sodium dodecyl sulfate film including bimetallic Pt–Cu nanoparticles and its application for formic acid oxidation

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Synthesis of poly (2-Methoxyaniline)/sodium dodecyl sulfate film including bimetallic PteCu nanoparticles and its application for formic acid oxidation Sayed Reza Hosseini a,*, Rahman Hosseinzadeh b, Shahram Ghasemi a, Nahid Farzaneh a a b

Nanochemistry Research Laboratory, Faculty of Chemistry, University of Mazandaran, 47416-95447 Babolsar, Iran Department of Organic Chemistry, Faculty of Chemistry, University of Mazandaran, 47416-95447 Babolsar, Iran

article info

abstract

Article history:

In this work, a facile, low cost and environmental friendly method without the use of any

Received 30 October 2014

linking chemicals, i.e. galvanic replacement (transmetalation) is used for preparation of

Received in revised form

bimetallic PteCu nanoparticles (NPs) onto highly porous poly (2-Methoxyaniline)/sodium

4 December 2014

dodecyl sulfate (P2MA-SDS) film. Electrochemical response of the prepared P2MA at the

Accepted 8 December 2014

presence of SDS in 0.5 M H2SO4 solution is, at least, 18 times higher than that obtained in

Available online xxx

the absence of SDS at the carbon paste electrode surface. The prepared bimetallic PteCu NPs are verified by scanning electron microscopy (SEM), energy dispersive spectroscopy

Keywords:

and electrochemical methods. The SEM images reveal that SDS strongly influences

Nanocomposite

morphology of the polymer film. The PteCu NPs formed on the P2MA-SDS film are with

Poly (2-Methoxyaniline)

average sizes about 65 nm. The obtained results from cyclic voltammetry and chro-

Sodium dodecyl sulfate

noamperometry reveal that the PteCu NPs demonstrate enhanced electrocatalytic activity

PteCu NPs

towards formic acid oxidation. Furthermore, the effects of several parameters such as

Electrocatalytic oxidation

P2MA thickness, 2 MA, and SDS concentrations towards formic acid oxidation as well as

Formic acid

long-term stability of the electrocatalyst have been investigated. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction One of the biggest challenges for twenty-first century is energy. Electrochemistry offers an efficient and clean route for energy production and storage. A promising energy source for transportation and portable electronic devices is direct formic acid fuel cell (DFAFC) [1e3]. Pt has been widely used in DFAFC for formic acid oxidation [4e6]. One of the major limitations

for commercialization of this technology is relatively slow reaction kinetics of formic acid oxidation and CO-poisoning of the Pt catalyst at room temperature [7e9]. To overcome these challenges, considerable researches have been made in developing novel Pt nanoparticles (NPs) with enhanced electrocatalytic performances. One tactic to improve the electrocatalysts durability is to prepare Pt nanocrystals with high-index facets [10]. Another tactic is to fabricate binary Pt alloy catalysts by adding

* Corresponding author. Tel.: þ98 1135302333; fax: þ98 1135302350. E-mail address: [email protected] (S.R. Hosseini). http://dx.doi.org/10.1016/j.ijhydene.2014.12.021 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Hosseini SR, et al., Synthesis of poly (2-Methoxyaniline)/sodium dodecyl sulfate film including bimetallic PteCu nanoparticles and its application for formic acid oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.12.021

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secondary noble metal to Pt such as PteAu [11e14], PtePd [15e22], PteRu [23,24], and PteBi [25,26] which significantly reduce the over-potential of formic acid oxidation. However, these solutions are not simple and cost-effective. Third tactic is to prepare Pt-based bimetallic structures (PteM; M]Cu, Ni, Sn, Fe, Co) as low cost anode catalysts due to their lower cost, greater abundance, and better resistance to CO-poisoning to reduce the expensive Pt cost and improve its catalytic activity [9,27e32]. Among the PteM catalysts, PteCu bimetallic catalyst is one of the most promising candidate due to its high stability against poisoning and abundant sources of copper. The enhanced catalytic activity can be attributed to the electronic and structural effects by addition of the certain metal in Pt [33,34]. Forth tactic is to disperse the particles on the conductive supports such as conducting polymers (CPs) due to their porous structures, high surface areas and access to many catalytic sites to reduce the catalyst loading by keeping high catalytic activity. A number of CPs such as polyaniline [35,36], poly (5-cyanoindole) [37], poly (p-anisidine) [38], poly (o-dihydroxy benzene) [39], polythiophene [40,41], polypyrrole [42,43] and polyindole [44] have been investigated as catalyst supports towards formic acid electrooxidation. The CP improve the properties of electrode-electrolyte interface and allow a facile flow of electronic charges during the oxidation of small organic molecules on the Pt particles [45,46]. Until recently, most of works on the catalysts prepared by transmetalation method has been in search of efficient methanol oxidation electrocatalysts, with only few papers dealing with formic acid oxidation in acid solution. Previously, poly (2-Methoxyaniline) (P2MA) film was prepared onto a glassy carbon electrode and used as a support for electrodeposition of Pt NPs and its application for formic acid oxidation [47]. P2MA film has good thermal stability, electroactivity, good conductivity, easy synthesis route and higher solution processability with respect to the other aniline family polymer films [48]. The presence of an electron donor group (methoxy, eOCH3) at ortho-position of aniline, its commercially availability at low cost and straightforward process at conversion of monomer to polymer make it as an appropriate monomer for these kinds of studies. Therefore, these studies explore the possibility of utilizing the P2MA as alternative to polyaniline for formic acid oxidation. Also, 2 MA has good solubility in water-acid and therefore its electrochemical polymerization may provide an alternative for reducing the use of hazardous chemicals as well as the cost of waste disposal. P2MA coatings have been synthesized onto various substrates by electrochemical polymerization of the 2 MA in aqueous solutions of oxalic acid [49], salicylate [50], dodecylbenzenesulfonate [51], and sodium dodecyl sulfate (SDS) [52]. All these reports reveal that these additives prepare new electrode materials with desired properties different from the normal polymers. Recently, we have prepared P2MA film in monomer solution containing SDS onto a glassy carbon electrode and used as a support for electrochemical deposition of Pt nano/microparticles for methanol and formaldehyde oxidation [53]. Surprisingly, the literature survey indicates that there is no report as yet on the application of P2MA-SDS modified popular CPE for deposition of bimetallic PteCu NPs and its application for electrocatalytic oxidation of formic acid in H2SO4 solution.

The objectives of the present study are: (i) to find a potentially good, low cost and easily available substrate for electrochemical polymerization of the 2 MA; (ii) to synthesize the uniform, porous and strongly adherent P2MA films onto carbon paste electrode (CPE) from aqueous solution containing SDS; (iii) to deposit bimetallic PteCu NPs on/in the P2MA-SDS film by electroless deposition/transmetalation method; and (iv) to explore capability of the P2MA-SDS film including bimetallic PteCu NPs for electrocatalytic oxidation of formic acid in acid medium.

Experimental Materials and instrumentation 2 MA (>98%, Fluka) was used as a monomer in electrochemical polymerization process. H2SO4 (98%, Merck) was used for the preparation of supporting electrolyte. K3Fe(CN)6, K4Fe(CN)6 (>99%, Merck) and KCl (>99%, Merck) were used for the electrochemical impedance studies. SDS (>85%, Merck), K2PtCl6 (Merck), CuSO4.5H2O (99%, Merck) and formic acid (>98%, Merck) were used as received. High viscosity paraffin oil (density: 0.86 g cm3, Merck) was used as pasting liquid in preparation of the CPE. Graphite powder (particle diameter: 0.1 mm, Merck) was used as the working electrode substrate. The solvent for preparation of the reagents solutions was double distilled water. The surface morphologies of the samples were examined by using scanning electron microscopy (SEM, KYKY-EM3200, China). Energy dispersive X-ray spectroscopy (EDS, VEGATescan, Razi Metallurgical Research Center, Iran) analysis was used to identify the sample elemental composition. The electrochemical measurements were conducted at room temperature by using a standard three-electrode cell connected to a potentiostat/galvanostat Autolab (Nova software model PGSTAT 302N, Metrohm, Netherlands) coupled with a personal computer. Electrochemical impedance spectroscopy was carried out by potentiostat/galvanostat Palmsense (PSTrace software version 4.2.2, Netherlands). The threeelectrode system consists of the CPE (3.4 mm in diameter) as working electrode, AgjAgCljKCl (3 M) as reference electrode and Pt wire as auxiliary electrode.

Preparation of the PteCu/P2MA-SDS/CPE Fabrication of the CPE (graphite powder and paraffin oil in a ratio of 70:30 (%W/W)) was performed according to our previous works [54e56]. Then, cyclic voltammetry was performed between 0.0 and 1.0 V at 50 mV s1 in 0.5 M H2SO4 solution, until stable voltammograms were obtained. Later modifications at the CPE surface were performed as following: (a) Electrochemical polymerization of 5.0 mM 2 MA monomer in 0.5 M H2SO4 solution containing 5.0 mM SDS by using consecutive potential cycling (6 cycles between 0.0 and 1.0 V at y ¼ 50 mV s1). (b) Electrochemical deposition of Cu NPs onto the P2MASDS/CPE at 0.17 V for 60 s in 0.1 M H2SO4 solution containing 20 mM CuSO4.

Please cite this article in press as: Hosseini SR, et al., Synthesis of poly (2-Methoxyaniline)/sodium dodecyl sulfate film including bimetallic PteCu nanoparticles and its application for formic acid oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.12.021

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(c) Partial galvanic replacement of the Cu film for deposition of the Pt NPs by immersing the electrode surface in 10 mM K2PtCl6 þ 0.1 M H2SO4 stirred solution under open circuit condition for 10 min. Following Cu electrodeposition and immersion into K2PtCl6/H2SO4 solution, the as-prepared PteCu modified electrode was scanned repeatedly, (typically, 5 times between 0.25e1.2 V at 50 mV s1) in the cleaning 0.5 M H2SO4 solution, ensuring that any unreacted surface Cu was anodically dissolved during exposure to positive potentials (active dissolution). Table 1 summarizes the surface parameters such as geometric surface area, polymer mass, film thickness, Cu loading, Pt real surface area and roughness factor for the PteCu/P2MASDS/CPE. The geometric surface area of the electrode was used to calculate the current density, except the comparison of formic acid oxidation. All experiments were conducted at ambient temperature.

Results and discussion Electrochemical properties The electrochemical polymerization is a simple, environmentally favorable, relatively low production cost and most convenient method for the preparation of a thin film with excellent control of thickness and morphology. Also, the technique combines the formation of polymer and deposition of coating in one process and this process can be easily automated. By adding 5.0 mM of SDS to 2 MA solution, onset oxidation potential of the monomer was shifted to more negative value and rate of the polymerization increased considerably. Furthermore, under successive potential cycling, P2MA peaks current increased and their growth continued (Fig. 1A and B). The SDS may be affect the preparation of P2MA in three ways: (i) the presence of SDS micelles

Table 1 e Surface parameters of the PteCu/P2MA-SDS/ CPE. Surface parameters Geometric area, Ag/cm2 (pr2) Polymer mass/mg (h Qdep M/F n) Film thickness, dn/mm (h Qdep M/r Ag F n) Cu loading, WCu/mg cm2 (Qnet M/F Z) Pt loading, WPt/mg cm2 Pt real surface area, Ar/cm2 (Q(e) H /0.21 mC) Roughness factor, RF (Ar/Ag)

Values 0.09 2.39 0.36 0.031(a), 0.028(b) 0.047(c), 0.042(d) 0.66 7.33

(a) As determined from chronoamperometry experiments. (b) The calculated loading obtained from stripping of Cu in 0.5 M H2SO4 solution. (c) and (d) as maximum possible loaded Pt with assuming that all of the Cu atoms are galvanically replaced by Pt from (a) and (b), respectively. (e) QH ¼ 0.5 Q, Q is charge in the hydrogen adsorption/desorption area obtained after double layer correction. 0.21 mC is conversion factor for the adsorption of a monolayer of hydrogen on Pt surface.

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controls the electrode-electrolyte interface, thus altering the locus and course of electropolymerization, (ii) DS may act as a counter ion for the CP polycations, and (iii) the hydrophobic part of the surfactant molecules may adsorb on the produced conducting polymer, SDS thus becoming a part of the resulting material [53,57]. Fig. 1C presents the redox behavior (typical oxidation and reduction peaks) of the prepared polymer films (i.e., P2MASDS/CPE and P2MA/CPE) in 0.5 M H2SO4 solution. When polymer is synthesized by anodic oxidation of monomer, the anions of supporting electrolyte function as doping anions and in turn influences overall properties of the conducting polymers. We also consider the interaction of the cationic radical (2 MAþ) with monomeric DS leads to formation of a 2 MAþ: DS pseudo complex [58]. The P2MA-SDS/CPE (a) films showed noticeably higher peak current density than that the P2MA/ CPE (b). The difference in redox currents reflects the effective surface areas, which are accessible to the electrolyte solution. The increase of surface area improves the doping-undoping rate which is benefit to ion diffusion and migration. Apparently, the porous P2MA-SDS film has higher effective surface area, which are desirable for supporting the electrocatalyst. The peak to peak potential separation, (DEp ¼ EpaeEpc, ¼ 30 mV) for the main redox peak is close to 59/ n mV at 25  C, which identifies that the number of involved electrons is 2 (n z 1.97). The ratio of anodic to cathodic peak current density (jpa/jpc) is almost equal to unity. Fig. 2 presents the cyclic voltammograms (CVs) of the P2MA-SDS/CPE at various potential sweep rates (y ¼ 5e1000 mV s1) in 0.5 M H2SO4 solution. The ja and jc are linearly proportional to the y. This result is attributed to the electrochemical activity of an immobilized redox couple at the electrode surface. From the slopes of these lines and by using equation (1), where Ip, A and G are peak current, geometric surface area and surface coverage, respectively, the G was derived at about 8.8  107 mol cm2, which further confirmed the immobilized state of the P2MA-SDS film. In a similar way, the G for the P2MA/CPE was derived at about 4.0  108 mol cm2.

Ip ¼ n2F2yAG/4RT

(1)

Fig. 3A shows the electrochemical responses of the P2MASDS (a), CPE (b) and P2MA/CPE (c) in 0.1 M KCl solution containing 5.0 mM K4[Fe(CN)6]. The DEp at the P2MA-SDS film was obtained about 90 mV, while on the P2MA-CPE, it was about 330 mV. The DEp is greater than that to 59/n mV expected for a reversible system. At the same time, the redox peak current density at the P2MA-SDS surface was 1.5-fold greater than the P2MA-CPE, which suggested that SDS could greatly promotes the electron transfer rate. Also, background current of the P2MA-SDS is much larger than the CPE. The enhancement at the background current is ascribed to double layer charging with the increased surface roughness. Electrochemical impedance spectroscopy is one of the best techniques for analyzing the properties of conducting polymer electrodes. Fig. 3B presents the Nyquist plot for the PteCu/P2MA-SDS/CPE (a), P2MA-SDS/CPE (b) and P2MA/CPE (c) in 1.0 mM K4[Fe(CN)6]/K3[Fe(CN)6] þ 0.1 M KCl solution. As

Please cite this article in press as: Hosseini SR, et al., Synthesis of poly (2-Methoxyaniline)/sodium dodecyl sulfate film including bimetallic PteCu nanoparticles and its application for formic acid oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.12.021

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Fig. 1 e Electropolymerization of 2 MA monomer in 0.5 M H2SO4 solution in the presence (A) and in the absence (B) of SDS at y ¼ 50 mV s¡1. The arrows indicate the trends of current density during the cyclic voltammetric experiments. (C) Electrochemical responses of the P2MA-SDS/CPE (a) and P2MA/CPE (b) in 0.5 M H2SO4 solution at y ¼ 50 mV s¡1.

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 charge transfer resistance (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 PteCu NPs have been incorporated into/onto the polymeric film, Rct is not significantly changed by existence of these 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 due to increase the mass diffusion. Advantages of the galvanic replacement technique include the fact that it is a fast and room temperature process, employs low concentration solutions of the precious metal and can lead to the formation of thin precious metal deposits that may decrease its loading. Fig. 4A presents the CVs of the

P2MA-SDS/CPE (a), Cu/P2MA-SDS/CPE (b) and PteCu/P2MASDS/CPE (c) in 0.5 M H2SO4 solution. As can be seen in trace (c), three apparent peaks at about 0.15, 0.38 and 0.65 V are assignable to the electrochemical response of the P2MA-SDS film and the broad peak in positive potential range corresponds to the formation of Pt oxides. It should be noted that the reduction peak of the P2MA-SDS is overlaid on reduction peak of Pt oxide. Fig. 4B shows the CVs of the different PteCu modified electrodes in 0.5 M H2SO4 solution at potential range of 0.25e1.2 V. Larger peaks in the hydrogen adsorption/ desorption regions were observed on the PteCu/P2MA-SDS/ CPE, which reflect the higher surface areas. The Ar for the PteCu/CPE, PteCu/P2MA/CPE and PteCu/P2MA-SDS/CPE catalysts are about 0.47, 0.50 and 0.66 cm2, respectively. Consequently, such PteCu NPs at the P2MA-SDS/CPE enhance the active sites towards the electrocatalytic oxidation of formic acid in H2SO4 solution.

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Fig. 2 e (A) CVs of the P2MA-SDS/CPE in 0.5 M H2SO4 solution at: (a) 0.005, (b) 0.01, (c) 0.02, (d) 0.03, (e) 0.05, (f) 0.08, (g) 0.10, (h) 0.20, (i) 0.40, (j) 0.60, (k) 0.80 and (l) 1.0 V s¡1. (B) The dependency of anodic (a) and cathodic (b) peak current densities on the potential scan rate.

Surface morphology and elemental analysis In order to surface characterization, the micrographs of the CPE (a), P2MA/CPE (b), P2MA-SDS/CPE (c), Cu/P2MA-SDS/CPE (d, e) and PteCu/P2MA-SDS/CPE (f, g) have been investigated by SEM (Fig. 5). As can be seen on the CPE (trace a), layers of irregular flakes of graphite powder and some holes/cavities were presented because carbon paste is a porous material and its surface structure is very complex due to many feasible factors. The P2MA at the CPE surface (trace b) is completely isolated from each other and its morphology is compact and presents less surface defects than those deposited in the presence of SDS. Electropolymerization in the presence of SDS

leads to formation a homogenous thick P2MA film and relatively uniform coverage (trace c). In this case, more polymers successfully are distributed at the electrode surface, indicating a larger and rougher area compared to the normal P2MA film and present a secondary germination. These results indicate that the surface morphology of the P2MA film is significantly influenced by the presence of SDS. This structure enhances the electrolyte constituent access to interior of the polymer film. This difference in thickness is in good agreement with the values of the anodic/cathodic peaks current densities in the CVs of electropolymerization (Fig. 1). Also, the structure provides a larger available surface area and hence serves as a beneficial support for dispersion and/or

Fig. 3 e (A) Electrochemical responses of the P2MA-SDS/CPE (a), CPE (b) and P2MA/CPE (c) in 0.1 M KCl solution to 5.0 mM K4[Fe(CN)6] at y ¼ 50 mV s¡1. (B) Electrochemical impedance spectra for the PteCu/P2MA-SDS/CPE (a), P2MA-SDS/CPE (b) and þ 0.1 M KCl solution. P2MA/CPE (c) in 1.0 mM Fe(CN)3¡/4¡ 6 Please cite this article in press as: Hosseini SR, et al., Synthesis of poly (2-Methoxyaniline)/sodium dodecyl sulfate film including bimetallic PteCu nanoparticles and its application for formic acid oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.12.021

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Fig. 4 e (A) CVs of the P2MA-SDS/CPE (a), Cu/P2MA-SDS/CPE (b) and PteCu/P2MA-SDS/CPE (c) in 0.5 M H2SO4 solution at y ¼ 50 mV s¡1. (B) Electrochemical response of the PteCu/CPE (a), PteCu/P2MA/CPE (b) and PteCu/P2MA-SDS/CPE (c) in 0.5 M H2SO4 solution in potential range ¡0.25e1.2 V at y ¼ 50 mV s¡1.

distribution of Cu NPs. The mechanism of formation and properties of the catalyst are largely determined by the dispersion degree of the initial less noble metal deposit. Traces (d and e) show morphologies of the composite consisting of Cu NPs with size distribution in the range of 40e90 nm. The polymeric film prevents the Cu agglomerating and provides high degree of distribution during the electrodeposition. A bimetallic PteCu catalyst can be generated by partial galvanic replacement (an irreversible and spontaneous redox process) of the Cu NPs by Pt. Traces (f and g) show the structure of PteCu/P2MA-SDS/CPE, which is decorated with PteCu NPs with average sizes at about 65 nm. The Pt/Cu NPs have a narrower size distribution and smaller size. This indicates that large quantities of Cu (especially those organized at large particles) are not contained in the final catalyst. Fig. 6 shows the EDS experiment for elemental analysis of the PteCu/P2MA-SDS/CPE composite electrode. The EDS gives evidences for the presence of Pt and Cu in the nanocomposite and content of the Pt and Cu in the prepared catalyst were 2.76 and 1.58 Wt%, respectively. Based on elemental composition of the PteCu NPs, we can estimate that only small amount of Cu remains in the final product, whereas a larger numbers of them have been exchanged with Pt NPs (partial galvanic replacement). The replacement reaction and thus deposition of the Pt NPs is limited by amount of the Cu deposited on the surface. The Cu NPs at the film surface are replaced with Pt during the galvanic replacement which leads to deposition of PteCu NPs. The obtained results also confirm that the galvanic replacement method is a surface limited process.

Electrocatalytic oxidation of formic acid Fig. 7 presents the electrocatalytic activity of the prepared electrodes for formic acid oxidation in 0.5 M H2SO4 solution. Formic acid oxidation at the Pt surface showed the

characteristic voltammetry peaks that appear in the forward and reverse scans [23,59,60]. The mechanism of formic acid oxidation has been well studied, involving the so-called dual pathways that are dehydrogenation (equation (2)) and dehydration (equation (3)).

HCOOH / CO2 þ 2Hþ þ 2e

(2)

HCOOH / COads þ H2O / CO2 þ 2Hþ þ 2e

(3)

The preferred reaction for the complete oxidation of formic acid is the dehydrogenation reaction, in which CO2 is directly formed through the formation of adsorbed active intermediates such as COOHads (i.e. direct pathway). In contrast, formic acid oxidation via a dehydration reaction produces COads which acts as a poisonous intermediate and requires a higher electrode potential to be further oxidized into CO2 (i.e. indirect pathway). In this work, the formic acid oxidation at the PteCu modified electrodes occurs mainly through the dehydration pathway (a broad small peak (I) at about 0.38 V with small current density, 0.43 mA cm2) and (an obvious peak (II) at about 0.72 V with large current density, 1.1 mA cm2). It was also revealed that oxidation reaction on the bimetallic PteCu NPs takes place through both of dehydrogenation and dehydration paths. Moreover, in the case of PteCu/P2MA-SDS/CPE, the catalytic ability is improved as the current density is enhanced. It is suggested that the PteCu/P2MA-SDS/CPE leads to the less poisoning of Pt by formed poisoning intermediates during the formic acid activation. The possible synergistic effect between the PteCu NPs and P2MA-SDS, introduction of Cu into Pt probably due to changed electronic ligand-effect and surface crystalline orientation of Pt particles are main factors for the enhanced catalytic activity.

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Fig. 5 e SEM images of the CPE (a), P2MA/CPE (b), P2MA-SDS/CPE (c), Cu/P2MA-SDS/CPE (d, e) and PteCu/P2MA-SDS/CPE (f, g). (e) is image with higher magnification of (d).

The effect of formic acid concentration Fig. 8 presents the effect of various formic acid concentrations on the electrooxidation current density at the PteCu/P2MASDS/CPE. As shown in this figure, when the excessive concentrations are added, the oxidation peak currents densities increase and drop afterward at concentrations higher than 1.7 M. This effect might be due to the saturation of the Pt active sites by formic acid molecules and also contamination

of the catalyst surface which is mainly arisen from the COads intermediate during the formic acid oxidation. Also, jpII is proportional to y1/2 (equation (4)), indicating that the formic acid oxidation is a diffusion-controlled process (Figure not shown).

jpII/mA cm2 ¼ 3.0562 y1/2/(V s1)1/2 þ 0.8408, r2 ¼ 0.9942

(4)

Please cite this article in press as: Hosseini SR, et al., Synthesis of poly (2-Methoxyaniline)/sodium dodecyl sulfate film including bimetallic PteCu nanoparticles and its application for formic acid oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.12.021

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Fig. 6 e Energy dispersive spectrum of the PteCu/P2MA-SDS/CPE.

Parameters affecting the electrode modification The electrocatalytic properties of the nanocomposite greatly can be controlled by varying the parameters such as SDS concentration (CSDS), film thickness, Cu electrodeposition time (td), 2 MA concentration (C2MA), and replacement time (tr).

Fig. 7 e CVs of the PteCu/CPE (a), PteCu/P2MA/CPE (b) and PteCu/P2MA-SDS/CPE (c) in the presence of 1.7 M formic acid in 0.5 M H2SO4 solution at y ¼ 50 mV s¡1.

Thus, in order to improve the properties of nanocomposite suitable for the electrocatalytic application, it is necessary to critically control and optimize the various synthesis parameters. In order to evaluate the effects of various parameters on formic acid electrooxidation, the jpII was monitored as an

Fig. 8 e Current density-potential curves of the Pt/P2MASDS/CPE in 0.5 M H2SO4 solution with different formic acid concentrations at y ¼ 50 mV s¡1: (a) 0.50, (b) 0.63, (c) 0.75, (d) 1.0, (e) 1.23, (f) 1.7, (g) 1.92 and (h) 2.15 M. Inset of plot: jpII as a function of formic acid concentration.

Please cite this article in press as: Hosseini SR, et al., Synthesis of poly (2-Methoxyaniline)/sodium dodecyl sulfate film including bimetallic PteCu nanoparticles and its application for formic acid oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.12.021

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Comparison of the electrocatalyst stability

Table 2 e Effects of various parameters on jpII at the PteCu/P2MA-SDS/CPE for 0.63 M formic acid þ0.5 M H2SO4 solution at y ¼ 50 mV s¡1. CSDS/mM jpII/mA cm2 Cycle number jpII/mA cm2 C2MA/mM jpII/mA cm2 td/s jpII/mA cm2 tr/min jpII/mA cm2

9

1

2

5

10

15

2.46 2 3.05 1 2.42 10 1.6 1 3.08

4.11 4 5.13 2 3.6 30 4.11 5 4.72

5.70 6 5.70 5 5.70 60 5.70 10 5.70

4.89 10 1.8 10 2.14 70 4.8 15 5.2

3.7 15 2.5 15 1.97 80 4.38 20 4.31

index for finding an optimum conditions and obtained results were summarized in Table 2. The data indicate that the jpII increases extensively for CSDS up to 5.0 mM, cycle number up to 6, C2MA up to 5.0 mM, tr up to 10 min and td up to 60 s and drop afterward.

The stability of the PteCu/P2MA-SDS/CPE towards formic acid oxidation were investigated by using various electrochemical methods. At first, stability of the electrocatalyst was checked by using successive potential cycling in 0.5 M H2SO4 þ 1.7 M formic acid (Fig. 9A). The peak current density decreases gradually by continuous potential cycling and its value at the 50th scan is about 76% than the 1st scan. For further evaluation of the electrocatalytic activity and stability of the catalysts, chronoamperograms were conducted for the PteCu/ P2MA-SDS/CPE (a), PteCu/P2MA/CPE (b) and PteCu/CPE (c) at peak potential value in continuous operation (Fig. 9B). As can be seen from this figure, a decrease in current density with time is found on the each electrode. This stability is attributed to less poisoning effect of the COads at the electrode surface. Also, stability of the catalyst was verified by measuring of its response to formic acid oxidation after ten days of storage in the laboratory atmosphere conditions (Fig. 9C). The peak current density at the 10th day is about 70% than the 1st day.

Fig. 9 e (A) CVs of the PteCu/P2MA-SDS/CPE in the presence of 1.7 M formic acid in 0.5 M H2SO4 solution: 1st cycle (a); 50th cycle (b) at y ¼ 50 mV s¡1. (B) Chronoamperograms of the PteCu/P2MA-SDS/CPE (a), PteCu/P2MA/CPE (b) and PteCu/CPE (c) in 1.7 M formic acid þ 0.5 M H2SO4 solution at peak potential values. (C) CVs of the PteCu/P2MA-SDS/CPE in 0.5 M H2SO4 solution in the presence of 1.7 M formic acid at y ¼ 50 mV s¡1: 1st day (a) and 10th day (c). Please cite this article in press as: Hosseini SR, et al., Synthesis of poly (2-Methoxyaniline)/sodium dodecyl sulfate film including bimetallic PteCu nanoparticles and its application for formic acid oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.12.021

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Conclusions We have successfully synthesized uniform and strongly adherent P2MA-SDS coating on the CPE as a suitable substrate by electrochemical polymerization of 2 MA from aqueous SDS medium. Bimetallic PteCu NPs with good stability has been achieved on the P2MA-SDS film by transmetalation to prepare a novel electrode. It has been proved to be a facile, effective and eco-friendly method. Good electrochemical performance for formic acid can be achieved by the presence of the film are following as small crystallite size of PteCu NPs and much higher real surface area. The proposed electrode exhibited high electrocatalytic activity towards formic acid oxidation, showing a satisfactory stability and reproducibility when stored in ambient conditions or continues cycling. These observations confirmed positive effect of the additive on the formic acid oxidation. The method can be used as a simple, easy and low cost preparation for effective electrodes based on the cheap carbon paste electrode.

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Please cite this article in press as: Hosseini SR, et al., Synthesis of poly (2-Methoxyaniline)/sodium dodecyl sulfate film including bimetallic PteCu nanoparticles and its application for formic acid oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.12.021