Electrochemical characterization of the redox couple of Fe(III)/Fe(II) mediated by grafted polymer electrode

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1 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 Res Chem Intermed DOI /s

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3 Res Chem Intermed DOI /s 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 K 3 Fe(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 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), Serdang, Selangor D.E., Malaysia

4 M. M. Radhi et al. 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 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 Scm -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

5 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 Fe 3? in the presence (Fig. 1a) and absence of GP (Fig. 1b) shows the enhancement of the redox current of Fe 3?. 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

6 M. M. Radhi et al. Fig. 1 Cyclic voltammetry of different electrodes a GP/Pt electrode, b Pt wire electrode, and c GCE in 1mMK 3 Fe(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) 6 3- 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 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) 6 3- 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.

7 Electrochemical characterization of the redox Fig. 2 Cyclic voltammetry of GPE in different concentrations of 1 mm K 3 Fe(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).

8 M. M. Radhi et al. Fig. 4 a Cyclic voltammetry of different electrodes a GPE, b GCE, c Au-electrode, d Pt-electrode in 1mMK 3 Fe(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 K 3 Fe(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: KClO 4 [ K 2 SO 4 [ K 2 HPO 4 [ KCl [ KNO 3 Reduction peak: KClO 4 [ KNO 3 [ K 2 SO 4 [ K 2 HPO 4 [ KCl

9 Electrochemical characterization of the redox Table 1 Cyclic voltammetry of 1 mm potassium ferricyanide in different electrolytes: 0.1 M of KCl, KNO 3,K 2 HPO 4,K 2 SO 4, and KClO 4 at scan rate 100 mv s -1 for the GCE and GPE Electrolyte Ipa (la) GCE Ipa (la) GPE Enhancement Ipc (la) GCE Ipc (la) GPE Enhancement KCl KNO K 2 HPO K 2 SO KClO 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 K 3 Fe(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 = ? with R 2 = 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 K 3 Fe(CN) 6 was studied on GPE. The current increases gradually at a temperature of C. Figures 7 and 8 show plots of log (oxidation current) and log (reduction current) of K 3 Fe(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 r 0 Exp ð E a =RTÞ ð1þ D ¼ D 0 Exp ð E a =RTÞ ð2þ where r/d are conductivity/diffusibility and r 0 /D 0 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 = kj/mol for Fe(III)/Fe(II) and Ea = 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].

10 M. M. Radhi et al. Fig. 5 Cyclic voltammetry of GPE in 0.1 M KCl and 1 mm K 3 Fe(CN) 6 at different scan rates (5,10, 50, 100, 250 and 500 mv s -1 ) versus Ag/AgCl and 100 mv s -1 LogIpa y = x R 2 = logv Fig. 6 Plot log Ipa (anodic current) versus log SR (scan rate) of GPE in 0.1 M KCl and 1 mm K 3 Fe(CN) 6 versus Ag/AgCl Effect of varying K 3 Fe(CN) 6 concentration Figure 9 shows the linear current dependent on K 3 Fe(CN) 6 concentration observed at the concentration range of 1 10 mm which is described by the equation of y = with R 2 = The slope of the linear line for K 3 Fe(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

11 Electrochemical characterization of the redox y = x R 2 = LogIpa /TX0.001 Fig. 7 Plot of log(ipa) versus 1/T910-3 of 1 mm K 3 Fe(CN) 6 in 0.1 M KCl at different temperatures (10 90 C) using GPE versus Ag/AgCl and 100 mv s y = x R 2 = Log(Ipc) /TX0.001 Fig. 8 Plot of log(ipc) versus 1/T910-3 of 1 mm K 3 Fe(CN) 6 in 0.1 M KCl at different temperatures (10 90 C) using GPE versus Ag/AgCl and 100 mv s -1 Current, ua y = x R 2 = Concentration, mm Fig. 9 Plot Ipc (cathodic current) versus different concentration K 3 Fe(CN) 6 (1 10 mm) in 0.1 M KCl at scan rate 100 mv s -1 using GPE versus Ag/AgCl

12 M. M. Radhi et al. Fig. 10 Chronoamperograms or Cottrell plot obtained for 1 mm K 3 Fe(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 K 3 Fe(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 cm 2 s -1. It was found that the GPE has a total charge transferred of 12.8 lc m -2 in Fe(CN) 6 3- ion as in Fig. 11.

13 Electrochemical characterization of the redox Fig. 12 Cyclic voltammetry of GPE in 0.1 M KCl and 1 mm K 3 Fe(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 K 3 Fe(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) 6 3- increases at a 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.

14 M. M. Radhi et al. Fig. 13 Scanning electron micrographs of mv s -1 grafted 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 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 K 3 Fe(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

15 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) 6 3- solution, and although many of the 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 K 3 Fe(CN) 6 solution. Redox peaks of Fe(CN) 3-6 /Fe(CN) 4-6 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 K 3 Fe(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. References 1. S.H. Hosseini, J. Simiari, B. Farhadpour, Iran. Polym. J. 18, 13 (2009) 2. E.A.J. Al-Mulla, Korean J. Chem. Eng. 28, 620 (2011) 3. G. Natta, P. Corradini, I.W. Bassi, Nuovo Cimento 15, 82 (1960) 4. E.A.J. Al-Mulla, Polym. Sci. Ser. A 53, 149 (2011) 5. H. Hosseini, M.A. Dabiri, J. Polym. Int. 55, 1081 (2006) 6. E.A.J. Al-Mulla, N.A.B. Ibrahim, K. Shameli, M.B. Ahmad, W.M.W. Yunus, Res. Chem. Intermed. (2012). doi: /s J.X. Thomas, C.A. Wilkie, Polym. Degrad. Stab. 56, 109 (1997) 8. E.A.J. Al-Mulla, Fiber Polym. 12, 444 (2011) 9. A.M. Jendrychowska, C. Michoi, Euro. Polym. J. I, 251 (1977) 10. C. Chuy, J. Ding, E. Swanson, S. Holdcroft, J. Horsfall, K.V. Lovellb, J. Electrochem. Soc. 150, 271 (2003) 11. E.D. Labaye, C. Jérôme, V.M. Gueskin, P. Louette, R. Lazzaroni, L. Martinot, J. Robert, Am. Chem. Soc. 18, 5222 (2002) 12. B. Noëlle, M. Lucien, J. Robert, J. Electroanal. Chem. 472, 83 (1999) 13. A.A. Abdullah, E.A.J. Al-Mulla, S.A. Aowda, Res. Chem. Intermed. (2012). doi: /s x 14. H.S. Hossein, M. Dabiri, M. Ashrafi, Polym. Inter. 55, 1081 (2006) 15. W.T. Tan, M.M. Radhi, M.Z.B. Ab Rahman, A.B. Kassim, J. Appl. Sci. 10, 139 (2010) 16. W.T. Tan, J.K. Goh, J. Electroanal. 20, (2008) 17. S.R. Jacob, Q. Hong, B.A. Coles, R.G. Compton, J. Phys. Chem. 103, 2963 (1999)

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