Triton X-100 composite catalyst at the surface of carbon nanotube paste electrode and its application for methanol oxidation

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Electrochemical fabrication of novel Pt/poly (m-toluidine)/ Triton X-100 composite catalyst at the surface of carbon nanotube paste electrode and its application for methanol oxidation Jahan-Bakhsh Raoof*, Reza Ojani, Sayed Reza Hosseini Electroanalytical Chemistry Research Laboratory, Department of Analytical Chemistry, Faculty of Chemistry, Mazandaran University, 3rd Kilometer of Air Force Road, Postal Code: 47416-95447, Babolsar, Iran

article info

abstract

Article history:

In this work, for the first time an aqueous solution of Triton X-100 (TX-100) non-ionic

Received 5 July 2010

surfactant is used as an additive for electropolymerization of m-toluidine (MT) onto multi-

Received in revised form

walled carbon nanotube paste electrode (CNTPE), which is investigated as a novel matrix for

6 September 2010

dispersion of platinum particles. Electrochemical response of the poly (m-toluidine) film

Accepted 8 September 2010

prepared in the presence of TX-100 (PMT/TX-100) is, at least, 20 times higher than that

Available online 23 October 2010

obtained in the absence of the surfactant. The as-prepared substrate is used as porous matrix for dispersion of platinum particles by potentiodynamic method. As-formed novel support

Keywords:

and composite catalyst are characterized by scanning electron microscopy (SEM), energy

Triton X-100

dispersive spectrum (EDS), and electrochemical methods. The SEM images reveal that the

Poly (m-toluidine)

deposits are composed of spherical Pt particles. The EDS confirms the presence of Pt on the

Carbon nanotube paste electrode

modified electrode. The electrochemical methanol oxidation reaction (MOR) is studied at the

Methanol

surface of this modified electrode. It has been shown that the PMT/TX-100 at the surface of

Platinum particles

CNTPE improves the catalytic efficiency of the deposited platinum particles toward MOR. Then, the influence of scan rates of potential and switching potential on the MOR as well as long-term stability of this modified electrode have been investigated by chronoamperometric and cyclic voltammetric methods. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Direct methanol fuel cells (DMFCs) are potential alternative energy sources for portable electronic devices because of their high energy-conversion efficiency, low pollution emission, and safe fuel handling. However, the electrooxidation of methanol is a very complex reaction, during which many intermediates and poisoning species are involved [1]. In an acidic medium, the most efficient catalyst for this reaction is platinum and its alloys [2e5]. The electrocatalytic activity of

platinum particles for methanol oxidation is dependent on many factors [6,7]. Of these, the supporting materials and their surface conditions are essential for the Pt catalyst to produce high catalytic activity [8]. The supporting materials with high surface area are essential to reduce the metal loading under the condition of keeping the high catalytic activity. Conducting polymers (CPs) are very important materials which are currently being investigated with regard their applications in energy storage, microelectronics, electrochromic displays, electrocatalysis, chemical sensors, etc. [9].

* Corresponding author. Tel.: þ98 112 5342392; fax: þ98 112 5342350. E-mail address: [email protected] (J.-B. Raoof). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.09.022

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CPs matrixes have been employed as catalyst support materials for the oxidation of small organic molecules in place of conventional support because when particle catalyst is dispersed in carbon black, a part of the active sites remains inaccessible to the reactant molecules [10,11]. However, metal particles dispersed into conducting polymer support, not only provide access to large number of catalytic sites, but also offer the possibilities of spent catalyst recovery. On the other hand, CPs offer great advantages over the other materials since they are permeable to electroactive species, readily modified by different techniques, and easy to coat on various substrates. Also, it should be noticed that the nature of working electrode substrate in electropreparation of polymeric film is very important because the properties of polymeric films depend on the working electrode material. The ease and fast preparation and obtaining a new reproducible surface, the low residual current, porous surface, and low cost of carbon paste are some advantages of carbon paste electrode (CPE) over all other solid electrodes [12,13]. On the other hand, carbon nanotubes (CNTs), as a new form of carbon, have received numerous theoretical and experimental studies [14]. Due to their nanometer size and interesting properties, CNTs are also of great interest for many applications. Furthermore, high accessible surface area, low resistance, and high stability [15] suggest that CNTs are suitable materials for electrodes and catalyst supports in fuel cell applications. It has been seen that surfactants (surface active agents) play a very important role in electrode reactions, not only in solubilizing organic compounds, but also by providing specific orientation of the molecules at the electrode interface [16]. These molecules can give rise to adsorbed layers of varying thickness monolayers, bilayers or multilayer of a very complex structure [17], thus affect the rate of electrode reaction [18]. Triton X-100 (C14H22O(C2H4O)n, n z 10) is a non-ionic surfactant which has a hydrophilic polyethylene oxide group

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and a lipophilic or hydrophobic group. The hydrophobic group is a 4-(1,1,3,3-tetramethylbutyl)-phenyl group. Our literature survey indicates that, there is no report about the electropolymerization of MT at the surface of multiwalled carbon nanotube paste electrode in the presence of TX100 as an additive. Therefore, in the present work, with respect to advantages of carbon paste, carbon nanotube and surfactants, we have decided to investigate the effect of the surfactant on the electropolymerization and on growth of poly (m-toluidine) films. Therefore, poly (m-toluidine)/TX-100 modified CNTPE, which is a conductive organic matrix was prepared and allowed a better dispersion of platinum particles as a catalyst for methanol electrooxidation which has a significant attraction in DMFCs.

2.

Experimental

2.1.

Materials

The solvent used in this work was double distilled water. Sulfuric acid (from Fluka) was used as supporting electrolyte. The MT monomer (99%), H2PtCl6.6H2O and Triton X-100 (99%) from Merck were used as received. Methanol (from Merck) used in this work was analytical grade. High viscosity paraffin (density: 0.88 g cm3) (from Fluka) was used as the pasting liquid for CNTPE. Graphite powder (particle diameter: 0.10 mm, from Merck) and multi-walled carbon nanotube (with purity >95%, diameter 54 nm, length 1e10 mm, number of walls 3e15, from Nanostartech. Co., Tehran, Iran) were used as the working electrode substrates. The as received multiwalled carbon nanotubes were treated with concentrated acids (H2SO4/HNO3: 3/1) for purification and generation of oxygen functionalities on the surface of MWCNTs. All other reagents are of analytical grade.

Fig. 1 e Electropolymerization of MT in a 0.02 M monomer D 1.0 M H2SO4 solution at the surface of CNTPE, (A) in the presence 6.0 3 10L3 M of Triton X-100 and (B) in the absence of the surfactant. The arrows indicate the trends of current during of cyclic voltammetry, y [ 0.05 VsL1.

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The amount of Pt particles loaded (m) onto the PMT (TX100)/MCNTPE or deposited onto other modified electrodes is calculated from the equation: m ¼ QdepM/FZ, where m is calculated by using the charge (obtained through graphical integration of cyclic voltammetric curves) utilized for the deposition of Pt particles. M is the atomic weight of Pt (195.078 g mol1), F (96485 C mol1) is the Faraday constant, and Z is the number of electrons transferred (taken as four for the formation of Pt). In this work, the amount of Pt deposited in all the cases was controlled about 0.02 mg.

Fig. 2 e Proposed mechanism for the electropolymerization of MT in the presence of TX-100 at the surface of carbon nanotube paste electrode.

2.2.

Instrumentation

Electrochemical experiments were performed with a computer controlled potentiostat/galvanostat m-Auto lab type III modular electrochemical system (Eco Chemie BV, Netherlands), driven with general purpose electrochemical system (GPES) software (Eco Chemie). A conventional three electrode cell was used with a AgjAgCljKCl (3 M) as reference electrode, a platinum wire as counter electrode, unmodified carbon paste and modified carbon nanotube paste as working electrode. The surface morphology and chemical composition of the deposits were evaluated by scanning electron microscope (Leo1455VP, Oxford Instrument) equipped with an energy dispersive spectrometer.

2.3.

After Pt particles incorporation, the electrode was rinsed with distilled water. At the beginning of experiments, the Pt/ PMT (TX-100)/MCNTPE was immersed in supporting electrolyte and the potentials were cycled between 0.3e1.3 V in 0.5 M H2SO4 solution at y ¼ 50 mV s1 until a reproducible cyclic voltammogram was attained (3e5 cycles).

Electrode modification

3.

Results and discussion

3.1.

Electrochemical polymerization

The poly (m-toluidine) film was prepared on the surface of carbon nanotube paste electrode in the absence and presence of TX-100. Fig. 1A shows the typical multi-sweep cyclic voltammograms during the electropolymerization of MT in the presence of TX-100. As can be seen in this figure, in the first anodic sweep, the oxidation of MT occurs as a distinct irreversible anodic peak (Ep ¼ 0.93 V). A part of the oxidation products of MT is deposited on the electrode, as a poly (mtoluidine) film. In the first reverse cycle, the new cathodic peak is appeared at a potential around 0.39 V, confirming the initial

A mixture of graphite powder (0.90 g) plus multi-walled carbon nanotubes (0.10 g) were blended by hand mixing with a mortar and pestle. Using a syringe, 0.65 g paraffin was added to the mixture and mixed well until a uniformly-wetted paste is obtained. The resulting paste was then inserted in the bottom of a glass tube (internal radius: 1.7 mm). The electrical connection was implemented by a copper wire lead fitted into the glass tube. A fresh electrode surface was generated rapidly by extruding a small plug of the paste out of the tube and smoothing the resulting surface on white paper until a smooth shiny surface is observed. The unmodified carbon paste electrode was prepared in the same way without adding carbon nanotubes to the mixture to be used for comparison purposes. Later modifications of the mentioned electrodes were performed in two steps: a Electropolymerization of MT monomer by using potential cycling (10 cycles at a scan rate of potential 0.05 V s1) between 0.0 and 1.2 V vs. AgjAgCljKCl (3 M) in aqueous solution containing 1.0 M H2SO4, 0.02 M MT and 6.0  103 M of TX-100 for construction of PMT (TX-100)/MCNTPE. b Incorporation of platinum particles to the polymeric film with electrochemical deposition from an aqueous sulfuric acid solution containing 4.0 mM H2PtCl6 by using potential cycling between 0.4 and 0.25 V vs. AgjAgCljKCl (3 M) at y ¼ 0.05 V s1 for fabrication of Pt/PMT (TX-100)/MCNTPE.

Fig. 3 e Typical Cyclic voltammograms recorded of the PMT (TX-100)/MCNTPE in 1.0 M H2SO4 solution: (a) first cycle and (b) 10th cycle. Inset: a CV of the electrode after its storage for five days in acid solution, y [ 0.05 V sL1.

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Fig. 4 e (A) Cyclic voltammograms of PMT (TX-100)/MCNTPE in 1.0 M H2SO4 solution at various sweep rates of potential: (a) 0.005, (b)0.01, (c)0.02, (d)0.05, (e)0.08, (f)0.10, (g)0.20, (h)0.40, (i)0.60, (j)0.80 and (k)1.0 V sL1. (B) Plot of Ep vs. log y for cyclic voltammograms depicted in the (A) for anodic peaks (a) and cathodic peaks (b). (C) The dependency of anodic and cathodic peak currents on the y at lower values (0.005e0.10 V sL1) and (D) on the y1/2 at higher values (y > 0.10 V sL1).

deposition of electrooxididized products. In the second positive scan of potential, an anodic peak is appeared at a potential around 0.44 V. The oxidation peak current of monomer is decreased with increasing of the number of potential cycles up to 3rd cycle. The decreasing of oxidation current is due to the loss of activity of the electrode surface when covered with newly formed polymer film. In the presence of TX-100, the rate of polymerization is considerably increased. Furthermore, under successive potential cycling, the peak currents related to the polymer are significantly increased, and their growth continued up to 20th cycle. Additionally in the 3rd cycle, the monomer oxidation potential is shifted to less positive potentials (by almost 0.03 V) and its oxidation current almost unchanged. These results show that, in the presence of TX-100, the monomers can easily reach toward the electrode surface and produce more monocation radicals. TX-100 improves the MT solubility and allows a well-defined polymer

growth on the working electrode. Moreover, their polymers show a better stability than those obtained in the absence of TX-100. For comparison, Fig. 1B shows typical cyclic voltammograms during polymerization in the absence of TX-100. Electrochemical signal of the conducting poly (m-toluidine) film prepared from the solution with TX-100 is greatly improved in comparison with that from the solution without the surfactant. The effect of the additive can be analyzed by looking into its molecular structure. The head group of TX-100 (PEO chains) is polar. TX-100 molecules may be adsorbed on the electrode surface and change the interface structure between the electrode and the electrolyte solution, which benefits the electropolymerization of this monomer. The proposed mechanism for the electrochemical polymerization of MT with TX-100 is shown in Fig. 2. The oxygen atoms in the poly oxyethylene group of TX-100 may attract the positive charge of monomer radical, resulting in MTþ.-TX-100 form.

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Therefore, TX-100 makes easier electropolymerization for MT. Indeed, the presence of TX-100 decreases the monomer oxidation potential and accelerates of the polymerization.

3.2. Electrochemical behavior of the PMT (TX-100)/ MCNTPE

Fig. 5 e Cyclic voltammograms of PMT/MCNTPE (a) and PMT (TX-100)/MCNTPE (b) in 0.5 M H2SO4 solution at scan rate of 0.05 V sL1.

The stability of prepared films is checked with CV in 1.0 M H2SO4 solution, which shows decreasing in current at initial ten cycles and the current almost remains constant afterward (Fig. 3). After potential cycling in 1.0 M H2SO4 solution, the electrode was stored in acid solution for five days; then CV experiment was carried out, and the peak currents were found to be around 62% of initial values (inset of Fig. 3). This indicates that the PMT (TX100)/MCNTPE has good stability and storage properties in acid solutions. Fig. 4A represents the cyclic voltammograms of PMT (TX-100)/MCNTPE recorded at different potential sweep rates, y, in a wide range of 0.005e1.00 V s1. A pair of well-defined peaks with a half wave potential of 405 mV appears in the voltammograms, and the peak-to-peak potential separation (at the potential sweep rate of 10 mV s1) is 70 mV. The peak-to-peak potential separation is deviated from the theoretical value of

Fig. 6 e SEM images of the CNTPE (a), PMT/MCNTPE (b) and PMT (TX-100)/MCNTPE (c, d).Scale bar for (a), (b), (c) and (d) are 1 mm, 2 mm, 2 mm and 1 mm, respectively.

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Fig. 7 e Typical SEM images of the Pt/PMT/MCNTPE (a) and Pt/PMT (TX-100)/MCNTPE (b, c). Scale bars for (a), (b) and (c) are 1 mm, 1 mm and 200 nm, respectively.

zero and increased at higher potential sweep rates. Fig. 4B shows the plot of Ep with respect to the logarithm y from cyclic voltammograms recorded at potential sweep rates 0.005e1.0 V s1 for anodic (a) and cathodic (b) peaks. It can be observed that the values of Ep are proportional to the logarithm y at y > 0.10 V s1. As shown, the peak currents increase with increasing of y and are proportional to y below 0.10 V s1 (Fig. 4C), which indicates a surface-confined redox process. In the higher range of y (y > 0.10 V s1, Fig. 4D), the peak currents depend on y1/2, signifying the dominance of a diffusion process as the rate limiting step in the total redox transition of the polymeric film [19e24]. For comparison, cyclic voltammetry behaviors of PMT/ MCNTPE and PMT (TX-100)/MCNTPE are shown in Fig. 5. The electrodes demonstrated their electrochemical activities, which are characterized by typical oxidation and reduction responses. Poly (m-toluidine)/TX-100 films have considerably higher redox current than the normal poly (m-toluidine) film. The difference in redox currents reflect the effective active surface areas that are accessible to the electrolytes at PMT (TX-100)/MCNTPE. Apparently, the porous PMT/TX-100 films have higher effective surface areas. This improvement of doping-undoping rate results from the increase of surface area and porous structure,

Fig. 8 e Energy dispersive spectrum (EDS) of the Pt/PMT (TX-100)/MCNTPE.

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Fig. 9 e (A) Electrochemical responses of Pt/PMT (TX-100)/MCNTPE in 0.5 M H2SO4 (solid line) in the absence and (dash line) the presence of 1.4 M methanol. (B) Cyclic voltammograms of (a) Pt/PMT/MCNTPE and (b) Pt/PMT (TX-100)/MCNTPE in 0.5 M H2SO4 D 1.4 M CH3OH at y [ 0.05 V sL1. (C) Cyclic voltammograms of methanol electrooxidation at (a) Pt/MCNTE and (b) Pt/ PMT (TX-100)/MCNTPE in 0.5 M H2SO4.

which are of benefit to the ion diffusion and migration [25]. Furthermore, the PMT/TX-100 film shows large background current. This is also attributed to the large surface area of porous structure of the film immobilized on the surface of CNTPE.

3.3.

Surface morphology and electrode composition

From the above discussion, it is conceivable that surface morphology of PMT is influenced by the incorporation of TX100 into PMT structure. The morphology of bare CNTPE, PMT/ MCNTPE and PMT (TX-100)/MCNTPE is characterized by scanning electron microscopy (Fig. 6). Fig. 6a shows the morphology of the bare CNTPE surface after smoothing the surface on white paper. As can be seen in this image, there are some holes or cavities on the electrode surface because carbon paste is porous. The SEM image of PMT film on the CNTPE (Fig. 6b) shows a scaly structure. Fig. 6(c) and (d) show

the structure of the PMT (TX-100)/MCNTPE surface. The surface of the PMT film prepared in the presence of TX-100 is smoother and larger than that of the PMT film without the surfactant (scale bar in both cases is 2 mm). Also, the presence of TX-100 improves the uniformity of the coverage. This structure allows the electrolyte constituent better access to the interior of the PMT. Furthermore, it yields a larger available area and, in the case of introducing of Pt catalyst, with better dispersion and/or distribution. As typical examples, images (a), (b) and (c) in Fig. 7 show the morphology of the Pt/PMT/MCNTPE and Pt/PMT (TX-100)/ MCNTPE. As can be seen in image (a), the large spherical aggregates may be formed through the settlement of several small particles of Pt. Thus the effective surface area of the aggregate is due to distance between settled particles. Figs. 7 (b) and (c) related to the platinum particles dispersed into the PMT (TX-100)/MCNTPE show that the presence of the PMT

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Fig. 10 e Histograms of mass activities for, (1) Pt/CPE, (2) Pt/ PMT/MCPE, (3) Pt/PMT (TX-100)/MCPE, (4) Pt/CNTPE, (5) Pt/ PMT/MCNTPE and (6) Pt/PMT (TX-100)/MCNTPE in a solution containing 1.4 M methanol D 0.5 M H2SO4 at y [ 0.05 V sL1.

(TX-100) gives rise to decrease the aggregation of the small Pt particles, relatively. Furthermore, in this case the distance within the particles is increased, thus the effective surface area of the aggregates is improved. These results are in agreement with electrochemical experiments. A typical EDS for determination of bulk composition of the Pt/PMT (TX-100)/MCNTP electrode is presented in Fig. 8. From the EDS results shown in this Figure, Pt and C are the major elements. Electrochemical evidence for the available Pt deposit was also obtained by cycling the Pt/PMT (TX-100)/ MCNTPE in the supporting electrolyte in the hydrogen adsorption/desorption region. Oxygen may come from the surfactant used in the electropolymerization of MT monomer or working electrolyte solution.

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reversed, a reduction peak current III appear, which is due to the reduction of platinum oxide to Pt and clean platinum is available. The methanol oxidation takes place more easily at the surface of clean platinum and therefore the peak current IV for methanol oxidation appears. The height of this peak depends on the residue poisoning species on platinum surface that can be removed. For comparison, the cyclic voltammograms of methanol oxidation at Pt/PMT/MCNTPE and Pt/PMT (TX-100)/MCNTPE are shown in Fig. 9B. As can be seen in the figure, Pt/PMT (TX100)/MCNTPE yields much higher mass activity (i.e. current normalized per Pt load/A g1 Pt ) than the other electrode. The differences between cyclic voltammogram (a) with (b) may be attributed to the large real surface area of the Pt particles in the PMT (TX-100) film immobilized on the CNTPE. In addition, the polymeric structure prevents the particle agglomerating and coalescing during accumulation and also stabilizes them on the electrode. These observations can clearly explain the role of the PMT (TX-100) film on the enhancement of electrocatalytic oxidation current of methanol. Indeed, the PMT (TX100) film is a good and proper bed for immobilization of Pt particles. It seems that the main and plausible reason for such an enhancement is the formation of a polymer film backbone at the surface of CNTPE that provides the facile arrival of methanol on Pt catalytic centers. Fig. 9C shows the CVs of methanol electrooxidation on (a) Pt/MCNT electrode and (b) Pt/PMT (TX-100)/MCNTP electrode in 0.5 M H2SO4 þ 1.4 M methanol solution. This Figure shows that the Pt/PMT (TX100)/MCNTPE has significantly greater electrocatalytic activity than the other electrode. For example, at the anodic peak potential, the Pt/PMT (TX-100)/MCNTPE has a mass activity about 117 A g 1 Pt , nearly eight times higher than that the Pt/ MCNT electrode (14.1 A g 1 Pt ). The mass activities for methanol electrooxidation under the same conditions at the surface of various electrodes are

3.4. Electrocatalytic oxidation of methanol at the Pt/PMT (TX-100)/MCNTPE The electrochemical behavior of Pt/PMT (TX-100)/MCNTPE was studied in 0.5 M H2SO4 solution using cyclic voltammetry (Fig. 9). As can be seen in Fig. 9A (solid line), three pair peaks (a/a’ corresponding to the adsorption/desorption of hydrogen, b/b’ corresponding to the oxidation/reduction of polymer and c/c’ corresponding to formation/reduction of Pt oxide) are discernible. It should be noted that the reduction peak of PtO superimposed on the reduction peak of polymeric film. Fig. 9A (dash line) shows a typical voltammogram of Pt/PMT (TX-100)/ MCNTPE in the presence of methanol. Like the other platinum dispersed in electrode substrates, the anodic current of methanol oxidation is started from þ0.05 V with low anodic current and by sweeping to the more positive potentials, the anodic peak current is maximum at about 0.65 V (peak I). The oxidation peak current decreases at more positive potential than region I due to the formation of platinum oxide and broad anodic peak (II) appears. When the potential scan is

Fig. 11 e The effect of upper limit of potential scanning region on the electrooxidation of 1.4 M methanol in 0.5 M H2SO4 solution at the Pt/PMT (TX-100)/MCNTPE at y [ 0.05 V sL1 (1) 0.9, (2) 1.0, (3) 1.1, (4) 1.2 and (5) 1.3 V.

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Fig. 12 e Cyclic voltammograms of the Pt/PMT (TX-100)/MCNTPE in 0.5 M H2SO4D1.4 M methanol aqueous solutions; (A) at lower values of y: (a)0.005, (b)0.010, (c)0.020, (d)0.050 V sL1 and (B) at higher values of y: (e) 0.20, (f) 0.40, (g) 0.60, (h) 0.80 V sL1. (C) The Dependency of the anodic peak (I) current obtained from (A) and (B) vs. y1/2.

given in Fig. 10. Methanol doesn’t undergo oxidation prior to the discharge of supporting electrolyte at CPE and CNTPE in potential window (0.3e1.3 V) in 0.5 M H2SO4 solution, while a small current is observed in the case of the Pt/CPE. Following the dispersion of platinum particles at the PMT/MCPE surface, methanol oxidation occurs at this electrode with higher current which may be attributed to the presence of polymeric film. Pt/PMT (TX-100)/MCPE shows higher mass activity with respect to the previous electrodes. Also, utilization of CNT as incorporated materials into a carbon paste electrode improves the electrochemical signal of methanol oxidation and thus the highest mass activity (about 117A g1 Pt ) is obtained at the surface of Pt/PMT (TX-100)/MCNTPE.

According to the literature evidences [26e30], it is wellknown that peak ‘I’ involves the progress of various steps that results in the formation of carboxyl intermediates together with formation of strongly adsorbed CO spices as follows: (CH3OH)solution / Pte(CH3OH)ads

(1)

Pte(CH3OH)ads / Pte(CH3O)ads þ Hþ þ e

(2)

Pte(CH3O)ads / Pte(CH2O)ads þ Hþ þ e

(3)

Pte(CH2O)ads / Pte(CHO)ads þ Hþ þ e

(4)

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and the cyclic voltammograms obtained are shown in Fig. 11. The fixed lower limit is 0.3 V. As can be seen, the decrease of upper limit potential cycling causes: (a) an increase in the current of peak IV; (b) a positive shift of the potential of peak IV and (c) the peak currents ratio of two peaks I and IV approaches to the unity. Additionally with increasing the scan upper limit, peak I almost remains unchanged. These may be explained by the preventing of platinum oxide formation by lowering the upper limit potential cycling and consequently maintaining the electrode surface relatively clean.

3.6. The effect of potential sweep rates on the electrocatalytic property of Pt/PMT (TX-100)/MCNTPE

Fig. 13 e Chronoamperograms of 1.4 M CH3OH at the surface of various electrodes: (a) Pt/CPE, (b) Pt/CNTPE, (c) Pt/ PMT/MCNTPE and (d) Pt/PMT (TX-100)/MCNTPE in 0.5 M H2SO4 solution for a period of 500 s and applied potential of 0.65 V vs. AgjAgCljKCl (3 M). Inset: Chronoamperogram of the Pt/PMT (TX-100)/MCNTPE for 10000 s (i.e. 2.78 h).

Pte(CHO)ads / Pte(CO)ads þ Hþ þ e

(5)

Reactions from (1)e(5) can be denoted by total dissociative adsorption reaction: (CH3OH)solution / Pte(CO)ads þ 4Hþ þ 4e

(6)

Because of releasing four electrons in reaction (6), the anodic peak ‘I’ appears in origin considerably high and also depending on the amount of clean active sites available on Pt particles surface, at the same time and nearly same potentials, reactions (7) and (8) occurred [31,32].

2Pt þ H2O / Pte(OH)ads þ PteHads

(7)

Pte(OH)ads þ Pte(CO)ads/CO2 þ Hþ þ 2Pt þ e

(8)

The overall oxidation reaction of methanol (reaction (9)) can be occurred and caused a sharp increase in current of peak ‘I’ [33].

CH3OH þ H2O/CO2 þ 6Hþ þ 6e

(9)

The main difference with respect to the mechanism on the bulk platinum electrode in acid medium is that the amount of COads formed on dispersed platinum particles in polymer matrix is smaller leading to a greater electrocatalytic activity.

3.5. The effect of switching potentials on the electrooxidation of methanol at Pt/PMT (TX-100)/MCNTPE The effect of anodic limit of potential scanning on the methanol electrooxidation at Pt/PMT (TX-100)/MCNTPE is studied

The effect of potential sweep rates is studied on the cyclic voltammetry behavior of methanol at the surface of Pt/PMT (TX-100)/MCNTPE. As can be seen in Fig. 12, it is of interest to note that the curve exhibits a dual linear region. This phenomenon is also observed in the related literatures [34,35]. The possible explanation for this phenomenon is as follows: at higher scan rate, the reaction product accumulates in the vicinity of the electrode due to the higher reaction rate, and it will, in turn, decrease the adsorption of methanol molecules. In other words, the rate of methanol diffusion to the electrode surface can keep up with the lower scan rates. Additionally, since the rate of CO2 production increases with increasing of potential scan rate, more electrolytes will be expelled from the pores of the electrode, thus decrease the available surface of catalyst for electrochemical reaction.

3.7. Comparison of long-term stability of the Pt/PMT (TX-100)/MCNTPE In the practical view, long-term stability of the electrode is important. The long-term stability of Pt/PMT (TX-100)/MCNTP electrode is examined in 0.5 M H2SO4 þ 1.4 M CH3OH solutions using cyclic voltammetry method (Figure not shown). The anodic peak current decreases gradually with potential cycling. In general, the loss of the catalytic activity with successive scans of potential may result from the consumption of methanol during the scan of potential in cyclic voltammetry. It also perhaps due to poisoning and the structure change of the platinum particles as a result of the perturbation of the potentials during the scanning in aqueous solutions, especially in the presence of organic compound [36]. Another factor may be due to the diffusion process occurring between surface of the electrode and the bulk solution. With increasing of scan number, methanol diffuses gradually from the bulk solution to the electrode surface. In 0.5 M H2SO4, the electrode gets accumulated by COads poison and hence the anodic current decreases. It was reported by Jiang et al. [37] that the methanol oxidation current on Pt polycrystalline electrode decays rapidly with the time, and the current in jet curve is almost zero after 400 s. The rapid current decay has been interpreted as the ‘‘self-poisoning’’ of the adsorbed species derived from the dissociative adsorption of methanol. In this work, for further evaluate the activity and stability of the catalysts, chronoamperograms are recorded for a period of time (500 s) for the oxidation of 1.4 M methanol at the surfaces of (a) Pt/

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CPE, (b) Pt/CNTPE, (c) Pt/PMT/MCNTPE and (d) Pt/PMT (TX100)/MCNTPE at the peak potential values (Fig. 13). Also, inset of this figure shows typical chronoamperogram curve for very long period of time (e.g. 104 s) for methanol oxidation at the surface of Pt/PMT (TX-100)/MCNTPE and it can be seen that mass activity at the end of the long time is about 913 mA g1 Pt . The relative activities of the electrodes slightly changed and the activity decreased in the sequence: d > c > b > a, which is in agreement with our cyclic voltammetry results. From the data, the maximum oxidation current is obtained at the surface of Pt/PMT (TX-100)/MCNTPE. This suggests that the poisoning effect of generated intermediate species of methanol oxidation at the peak potentials is least on (d) when compared to that on (a), (b) and (c). The response of Pt/PMT (TX-100) film to methanol oxidation at the peak potential shows lesser sensitivity to poisoning by CO compared to the other modified electrodes. Also, stability of the modified electrode is verified by measuring its response to methanol oxidation after ten days of storage in the laboratory atmosphere conditions.

4.

Conclusion

In this work, a novel poly (m-toluidine)/Triton X-100 film was prepared by electropolymerization of MT at the surface of carbon nanotube paste electrode in the presence of TX-100. Addition of TX-100 to the monomer solution leads to an increase in the polymer growth rate. Also, TX-100 in the electrolyte solution may be adsorbed on the electrode and changes the interfacial structure between electrode and electrolyte, which benefits the preparation of poly (m-toluidine) film with more current density. The electrochemical behavior of PMT (TX-100)/MCNTPE shows that, apart from the higher polymerization rate in the presence of TX-100, the resulting polymer has good electrical conductivity, which can be due to the different morphology of poly (m-toluidine) film in the PMT (TX-100)/MCNTPE. Platinum particles incorporated to the PMT (TX-100) films show higher catalytic activity towards methanol oxidation than that the other platinum modified electrodes. Furthermore, long-term stability of the modified electrodes is studied and it is found that Pt/PMT (TX-100)/MCNTPE shows a maximum stability toward methanol oxidation. On the other hand, the linear relationship between the anodic peak current and y1/2 can be observed. This implies that the electrooxidation of methanol at the surface of this modified electrode may be controlled by diffusion process. Also, utilization of MWCNTs as incorporated materials into a carbon paste electrode improves the electrochemical signal of methanol oxidation.

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