Investigation of methanol oxidation on a highly active and stable Pt–Sn electrocatalyst supported on carbon–polyaniline composite for application in a passive direct methanol fuel cell

Accepted Manuscript Title: Investigation of Methanol Oxidation on a Highly Active and Stable Pt-Sn Electrocatalyst Supported on Carbon–Polyaniline Composite for Application in a Passive Direct Methanol Fuel Cell Author: Mitra Amani Mohammad Kazemeini Mahboobeh Hamedanian Hassan Pahlavanzadeh Hussein Gharibi PII: DOI: Reference:

S0025-5408(15)00138-5 http://dx.doi.org/doi:10.1016/j.materresbull.2015.02.053 MRB 8062

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MRB

Received date: Revised date: Accepted date:

16-2-2014 11-2-2015 18-2-2015

Please cite this article as: Mitra Amani, Mohammad Kazemeini, Mahboobeh Hamedanian, Hassan Pahlavanzadeh, Hussein Gharibi, Investigation of Methanol Oxidation on a Highly Active and Stable Pt-Sn Electrocatalyst Supported on CarbonndashPolyaniline Composite for Application in a Passive Direct Methanol Fuel Cell, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.02.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Investigation of Methanol Oxidation on a Highly Active and Stable Pt-Sn Electrocatalyst Supported on Carbon–Polyaniline Composite for Application in a Passive Direct Methanol Fuel Cell

Mitra Amania, Mohammad Kazemeinib, Mahboobeh Hamedanianc, Hassan Pahlavanzadeha, Hussein Gharibi*c,d a

Department of Chemical Engineering, Tarbiat Modares University, P.O. Box 14115-175,

Tehran, Iran b

Department of Chemical and Petroleum Engineering, Sharif University of Technology,

Tehran, Iran c

Department of Chemistry, Faculty of Science, Tarbiat Modares University, P.O. Box 14115-

175, Tehran, Iran d

Department of Material Science & Engineering, 122 S Campus Drive, University of Utah,

Salt Lake City, UT 84112, United States *

Corresponding author: E-mail addresses: [email protected]; [email protected]; [email protected] Phone Number: +98 09126147981

Highlights

► PtSn/C-PANI performed superior in the MOR compared with a commercial PtRu/C. ► Catalytic activity of PtRu/C was highly reduced during the accelerated durability test. ► Anode of the PtSn/C-PANI in a passive DMFC lowered methanol crossover by 30%.

Abstract Polyaniline fiber (PANI) was synthesized and utilized to fabricate a Vulcan-Polyaniline (CPANI) composite. Pt/C-PANI and PtSn/C-PANI electro-catalysts with different Pt:Sn atomic ratios were prepared by the impregnation method. These electro-catalysts, along with commercial PtRu/C (Electrochem), were characterized with respect to their structural and electrochemical properties in methanol oxidation reaction (MOR). PtSn(70:30)/C-PANI showed excellent performance in MOR, the obtained maximum current density being about 40% and 50% higher than that for PtRu/C and Pt/C-PANI; respectively. It was also found that the CO tolerance and stability of PtSn(70:30)/C-PANI was considerably higher than that of PtRu/C. Finally, the performance of these two materials was compared in a passive direct methanol fuel cell (DMFC). The DMFC test results demonstrated that the membrane electrode assembly (MEA) prepared using PtSn(70:30)/C-PANI anode catalyst performed more satisfactorily in terms of maximum power density and lower methanol crossover. KEYWORDS: A. Composites; A. Nanostructures; D. Electrochemical properties; D. Catalytic properties.

1. Introduction The direct methanol fuel cell (DMFC) is one of the alternative candidates for fuel cell vehicles, stationary applications and portable power sources such as mobile phones and laptops due to its better economics and high theoretical energy density (6100 Wh kg-1 at 25ºC) of methanol [1]. Nonetheless, various problems including the contamination of the cathode catalyst by methanol crossover, the high cost and low durability of the electro-catalysts and the poisoning of the Pt anode electro-catalyst prohibited DMFC from a wider practical usage [2-

3]. The methanol oxidation reaction (MOR) by itself is rather slow and requires active catalytic sites for adsorption and oxidation of methanol, as well as oxidation and repulsion of the adsorbed intermediates. On the other hand, platinum (Pt) is not the best catalyst for the electrochemical oxidation of small organic molecules such as methanol. In other words, catalytic activity of Pt is still too low, while its loading is too high to consider DMFC as a practical power source. The poor performance of pure Pt for MOR results from the strong adsorption of the CO-containing species on the active Pt sites. This implies that the formation of OHads on Pt takes place at too high potentials to be of practical interest. In addition, Pt is a rare metal and is extremely expensive, therefore it is necessary to reduce the cost-toefficiency ratio [4-5]. In order to improve the CO tolerance and decrease the electro-catalyst price, Pt-based bimetallic or multi-metallic electro-catalysts have been widely studied, such as PtRu [2, 6-7], PtSn [8-9], PtPd [10-11], PtCo[12]-[13], PtRuSn [14], PtRuCo [15] PtRuNi [16] and PtSnCo [17], amongst which PtRu and PtSn are the most active ones. The superior performance of these binary electro-catalysts for MOR in comparison with Pt alone is explained mainly in terms of the so called bi-functional mechanism, according to which the formation of oxygen-containing species on the second metal takes place at lower potentials in comparison to pure Pt [18]. However, the electronic (i.e. ligand) effect cannot be ruled out since the adsorption energies of CO on these catalytic materials is different in comparison with pure Pt as suggested by thermal programmed desorption experiments [19] as well as theoretical calculations [20]. These works clearly indicated that, the binding energy of CO on Pt sites available on PtRu and PtSn bimetallic surfaces was lower than that on pure Pt. The PtRu alloy electro-catalyst with a 50% Ru atomic composition is indeed one of the most popular DMFC electro-catalysts utilized to date. However, the CO tolerance of this material at the higher CO concentrations expected upon the system’s start-up or during loading

changes is not acceptable. On the other hand, problems related with Ru leaching from the PtRu anode electro-catalyst, the high price of PtRu and the limited availability of Ru can limit its usage in DMFCs [3, 21]. From an economic viewpoint, the actual cost of Sn is much lower than that of Pt or Ru. Therefore, Pt–Sn/carbon nano-composites have been studied for decades as anode catalysts for electro-oxidation of methanol and other small fuel molecules [22]. The catalytic activity of a PtSn electro-catalyst for MOR varies considerably depending upon the Pt:Sn molar ratio, the support structure and its synthesis method [23-24]. Different support materials such as carbon Vulcan [25-26], carbon nanotubes [27-28], carbon fibers[29] and different metal oxides composites [30-31] have been proposed for PtSn catalysts. A higher catalyst dispersion on a suitable support results in decrease of the amount of Pt used in the DMFC active layer without performance loss. A better dispersion of catalyst also results in decrease of the amount of adsorbed linear CO species [32]. One way of getting a better distribution of the catalytic particles is to disperse the material within a convenient electron conducting matrix providing a three-dimensional array of the catalyst, thus achieving efficient transport of charge from the underlying electrode support to the reaction sites. Polyaniline (PANI) with porous structure and high surface area is a particularly attractive material as catalyst support because this polymer adheres strongly to the electrode surface and also has a high conductivity and durability under operating conditions of fuel cells employing aqueous acidic electrolytes [33]. Pt dispersion inside such a support leads to decrease in the amount of noble metal used, allowing improvement of the catalytic activity for methanol oxidation via better utilization of the platinum crystallites as well as via hindering their poisoning. PANI films modified with nano-sized Pt particles have already been used to electro-oxidize formaldehyde [34], methanol [35] and, even more importantly, for carbon monoxide [36]. Of course, there has been some previous research done on the

simultaneous use of PANI and Pt alloys, such as PtSn, for the MOR [37-39]. Nonetheless, to the best of our knowledge the followings issues are noteworthy: (i) The atomic ratio of the Pt:Sn in the presence of PANI has not been optimized, (ii) The activity and stability of these composites has not been compared with those of the commercial Pt-Ru catalyst; (iii) The application of PtSn and carbon Vulcan-Polyaniline composite (C-PANI) in the anode catalyst layer for a real DMFC has not been reported to date. All these open issues were investigated in the present work. In this work PANI was synthesized chemically by an interfacial method and doped with para toluene sulfonic acid (PTSA). PtSn nano particles with Pt:Sn atomic ratios of (85:15), (70:30) and (65:35) were deposited on a composite of carbon Vulcan (XC-72) and 20% PANI via impregnation. The electrochemical activity of the different synthesized PtSn/C-PANI catalysts was compared with that of synthesized Pt/C-PANI and commercial PtRu/C catalysts and the optimum Pt:Sn ratio was determined. In order to investigate the synergism between Sn and PANI and the positive effect of PANI on catalytic activity of alloy catalysts, the optimized PtSn/C electrocatalyst was compared with other electro-catalysts as well. Finally, the performance and methanol crossover of the optimum PtSn/C-PANI and commercial PtRu/C as anode catalysts of a passive direct methanol fuel cell were investigated and the maximum power density obtained with the optimized catalyst was compared with power density values reported in the open literatures [40-48].

2. Experimental 2.1. Materials Analytical grade aniline, chloroform, ammonium peroxydisulfate (APS), H2SO4, para toluene sulfonic acid (PTSA), H2PtCl6.6H2O, SnCl2.2H2O, isopropyl alcohol (IPA), CH3OH and

ethylene glycol were purchased as analytically pure reagents from Merck. Nafion solution 5% (Aldrich) and carbon Vulcan XC-72 (Cabot) were used as received. Aniline was purified by repeated vacuum distillation and stored under nitrogen gas at 0 ºC prior to use. 2.2. Polyaniline and Electro -catalyst synthesis PANI fibers were synthesized under ambient condition via interfacial polymerization. The oxidant solution, consisting of APS in H2SO4 solution (1M), was carefully added to the organic phase, consisting of aniline in chloroform. After 24hr, the aqueous layer was filled with a dark green colored emeraldine salt of PANI that was washed repeatedly with distilled water and methanol. The sulfate-ion-doped PANI was converted into emeraldine base form by treatment with ammonium hydroxide and then this material was redoped with PTSA for the electro-catalyst synthesis. The electrical conductivity of this material was in order of 10-1 S cm-1, higher than that of synthesized by bulk polymerization or electrochemical methods [49]. For the PtSn/C-PANI electro-catalysts synthesis, carbon Vulcan XC-72 (C) and 20wt% PANI were mixed with ethylene glycol ultrasonically for 30 min. The C-PANI composite was impregnated with Pt and Sn nano particles at different atomic ratios to a total metal loading of 10 wt%, by addition of H2PtCl6.6H2O aqueous solution and SnCl2.2H2O powder to the above suspension followed by sonication for 30 min and subsequent magnet stirring of the mixture for 2 h at room temperature. The suspension was then mixed ultrasonically with an excess of ethylene glycol and afterwards stirred at 130°C under Ar atmosphere for 16 h. Finally, the resulting mixture was filtered, washed with distilled water several times and then dried in an oven at 80°C to obtain the PtSn/C-PANI electro-catalyst. For comparison purposes, Pt/CPANI and PtSn(70:30)/C electrocatalysts were synthesized following the same procedure.

2.3. Electrode preparation The glassy carbon (GC) working electrode with the area of 0.0314 cm2 was polished with aluminium oxide paste and then rinsed with distilled water. To prepare the electro-catalyst ink, a suspension containing the desired amount of a given electro-catalyst, Nafion solution and a sufficient volume of IPA: H2O (2:1) solution was sonicated for 20 min with a sonicator (Misonix model S-3000) to produce a homogeneous ink-like solution. The homogenous ink was spread onto the glassy carbon substrate using a micro-syringe and the resulting composite structure was dried in an oven at 80°C for 2 h. The electro-catalyst loading on GC was 0.1 mgmetal cm-2 in all tests.

2.4. Physical characterization The morphology of the synthesized PANI was examined via the scanning electron microscopy (SEM) performed using a Philips model XL 30 system. The atomic ratios of the Pt–Sn/C-PANI electro-catalysts were determined using the energy dispersive X-ray analysis (EDX) technique, coupled with scanning electron microscopy. The amount of Pt and PtSn electro-catalysts deposited on the C-PANI composite was determined using the inductively coupled plasma optical emission spectroscopy technique (ICPOES, Varian Vista-PRO, Australia). For this purpose, 5mg of each electro-catalyst were dissolved in a mixture of HCL and HNO3 (3:1) and refluxed at 150ºC for 8hr. The cooled mixture and standard samples of Pt and Sn were then used. X-ray diffraction (XRD) measurements were performed using an XPERT MPD Phillips diffractometer with a Co X-ray source operating at 40kV and 30mA. The XRD patterns were obtained with a scanning rate of 1ºmin-1, with a step size in the 2θ scan of 0.02º over the range of 10-100º. The morphology and metal distribution of the optimized electro-catalyst was also investigated by transmission electron microscopy (TEM) using a Phillips model EM

280 system.

2.5. Electrochemical measurements All electrochemical experiments were performed using a conventional double wall threeelectrode glass cell and an EG&G Model 273A Potentiostat / Galvanostat. A flat platinum electrode (1 cm2 area) was used as counter electrode. A Ag/AgCl (in saturated KCl) reference electrode was placed close to the working electrode surface by using a Luggin capillary. All potentials reported in this paper are referenced with respect to this Ag/AgCl reference electrode. Prior to each electrochemical test, the surface of the working electrode was cleaned by potential cycling in N2 purged 0.5M H2SO4 electrolyte solution. The electrolyte solution was static in all the tests. CO stripping voltammetry experiments were performed passing over the working electrode high purity CO gas for 20 min. The electrode potential was kept at -0.1 V to form a COads layer on the surface of the electro-catalyst. Subsequently, the gas feed was switched to N2 to remove the dissolved CO gas from the electrolyte solution. After N2 purging for 30min, the COads was stripped by scanning the potential between -0.2 to 1.1V at a scan rate of 25mVs-1.

2.6. Passive direct methanol fuel cell studies An in-house prepared oxygen-breathing DMFC with stainless steel (SS316) current collectors was used to assess the electrochemical performance of the optimized PtSn(70:30)/C-PANI electro-catalyst in comparison with a commercial PtRu/C (provided by Electrochem Inc.). A pre-treated Nafion 115 membrane with a thickness of 125µm was employed in this work. The pre-treatment procedures included boiling of the membrane in 5vol. % H2O2, washing in distilled water, boiling in 0.5M H2SO4 and washing in distilled water, each step for 1.5h at

80ºC. The pre-treated membrane was kept in distilled water prior to fabrication of the membrane electrode assembly (MEA). Carbon paper was used as backing support of the anode and cathode electrodes. The catalyst loading on the cathode electrode of MEA-1 and MEA-2 was 3mg Pt cm-2, using 20wt. % Pt on carbon Vulcan (XC-72). The catalyst loading on the anode electrode was 4mgmetal cm-2 , using 20wt. % PtRu on carbon Vulcan (XC-72) for MEA-1 and 20wt. % PtSn (70:30) on C-PANI composite for MEA-2. The MEAs were prepared by mechanically pressing anode, cathode and membrane, while silicone–rubber gaskets were employed to seal the system. The active area of MEAs and the volume of the anode compartment were 4.75cm2 and 12.5mL, respectively.

3. Results and discussion 3.1. Determination the optimum Pt:Sn ratio in PtSn/C-PANI electro-catalyst 3.1.1. Structural characterization A typical SEM micrograph of the synthesized PANI is presented in Fig.1. As can be seen in this figure, the synthesized PANI has a fibular structure. Fibular PANI possesses much better properties than those of its granular form, higher electrical conductivity and relatively higher specific surface area, which is beneficial for achieving more homogenous Pt distribution and CO tolerance [50]. The composition of the synthesized PtSn and commercial PtRu alloy electro-catalysts was evaluated by ICPOES and EDX analysis. The obtained results are presented in Table 1. They show that in all cases the determined percentage of each metal was close to the nominal value, which indicates that Pt and Sn can be co-deposited. As discussed below, Sn may be alloyed with Pt in the course of the followed synthesis method.

Fig.2 shows the XRD patterns of the synthesized Pt/C-PANI and PtSn/C-PANI electrocatalysts with different Pt:Sn atomic ratios as well as of the commercial PtRu/C electrocatalyst. All XRD patterns clearly exhibit the three main characteristic peaks of the face-centered cubic (fcc) crystalline Pt, namely the (1 1 1), (2 0 0) and (2 2 0) peaks. Moreover, the diffraction peak at 2θ ~ 25º is attributed to the (0 0 2) plane of the hexagonal structure of carbon Vulcan XC-72 support material. No additional peaks appear in the XRD patterns of the synthesized PtSn and Pt electro-catalysts supported on the C-PANI composite, due to the amorphous PANI structure [51]. Also, no peaks for pure Sn, Ru or their oxides were observed, but their presence could have not been disregarded since they were presumably present in the samples used for XRD in amounts less than the detection limit or in amorphous form.[7].It is obvious from Fig.2 (and Table 2) that the diffraction peaks of the binary electro-catalysts were shifted to lower 2θ values in the case of PtSn and to higher 2θ values in the case of PtRu with respect to the corresponding peaks for pure Pt. The shift of diffraction peaks reveals alloy formation between Pt and Sn or Ru, caused by the incorporation of the second metal in to the fcc structure of Pt. The width of the Pt(1 1 1) peak was used to calculate the average crystallite size via Scherrer’s equation [52]:

where d is average crystallite size (nm), λ the X-ray wavelength (0.17889 nm for Co Ka radiation), θ the angle corresponding to the peak maximum and B2θ the full width at half maximum, estimated by a Lorentzian function fitted to the peak. In addition, the lattice parameter (afcc) values were computed using the Pt (220) peaks according to Vegard’s law [52]:

The calculated average crystallite size and lattice parameter from the XRD patterns are summarized in Table 1. It is clear that the electro-catalyst crystallite size increased with increasing Sn content. Also, the lattice parameter of the PtSn electro-catalysts became larger than that of Pt due to a lattice expansion caused by incorporation of the bigger Sn atom (rSn = 1.61ºA) into the fcc structure of Pt (rPt = 1.39 ºA). On the contrary, the lattice parameter of the PtRu alloy electro-catalyst was smaller than that of Pt due to a lattice contraction caused by incorporation of the smaller Ru atom (rRu = 1.34ºA) into the fcc structure of Pt. The change of the lattice parameter and the absence of XRD peaks for Sn or Ru and their oxides can be explained by alloy formation and interaction between Pt and Sn or Ru. In the case of the PtSn alloy electro-catalyst, the value of the lattice parameter was in good agreement with that for the bulk PtSn alloy phase (0.400 nm), indicating a high degree of alloying from which it may be deduced that the amount of metal oxide species in this material was rather small [53]. On the contrary, for the PtRu alloy electro-catalyst, the value of the lattice parameter was remarkably larger than those for the bulk PtRu alloy phase (0.3863 nm) or carbon supported PtRu (0.3853 nm) [54]. This indicates a low degree of alloying and the presence of non-alloyed Ru and/or its oxides. Additional information on particle size and dispersion status of the PtSn(70:30) on C-PANI composite was obtained from TEM analysis. Fig.3 (a) depicts a high magnification TEM image whereas Fig.3 (b) shows the histogram of size distribution of the optimized PtSn(70:30)/C-PANI electro-catalyst. Heavy black dots correspond to spherical metal nano particles uniformly dispersed on the C-PANI composite. The particle size of this electrocatalyst was in the range of 3.5 nm to 6 nm with an average diameter of 5.2 nm, perfectly matching the XRD results. The completely homogeneous dispersion of metal nano particles on the C-PANI composite was attributed to the advantage of utilizing ethylene glycol in preparing the electro-catalyst [55]. Furthermore, the structure of the PANI facilitated the

better dispersion of PtSn particles on C-PANI composite. Moreover, it is anticipated that the SO3H group in the PTSA doped PANI may act as a fixator for the Pt particles, preventing their aggregation.

3.1.2. Electrochemical studies 3.1.2.1. Electrochemical active surface area Electrochemical activities of the synthesized PtSn/C-PANI with different Pt:Sn atomic ratios, Pt/C-PANI, PtSn(70:30)/C and the commercial PtRu/C electro-catalysts were compared using cyclic voltammetry (CV). Fig.4 depicts the related voltammograms recorded in N2 purged H2SO4 (0.5 M) solution at 50 mVs-1 and in the potential range of -0.2 to 1.1 V vs. the Ag/AgCl electrode. Well-established features of hydrogen adsorption/desorption, double layer charging, oxide formation and oxide reduction are evident in the voltammograms of the PtSn and PtRu samples which are similar to those of the Pt-based electro-catalysts supported on carbon. However, some differences are apparent, mainly the following: (i) The characteristic hydrogen adsorption/desorption peaks of alloy electro-catalysts became ill-defined as the Sn content increased. According to the obtained XRD results in this work, the Pt and Sn species in the synthesized Pt-Sn catalysts formed alloys with a high degree of interaction between them. This alloy formation altered the crystalline structure and lattice parameter of the Pt species as well as decreased the available amount of Pt catalytic sites on the surface of the electrode as the Sn concentration increased. Hence, the hydrogen adsorption/desorption features became ill-defined. Additional explanations related to the changes of lattice constant or electronic interactions between Pt and Sn have been provided in the open literature as other researchers encountered the same effect for these species [52, 56-57]. (ii) Increase of Sn content in the bimetallic PtSn electro-catalysts led to enhancement of the double layer charge. Hence, the total current density in this region for the PtSn and PtRu electrodes was larger

than that corresponding to pure Pt. This behaviour is characteristic of bimetallic electrocatalysts containing oxophilic transition metals such as Sn and Ru, ascribed to the activation of H2O on Ru and Sn in the PtRu and PtSn electro-catalysts [9]. Electrochemically active surface area (ECSA), being an important measure of the number of electrochemically active sites per gram of electro-catalyst, was calculated by dividing the mean value of hydrogen adsorption/desorption coulombic charge, QH (µC cm-2), by the charge required to oxidize an adsorbed monolayer of hydrogen (210 µC cm-2) on Pt sites [21]. The calculated ECSA values at the 50th cycle were found to decrease in the following order: PtSn(70:30)/C-PANI (73.4 m2gPt-1) > PtSn(85:15)/C-PANI (68.6 m2gPt-1) > Pt/C-PANI (62.1 m2gPt-1) > PtSn(65:35)/C-PANI (59.5 m2gPt-1) > PtSn(70:30)/C (58.2 m2gPt-1) > PtRu/C (55.9 m2gPt-1). The slightly extended bonding distances in the local structure of Pt, the electronic modification via charge transfer from Sn to Pt and the more homogenous dispersion of the PtSn and Pt particles in the presence of PANI in the catalyst support structure may all be the reasons for the observed higher ECSA of the PtSn catalyst compared to that of the PtRu catalyst. Interestingly, a lower ECSA was calculated for PtSn(65:35)/C-PANI compared to Pt/C-PANI. This apparent discrepancy may be attributed to blocking by Sn of Pt sites available for hydrogen adsorption or to Pt segregation, as the Sn percentage in PtSn(65:35)/C-PANI exceeds the one in the PtSn(70:30)/C-PANI electrocatalyst, which exhibited optimum performance. However, it might also be explained by the standard error in ESCA calculation, which was around 5-8%.

3.1.2.2. Electro-oxidation of methanol Catalytic activity in the MOR was assessed by CV test in H2SO4 (0.5M) + CH3OH (1M) solution previously purged by N2 in order to avoid oxygen contamination. The results are

displayed in Fig.5. Hydrogen adsorption was inhibited (in the -0.2 to 0V region) due to the dissociative adsorption of methanol and production of some poisonous species such as COads. Thus, the methanol oxidation current increased slowly in the double layer region (between 0 to 0.4V). In principle, this process consisted of a pattern of parallel reactions, shown schematically as follows: COads

CO2

According first anodic process is attributed to the oxidative removal of CH3to OHFig.5, the Adsorbed intermediate adsorbed/dehydrogenated methanol fragments via oxygen-containing species on Pt (such as HCOOH, HCHO PtOH) and formation of CO2, HCOH and HCOOH. This process is followed by a decline in the current density due to surface blockage through formation of surface oxides and further oxidation of methanol on the free oxide surface upon increasing the potential up to 1.1V. During the negative potential sweep, methanol and other adsorbed species undergo reoxidation at about 0.5V on the clean Pt surface after the removal of the oxide layer. The onset potential for oxidation, the peak potential and the peak current density are all often used to evaluate the catalytic activity of an electro-catalyst. As shown in Fig.5, all the synthesized PtSn/C-PANI and PtSn(70:30)/C electro-catalysts had practically the same onset and peak potential with PtRu/C, which were about 40 mV lower than the onset potential and 100 mV lower than the peak potential for Pt/C-PANI. Also, the synthesized PtSn(70:30)/CPANI electrocatalyst exhibited the highest current density, which was about 1.5 times larger than that for the Pt/C-PANI electrocatalyst and 1.4 times larger than that for the PtRu/C catalyst. This means that methanol oxidation on PtSn/C-PANI and PtRu/C is much easier than on Pt/C-PANI while PtSn(70:30)/C-PANI has the highest electro-catalytic activity. It is obvious that alloying Pt with Sn and the presence of PANI fibers in the catalyst support in the PtSn(70:30)/C-PANI electrodes, both improved the activity for methanol oxidation. This catalytic enhancement may be due to the following reasons: (i) Increase of the lattice

parameter in the presence of the PtSn alloys enhanced the adsorption and dissociation of methanol while it also increased the activity of methanol oxidation at lower potentials. (ii) Formation of OH groups through water activation on the Sn species at lower overpotentials than on Pt, which is a key step for the oxidative removal of the adsorbed CO; thus, adsorbed CO oxidation on PtSn electro-catalysts is possible at lower potential than on pure Pt [58]. (iii) Modification of the electronic structure of the Pt atoms caused by charge transfer from the less electronegative Sn to the more electronegative Pt atoms in the solid solution. This characteristic can reduce the adsorption of CO and other intermediates on the Pt surface and consequently reduce the poisoning of the electro-catalyst [59]. (iv) Incorporation of the PTSA into PANI, which can prevent the formation of strongly absorbed poisoning species on the active Pt sites while alleviating the poisoning effect of adsorbed CO species due to its hydrophilic group (SO3H). Of course, increasing the Sn content above that corresponding to PtSn(70:30)/C-PANI leads to blockage of the Pt sites available for methanol oxidation and, thus, to decrease of the catalytic activity in PtSn(65:25)/C-PANI catalyst, as shown in Fig.5.

3.1.2.3. CO stripping analysis Since the CO species was the main poisoning intermediate in the MOR, a good electrocatalyst should possess strong CO electro-oxidizing ability reflected in the CO stripping test. In order to evaluate the ECSA and investigate the enhanced activity of the PtSn/C-PANI electro-catalysts for the MOR (as shown in Fig. 5), the CO elimination ability of the electrocatalysts was studied via COads stripping analysis. The forward scan of COads oxidation profiles for the Pt, PtSn and PtRu electro-catalysts, are depicted in Fig.6. The calculated ECSAs according to CO oxidation charges are as follows: PtSn(70:30)/C-PANI (74.9 m2gPt-1) > PtSn(85:15)/C-PANI (69.8 m2gPt-1) > Pt/C-PANI (63.5 m2gPt-1) > PtSn(65:35)/C-PANI (60.1 m2gPt-1) > PtSn(70:30)/C (59.2 m2gPt-1) > PtRu/C (57.1

m2gPt-1), which are in good agreement with calculated ECSAs in section 3.1.2.1. As shown in Fig.6, during the forward CO stripping cycle the hydrogen desorption features did not appear, reflecting a blockage of the Pt sites by adsorbed CO. The shape and position of the CO stripping peak depended largely upon the nature of the electro-catalyst, due to the structure sensitivity of the CO oxidation. The oxidation of the adsorbed CO monolayer on the Pt surface corresponds to a narrow current peak with a maximum at 630mV while, in the case of the PtRu and PtSn electro-catalysts the CO oxidation peak was broader than that for Pt alone, with a current maximum at 618mV for the PtRu and 550mV for the PtSn electrocatalysts. The potential corresponding to the CO stripping current peak was almost the same for all the PtSn alloy catalysts, independent of the Sn content in the catalyst structure. This result is in agreement with the work of Crabb et al.[60], who found that the onset potential of CO oxidation is lowered upon addition of a small amount of Sn, with little further effect in terms of potential shift as the amount of Sn was increased. This suggested that, only a small amount of Sn was required to provide good promotional effects. On the other hand, the surface of platinum nanoparticles included a wide range of active site types. In other words, this varied from strongly to weakly active sites interacting with the Sn. Moreover, it seemed that, lower loadings of the Sn species targeted the most active platinum sites while higher loadings of this material tended towards interacting with the less active platinum sites. As can be seen in the inset of Fig.6, CO oxidation on PtSn/C-PANI electro-catalysts started at about 120mV, this potential being about 80 and 60mV more negative than that for Pt/C-PANI and PtRu/C, respectively. The significant difference between the onset and peak potential for the synthesized PtSn/C-PANI electro-catalysts and those for Pt/C-PANI or even PtRu/C electrodes indicates the promotion effect on COads oxidation of alloying Pt with Sn in the presence of PANI. This shift of COads oxidation to lower potentials compared with the Pt electrode is usually explained by the ability of Sn to adsorb OH at more negative potentials

which react with neighbouring CO species adsorbed on the Pt sites (i.e through the bifunctional mechanism). On the other hand, the modification of the d-band electronic structure of Pt due to the Pt lattice expansion resulting through incorporation of the Sn atoms weakened CO bonding to Pt. This latter interaction caused oxidation of the weakly bonded COads fraction at lower overpotentials [61]. Also, it has been reported that the Sn surface sites are freely available for adsorption of OH species, as CO does not prefer binding to the Sn surface sites [62]. Both the positive effects of Sn on catalytic activity and CO tolerance of the PtSn electro-catalysts as well as the noticeable effect of the PTSA doped PANI may all be accounted for explaining the significant differences in the performance between the PtSn/CPANI and PtRu/C electro-catalysts. On the other hand, the defects in the PANI chains and the hydrophilic group of PTSA may be advantageous for producing hydroxyl radicals on the electro-catalyst surface, in turn promoting the oxidation of the adsorbed CO species.

3.1.2.4. Methanol diffusion coefficient Another parameter affecting methanol oxidation activity is the methanol diffusion coefficient, which was evaluated via linear sweep voltammetry (LSV) at different potential scan rates (1, 10, 30, 50, 70, 100 and 120mVs-1). As shown in Fig.7, a linear relationship between the peak current density (Ip) and the square root of scan rate (ν0.5) was obtained. This implies that methanol oxidation on the Pt and Pt alloy electrodes was controlled by a diffusion process. The methanol diffusion coefficient was calculated using the following equation [63]:

where Ip, C and D denote the peak current density (Acm-2), methanol concentration (molcm-3) and diffusion coefficient (cm2s-1), respectively, F denotes the Faraday constant (96485 Cmol1

), R denotes the universal gas constant (8.314 Jmol-1K-1) and T denotes the room temperature

(298 K). The calculated diffusion coefficients followed this trend: PtSn(70:30)/C-PANI (4.9*10-6 cm2s-1) > PtSn(85:15) /C-PANI (4.7*10-6 cm2s-1) > PtSn(65:35) /C-PANI (4.4*10-6 cm2s-1) > Pt (2.4*10-6 cm2s-1) > PtSn(70:30)/C (2.15*10-6 cm2s-1) ~ PtRu/C (2.1*10-6 cm2s-1). It is obvious that the lowest diffusion coefficients correspond to the PtRu/C and PtSn(70:30)/C electrocatalysts which is consistent with the ability of the PANI material to enhance the rate of methanol diffusion in the reaction layer due to its high porosity.

3.2. Comparison of the MOR mechanism and the stability of the optimized PtSn(70:30)/C-PANI and commercial PtRu/C electro-catalysts 3.2.1. Tafel slope and exchange current density In order to carefully investigate the catalytic activities of the PtSn(70:30)/C-PANI and PtRu/C electrodes, Tafel slopes and exchange current densities (i0) for the methanol oxidation reaction were calculated. The LSV studies for these electrodes were carried out in H2SO4 (0.5 M) and CH3OH (1M) solution with scan rate of 1mVs-1. The corresponding Tafel plots are shown in Fig.8. The open circuit potential is about 0.5V for PtRu/C and about 0.6V for PtSn(70:30)/C-PANI catalysts. The PtSn(70:30)/C-PANI electrocatalyst showed a better performance in the kinetic control region compared with the commercial PtRu/C electrocatalyst. Each plot can be fitted to the Tafel equation, divided into two linear regions corresponding to two different Tafel slopes. In the low overpotentials region (i.e. 0.3 to 0.43 V) the Tafel slopes were 103 mVdec-1 for the PtSn(70:30)/C-PANI and 114 mVdec-1 for the PtRu/C electrocatalyst, while they were 229 mVdec-1 for the PtSn(70:30)/C-PANI and 244 mVdec-1 for the PtRu/C in the high overpotentials region (i.e. 0.43 to 0.6 V). The latter Tafel slopes were almost twice the former ones, which indicate a possible change of reaction mechanism or, at least, a change of the rate determining step (RDS) at different potential

ranges. From kinetics point of view, a Tafel slope of approximately 118 mVdec−1 for the MOR indicates that, the first step involving the splitting of the first C-H bond of methanol molecules, where the first electron transfer occurs, is the RDS at low overpotentials [64]. The Tafel slopes for PtSn(70:30)/C-PANI and PtRu/C in the low overpotentials region were rather close to this theoretical value. According to in-situ infrared reflectance spectroscopy studies performed by Dubau et al [65] , the CO diffusion on Pt and the reaction between COads and OHads are very fast, thus methanol dehydrogenation is the RDS in the low overpotential region. With increasing potential, the large amounts of OHads formed on the Pt and Sn or Ru sites and part of the adsorbed CO-like residues are gradually oxidized. This results in cleaning up of the Pt active sites and enhances subsequent methanol adsorption and dehydrogenation. On the other hand, increased Tafel slope at higher potentials was related to strong adsorption of the OH species on the Pt surface resulting in lowering the coverage of the poisoning intermediates. This in turn implies that, the reaction between COads on the Pt sites and OH species on the Ru or Sn sites may be considered to be the rate determining step (RDS) at higher potentials, which justifies the increase of the Tafel slopes. Similar conclusions were drawn by other research groups who reported that the dehydrogenation of methanol and the oxidation reaction between COads and OHads are the RDS in the low and high potential ranges, respectively [64, 66]. The exchange current densities for the MOR were then calculated according to the Butler – Volmer equation and normalized to the geometrical electrode area. The determined exchange current density of PtSn(70:30)/C-PANI (2.8*10-6 A cm-2) was about 47% higher than that of PtRu/C (1.9*10-6 A cm-2).

3.2.2. Electrochemical Impedance Spectroscopy (EIS) study In order to analyze the electron transfer kinetics of the MOR, EIS study was performed with PtRu/C and PtSn(70:30)/C-PANI, at various potentials from 0.1V to 0.9 V with 0.1 V

increment, at a frequency range of 100 kHz to 100 mHz, amplitude of the sine wave equal to 10 mV and 30 experimental points are obtained in each run. Fig.9 shows representative Nyquist impedance plots for the PtRu/C (a1) and PtSn(70:30)/C-PANI (a2) electrocatalysts (a2) and corresponding bode plots (b1) and (b2). In all Nyquist plots, the high frequency intercept on the real axis corresponding to the solution (i.e. ohmic) resistance was negligible compared with the arc diameter which represents the charge transfer resistance. This was due to the use of Luggin capillary. As can be seen in Figs. 9(a) and (b), the Nyquist plots corresponded to distorted semicircles. Such impedance features may be attributed to coupling of interfacial charge transfer and diffusion as well as the roughness of the highly dispersed electrode surfaces. Nonetheless, the precise explanation of this behavior requires deconvolution of the individual contributions, which was not possible. The precise reason for this behaviour is still open to further discussion. Both electrodes showed the same behaviour in the potential range of 0.1 to 0.4V with the diameter of the impedance arcs decreasing with increasing electrode potential. This was attributed to double layer charging followed by methanol adsorption and splitting of the C-H bound with the first electron transfer. This behaviour may further reflect the decrease of the polarization resistance related to an anodic charge transfer step due to increase of the corresponding electro kinetic constant with increasing potential. The projected diameters of the impedance arcs were substantially smaller for PtSn(70:30)/C-PANI in comparison with that of PtRu/C, reflecting the higher electrocatalytic activity of the former electrocatalyst. The EIS behaviour was completely different for these two electrodes in the potential range from 0.5 to 0.7V. As can be seen in Fig. 9(a), the impedance arcs began to appear in the second quadrant instead of the first, for PtRu/C electrode. Such negative impedance behaviour has been observed in previous studies for methanol electro-oxidation being ascribed to the oxidative removal of COads from the catalyst surface and the recovery of the

catalytic active sites [51]. Unlike the observed behaviour for the PtRu/C electrode, in the case of the PtSn(70:30)/C-PANI electrocatalyst the impedance arcs became smaller with increasing potential in the potential range from 0.5 to 0.7V whereas negative impedance wasn’t observed. This remarkable difference may be related to the presence of additional active sites available for methanol oxidation on the PtSn(70:30)/C-PANI electrode. Also, this enhancement may be due to the homogenous dispersion of PtSn(70:30) nano particles on the C-PANI composite (as shown in Fig.3) as well as the significantly higher CO tolerance of PtSn(70:30)/C-PANI in comparison with PtRu/C, evident in Fig.6. By increasing the potential from 0.7 to 0.9V, the arcs diameter increased for PtSn(70:30)/CPANI and reversed to the first quadrant for PtRu/C. This behaviour may be related to surface blockage through formation of surface oxides, in accordance with the CV results shown in Fig.5.

3.2.3. Influence of temperature Methanol oxidation experiments were also performed at different temperatures (25, 30, 35, 40, 45 and 50ºC) in electrolyte containing H2SO4 (0.5M) + CH3OH (1M). The corresponding linear sweep voltammograms (not shown here) of PtSn(70:30)/C-PANI and PtRu/C catalysts, obtained at a scan rate of 2mVs-1, exhibited an increasing oxidation current with rising temperature. It is noted that, the system studied in the present work showed a threshold where two asymptotic behaviours intersected one another. The first one seemed to be a behaviour determined by diffusion limitations whereas the other seemed to be related with thermal activation. In other words, as two extremes, either mass transfer or thermal activation limited the entire process. The former limitation was observed in the present work whereas the later has also been reported by others in the open literature [67-68]. Enhanced electro-catalytic activities at elevated temperatures can be attributed to decrease of

the CO coverage as a result of the thermal activation of Pt-(OH)ads formation and the enhancement of OH adsorption on the electro-catalyst surface. This latter effect is due to the discharging of the interfacial water molecules necessary for COads oxidation [69]. Corresponding Arrhenius plots (Log I vs. 1/T) at 0.3 and 0.4V in the low overpotential region are shown in Fig. 10. The apparent activation energy was obtained from the slopes of these plots using the following Arrhenius-type equation:

where A is the pre-exponential constant, R is the universal gas constant and Ea is the activation energy of the reaction. It is reiterated that Ea is an apparent value due to dependence of adsorbed species coverage on electrolyte concentration, temperature, electrocatalyst type and synthesis method. The apparent activation energy for PtSn(70:30)/C-PANI (36 kJmol-1) was considerably lower than that for the PtRu/C (50 kJmol-1). It is noted that Ea at a specific potential corresponds to the RDS at that potential and varies as the potential changes. The calculated activation energies in the low overpotential region (practically the same for both 0.3V and 0.4V) are rather close to those obtained for the methanol adsorption on a smooth Pt surface [70]. This suggests that the adsorption of methanol is the RDS in that overpotential region, confirming the conclusion drawn on the basis Tafel slope value. The considerable difference between calculated activation energies for methanol oxidation on PtSn(70:30)/C-PANI and PtRu/C surfaces was attributed to several factors, including the capability of water absorption on the PANI [71], the existence of hydrophilic groups (SO3H) in doped PANI and the synergistic effect of PANI and Sn nano particles in the electrocatalyst structure.

3.2.4. Influence of methanol concentration

The influence of the methanol concentration was investigated using LSV with a scan rate of 2mVs-1 at room temperature in electrolytes containing H2SO4 (0.5M) and (0.2, 0.4, 0.6, 0.8 and 1M) methanol. In the entire concentration range, the reaction rate increased with increasing methanol concentration. The reaction order was calculated via analysis of the dependence of the anodic current on methanol concentration using the following equation:

Fig.11 (a) shows the Log (I) versus Log(C) dependence in the low overpotential region (i.e. 0.3 and 0.4V). The data fall on parallel lines with a slope of about 0.5 which implies that the MOR follows half-order kinetics with respect to methanol concentration, also reported elsewhere for the Pt, Pt black and unsupported PtRu electro-catalysts [67]. Furthermore, a reaction order less than unity in the low overpotential region indicates that the adsorbed organic species take part in the RDS [67]. This further suggests that the oxidation reaction between COads and OHads is the RDS in the low overpotential region. This implication contradicts the conclusions drawn on the basis of the calculated Tafel slopes and activation energies in this region. However, it is more likely that a mixed activation – adsorption control exists in this potential range which can explain this apparent contradiction. The dependence of methanol oxidation peak potential on methanol concentration is shown in Fig.11 (b). Increasing methanol concentration had no obvious effect on peak potential for the PtSn(70:30)/C-PANI electrocatalyst but caused a positive shift of it for PtRu (from 0.47V to 0.6V). Shifts of peak potential may be attributed to the difference in oxidation pathway of adsorbed CO on Pt–Ru [72]. Enhancement of the methanol concentration led to increasing of the adsorbed amount of poisonous species (in particular, CO) upon the catalyst’s surface. According to the CO stripping tests (see Figure 6 please), the CO oxidation on PtSn/C-PANI catalyst was much easier than that of the PtRu/C material. Thus, the lower CO tolerance of

the PtRu catalyst might have been a major reason for the positive shift of the peak potential for the methanol oxidation reaction.

3.2.5. Accelerated Durability test (ADT) Accelerated stability tests of the PtRu/C and PtSn(70:30)/C-PANI electro-catalysts were carried out by sweeping the potential from -0.2 to 1.2 V for 2500 consecutive potential cycles, at a scan rate of 50mVs-1 in a H2SO4 (0.5M) + CH3OH (1M) solution. The voltammograms of H2 adsorption/desorption were used to determine changes of the Pt active surface area after each 500 cycles. Finally, CO stripping analysis was performed to survey the change of CO tolerance of the electro-catalysts before and after 2500 potential cycles. The change of peak current density for the MOR (Ip) as well as the variation of ECSA during the ADT is presented in Fig.12 (a) and (b). As shown in Fig.12 (a), methanol oxidation on both electrodes corresponded to almost constant anodic peak current up to about 500th cycles. After-wards the peak current decreased, with a lower rate for PtSn(70:30)/C-PANI. The Ip value for PtSn(70:30)/C-PANI decreased slowly with the number of CV cycles and reached almost 75% of its initial value at the 2500th scan. The Ip value for PtRu/C dropped much more rapidly with the number of CV cycles and reached only around 20% of its initial value after 2500 cycles. This loss of catalytic activity may have resulted either from the consumption of methanol during the potential scan or from poisoning and structural changes of the electrocatalyst nano particles as a result of the applied potential during scanning in an aqueous solution, especially in the presence of an organic compound. As clearly shown in Fig.12 (b), ECSAs for both electrodes reduced to about 77% for PtRu/C and 30% for PtSn(70:30)/CPANI indicating a remarkable decrease of the Pt active surface area due to a variety of possible reasons, including Pt sintering and aggregation , dissolution of Pt metal and oxidation of the carbon support [52]. Due to adsorption of reactant species such as H2, CO or

methanol molecules only on the Pt sites, the active reaction sites in a composite of Pt with other metals (e.g. Sn) would still be the Pt sites. Therefore, the change of Pt structure surely changes the ECSA value of the catalyst. The aforementioned significant difference in the stability of the two electro-catalysts may be attributed to the positive effect of doped PANI in the electro-catalyst support. It was previously mentioned in this paper that, the presence of PANI in the catalyst support leads to a more homogenous distribution of the resulting catalyst particles. This further rationalizes the observed stability of the synthesized PtSn(70:30)/CPANI material in terms of the catalytic components delayed aggregation during the ADT, caused by the uniform dispersion of the catalytic active sites induced by the presence of PANI. Fig.12 (c1) and (c2) shows CO stripping voltammograms of the PtRu/C and PtSn(70:30)/C-PANI electro-catalysts before and after the ADT test. As can be seen, the location and pattern of CO stripping voltammograms of PtSn(70:30)/C-PANI did not change much. A minor shift of the onset potential was observed at the end of the ADT, but the location of the peak potential remained the same. In contrast, a remarkable change in the CO stripping voltammograms of PtRu/C was observed, specifically the peak potential for COads stripping shifted from 0.62V to about 0.8V after the ADT. The positive shift of the peak potential indicates that this electro-catalyst went through a change in its surface characteristics. According to the corresponding Pourbaix diagrams, Ru was subjected to phase transitions through RuO2– Ru(OH)3–Ru during the potential scans between 0 and 1.2 V in acidic solutions (pH=1). The repetitive phase transition of Ru component in the catalyst during the ADT is thought to weaken the bonding of Ru and facilitate its dissolution. In contrast, the Sn species converted into SnO2 were not subjected to other phase transition. This further emphasizes the better stability of this component in the optimized Pt–Sn catalyst prepared in the present study. The dissolution of Ru under the ADT electrochemical

conditions was a major cause for the change in the electrochemical surface characteristics and the consequent performance deterioration. The ADT test results show the PtSn(70:30)/C-PANI electro-catalyst has a better durability in the MOR in comparison with the commercial PtRu/C electrocatalyst, due to the coexistence of PtSn alloy and PTSA-doped PANI in the electro-catalyst structure.

3.3. Passive direct methanol fuel cell studies From a practical point of view, the single cell test was the final evaluation criterion for the novel electro-catalytic material developed. The only variable parameter investigated in this study, was the difference in the single cells performance of different anode catalysts, depicting to some extent the activities of these materials. Fig.13 shows the performance of MEA-1 (PtRu/C as anode catalyst) and MEA-2 (PtSn(70:30)/C-PANI as anode catalyst) in a passive DMFC filled with a 2M methanol solution. It is evident that MEA-2 had a better performance over the entire current density region producing a maximum power density 20% higher than that of MEA-1. The performance of the cell with MEA-2 was satisfactorily comparable to those of typical passive DMFCs, listed in Table 2 [40-48]. It was observed that the maximum power density produced by the cell with MEA-2 was similar to the previously reported ones, despite the use of more precious metals in both the anode and cathode electrodes of the corresponding cells. This means that a more economical power source based upon the PtSn(70:30)/C-PANI anode catalyst for a passive DMFC may be realized, compared to the one utilizing the commercial PtRu/C provided by the Electrochem. As shown in Fig.13, the open circuit voltage (OCV) of MEA-2 is significantly higher than that of MEA-1. Since the OCV is related to the concentration of methanol existing at the cathode, its higher value is indicative of a lower methanol crossover. Methanol crossover corresponds to diffusion of un-reacted methanol from the anode to the cathode through the

Nafion membrane. This causes a mixed potential effect as well as poisoning of the Pt electrocatalyst at the cathode. As a result, methanol crossover significantly reduces cell performance and fuel utilization. Therefore, it is essential to reduce methanol crossover in order to increase the efficiency and stability of DMFCs. This higher OCV may be related with the positive effect of PANI fibers in the anode structure as it concerns decreasing methanol crossover through the Nafion membrane. In order to survey the effect of PANI on MEA open circuit potentials, methanol crossover was measured using an electrochemical technique [21]. Nitrogen was injected into the cathode electrode, and a high positive voltage was applied to oxidize the diffused methanol on cathode catalyst surface. Finally, a limiting current was achieved. According to Faraday’s law, the methanol crossover rate (n (molcm-2s-1)) can be calculated by

, where i (mAcm-2) is the

crossover current and 6 is the number of transferred electron numbers per mole of oxidized methanol. The crossover current density versus the applied voltage is shown in Fig. 14. The data presented in the figure indicate that the presence of PANI in the anode electro-catalyst layer of MEA-2 reduced methanol crossover by up to around 30% in comparison with that for MEA-1. This may be due to the synergism of carbon Vulcan and PANI as well as to faster kinetics of methanol oxidation on the surface of the anode catalyst layer of the MEA-2 compared to MEA-1.

4. Conclusions In summary, a series of PtSn/C-PANI electro-catalysts with different atomic ratios of Pt:Sn = 85:15, 70:30 and 65:35 were prepared by impregnation and compared with the synthesized Pt/C-PANI and commercial PtRu/C (Electrochem) materials in terms of the CO tolerance, catalytic activity and durability for the MOR. The obtained results as it concerns the performance of these electro-catalysts as DMFC anodes are summarized as follows:

I. All PtSn/C-PANI synthesized electro-catalysts had practically the same onset and peak potentials in the MOR as those of the commercial PtRu/C electrocatalyst, being about 40mV and 100mV lower than the corresponding ones for the Pt/C-PANI electrocatalyst. II. The PtSn(70:30)/C-PANI electro-catalyst showed a superior performance for MOR, with a peak current density about 40% higher than that of the commercial PtRu/C. III. The oxidation of CO over the PtSn alloy electrocatalyst occurred at lower potentials than on pure Pt or even PtRu, by about 80mV and 68mV respectively. IV. The catalytic activity of PtRu/C decreased significantly during the accelerated durability test. This activity deterioration was related to changes in the electrochemical surface characteristics due to dissolution of the Ru species. V. Use of the PtSn(70:30)/C-PANI as anode catalyst, instead of commercial PtRu/C, in a passive DMFC resulted in decrease of methanol crossover by about 30%. Finally, the obtained results indicated that the synthesized PtSn(70:30)/C-PANI electrocatalyst displayed a significant CO tolerance as well as a high electro catalytic activity and stability toward methanol oxidation. All these imply that this material may be a good candidate to replace the commercial PtRu/C electrocatalyst.

Nomenclature PANI

Polyaniline

PTSA

Para Toluene Sulphonic Acid

C-PANI

Carbon Vulcan – 20% Polyaniline composite

MOR

Methanol Oxidation Reaction

MEA

Membrane Electrode Assembly

θ

The angle corresponding to the peak maximum in XRD

OHads

Adsorbed hydroxyl groups (H) on catalytic Sites

COads

Adsorbed CO on catalytic sites

afcc

Lattice parameter

ECSA Electrochemical active surface area

λ

X-Ray wavelength (0.17889 nm for Co Ka

ADT

Accelerated durability test

radiation) Ip

Peak current density for the MOR

RDS

Rate determining step

D

Diffusion coefficient of methanol (cm2s-1)

Ea

Apparent activation energy of the reaction

B2θ

Full width at half maximum

ν

Scan rate of the potential

Acknowledgment I wish to express my sincere gratitude to Professor Anil Virkar of the Management Department of Material Science & Engineering of the University of Utah for providing me an opportunity as a visiting professor at the university of Utah.

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Figure captions page Fig.1. SEM micrographs from the surface of synthesized polyaniline Fig.2. XRD patterns of the Pt/C-PANI, PtSn/C-PANIs and PtRu/C electrocatalysts. Each spectrum was arbitrarily shifted along the y-axis to facilitate comparison. Fig.3. (a) High magnification TEM micrographs of the PtSn (70:30)/C-PANI electro-catalyst (b) Size distribution Fig. 4. Cyclic voltammograms of the synthesized Pt and PtSn and the commercial PtRu electrodes obtained in the potential range of -0.2 to 1.1V vs. Ag/AgCl(sat.) with a scan rate of 50mV s-1, in 0.5 M H2SO4 and at 25ºC; The arrows show the scan direction. Fig.5. Cyclic voltammograms of the synthesized Pt and PtSn and commercial PtRu electrodes obtained in the potential range of -0.2 to 1.1V vs. Ag/AgCl(sat.) with a scan rate of 50mVs-1, in CH3OH (1M) + H2SO4 (0.5M) and at 25ºC; The arrows show the scan direction. Fig.6. Forward scan of the CO stripping voltammograms of the synthesized Pt and PtSn and the commercial PtRu electrodes in the potential range of -0.2 to 1.1V vs. Ag/AgCl(sat.) with a scan rate of 25mVs-1 in 0.5 M H2SO4 and 25ºC. Each spectrum was arbitrarily shifted along the y-axis to facilitate comparison; Inset: zoom of the CO stripping profile in the 0-0.3V range for comparison of the COads oxidation onset potential

Fig. 7. The relation between the peak current density (Ip) and square root of the scan rate (ν0.5); Potential range of -0.2 to 1.1V vs. Ag/AgCl(sat.), in CH3OH (1M) + H2SO4 (0.5M) solution and 25ºC. Fig. 8. Tafel plots for MOR on PtRu/C and PtSn(70:30)/C-PANI electrodes in H2SO4 (0.5M) + CH3OH (1M) electrolyte at 25ºC; scan rate: 1mVs-1. Fig. 9. EIS Nyquist plots in MOR for the PtRu/C (a1) and PtSn(70:30)/C-PANI (a2) electrocatalysts in H2SO4 (0.5M) + CH3OH (1M) obtained in the potential range of 0.1V0.9V (vs. Ag/AgCl). The corresponding bode plots (b1) and (b2). Fig.10. Arrhenius plots for the methanol oxidation on PtRu/C and PtSn(70:30)/C-PANI electrodes in the low overpotential region (0.3 and 0.4V); H2SO4 (0.5M) + CH3OH (1M). Fig.11. Logarithmic plot of MOR current vs. concentration in the low overpotential region (0.3 & 0.4V) for the PtSn(70:30)/C-PANI and PtRu/C electrodes in H2SO4 (0.5M) (a). Dependence of MOR peak potential on methanol concentration (b) Fig.12. Obtained results of ADT for the PtRu/C and PtSn(70:30)/C-PANI electrodes, scanning the potential between -0.2 to 1.2V in H2SO4 (0.5M) + CH3OH (1M): Variation of Ip in the MOR (a). Change in the ECSA (b). CO striping voltammograms before and after the ADT for PtR/C (c1) and PtSn(70:30)/C-PANI (c2). Fig.13. Polarization and power density curves of passive DMFCs with MEA-1 and MEA-2 at room temperature. Methanol solution 2M (12.5mL) Fig.14. Crossover current density versus potential in the potential range of 0 to 1V, Methanol solution 2M (12.5mL) Table 1: Structural parameters of Pt/C-PANI, PtSn/C-PANIs and commercial PtRu/C electrocatalysts

Electro-catalyst

Composition

Pt (1 1 1) peak

Metal

Lattice

ICP

EDX

EDX

position

particle size

parameter

(wt.%)

(wt.%)

(at.%)

2θ (º)

(nm)

( ºA)

Pt/C-PANI

100

100

100

46.60

4.2

3.94

PtSn(85:15)/C-PANI

93:7

92:8

87:13

46.38

5.0

3.99

PtSn(70:30)/C-PANI

83:17

80:20

71:29

46.33

5.2

4.00

PtSn(65:35)/C-PANI

77:23

74:26

64:36

46.20

5.8

4.00

PtRu/C (Electrochem)

----

70:30

54:46

47.03

3.8

3.92

Table 2: Comparison of the performance of passive DMFCs reported by other research groups

Anode catalyst

PtRu black

Anode catalyst Cathode loading catalyst -2 (mgmetal cm ) 4

Pt/C (40%)

Cathode catalyst loading (mgPt cm-2)

Membrane

2

N117 GEFC-10N perfluorinated ion exchange membrane GEFC-10N perfluorinated ion exchange membrane

Methanol Methanol solution solution concentration volume (M) (mL) 2

50

2

10.8

2

10.8

PtRu(1:1)

4

Pt

2

PtRu(1:1)

4

Pt

2

PtRu black

4

Pt black

4

N115

2

8

PtRu(1:1)

4

Pt

2

N115

2

10.8

PtRu black

4

Pt black

2

N115

2

10.8

PtRu (50%)

3

Pt/C (50%)

3

N115

2

4

PtRu black

4

Pt/C (40%)

2

N115

2

25

4.1

Pt/C (47.2%)

2.6

N112

2

5

4

Pt/C (20%)

3

N115

2

12.5

PtRu/C (30.1%Pt: 23.3%Ru) PtSn(70:30)/CPANI (20%)

Maxim de (mW

(stack c