Electrochemical Behavior and Enhanced Stability of a

Z. Phys. Chem. 217 (2003) 1369–1385  by Oldenbourg Wissenschaftsverlag, München

Electrochemical Behavior and Enhanced Stability of a Thin Film of Prussian Blue Deposited under Magnetic Field By Ali Eftekhari ∗ Electrochemical Research Center, P.O. Box 19395-5139, Tehran, Iran (Received January 18, 2003; accepted in revised form August 26, 2003)

Magnetic Field / Magnetohydrodynamics / Prussian Blue / Modified Electrode / Film Formation / Stability Magnetic field was employed as a useful tool to improve stability of electroactive films deposited on substrate surfaces. To this aim, electrochemical preparation/deposition of Prussian blue (PB) was carried out in the presence of an external magnetic field to form a thin film onto a Pt substrate electrode. The results were compared with a conventional PB modified electrode throughout the research. For the electrode prepared in the presence of the magnetic field, a stronger voltammetric behavior with higher peak current was observed. It was accompanied by higher chemical and electrochemical stabilities of the deposited electroactive film. Spectroscopic measurements were also performed to investigate the influence of the applied magnetic film on the properties of the electroactive film deposited. The results were indicative of the fact that deposition under magnetic filed is accompanied by stronger chemical bonds and different elemental composition of the electroactive material formed on the electrode surface. Based on the experimental results, it was concluded that both Lorentz and magnetic forces are responsible for the behavior observed.

1. Introduction It is well known that magnetic fields have significant effects on electrochemical processes. Thus, studies of electrochemical systems in the presence of magnetic fields (the so-called magnetoelectrolysis) are an advancing and rapidly growing subject in modern electrochemistry. The design of a rotating cell in magnetic field for magnetoelectrolysis has been described in the literature [1]. The magnetic field effects on various electrochemical processes such as electrodissolution (corrosion) [2–5], electrodeposition [6–8], electroorganic reactions [9, 10], electropolymerization [11, 12], photoelectrochemical * E-mail: [email protected]

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behavior [13] have been reported. In these cases, magnetic field aggravates the electrochemical processes to reach higher currents. However, this effect is related to the mass transport-limited regions. Indeed, the effect of magnetic field is to induce convection in the solution and it is equivalent to rotating the electrode or stirring the solution [14]. The magnetic field causes a transport of all ions due to the difference in their magnetic susceptibility in the solution at the electrode surface. Such effects are usually referred to as secondary effect of magnetic fields. The effects of static magnetic fields on electrochemical systems consisting of ferromagnetic electrodes immersed in paramagnetic solutions have been reported by Waskaas and Kharkats [15]. They have shown that the magnetic field tends to cause an additional convective transfer of all components of the solution, which would be generated in the vicinity of the electrode surface. Thus, the magnetic field effect increases by increasing the magnetic flux density and magnetic susceptibility of the electroactive species in the solution, and decreases by increasing the temperature and stirring rate. Devos et al. have discussed that magnetic fields have no effect on electrochemical kinetics [16]. Based on impedance spectroscopic studies of different electrochemical systems including mass transport controlled, kinetically controlled, and mixed systems, they have described that the charge transfer coefficient is independent of the applied magnetic field. Modification of electrode surfaces with Prussian blue (PB) and its analogues has achieved a considerable attention in the past two decades, since the report of electrodeposition of a thin film of PB onto electrode surface by Neff [17]. A comprehensive list of references regarding various hexacyanoferrate-based modified electrodes seems to be too long to present in this paper. However, electrochemical studies of this type of modified electrodes can be found in many reviews devoted to chemically modified electrodes [18– 20]. In the present manuscript, we would like to study the magnetic field effect on the modification of a Pt electrode surface with one of the most famous and interesting modifier materials namely PB (iron hexacyanoferrate). To our knowledge, this case (electrochemical formation and deposition of PB under magnetic fields) is not well documented in the literature, although it is expected due to the importance of PB thin solid films for the practical applications. Watanabe et al. have studied electrochemistry of [Fe(CN) 6 ] 3− and [Fe(CN 6 )] 4− ions in solution using potentiometric technique [21], as typical electrochemical systems. The magnetic properties of some polynuclear cyanides have also been reported [22–24].

2. Experimental The electrode preparation was the only process, which carried out under magnetic field. After that, the electrode was conditioned in a conventional

Electrochemical Behavior and Enhanced Stability of a Thin Film . . . WE

RE

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CE

Electrolyte solution Insulator

Pt electrode

Magnet

i

N F

B

S Fig. 1. Schematic of the cell design for the electrode preparation in the presence of a magnetic field.

electrochemical cell for further investigation of the electrochemical properties of the modified electrode. To this aim, experiments for the deposition of PB film onto a Pt electrode surface were performed in a magnetic electrochemical cell. The structure of the cell was similar to conventional cells with the difference that a permanent magnet (B = 0.5 T) was attached to the working electrode in the perpendicular direction. It is needed to provide a uniform magnetic field. In the present research, a superconducting magnet with a horizontal 30 mm diameter was employed to place the Pt electrode upon the magnet. The field inhomogeneity was 1% in 1 cm along the bore axis, and the maximum field gradient over the entire volume of the cell was 4 T m −1 . For simplicity, B max was used as the representative value of the magnetic field. To gain an ideal one-dimensional diffusion induced by the magnetic field, the electrode size was significantly smaller than the magnet size. A schematic of the electrochemical cell and the magnetic field provided by the permanent magnet are presented in Fig. 1. An electrode was constructed by the deposition of a PB film onto the substrate electrode in this magnetic cell. According to the presented scheme, the magnetic flux B is horizontal and parallel to the electrode surface. Another electrode was fab-

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ricated using common modification process (in the absence of the magnetic field). To investigate the influence different magnetic field effects on the system under investigation, another PB electrode was also used for comparison in Sect. 4.2. The latter PB electrode was also constructed in the presence of a magnetic field, which was perpendicular to the electrode surface and parallel to the faradaic current to eliminate magnetohydrodynamic effect. The experimental setup for this process was similar to that noted above and illustrated in the scheme (Fig. 1), but the magnet was rotated 90 ◦ . In this case, the magnetic flux (indicated by S–N lines) was perpendicular to the electrode surface. As it has been discussed by Itaya et al. [25], the best method for the preparation of a PB film with quite uniform thickness is cathodic polarization under a galvanostatic condition. Modification process was carried out from the modifier solution containing 20 mM FeCl3 ·6H2 O and 20 mM K 3 Fe(CN) 6 in 0.01 M HCl by applying current density of 40 mA cm −1 for 150 s. The experiments were carried out at a three-electrode conventional cell using a platinum rod as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The electrochemical measurements were performed using a homemade potentiostat connected to a computer running CorrView software. The absorption spectra were recorded using a Viga 740 spectrophotometer. A GBU 1260 X-ray photoelectron spectrometer was employed for the characterization of the stoichiometric composition of the film deposited.

3. Results 3.1 Voltammetric studies Voltammetric characteristic of PB film has been well documented in the literature. To the aim of this research, redox couples of PB are not discussed here and the voltammetric behavior of the electrodes were just compared to investigate the magnetic field effect on the electrode preparation. Fig. 2 shows typical cyclic voltammograms of two different electrodes. The voltammograms were recorded in the conventional electrochemical cell (in the absence of a magnetic field). As seen, both anodic and cathodic peak currents of the modified electrode prepared in the presence of a magnetic field is significantly higher than the conventional modified electrode, indicating higher electrochemical activity.

3.2 Electrochemical stability One of the most important problems in the preparation of modified electrodes is high chemical and electrochemical stability to gain acceptance for the practical applications. It has been described that the main drawback for chemically

Electrochemical Behavior and Enhanced Stability of a Thin Film . . .

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Fig. 2. Typical cyclic voltammetric characteristics of the modified electrodes prepared (a) in the presence of a 0.5 T magnetic field and (b) in the absence of magnetic field. The electrolyte solution was 1 M KCl (scan rate 100 mV/s).

modified electrodes based on transition metal hexacyanometallates is the gradual dissolution during potential cycling [26]. Some devices have been reported for improving the stability of electroactive films formed on electrode surfaces. Two reasons are responsible to reach highly stable films: (i) stability of the electroactive material and (ii) stable connection (deposition) of the electroactive film to the substrate surface. The first reason can be achieved by improving stability of the hexacyanoferrate lattice. Kulesza et al. [27] has claimed a better stability of NiHCF (nickel hexacyanoferrate) films deposited in the presence of Ag(I) ions. It has been suggested that the incorporation of Ag(I) provides an augmented crosslinking between microparticles of NiHCF, probably due to the lower solubility of the relevant silver-containing film. Cataldi et al. [26, 28] have reported that incorporation of ruthenium into the lattice of indium hexacyanoferrate film improves the stability of the film formed on a glassy carbon electrode. The second reason, connection of the electroactive film to the substrate surface, is related to the deposition conditions and the substrate electrode employed. Indeed, the deposition manner is very important to gain a stable connection between the electroactive film and the substrate surface. For example, aluminum is a suitable substrate electrode for the preparation of chemically modified electrodes to deposit highly stable electroactive films [29–31].

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Fig. 3. Electrochemical stability of the modified electrodes examined during potential cycling of the electrodes prepared in the presence ( ) and absence ( ) of the magnetic field. With the same conditions as noted in Fig. 2.

It is due to well-known behavior of aluminum in aqueous media viz. passivation to form a stable aluminum oxide layer on the electrode surface. Formation of a passive film on the aluminum surface causes the generation of more suitable surface for the deposition of electroactive films, thus the electroactive film deposited on the passive substrate is more stable in comparison with that deposited on conventional electrodes [29]. The effect of passive layer on stability of the deposited film has been discussed for the deposition of conductive polymers on Fe and passivated Fe substrate electrodes [32]. Similar effect has been reported for the direct modification process, as the self-passivation of the metallic film is responsible for high stability of the electroactive film generated on the substrate surface [33, 34]. Electrochemical stability of the PB film modified electrodes was examined during potential cycling. For this purpose, the potential was scanned between −0.2 and 0.6 V vs. SCE with scan rate of 100 mV s −1 . Fig. 3 shows the results obtained from repetitive voltammetric measurements of two different electrodes. The decrease observed in the peak current of the modified electrode prepared in the presence of the magnetic field is significantly lesser than that of the conventional modified electrode, indicating higher stability provided as the result of deposition in the presence of a magnetic field. It is observable that the conventional PB film electrode loses about 65% of its


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Fig. 4. Stability of the modified electrodes prepared in the presence ( ) and absence ( ) of the magnetic field during long-term storage.

electroactivity during 1000 cycles, whereas, the PB film electrode prepared in the presence of the magnetic field loses only 20% after the same number of potential cycling.

3.3 Chemical stability To investigate the influence of the applied magnetic field on chemical stability of the modified electrodes fabricated, electrochemical activities of the modified electrodes were investigated during long-term storage. Fig. 4 presents the results obtained from voltammetric measurements of the modified electrodes at different time of storage, as it shows the changes in the peak current as a function of the storage time. Both modified electrodes were conditioned in the same supporting electrolyte and cyclic voltammograms were recorded at different times of the storage. As expected, the results are similar to those obtained from studies of electrochemical stability. Indeed, the modified electrode prepared in the presence of the magnetic field is more stable in both electrochemical condition (potential cycling) and simple chemical condition (storage in the supporting electrolyte).

3.4 Diffusion towards the electrode surface It is well known that cation of the supporting electrolyte is involved in the redox reaction of PB. It is accompanied by a Nernstian behavior of the mid-

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Fig. 5. Dependence of the formal potential of the modified electrodes prepared in the presence ( ) and absence ( ) of the magnetic field on concentration of the supporting electrolyte.

peak potential (the formal potential) with respect to the potassium ion activity. As the potentiometric response to potassium ions is highly dependent on the film structure, this behavior was also investigated to show the difference between two modified electrodes (Fig. 5). The curve slopes for the electrodes prepared in the presence and absence of an external magnetic field were 53.7 and 56.1 mV/decade, respectively. It is recognizable that the electrode prepared in the presence of the magnetic field has more ideal response to the potassium ion activities, as its slope is closer to the theoretical Nernstian slope. As the potassium-involved redox of PB and its analogues have been widely used to prepare potassium-selective electrodes, the results are of interest from analytical performance point of view. The improvement is more obvious for the response time of the PBbased potassium-selective electrode. The modified electrodes were examined as chemical sensors for sensing potassium ions. A significant difference in the potentiometric response times of the sensors was observed, as they were about 45 and 65 seconds for the electrodes prepared in the presence and absence of the magnetic field, respectively. As the redox mechanism for other reactions of the electroactive film, e.g., electrocatalytic oxidation of different compounds, is similar to the original redox of the system, it is thought that the electrode prepared in the presence of the magnetic field has a higher activity for electrocatalytic purposes.


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Although, the results suggest a better analytical performance for the possible chemical sensors based on the PB modified electrode prepared in the magnetic field, however, investigation of this feature is out of the aim of this paper. Investigation of the electrolyte concentration effect on the voltammetric characteristics of the electrodes was performed to show that using an external magnetic field just improves the film deposition and has no significant effect on the nature of the electrochemical redox of the system under investigation. As seen, the curves obtained from relationship between the formal potentials and potassium ion activity is overlapped, suggesting the same electrochemical characteristics of the redox system. To understand that the results observed are due to the magnetic fields applied, not random phenomena affecting the deposition process, different electrodes were fabricated with the same procedure. The studies of different electrodes gave similar results, indicating that the enhanced stability achieved for the PB film was provided by the magnetic field applied.

3.5 Physical properties of the deposited films Although, the aim of the present report is to show the usefulness of magnetic fields for the preparation highly stable films for electrochemical systems, preliminary investigation of the physical properties of the PB films exhibited interesting results. (i) The amount of the electroactive film deposited on the substrate electrodes significantly increases when a magnetic field is applied. As it was calculated by weighing the electrodes before and after the deposition, the PB loading on the substrate electrode was 1.4 × 10−7 mol cm −2 , whereas 8.7 × 10−8 mol cm −2 PB was deposited in the absence of the magnetic field. (ii) The film thickness is approximately constant for both PB films deposited in the presence and absence of magnetic fields, which were about 80 nm. Whereas, the film density increases for the PB film deposited under magnetic fields, as simply can be determined according to the thickness and weight of the PB films. (iii) Conductivity of the PB film deposited in the presence of the magnetic field is higher than that prepared in the absence of a magnetic field. The PB deposition under magnetic field is accompanied by about 1.2 times higher conductivity for the electroactive film deposited. This can be attributed to the higher electronic conductivity at the interfaces of the particles squeezed as a result of the magnetic field applied. Such effects were mentioned briefly to show the magnetic field effects on the system under investigation. However, detailed studies of all of these effects are now under investigation.

3.6 Spectroscopic characterization To examine the results obtained from electrochemical measurements and to understand the reason for the stability enhancement of PB deposited under an applied magnetic field, the PB films deposited in the presence and absence of the

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Fig. 6. Electronic absorption spectra of the PB film deposited in the presence (a) and absence (b) of the magnetic field.

magnetic field were also investigated using non-electrochemical techniques. The absorption spectroscopy is a common technique for the investigation of electroactive films deposited on substrate surfaces. It is known that absorption spectra of Prussian blue and its analogues are accompanied by characteristics ferricyanide peaks in the range from 400 to 800 cm −1 . Although, investigation of the presence of –CN functional group appearing at about 2100 cm −1 is more important for the electrochemical studies to investigate the difference of the reduced and oxidized states, inspection of the maximum absorbance at 690 cm −1 for PB corresponding to the Fe–C band is of importance for the present investigation. The absorption spectra of two different PB films deposited in the presence and absence of the external magnetic field are illustrated in Fig. 6. It is recognizable that the Fe–C stretching frequency shifts to higher values for the PB film deposited in the presence of the applied magnetic field. This provides a strong evidence for an increase of the force constant, indicating an increase in the stability of the hexacyanoferrate unit.


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Fig. 7. Iron 2 p XPS spectra of the PB film deposited in the presence (a) and absence (b) of the magnetic field.

Table 1. Magnetic field

Fe2 p3/2 /eV

Fe2 p1/2 /eV

N/Fe

Cl/Fe

0.0 T 0.5 T

708.1 709.4

722.6 723.1

2.4 2.0

0.35 0.12

While, X-ray photoelectron spectroscopic measurements were performed for elemental analysis of the PB films, the Fe 2 p XPS spectra recorded display an interesting feature. Fig. 7 presents typical XPS spectra of the PB film deposited in the presence and absence of an external magnetic field. As the XPS spectra show signals at about 708 eV corresponding to Fe–CN of octahedral structure Fe(CN) 6 3− and at about 722 eV corresponding to Fe–O, the binding energy shifts towards higher values for the PB film deposited in the presence of the magnetic field. Although, this shift is slight, it is regardable, particularly for the peak corresponding to Fe2 p3/2 . However, the binding energy is still lower than the known value of 710.5 for Fe(CN) 6 3− [35]. The results obtained for the elemental analysis of the PB films deposited in the presence and absence of the magnetic field are summarized in Table 1. It is known when the PB film is precipitating on the electrode surface; it will be con-

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taminated by the electrolyte species. Incorporation of such ionic species can be a source of instability of the PB film deposited, as they increase solubility of the insoluble material. As seen, employing the magnetic field significantly decreases the amount of electrolyte species incorporated into the solid film. This phenomenon can explain the enhanced stability of the PB film achieved by the deposition under an external magnetic field. The results obtained from comparative studies of the PB film deposited in the absence and presence of an external magnetic field with two common non-electrochemical techniques viz. absorption spectroscopy and X-ray photoelectron spectroscopy obviously suggest the enhanced stability of the PB film concluded from the electrochemical experiments. Indeed, non-electrochemical techniques as well as electrochemical measurements suggested the usefulness of an external magnetic field to deposit highly stable PB film, which is of interest for the practical applications.

4. Review and discussion 4.1 Lorentz force According to the aim of this preliminary research reporting usefulness of magnetic fields for the preparation of more stable chemically modified electrodes, we do not attempt to formulate the physical aspect of the PB deposition under magnetic fields, as it needs deep investigations of the film deposited using non-electrochemical techniques. However, to understand the reasons affecting this phenomenon and finding the most possible theory to predict it, the available theories of magnetoelectrolysis were reviewed for the system under investigation. Both Fe3+ and Fe(CN) 6 3− are paramagnetic ions and each of them has an unpaired electron, whereas PB is a diamagnetic compound. However, as the electrochemical reaction to generate Prussian blue is occurred at the electrode surface, the magnetic field is effective on such paramagnetic ions to reach the electrode surface. In other words, PB generated at the electrode surface is not affected by the magnetic field, as the magnetic field has an effect electroactive species in the bulk solution (or diffusion layer). Indeed, the magnetic field effect on the system under investigation is a hydrodynamic process (i.e. it is called magnetohydrodynamic), not on the solid state. The behavior observed for the system under investigation can be attributed to the convection induced by the magnetic field. This phenomenon can be described simply according to the existence of the Lorentz force: FL = j × B

(1)

where j is the current density in A m −2 , B is the applied field in tesla and FL is the Lorentz force per unit volume. This force has an effect on the electroac-


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tive species, moving as the result of momentum transfer, in convective flow of the whole electrolyte solution. It is the classical magnetohydrodynamic (MHD) phenomenon [36]. As B and j are in perpendicular vectors (not parallel vectors), the value of the Lorentz force is not zero. It is well known that in such electrochemical systems, the Lorentz force is the main force induced by the magnetic field [4, 15, 37–39]. Therefore, it can be simply concluded that the results observed are due to hydrodynamic convection effect of the magnetic field. Indeed, the existence of the Lorentz force can be evidenced from the fast deposition process (judged from the weight of the electroactive material PB deposited per unit of time) due to the convection induced by the applied magnetic field. As stated above, more amount of the electroactive film would be deposited in the presence of the magnetic field under the same experimental condition. Electrodeposition is a mass transfer controlled process and thus it is highly dependent on the electrolyte convection.

4.2 Magnetic forces Although, it is obvious that the magnetic effect on the system under investigation is mainly due to the Lorentz force, however, other magnetic forces can also be effective. The influence of other forces induced by magnetic fields on electrochemical systems have been studied in the absence of the Lorentz force, where B and j are in parallel vectors [15, 40]. Thus, description of the system under investigation with other available models for the magnetic field effects on electrochemical systems such as the models proposed based on susceptibility of the magnetic ions is also possible. A local energy density E = −cχ m B 2 /2µ 0 can be created as the result of a uniform magnetic field due to the susceptibility of the magnetic ions, where c is the concentration in mol m −3 , χ m is the molar susceptibility of the ions in m 3 mol−1 and µ 0 is the permeability of free space, 4π × 10 −7 H m −1 . In the diffusion layer, where c is nonuniform, there is a force as [15]: Fm,∇c =

χ m B 2 ∇c . 2µ 0

(2)

The magnetic force due to concentration gradient Fm is effective on the movement of both paramagnetic and diamagnetic ions, but in opposite directions. There is another magnetic force similar to that introduced in Eq. (2), but due to magnetic field gradient instead of concentration gradient Fm,∇ B =

χ m cB∇ B . µ0

(3)

As there is a field gradient for the magnetic-electrochemical cell employed, this force can be significant. Unfortunately, we cannot divide these two simi-

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Table 2. Magnetic field

0T 0.5 T 0.5 T

Direction

– Parallel Perpendicular

Deposition ratea /nmol s−1

0.58 0.93 0.66

Elemental compositionb N/Fe

Cl/Fe

2.4 2.0 2.1

0.32 0.12 0.19

a Deposition rate was calculated from the amount of PB deposited onto the substrate electrode in “nano mol per second”. b Estimated from XPS measurements (similar to those reported in Table 1).

lar magnetic forces (introduced in Eqs. (2) and (3)), as both concentration and magnetic field gradients are appropriate for the system under investigation. Thus we considered the total magnetic force as Fm = Fm,∇c + Fm,∇ B . However, we can compare the magnetic field effects on different ions due to their magnetic susceptibilities, as both the magnetic forces introduced are dependent on χ m , but Lorentz force is not. The data presented in Table 2 obviously show that the deposition rate is increased even when the magnetic field is perpendicular to the electrode surface, where the Lorentz force is absent. Nevertheless, the improvement of the deposition rate provided by the parallel magnetic field was 61%, whereas that obtained for the perpendicular magnetic field was just 14%. This provides strong evidence for the hypothesis stated in Sect. 4.1 that the Lorentz force is the main force induced by the magnetic field to the electrochemical system under investigation. As the influence of the magnetic forces on the electrochemical system to increase the deposition rate is significantly lesser than that of the Lorentze force, this is not of interest. Indeed, the interesting feature of the magnetic force induced to the system is related to their ability to change the deposition structure. According to the data presented in Table 2, when the magnetic field is perpendicular and has no significant effect on the electrochemical system (to strengthen the electrochemical reaction) due to the absence of the Lorentz force, the magnetic field induced has a different effect to change the chemical composition of the electrodeposit generated. As this changed chemical composition contains higher amounts of elements with higher magnetic susceptibility (their corresponding ions), it can be concluded that the magnetic force is responsible for this action. The reduced ratio of Cl/Fe in the absence of the Lorentz force (Table 2) is due to the fact that the magnetic force increased the flux of a paramagnetic ion (i.e. Fe3+ ) and reduced the flux of a diamagnetic ion (i.e. Cl − ). The importance of the influence of a magnetic force on an electrochemical system will be clarified in the following section.


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4.3 Comparison with another mechanical force As an important effect of magnetic field is to increase the electrolyte convection, the results were compared with the literature dealing similar systems under another (similar) mechanical force. The aggravated flux of the electroactive species under the magnetic field results a controlled diffusion process towards the electrode surface. It causes the electrochemical reaction to occur in a regular manner. Thus, an ordered film is generated and deposited on the substrate surface. Formation of ordered surfaces in the presence of an external magnetic field has been described for various electrodeposition processes [41, 42]. It is similar to the perpendicular centrifugal forces induced to the electrodeposition Au electrodeposits, as ordered surface with lower fractal dimensions were obtained under stronger centrifugal fields [43], or Cu metallization of silicon surface in the presence of an external centrifugal force, which is accompanied by the formation of smoother metallic surfaces [44]. Similar to the results reported here for the deposition of PB, we have found higher stability of a PB analogue namely nickel hexacyanoferrate (NiHCF) deposited under centrifugal fields [45]. For electropolymerization, which is an electrochemical formation/deposition process like the system under investigation, a similar phenomenon has been reported in the presence of an applied centrifugal force. It has been reported that electropolymerization of aniline under centrifugal fields is accompanied by higher activity of the conductive polymer deposited on the substrate surface [46]. Even for an electrodeposition in the absence of any chemical formation of electroactive under centrifugal fields, such phenomena have been reported. Higher stability and activity of LiMn2 O4 deposited under centrifugal forces (a process which ions are not mainly involved) has been described [47]. Interestingly, LiMn2 O4 cathodes prepared under stronger centrifugal fields provided better properties for the practical applications in lithium secondary batteries. According to the results reported in the literature, a centrifugal (or gravitational) force acts similar to the Lorentz force induced by a magnetic field to increase the convection of the electrolyte solution. This action aggravates the electrochemical process. However, the magnetic force (as shown in the preceding section) has a different effect. For the electrodeposition of CoNiFe film under centrifugal forces, it has been reported [48] that the gravitation force induced increases the deposition rated (aggravate the electrochemical process); however, it has no significant effect on the amounts of three elements deposited. Indeed, the unique advantage of the magnetic field due to the corresponding magnetic force can be used for specified purposes to prepare materials with different composition. For example, it was shown for the system under investigation that employing a magnetic field during the electrochemical formation/deposition of PB leads to the formation of PB film with lesser amounts of chlorine element. It is known that chlorine element incorporates into the de-

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positing electroactive material from the supporting electrolyte, and this is an unfavorable action which should be avoided.

5. Conclusion Electrochemical generation and deposition of a thin film of PB was investigated in the presence of a magnetic field. Electrochemical behavior of the modified electrode fabricated was studied and compared with a conventional PB modified electrode (prepared in the absence of a magnetic field). Based on the experimental results, it was shown that modification of electrode surface with PB could be improved by employing an external magnetic field. It is accompanied by stronger electrochemical activity, higher chemical and electrochemical stabilities, etc. This improvement was reached as the result of stronger and ordered deposition of the electroactive film in the presence of the magnetic field.

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