poly (2-Methoxyaniline)-sodium dodecyl sulfate composite and its application for electrocatalytic oxidation of methanol and formaldehyde

Electrochimica Acta 141 (2014) 340–348

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Preparation of Pt/poly (2-Methoxyaniline)-sodium dodecyl sulfate composite and its application for electrocatalytic oxidation of methanol and formaldehyde Jahan-Bakhsh Raoof a,∗ , Sayed Reza Hosseini b , Sharifeh Rezaee a a Electroanalytical Chemistry Research Laboratory, Department of Analytical Chemistry, Faculty of Chemistry, University of Mazandaran, Postal Code: 47416-95447, Babolsar, Iran b Nanochemistry Research Laboratory, Faculty of Chemistry, University of Mazandaran, Postal Code: 47416-95447, Babolsar, Iran

a r t i c l e

i n f o

Article history: Received 4 February 2014 Received in revised form 1 July 2014 Accepted 7 July 2014 Available online 21 July 2014 Keywords: Poly (2-Methoxyaniline) Sodium dodecyl sulfate Platinum particles Electrocatalytic oxidation Methanol

a b s t r a c t In this work, poly (2-Methoxyaniline) (P2MA) film was prepared by using successive potential cycling in monomer solution in the presence of sodium dodecyl sulfate (SDS) as an anionic surfactant at the surface of glassy carbon electrode (GCE). Then, electrodeposition at a constant potential (-0.20 V vs. Ag|AgCl|KCl (3 M)) in 0.5 M H2 SO4 solution containing 5.0 mM H2 PtCl6 was performed for dispersion of Pt nano/microparticles for fabrication of the Pt/P2MA-SDS composite. The as-formed Pt/P2MA-SDS/MGCE was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and electrochemical methods. The SEM studies revealed that the presence of SDS strongly influences the morphology of the polymer film. The electrochemical methanol oxidation reaction was investigated at the Pt/P2MA-SDS/MGCE by cyclic voltammetry and chronoamperometric techniques. The results indicated that the Pt/P2MA-SDS/MGCE in comparison with Pt/MGCE and Pt/P2MA/MGCE exhibits excellent electrocatalytic activity towards the methanol oxidation. The improved performance showed that the P2MA-SDS films significantly increase the effective active surface area of the Pt particles which is due to the synergistic effect between Pt and P2MA-SDS at the electrode surface. Furthermore, the effect of several parameters such as P2MA thickness, 2MA, SDS and CH3 OH concentrations towards the methanol oxidation as well as long-term stability of the modified electrode has been investigated. The electrocatalytic oxidation of formaldehyde as an intermediate in the methanol oxidation was also investigated. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction The choice of a suitable supporting material is an important factorthat may affect the electrochemical performance of the supported electrocatalyst owing to the support-catalyst interaction and surface reactivity. Recent researches indicate that modifying the electrode surface by polymeric films in place of conventional supports give considerable significance to develop the practical application of electrochemically modified electrodes [1,2]. Conducting polymers (CPs) as novel organic semiconducting materials have received good deals of attentions due to their potentially wide speared applications in catalysis, electrochromic, electronic device, anticorrosion coating and sensors [3–5]. The CPs have superiority applications due to their good synthetic flexibility, environmental

∗ Corresponding author. Tel.: +98 112 5342392; fax: +98 112 5342350. E-mail address: [email protected] (J.-B. Raoof). http://dx.doi.org/10.1016/j.electacta.2014.07.054 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

stability, good conducting properties and high adhesion to the electrode surface especially in contact with aggressive media such as strong acids or bases. One of the main potent properties of the CP is that it can be considered as a redox mediator between the electro-active reactant and electrode by electron-proton transfer [6]. It is obvious that the CPs matrixes with excellent stability, porous structures and high effective surface areas are attractive and favorable supports for incorporation of the catalyst particles. Also, this support structure avoids the agglomeration and reduces the Pt loading [7,8]. High accessible surface area and synergistic effect between the Pt particles and CPs improve the efficiency of the electro-catalyst, decrease the surface poisoning and enhance the stability of the Pt particles for oxidation of small organic molecules [9,10]. Another reason for introduction of Pt particles into the CPs is the stronger tolerance against COads species which are formed during the methanol electrooxidation. The composites of the CPs with Pt particles provide a rapid electron transfer pathway through

J.-B. Raoof et al. / Electrochimica Acta 141 (2014) 340–348

the polymer matrix during the electrochemical process. So, electrodeposition of metals on/in the CPs can provides a cheap and convenient route for the electrode modification [11–13]. On the other hand, surfactants are usually organic amphiphilic ions/molecules with hydrophilic heads on one side and a long hydrophobic tails on the other side [14]. It has been seen that surfactants fulfill a vital role in the electrode reactions by adsorption at the interface or aggregation into supramolecular structures. Thus, they can change the electrical properties of the electrode interface and the electrochemical process. Some innovative attempts have been recently devoted to use aqueous anionic micellar media, based mainly on sodium dodecyl sulfate (SDS) for preparation of the CPs [15–18]. Adding SDS to the monomer solution leads to an increase of the polymer growth rate which can be attributed to association between the SDS and monomer. In addition, the presence of SDS causes a decrease in monomer concentration which is needed for the electropolymerization. Also, it has been seen that the presence of SDS in the electropolymerization process leads to an increase of monomer solubility and lowering of itsoxidation potential.The surfactants also can improve physical and mechanical properties of the polymer and produce adherent and well-organized films on the electrode surface[19,20]. Poly (2-Methoxyaniline) (P2MA) has considerable features such as excellent electrochromism, fair solubility, good electroactivity and high thermal stability[21]. Therefore, it can be expected that, we can combined the advantageous properties of SDS with P2MA as support for catalyst particles. To the best of our knowledge, until now, some works have been directed in the use of P2MA and Pt particles towards methanol and formic acid oxidation [22,23]. Previous study has explored the incorporation of nickel ions onto the P2MA/multi-walled carbon nanotube modified glassy carbon electrode (GCE) for electrocatalytic oxidation of methanol in NaOH solution [24]. Also, recently we have combined the advantageous features of the P2MA films in the presence of SDS with dispersion of Ni species by construction of P2MA/Ni(OH)2 modified carbon paste electrode as a sensor for cephalosporins [25]. Our literature survey indicates that the P2MA prepared in the presence of SDS (P2MA-SDS) modified GCE (P2MA-SDS/MGCE) as a support for Pt particles for the electrocatalytic applications has not been reported. Hence, in the present investigation, we have prepared the P2MA-SDS film and then Pt particles were electrodeposited onto the composite electrode and its efficiency for electrocatalytic oxidation was investigated. The experimental data reveal that the Pt particles dispersed into/onto the P2MA-SDS (Pt/P2MA-SDS) shows excellent performance in the electrocatalytic oxidation of methanol and formaldehyde in acid solution. 2. Experimental 2.1. Reagents and materials 2MA (>98%, Fluka) was used as a monomer at electrochemical polymerization process. SDS (99%, Fluka), H2 PtCl6 .6HO (Merck) and formaldehyde (Merck)were used as received. Methanol (Merck) used in this work was analytical grade. Sulfuric acid (98%, Fluka) was used for preparation of the supporting electrolyte. The solvent used in this work to prepare the reagents solutions was double distilled water. 2.2. Electrochemical measurements The electrochemical experiments were carried out using a potentiostat/galvanostat (SAMA 500-C Electrochemical analysis system, Sama, Iran) coupled with a PC through an interface with SAMA-500 software version 2.7. Electrochemical impedance

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spectroscopy (EIS) was performed by an Autolab model PGSTAT 30 with FRA software version 4.9 (Eco Chemie, the Netherlands). The three-electrode system consists of GCE, (1.5 mm in diameter) as working electrode substrate, Ag|AgCl|KCl (3 M) as reference electrode and a Pt wire as an auxiliary electrode. The surface morphology and elemental analysis of the deposits were evaluated by scanning electron microscopy (SEM, model VEGA-Tescan, Razi Metallurgical Research Center, Tehran, Iran) equipped with an energy dispersive spectrometer (EDS). 2.3. Preparation of the Pt/P2MA-SDS composite electrode Prior to the GCE modification, its surface was hand-polished successively with alumina slurry in water by using a polishing cloth until the electrode surface had a mirror finish. The polished electrode was placed in ethanol and sonicated for 5 min to remove the alumina residues that might be trapped at the surface.Then, the electrode was rinsed thoroughly with distilled water. For fabrication of the P2MA-SDS/MGCE, electropolymerization was performed by using successive potential cycling technique (15 cycles at  = 50 mV s−1 ) between -0.3 and 0.9 V vs.Ag|AgCl|KCl (3 M) in 0.5 M H2 SO4 solution containing 0.02 M 2MA monomer in the presence of 0.015 M SDS. Once polymerization was accomplished, the P2MA-SDS/MGCE was thoroughly rinsed with 0.5 M H2 SO4 solution and 15 potential cycles were conducted until stable voltammograms were obtained. Then, for adsorption of hexachloroplatinate anions (PtIV ) onto the polymeric film, P2MASDS/MGCE was immersed in 0.5 M H2 SO4 solutions containing 5.0 mM H2 PtCl6 for 10 min. In order to incorporation of Pt particles (Pt0 ) at the P2MA-SDS film for fabrication of the Pt/P2MA-SDS composite, electrochemical deposition was performed by keeping the electrode potential at -0.20 V vs. Ag|AgCl|KCl (3 M). After this, the modified electrode was removed and rinsed with double distilled water. The modified electrode was then conditioned in 0.5 M H2 SO4 solution by potential cycling between -0.3 and 1.3 V at  = 50 mV s−1 for 3-5 cycles until reproducible voltammograms were achieved. The mass of the deposited Pt was calculated by quantification of the electrical charge consumed during the electrodeposition process by faraday law. It was also assumed that the current efficiency for the PtIV -Pt0 was 100%. The net charge consumed during the Pt electrodeposition can be determined by subtraction of the electrical charges obtained by integrating the experimental chronoamperograms for 5.0 mM H2 PtCl6 and blank (i.e., 0.5 M H2 SO4 ) solutions. The quantity of the Pt loaded onto the P2MA-SDS film (WPt ) was determined by the following Eq (1): WPt = Qnet M/F Z

(1)

Where WPt is calculated by using the integrated charge (Qnet /C) passed during the Pt deposition assuming a 100% current efficiency [26]. M, F and (Z = 4) are Pt atomic weight, faraday constant, and number of electrons transferred, respectively. In this work, amount of the deposited Pt in all cases was controlled at about 0.01 mg. Moreover, the mass (WP2MA ) and thickness (dn ) of the P2MA films were calculated from the total charges passed through the cell during the film growth process assuming a 100% current efficiency (␩), according to the Eqs (2 and 3) [27,28]. WP2MA = ( Qdep ) M/F n

(2)

dn = ( Qdep ) M/ A F n

(3)

Where, Mis the molecular weight of 2MA (126.16 g/mol), n is the number of electrons transferred per monomer attached to the polymer (n = 2). F is the faraday constant,  is the P2MA density (1.50 g/cm3 ) and Ag is the GCE geometric surface area (0.018 cm2 ). In this work, the mass and thickness of the P2MA-SDS film were obtained at about 1.83 ␮g and 0.67 ␮m, respectively. The Ag

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Fig. 2. Cyclic voltammograms of the P2MA/MGCE (a) and P2MA-SDS/MGCE (b) in 0.5 M H2 SO4 solution at  = 50 mV s−1 . Inset: 1st cycle of the P2MA-SDS/MGCE after immersion in 0.5 M H2 SO4 solution at  = 50 mV s−1 .

Fig. 1. Electropolymerization of 20 mM 2MA monomer in 0.5 M H2 SO4 solution in the absence (A) and in the presence of 15 mM SDS (B) at  = 50 mV s−1 . The arrows indicate the trends of current density during the cyclic voltammetric experiments.

was used to calculate current density, except the comparison of the methanol and formaldehyde electrooxidation. All experiments were conducted at ambient temperature. 3. Results and discussion 3.1. Electrochemical polymerization In this work, the electrochemical polymerization of the 2MA monomer was investigated at the GCE surface in the absence and presence of SDS. The consecutive cyclic voltammograms recorded during the electropolymerization of 2MA in the absence of SDS were presented in Fig. 1A. As can be seen in this figure, in the first positive scan, the oxidation of 2MA starts at about 0.63 V and occurs at about 0.87 V and during the reverse scan any distinct reversible anodic peak is not observed. Deposition a part of oxidation products at the electrode surface leads to formation of the P2MA film. Also, in the first negative scan, a cathodic peak appears at about 0.30 V, which is corresponds to the reduction of the anodically formed species. This peak can proves the initial deposition of the electrooxidized product at the electrode surface. As well it is obvious, by continuous potential cycling, the peak height related to the monomer oxidation decreases and its position is shifted to more positive potentials (rigid oxidation). This can reflect the loss of surface activity when it is covered by a polymeric film. At the subsequent positive cycles, a new oxidation peak at a potential around 0.34 V can observed which is ascribable to build-up the electroactive P2MA film and this peak not increase considerably with potential cycling. This can be attributed to this fact that the soluble products adsorb and/or react on the electrode surface, do not allow the monomer to reach the surface and produce the more cations radical. By adding 15 mM SDS to the monomer solution, rate of polymerization is significantly increased (Fig. 1B). As the cyclic voltammetry continuous, the peaks currents densities

related to the oxidation/reduction of the polymer were gradually increased. This result indicates that the P2MA growth was greatly facilitated in the presence of SDS. It is worthwhile to note that in the micellar media, the 2MA oxidation potential was shifted to less positive potential and started at about 0.57 and occurred at 0.72 V. This result can be due to the specific interactions between radical-cations which were formed during the first step of the electropolymerization and SDS hydrophilic head [29,30]. Also, it should be noted that the potential of 2nd cycle of the 2MA oxidation was shifted to more negative value than the 1st cycle indicating facile oxidation in the presence of the P2MA-SDSfilm. As a comparison, their peaks potentials and shapeare not the same and electrochemical polymerization pattern of the 2MA in the presence of SDS is different. This result shows that the polymerization mechanism of the 2MA is significantly affected by existence of the SDS in solution. On the other hand, SDS molecules enhance the local concentration of the 2MA at the electrode-electrolyte interface in the former media. Also, the polymer film electro-synthesized in the micellar media had well-defined structures and stability [31]. For comparison, the redox behavior of the prepared polymeric films in the absence and presence of SDS in the electrolyte solution was presented in Fig. 2. Typical oxidation and reduction peaks indicate the electrochemical activities. The P2MA-SDS films have noticeably higher anodic and cathodic peakcurrents densitythan the normal P2MA. These results reveal that the SDS increases the effective active surface area of the polymeric film which are accessible to the electrolyte solution. The increase of the surface area improves the doping-undoping rate which is benefit to the ion diffusion and migration [32]. 3.2. Surface morphology and elemental analysis In order to surface characterization, the micrographs of the bare GCE (a), P2MA/MGCE (b), P2MA-SDS/MGCE (c) and Pt/P2MA-SDS/MGCE (d) have been investigated by SEM and the corresponding results were shown in Fig. 3. As shown in trace (a), the surface of the bare GCE after polishing shows some defects which are obtained during the cleaning process. From the trace (b), P2MA film on the GCE shows an almost even and smooth surface without any holes or cavities. Fig. 3(c) exhibits the formed P2MA film in the presence of SDS which has amorphous like surface with polydisperse morphology. On the other hand, P2MA-SDS film is thicker and shows a larger and rougher surface area compared to the normal P2MA. This structure enhances the electrolyte constituent access to interior of the polymeric film. This result indicates

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Fig. 3. SEM images of the bare GCE (a), P2MA/MGCE (b), P2MA-SDS/MGCE (c), Pt/P2MA-0SDS/MGCE (d) and EDS spectrum of the Pt/P2MA-SDS/MGCE (e).

that the surface morphology of the P2MA is significantly influenced by incorporation of the SDS into the P2MA structure. This is due to that the SDS facilitates the electron exchange between the electrode and 2MA, so it can increase the polymer growth

and improves uniformity of the coverage. This structure provides a larger available surface area and hence servesas a beneficial support for dispersion and/or distribution of the Pt particles. Image 3(d) shows the morphology of the composite consisting of the

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J.-B. Raoof et al. / Electrochimica Acta 141 (2014) 340–348 Table 1 Surface parameters of the Pt deposited on the different modified GCE. Surface parameter

Pt/MGCE

Pt/P2MA/MGCE

Pt/P2MA-SDS/MGCE

Pt loading/mg QH /␮C Ar /cm2 RF EASA/m2 g−1 Ag /cm2

0.01 176 0.83 46 8.3 0.018

0.01 215 1.02 56 10.2 0.018

0.01 360 1.71 76 17.1 0.018

Fig. 4. Cyclic voltammograms of the Pt/MGCE (a), Pt/P2MA/MGCE (b) and Pt/P2MASDS/MGCE (c) in 0.5 M H2 SO4 solution at  = 50 mV s−1 .

spherical Pt nano/micro-particles which are homogeneously distributed onto/into the polymeric film. The polymeric film prevents the Pt coalescing and provides high degree of distribution during the electrodeposition, hence remarkably enhance the active surface area and increase the electrocatalytic activity. Furthermore, existence of the Pt particles incorporated into the composite matrix was confirmed by EDS (trace 3 (e)). From the EDS results, Pt is the major element. Carbon is derived from the GCE, P2MA film and/or SDS. Nitrogen is from the P2MA. Oxygen is from P2MA, SDS and/or working electrolyte solution. 3.3. Cyclic voltammetry of the Pt/P2MA-SDS composite After deposition of the Pt particles, to understand the electrochemical behavior of the modified electrodes, cyclic voltammograms of the Pt/MGCE (a), Pt/P2MA/MGCE (b) and Pt/P2MA-SDS/MGCE (c) were recorded in a potential window of -0.3 to 1.0 V in 0.5 M H2 SO4 solution (Fig. 4). The larger peaks in hydrogen adsorption/desorption regions were observed on the Pt/P2MA-SDS/MGCE, reflecting higher surface area of the Pt particles. Consequently, such Pt particles enhance the active sites towards the electrocatalytic oxidation of methanol and formaldehyde. The experiments also support that the Pt particles were successfully deposited on the polymer matrix. On the other hand, Pt real surface area (Ar /cm2 ) has clear physical significance. So, electrocatalytic properties are depended on the number of the available surface area. The Ar of the Pt catalysts can be estimated from the integrated charges in the hydrogen adsorption region according to the following Eq (4). Ar = QH /QO

(4)

Where QO has been commonly taken as 0.21 mC/real cm2 and QH is the charge consumed for hydrogen adsorption [33]. Similarly, the roughness factor (RF) can be estimated by dividing Ar to Ag . The electrochemically active surface area (EASA/m2 g−1 ) was also estimated as follows, wheremPt is the Pt loading (␮g) [34]: EASA = 100 Ar /mPt

(5)

Table 1 summarizes the surface parameters for the different modified electrodes. 3.4. EIS studies EIS is an effective tool for studying the interface properties by evaluating of the characteristic of the film structure, conductivity, mechanism, kinetics of ion transport and charge transfer

Fig. 5. Nyquist plots for the faradaic impedance measurements in 0.5 M H2 SO4 solution performed on the Pt/P2MA-SDS/MGCE (a), P2MA-SDS/MGCE (b) and P2MA/MGCE (c).

in composite film/electrolyte interface [35,36]. Impedance spectrum (presented in the form of Nyquist plot) was composed of semicircle over the high frequency and straight line in the low frequency regions. The diameter of the semicircle is attributable to the charge transfer resistance (Rct ) and representing the rate of charge exchange process between the polymer-electrolyte interface [37]. Hence, the specific propose of the EIS is to describe the interface properties in term of Rct . Fig. 5 presents the Nyquist plot for the Pt/P2MA-SDS/MGCE (a), P2MA-SDS/MGCE (b) and P2MA/MGCE (c) in 0.5 M H2 SO4 solution. As can be seen in this figure, there is an obvious difference between the shape of the two impedance spectra (b and c). This result indicates that (b) has much smaller Rct than (c) and suggests that (b) has higher surface area and active site for faradaic reaction and easier charge transfer. The comparison of(a) and (b) in this figure shows that although the Pt particles have been incorporated into/onto the polymeric film, Rct is not significantly changed by existence of the Pt particles. Both of the impedance spectra of (a) and (b) have almost similar forms but the slope of (a) is higher than (b) in low-frequency regions. This reveals that dispersion of the Pt particles in the polymer film helps to increase the mass diffusion [38]. The aforementioned results show an advance in diverse chemical and physical properties of the supports such as charge transfer andconductivity. 3.5. Electrocatalytic oxidation of methanol Fig. 6 depicted the cyclic voltammograms of the methanol oxidation onto the Pt/MGCE (a), Pt/P2MA/MGCE (b) and Pt/P2MASDS/MGCE (c) in 0.5 M H2 SO4 + 1.78 M CH3 OH solution. Methanol electrooxidation at the Pt surface exhibits the characteristic double voltammetric peaks that appear in the forward and reverse scans [39]. As can be seen in curve (c), the onset potential of methanol oxidation is about 0.10 V and an enormous anodic peak appears approximately at 0.86 V in the forward scan which is generated by oxidation of the CO to CO2 and another peak observed at about 0.66 V in the backward scan is attributed to complete oxidation of

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Table 3 Effect of P2MA film thickness, 2MA and SDS concentrations on the peak current density of methanol electrooxidation. Cycle number

5

10

15

20

25

-

j/mA cm−2 C2MA /mM j/mA cm−2 CSDS /mM j/mA cm−2

20.5 5 24.05 5 29.33

34.16 10 44.44 10 80.55

44.44 15 68.61 15 100

35.94 20 80.83 20 61.66

29.33 25 49.72 25 40.83

30 32.72 -

Fig. 6. Electrochemical responses of the Pt/MGCE (a), Pt/P2MA/MGCE (b) and Pt/P2MA-SDS/MGCE (c) in 1.78 M CH3 OH + 0.5 M H2 SO4 solution at  = 50 mV s−1 .

Table 2 ElectrocatalystsCH2SO4 /CMethanol (M) /mV s−1 Eop vs. SCE/V Mass activity/A gPt −1 Ref. Pt/PoAP/Al

0.1/0.1

20

0.20

520

[28]

Pt/PPy/Au Nano-Pt/MGCE Pt/SBA-15/MGCE Pt/PANI/MGCE Pt/Nano-PDAN/MGCE Pt/PAANI/MGCE Pt/MGCE Pt/SWCNT Pt/PMT (TX-100)/MCPE Pt nanorod/Ti Pt/P2MA-SDS/MGCE

0.1/1.0 0.5/1.4 0.1/0.05 0.5/0.5 0.5/2.4 0.5/0.5 0.5/0.5 1.0/2.0 0.5/1.4 0.5/2.0 0.5/1.75

50 50 50 5 50 10 50 50 50 50 50

0.40 0.20 0.15 0.30 0.20 0.30 0.20 0.30 0.05 0.36 0.14

52.6 74.2 32 27.8 110 117 54 250

[40] [41] [42] [43] [44] [45] [46] [47] [48] [49] Thiswork

CH3 OH to CO2 [39]. The most visible difference founded for cyclic voltammetry on the (c) in comparison with (a) and (b) is a higher peak current densityand lower onset oxidation potential. The current density in this figure is defined as current normalized per Pt real surface area. This result can be due to the presence of SDS which enhances the conductivity and surface area of the P2MA film, representing good and proper bed for fine dispersion, efficient adhesion and prevention of the Pt coalescing. Therefore, these results clearly reveal that at the Pt/SDS-P2MA/MGCE, the kinetics of methanol oxidation is tremendously improved and methanol oxidation becomes much easier. For evaluation of the electrocatalytic activity, the onset potential and mass activity were listed in Table 2 and the methanol electrooxidation herein has been compared with the some previous works. The results illustrate that the P2MA-SDS/MGCE could dramatically enhances the electrocatalytic activity of the Pt catalyst, so thereby proving to be an efficient catalyst support.

3.6. Parameters affecting the electrode modification In order to evaluate the effect of various parameters such as P2MA film thickness, 2MA and SDS concentrations on the methanol electrooxidation, the amount of anodic peak current density was monitored as an index for finding the optimum conditions and the obtained results were presented in Table 3. The results indicate that the peak current density increases extensively for the cycle numbers up to 15 cycles, 2MA concentration up to 20 mM and SDS concentration up to 15 mM and drop afterwards.

Fig. 7. Current density-potential curves of the Pt/P2MA-SDS/MGCE in 0.5 M H2 SO4 solution with different methanol concentrations at  = 50 mV s−1 : (a) 0.24 M, (b) 0.47 M, (c) 0.70 M, (d) 0.92 M, (e) 1.15 M, (f) 1.36 M, (g) 1.58 M, (h) 1.78 M, (i) 1.99 M and (j) 2.19 M. Inset of plot: jpI as a function of methanol concentration.

3.7. The effect of methanol concentration Fig. 7 presents the effect of various CH3 OH concentrations on the electrooxidation current density at the Pt/P2MA-SDS/MGCE. As shown in this figure, when the excessive concentrations are added, the oxidation peak currents densities increase and drop afterward at concentrations higher than 1.78 M. This effect might be due to the saturation of Pt active sites by methanol molecules and also contamination of the catalyst surface which is mainly arisen from the COads intermediate during the methanol oxidation. In accordance with this result, the optimum methanol concentration to achieve a higher peak current density might be considered as about 1.78 M. 3.8. Effect of potential scan rates and switching potentials on the methanol oxidation The influence of various potential scan rates was examined on the electrocatalytic behavior of the Pt/P2MA-SDS/MGCE in the presence of methanol (Fig. 8). It is founded that there is dual linear regions between the lower and higher values of the potential scan rates. It could be understood that by increasing of the potential scan rates, the methanol oxidation is improved. Thus, the reaction products accumulate at the electrode surface, so it can decrease the adsorption of methanol molecules for electrooxidation process. Meanwhile, it should be noted that by enhancing the production rate of CO2 molecules from methanol oxidation with increasing of potential scan rates, electrolyte expelled from the pores of the modified electrode. Therefore, it can decreases the accessible surface area and effective active site for electrochemical oxidation. So, the rate of methanol diffusion to the electrode surface can keep up with the lower potential scan rates. In order to get insight for understanding the relation between Pt oxide species and methanol oxidation, the influence of upper limit potentials were investigated at 1.78 M CH3 OH in 0.5 M H2 SO4 solution (Fig. 9). As can be seen from this figure, by increasing the final

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Fig. 10. (A)Cyclic voltammograms of the Pt/P2MA-SDS/MGCE in the presence of 1.78 M methanol in 0.5 M H2 SO4 solution: (solid line) 1st cycle; (dotted line) 50th cycle of the potential sweeping at  = 50 mVs−1 .(B) Chronoamperograms of the Pt/MGCE (a), Pt/P2MA/MGCE (b) and Pt/P2MA-SDS/MGCE (c) in 0.5 M H2 SO4 solution + 1.78 M CH3 OH at peak potential values. Inset of plot (B): Chronoamperograms of the Pt/P2MA-SDS/MGCE for the methanol oxidation upon application of 0.45 (a), 0.60 (b) and 0.86 V (c).

Fig. 8. Cyclic voltammograms of the Pt/P2MA-SDS/MGCE in 0.5 M H2 SO4 + 1.78 M CH3 OH solution: (A) at lower values of : (a) 0.002 V s−1 , (b) 0.005 V s−1 , (c) 0.01 V s−1 , (d) 0.02 V s−1 , (e) 0.05 V s−1 , (f) 0.08 V s−1 , and (B) at higher values of : (g) 0.1 V s−1 , (h) 0.2 V s−1 , (i) 0.4 V s−1 , (j) 0.6 V s−1 , (k) 0.8 V s−1 . (C) The dependency of the anodic peak (I) current densityobtained from (A) and (B) versus 1/2 .

positive potential, oxidation currentdensity and potential of peak (I) remain almost invariable, but the oxidation current densityof peak (II) decreases and its potential shifts to the negative directions. Based on the above results, a possible interpretation may be explained as following as: by increasing the upper limit potential, formation of the PtO is accelerated and poisoning of the electrode surface increases and this reaction can decreases the oxidation current in the negative sweep. Thus, on the clean surface, the oxidation process through intermediate becomes much easier, which can be seen from the negative shift of the peak (II). 3.9. Long-term stability

Fig. 9. Effect of upper limit potential scanning on the oxidation of 1.78 M CH3 OH onto the Pt/P2MA-SDS/MGCE in 0.5 M H2 SO4 solution at = 0.05 V s−1 : (a) 0.0-1.0 V, (b) 0.0-1.1 V, (c) 0.0-1.2 V, (d) 0.0-1.3 V and (e) 0.0-1.4 V.

From practical view, long-term stability of the electrode is important. Hence, the stability of the Pt/P2MA-SDS/MGCE was checked by measuring the anodic peak current densityfor methanol oxidation after 50 cycles in 1.78 M CH3 OH + 0.5 M H2 SO4 solution (Fig. 10A).It can be observed that the anodic currents densitydecrease gradually by consecutive potential scanning.The peak current densityof the 50th scan is about 74.4% than that the first scan for the modified electrode. This result can be attributed to the poisoning of the Pt particles by COads species which were produced during the oxidation. So, the Pt/P2MA-SDS/MGCE has good stability and durability during the methanol oxidation. Also, Fig. 10 B depicted the typical chronoamperometric response for a period of time 1000 s at the methanol oxidation peak potential values for the Pt/MGCE(a), Pt/P2MA/MGCE (b) and Pt/P2MA-SDS/MGCE (c) in 1.78 M CH3 OH + 0.5 M H2 SO4 solution. As can be seen in this figure, there is decay in activity with time and after that reaches to a stable plateau for the electrodes. This decrease in current

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Scheme 1. Schematic representation of formaldehyde oxidation on the Pt/P2MA-SDS/MGCE.

density can be due to the intermediate poisoning species which was generated during the methanol oxidation. The data implies that the maximum oxidation current densityis obtained at the surface of the Pt/P2MA-SDS/MGCE. Thus, in comparison with (a) and (b), the proposed modified electrode has better poisoning-tolerance ability. Also, inset of Fig. 10 B shows chronoamperograms of the modified electrode at different constant potentials. Although, as expected, the largest currents are observed at the most positive potential applied (curve c), it is noteworthy that the steady state currents were still developed at more negative potentials (curves a and b). 3.10. Electrocatalytic oxidation of formaldehyde Recently formaldehyde has attracted a great attention as one of the intermediate in methanol oxidation and like formic acid can be activated to decompose to smaller fragments, electrons, protons and CO2 at high efficiency [50]. Therefore, survey of formaldehyde electrooxidation is a basis for complete understanding of methanol oxidation. The electrochemical oxidation of formaldehyde is schematically shown in Scheme 1. Fig. 11 displays the cyclic voltammograms of the Pt/MGCE (a), Pt/P2MA/MGCE (b) and Pt/P2MA-SDS/MGCE (c) in 0.5 M H2 SO4 solution containing 0.52 M formaldehyde. The current density in this figure is defined as current normalized per Pt real surface area. From the trace (c), in comparison with (a) and (b), it can be observed that there is an increment in the peak current density and a decrease in oxidation onset potential which confirm that the Pt/P2MA-SDS/MGCE

Fig. 11. Cyclic voltammograms of the Pt/MGCE(a), Pt/P2MA/MGCE (b) and Pt/P2MASDS/MGCE(c) in0.5 M H2 SO4 solution + 0.52 M formaldehyde at  = 50 mV s−1 .

has better catalytic abilityfor formaldehyde oxidation. These data can be attributed to the faster electropolymerization, higher conductivity and surface areas which are obtained in the presence of SDS. The interaction between this support and Pt catalysts improves the Pt dispersion, accessible surface area and effective sites. In this section, a comparative study was done for the present electrode and some previous works in terms of onset and peak potential of formaldehyde oxidation (Table 4). From the data, it is evidence that

Table 4 Comparison of the electrocatalytic oxidation of formaldehyde at the Pt/P2MA-SDS/MGCE with some chemically modified electrodesat  = 50 mV s−1 . Modified electrode

CElectrolyte /CFormaldehyde (M)

Eop vs. SCE/V

Ep vs. SCE/V

Ref.

Pt/SWCNT/PANI Pt/Nano-PDAN/MGCE Pt-Pd/Nf/MGCE Pt-Pd/CNT Pt/PAANI/MWCNTs/MGCE Pt-PPy/CNT Pt-Pd/PPy-CNT Pt/Carbon-Ceramic Pt/P2MA-SDS/MGCE

0.5 M H2 SO4 , 0.26 0.5 M H2 SO4 , 0.26 0.5 M H2 SO4 , 0.001 0.5 M H2 SO4 , 0.50 0.5 M H2 SO4 , 0.50 0.5 M H2 SO4 , 0.50 0.1 M HClO4 , 0.75 0.1 M H2 SO4 , 0. 75 0.5 M H2 SO4 , 0.52

0.30 0.20 0.30 0.20 0.20 0.20 0.20 0.20 0.24

0.66 0.75 0.58 0.67 0.72 0.67 0.64 0.85 0.77

[38] [44] [51] [52] [53] [54] [54] [55] Thiswork

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the Pt/P2MA-SDS/MGCE has a good performance and can it act as an active electrocatalyst for formaldehyde oxidation. 4. Conclusion In this work, the P2MA-SDS film was prepared by using electropolymerization of the 2MA in the presence of SDS on the GCE surface. Adding SDS to the solution led to increase the polymer growth rate and produced an important change in its oxidation mechanism. After that, Pt nano/micro particles were dispersed onto the polymer film by the electrodeposition process for construction of the Pt/P2MA-SDS/MGCE. It was observed that the Pt/P2MASDS/MGCE was capable to catalyze efficiently the methanol and formaldehyde electrooxidation as evidenced by lower onset potentials and higher peak current densities. It might be due to the presence of SDS during the electropolymerization process which improves the polymer surface area and make it as effective and suitable support for high and uniform dispersion of Pt particles. It has been found out that the P2MA-SDS film enhanced the effective sites and accessible surface areas of the Pt catalysts and improved the electrooxidation performance. Also, stability of the Pt/P2MASDS/MGCE showed the satisfactory result. Furthermore, the linear relation between the anodic peak current densityand square root of potential scan rates displayed that the methanol oxidation reaction was a diffusion controlled process. Thus, the Pt/P2MA-SDS/MGCE was anticipated to found a potential application in direct methanol fuel cells. References [1] C.W. Kuo, L.M. Huang, T.C. Wen, A. Gopalan, J. Power Sour. 160 (2006) 65. [2] P. Santhosh, A. Gopalan, T. Vasudevan, K.P. Lee, Appl. Surf. Sci. 252 (2006) 7964. [3] K. Kaneto, J. Nakajima, M. Nakagawa, W. Takashima, Thin solid films 438 (2003) 195. ˙ A. Malinauskas, Electrochim. Acta 51 [4] A. Ramanaviˇcius, A. Ramanaviˇciene, (2006) 6025. [5] T. Rajesh, D. Ahuja, Kumar, Sens.Actuat B.chem 136 (2009) 275. [6] E. Antolini, E.R. Gonzalez, Appl. Catal. A: Gen. 365 (2009) 1. [7] A. Adhikari, S. Radhakrishnan, R. Patil, Synth. Met. 159 (2009) 1682. [8] S.M. Choi, J.H. Kim, J.Y. Jung, E.Y. Yoon, W.B. Kim, Electrochim. Acta 53 (2008) 5804. [9] K.M. Kost, D.E. Bartak, B. Kazee, T. Kuwana, Anal. Chem. 60 (1988) 2379. [10] W. Zhou, Y. Du, F. Ren, C. Wang, J. Xu, P. Yang, Int. J. Hydrogen Energy 35 (2010) 3270. [11] Y.C. Liu, K.H. Yang, M.D. Ger, Synth. Met. 126 (2002) 337. [12] Y. Li, G. Shi, J. Phys. Chem. B 109 (2005) 23787. [13] M. Pournaghi-Azar, B. Habibi, J. Electroanal. Chem. 601 (2007) 53.

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