process: Fe(III) Fe(II), and Fe(II) Fe(0). Interestingly, the redox reaction of Fe(III)
solution using GPE remained constant even after 15 cycles. It is therefore evident
that the GPE possesses some degree of stability. The potential use of the grafted
polymer as a useful electrode material is therefore clearly evident.
Keywords Grafted polymer electrode Redox couple Fe(III)/Fe(II) Electrocatalysis Cyclic voltammetry
M. M. Radhi (&)
Department of Radiological Techniques, College of Health and Medical Technology, Baghdad, Iraq
e-mail: mmradhi@yahoo.com
E. A. Jaffar Al-Mulla
Department of Chemistry, College of Science, University of Kufa, P.O. Box 21,
An-Najaf 54001, Iraq
E. A. Jaffar Al-Mulla W. T. Tan
Department of Chemistry, Faculty of Science, University Putra Malaysia (UPM),
43400 Serdang, Selangor D.E., Malaysia
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Introduction
Working electrodes act as a source or sink of electrons for exchange with molecules
in the interfacial region, and must also be electronic conductors and electrochemically inert. Commonly used working solid electrode materials for cyclic
voltammetry include platinum, gold, and glassy carbon. Other materials (e.g.,
semi-conductors, for example, ITO, indium-tin oxide, conductive polymers or
grafted polymers) are also used for more specific applications. Polystyrene is a
polymer made from the monomer styrene. At room temperature, polystyrene is
normally a solid thermoplastic, colorless and harder plastic, and it has low thermal
conductivity 0.08 W/(mk), while electrical conductivity (r) is 10–16 S m-1. A
common thermoplastic is a copolymer of acrylonitrile and styrene, toughened with
polybutadiene. Conductive polymers are good candidates for preparation of
conducting grafted copolymers [1–6]. The thermal degradation of a grafted
copolymer in which acrylonitrile is grafted on to polystyrene has been studied by
thermogravimetric analysis [7, 8]. Polystyrene–polyacrylonitrile copolymers have
been prepared by the direct radio-induced grafting method [9]. Grafted copolymers
were prepared by copolymerization of acrylonitrile with poly (sodium styrene
sulfonate) macromonomers. Macromonomers were prepared by stable free radical
polymerization, and then the ionic conductivity of the new copolymer was studied
by electrochemical reduction [10, 11]: electrochemical synthesis of electrically
conducting polymers chemically grafted to conducting surfaces (e.g., glassy carbon,
stainless steel, nickel, gold). This is based on new functional acrylate monomers,
i.e., 3-(2-acryloyloxyethyl) thiophene and N-(2-acryloyloxyethyl) pyrrole. Cyclic
voltammetry was used to confirm that the two-component film is conducting and
electrochemically active (reversible doping and dedoping) [12]. The cathodic
electropolymerization of acrylonitrile (AN), ethylacrylate (EA), and methylmethacrylate (MMA) has been monitored for the first time by coupled electrochemical
quartz crystal microbalance (QCM) and cyclic voltammetry analyses [13]. A new
conducting copolymer, polyacrylonitrile-graft-polyaniline (PAN-g-PANi), has been
prepared by chemical and electrochemical methods from a precursor polymer.
Electrical conductivity of the copolymer was studied using the four-probe method,
which gave a conductivity of 4.5 9 10-3 S cm-1 with 51.4 % PANi [14].
In this work, a grafted polymer electrode (GPE) was fabricated from 15 % grafted
polystyrene with acrylonitrile using gamma irradiation. This new working electrode was
successfully used in cyclic voltammetry with large advantages in comparison with other
working electrodes like GCE, Pt-E, and Au-E. GPE was characterized electrochemically
in aqueous electrolyte in which it proved successful with good results.
Experimental
Synthesis of grafted polymer (GP)
Polystyrene as a polymer was grafted using acrylonitrile as a monomer using
gamma-irradiation, with chloroform as a solvent and ferrous ammonium sulphate
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Electrochemical characterization of the redox
(FAS) as a catalyst. The new grafted polymers obtained in different percentages of
grafting have been investigated and characterized [11].
Instrument and electroanalytical methods
Electrochemical workstations of Bioanalytical System, USA (Model BAS 50 W
with potentiostat driven by electroanalytical measuring software) were connected to
a PC computer in order to perform cyclic voltammetry (CV), chronoamperometry
(CC), and chronoamperometry (CA). An Ag/AgCl (3 M NaCl) and platinum wire
(1 mm diameter) were used as the reference and counter electrodes, respectively.
The working electrode used in this study was a grafted polymer electrode (GPE)
(see below). The voltammetric experiments were carried out with 0.1 M KCl as
supporting electrolyte. The solution was degassed with nitrogen for 10 min prior to
recording the voltammogram.
Reagents
All reagents were analytical reagents or electrochemical grade purity. All solutions
were prepared using distilled water. Unless otherwise specified, the supporting
electrolyte was used 0.1 M KCl in aqueous media at room temperature.
Preparing the new grafted polymer electrode (GPE)
The grafted polymer electrode (GPE) has been fabricated from grafted polymer. The
percentage of grafted polymer and the diameter of electrode were 15 % and 3 mm,
respectively. A hole was bored (1 mm) to allow 1 cm length of platinum wire to
come out from the other side of the electrode. A copper wire was then joined with
the platinum wire. After that, all parts of the fabricated electrode was covered with a
glass tube and then fixed with epoxy resin.
Results and discussion
Effect of GPE on the redox reaction of potassium ferricyanide during CV
Potassium ferricyanide is commonly used as a reference standard for the purpose of
calibrating a voltammetric system in aqueous solutions. During the calibration
process of the electroanalytical workstation (BAS Model 50 W) using GCE, Ptwire, and GPE, the oxidative current of the Fe(III)/Fe(II) redox couple appeared to
be significantly enhanced by the GPE as in Fig. 1.
Comparison of the cyclic voltammetric of Fe3? in the presence (Fig. 1a) and
absence of GP (Fig. 1b) shows the enhancement of the redox current of Fe3?.
It was also observed that the redox potential with Epa = ?227 mV and
Epc = ?320 mV with peak separation of 93 mV (for n = 1) in 0.1 M KCl
electrolyte indicates the reversible reaction of the Fe(III)/Fe(II) couples in KCl
solution, and agrees well with the accepted values. In subsequent studies, various
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Fig. 1 Cyclic voltammetry of different electrodes a GP/Pt electrode, b Pt wire electrode, and c GCE in
1 mM K3Fe(CN)6 with 0.1 M KCl as supporting electrolyte versus Ag/AgCl and 100 mV s-1
chemical and physical effects were assessed in order to determine the optimum
conditions under which maximum current response at the GPE can be obtained.
Figure 2 shows the two redox peaks of Fe(CN)36 when the working electrode
GPE is used as the working electrode in 0.1 M KCl as the supporting electrolyte.
The first redox process of Fe(III)/Fe(II) produced ?0.1 and ?0.3 V for reduction
and oxidation peaks, respectively, while the second redox process of Fe(II)/Fe(0) at
-0.3 V can be assigned for the reduction and -0.05 V for oxidation.
Potential window of different electrodes
The working potential window of GCE (where electroactivity due to electrode
materials and electrolyte is negligible) was found to be in the range (-1.5 to
?1.2 V) with small significant peaks appearing at -0.4 and ?1.0 V. A considerable
extension of the working window with the absence of each, especially at the anodic
region, is observed when GPE is used. Although the working window at the anodic
and cathodic regions deteriorates, this is due to the ‘‘electrocatalyzed’’ oxidation–
reduction of the aqueous electrolyte or the reactivity of GPE giving rise to a limiting
current appearing at about -1.5 to ?2.0 V. This limiting current, however,
diminishes in subsequent cycles allowing the potential working window to be
widened at the anodic region to near 2.0 V as shown in Fig. 3. This shows that GPE
is useful for redox reaction studies appearing at a more negative potential of up to at
least -2.0 V especially if the 2nd cycle is used. Figure 4a, b shows the CV of
Fe(CN)36 in 0.1 M KCl is taken at varying working electrodes. In general, degrees
of sensitivity of response that is evident for various solid electrodes appear as:
GPE [ GCE [ Au-E [ Pt-E.
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Fig. 2 Cyclic voltammetry of GPE in different concentrations of 1 mM K3Fe(CN)6 and 0.1 M KCl two
redox peaks of Fe versus Ag/AgCl and 100 mV s-1
Fig. 3 Cyclic voltammogram of (a) GCE, (b) GPE in 0.1 M KCl. SR = 100 mV s-1 versus Ag/AgCl
It is interesting to note that GPE has a quite similar enhancement in capacitance
current as compared with the GC electrode (3 mm diameter) followed by Au and Pt
(1 mm diameter) due to area effect over potential.
Effect of varying supporting electrolyte
The different type of supporting electrolyte exerts slight influence on the redox peak
potential of Fe(III) as expected, especially in non-complexing solutions (Table 1).
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Fig. 4 a Cyclic voltammetry of different electrodes a GPE, b GCE, c Au-electrode, d Pt-electrode in
1 mM K3Fe(CN)6 with 0.1 M KCl as supporting electrolyte versus Ag/AgCl and 100 mV s-1. b Cyclic
voltammetry of different electrodes a Pt-electrode, b Au-electrode, c GCE in 1 mM K3Fe(CN)6 with
0.1 M KCl versus Ag/AgCl and 100 mV s-1
In general, redox peaks of Fe(III) appear to have a similar pattern of peak formation
in various supporting electrolytes. However, the redox current peak of the Fe(II)/
Fe(III) couple shows that the greatest enhancement effect is obtained when
potassium perchlorate is used as electrolyte. In general, the degree of redox current
enhancement in varying electrolyte varies in the following order:
Oxidation peak: KClO4 [ K2SO4 [ K2HPO4 [ KCl [ KNO3
Reduction peak: KClO4 [ KNO3 [ K2SO4 [ K2HPO4 [ KCl
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Table 1 Cyclic voltammetry of 1 mM potassium ferricyanide in different electrolytes: 0.1 M of KCl,
KNO3, K2HPO4, K2SO4, and KClO4 at scan rate 100 mV s-1 for the GCE and GPE
Electrolyte
Ipa (lA)
GCE
Ipa (lA)
GPE
KCl
51.10
253.2
KNO3
28.60
136.7
K2HPO4
26.20
K2SO4
17.03
KClO4
20.50
Enhancement
Ipc (lA)
GCE
Ipc (lA)
GPE
Enhancement
4.95
97.2
146.8
1.51
4.78
24.4
305.3
12.51
168.6
6.44
30.9
335.7
10.86
142.8
8.39
22.6
282.5
12.50
435.4
21.24
30.9
900.9
29.16
Effect of varying scan rate
The effect of varying scan rates on the cyclic voltammograms using GPE as
working electrode in 0.1 M KCl supporting electrolyte was studied with 1 mM
K3Fe(CN)6 over a scan rate range of 5–1,000 mV s-1. Oxidation and reduction
currents of the Fe(III)/Fe(II) couple was observed to increase with scan rate due to
heterogeneous kinetics and IR effect as shown in Fig. 5.
Based on a plot of log(Ipa) versus log (scan rate) for the oxidation current of the
first cycle, a straight line was obtained in Fig. 6 fulfilling the equation
y = 0.4388 9 ?1.7912 with R2 = 0.9988. A slope of 0.44 is quite comparable
with the theoretical slope of 0.5 for the diffusion controlled process.
Effect of varying temperature
Effect of temperature on the redox process of K3Fe(CN)6 was studied on GPE.
The current increases gradually at a temperature of 10–90 °C. Figures 7 and 8
show plots of log (oxidation current) and log (reduction current) of K3Fe(CN)6
versus the reciprocal of temperature, respectively, which is found to be fairly
linear in agreement with the thermodynamic expectation of Arrhenius (Eqs. 1, 2)
[15, 16]:
r ¼ r0 Exp ðEa =RTÞ
ð1Þ
D ¼ D0 Exp ðEa =RTÞ
ð2Þ
0
0
where r/D are conductivity/diffusibility and r /D are standard conductivity/the
initial diffusibility.
From the slope of the linear relationship, the value of activated energy of
reduction Fe(III)/Fe(II) is Ea = 23.3 kJ/mol and for oxidation Fe(II)/Fe(III) is
Ea = 1.75 kJ/mol compared with the GCE electrode which is Ea = 33.59 kJ/mol
for Fe(III)/Fe(II) and Ea = 57.894 kJ/mol for Fe(III)/Fe(II). The conductivity of
GPE with increasing temperature also has a significant influence on the activation
energy for diffusion of the substrate of interest, Ea. Also, the stability of GPE at
high temperature is better than other working electrodes [17].
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LogIpa
Fig. 5 Cyclic voltammetry of GPE in 0.1 M KCl and 1 mM K3Fe(CN)6 at different scan rates (5,10, 50,
100, 250 and 500 mV s-1) versus Ag/AgCl and 100 mV s-1
3.1
y = 0.4388x + 1.7912
3
2
2.9
R = 0.9988
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
0.69
1.19
1.69
2.19
2.69
3.19
logV
Fig. 6 Plot log Ipa (anodic current) versus log SR (scan rate) of GPE in 0.1 M KCl and 1 mM
K3Fe(CN)6 versus Ag/AgCl
Effect of varying K3Fe(CN)6 concentration
Figure 9 shows the linear current dependent on K3Fe(CN)6 concentration observed
at the concentration range of 1–10 mM which is described by the equation of
y = 41.802 9 –69.113 with R2 = 0.9484. The slope of the linear line for
K3Fe(CN)6 showed that a considerably high sensitivity response of 41.8 lA/mM
is readily obtained at the GP electrode during cyclic voltammetry.
Chronoamperometry and chronocoulometry studies
Figure 10 shows the monotonous rising and decaying current transient in
accordance with the theoretical expectation of the Cottrell equation [18–20] based
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Electrochemical characterization of the redox
2.75
y = -0.2098x + 3.2967
2
R = 0.9937
LogIpa
2.7
2.65
2.6
2.55
2.5
2.6
2.8
3
3.2
3.4
3.6
1/TX0.001
Fig. 7 Plot of log(Ipa) versus 1/T910-3 of 1 mM K3Fe(CN)6 in 0.1 M KCl at different temperatures
(10–90 °C) using GPE versus Ag/AgCl and 100 mV s-1
3.5
3.4
y = -2.8113x + 10.708
3.3
R = 0.9859
Log(Ipc)
2
3.2
3.1
3
2.9
2.8
2.7
2.6
2.65
2.7
2.75
2.8
2.85
1/TX0.001
Fig. 8 Plot of log(Ipc) versus 1/T910-3 of 1 mM K3Fe(CN)6 in 0.1 M KCl at different temperatures
(10–90 °C) using GPE versus Ag/AgCl and 100 mV s-1
390
y = 41.802x - 69.113
2
R = 0.9484
Current, uA
340
290
240
190
140
90
40
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
Concentration, mM
Fig. 9 Plot Ipc (cathodic current) versus different concentration K3Fe(CN)6 (1–10 mM) in 0.1 M KCl at
scan rate 100 mV s-1 using GPE versus Ag/AgCl
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Fig. 10 Chronoamperograms or Cottrell plot obtained for 1 mM K3Fe(CN)6 and 0.1 M KCl as
supporting electrolyte using GPE versus Ag/AgCl. Potential was scanned in a negative direction from
-1,800 to ?1,800 mV with 250 ms pulse width
Fig. 11 Choronocoulomogram or Anson plot of charge, versus t1/2 obtained for the redox of 1 mM
K3Fe(CN)6 and 0.1 M KCl as supporting electrolyte using GPE versus Ag/AgCl
on the diffusion process to a planar electrode. The diffusion coefficient (D) of 0.1 M
KCl using GPE as a working electrode is 3.2 9 10-7 cm2 s-1. It was found that the
GPE has a total charge transferred of 12.8 lC m-2 in Fe(CN)36 ion as in Fig. 11.
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Fig. 12 Cyclic voltammetry of GPE in 0.1 M KCl and 1 mM K3Fe(CN)6 at different pH (1, 2, 3, and 4)
versus Ag/AgCl and 100 mV s-1
Electrochemical characterization in electrolyte of varying pH
Figures 12 and 13 are cyclic voltammograms for studying the effect of different pH
of 1 mM K3Fe(CN)6 in 0.1 M KCl solution using HCl or NaOH solution on the
working GPE. It was found that the redox current of Fe(CN)3increases at a
6
solution of highly acidic pH, and 50 mV potential shifted to higher potential. For the
pH 11 electrolyte solution, redox peaks were shifted to the original potential with a
decrease of the redox current.
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Fig. 13 Scanning electron micrographs of mV s-1grafted polymer as electrode a before and b after
electroanalysis with potassium ferricyanide in aqueous solution of 0.1 M KCl
Scanning electron microscopy (SEM)
Scanning electron microscopy of the grafted polymer GP was studied using a JEOL
attached with Oxford Inca Energy 300 EDXFEL scanning electron microscope
operated at 20–30 kV. The scanning electron photographs were recorded at a
magnification of 100k9 depending on the nature of the sample. Samples were
dehydrated for 45 min before being coated with gold particles using the Baltec
SC030 Sputter Coater. SEM was used to examine the morphology of the grafted
polymer (GP) before (Fig. 13a) and after (Fig. 13b) electrolysis in K3Fe(CN)6 by
cyclic voltammetry. Before electroanalysis, the GP surface appears compact and
nonporous, while the uniformity of the GP surface is marred by a wavelike
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Electrochemical characterization of the redox
protrusion, but this slightly increases as the occurrence of the protrusion is an
observed phase of an absence of microcrystalline structure as shown in Fig. 13a.
After electroanalysis, the GPE was subjected to a continuous 10 potential cyclings
during CV in the presence of the Fe(CN)3solution, and although many of the
6
micro-particles still remained at about \1 lm, there were some of the GP/Fe(0)
micro-particles that appeared to increase in size. The slight increase in size could be
due to the presence of Fe(0) as shown in Fig. 13b.
Conclusion
A GPE has an extended potential working region as compared to GCE. The stability
of GPE as a working electrode was evaluated by using a K3Fe(CN)6 solution. Redox
4peaks of Fe(CN)36 /Fe(CN)6 obtained at GPE appears as a high current compared
with GCE. Although the oxidative and reductive region is limited with the high
range of ?2.0 to -2.0 V, the use of potential cycling helps to significantly widen
the redox working potential range. Electrocatalytic activity of GPE is therefore
evident in this study.
A new grafted polymer electrode (GPE) was studied by the redox process of
K3Fe(CN)6 during cyclic voltammetry, and the redox peaks potential shifted
slightly to a less negative value by about 100 mV for the oxidative peak and 50 mV
for the reductive peak with current enhancement of about three–fivefold. The
sensitivity under conditions of cyclic voltammetry is significantly dependent on the
concentration, pH, temperature, electrolyte, and scan rate.
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