Catalytic Intervention of MoO3 toward Ethanol Oxidation on PtPd

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The Catalytic Intervention of MoO3 towards Ethanol Oxidation on PtPd nanoparticles decorated MoO3-Polypyrrole Composite Support Abhishek De, Jayati Datta, Ipsita Haldar, and Mukul Biswas ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07455 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016

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ACS Applied Materials & Interfaces

The Catalytic Intervention of MoO3 towards Ethanol Oxidation on PtPd nanoparticles decorated MoO3Polypyrrole Composite Support Abhishek De †, Jayati Datta †*, Ipsita Haldar‡ and Mukul Biswas ‡ †

Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah-711103, India ‡

Department of Chemistry, Presidency University, Kolkata, West Bengal, India

ABSTRACT. Ethanol oxidation reaction has been studied in acidic environment over PtPd Nps grown on the molybdenum oxide-polypyrrole composite (MOPC) support. The attempt was focused on using reduced Pt loading on non carbon support for direct ethanol fuel cell (DEFC) operated with proton exchange membrane (PEM). As revealed in SEM study, molybdenum oxide network exist in polypyrrole caging and the presence of metal NPs over the composite matrix is confirmed by TEM analysis. Further physicochemical characterizations like XRD, EDAX and XPS are followed in order to understand the surface morphology and composition of the hybrid structure. Electrochemical techniques like voltammetry, choroamperometry and impedance spectroscopy along with performance testing of in-house fabricated fuel cell are carried out to evaluate the catalytic activity of the materials for DEFC. The reaction products are 1

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estimated by ion chromatographic analysis. Considering the results obtained from the above characterization procedures, the best catalytic performance is exhibited by the Pt-Pd (1:1) on MOPC support. A clear intervention of the molybdenum oxide network is strongly advocated in the EOR sequence which increases the propensity of the reaction by making the metallites more energy efficient in terms of harnessing sufficient numbers of electrons than with the carbon support.

KEYWORD: PtPd nano particles, molybdenum oxide, polypyrrole, non graphitic support, electrocatalysis, direct ethanol fuel cell.

1. INTRODUCTION Fuel cell is one of the promising energy conversion devices that cater to the needs of future clean energy demands. In the category of direct oxidation fuel cell (DOFC), during the recent past, ethanol has been the target fuel due to its ease of production, storage, transport and favorable power capacity and exceptionally high energy density ∼29.7 MJ/kg compared to the other organic fuels.1-3 However, due to complexity of the C-2 molecule, stringent catalyst requirement has become a challenging issue in case of electro-oxidation of ethanol.4 Although a common choice, Pt alone limits its usage as efficient catalyst due to slower kinetics, self- poisoning of surface by adsorbed CO and other reaction intermediates.5 Alloying Pt with a second or third metal like (Pd, Au, Ni, Co, Ru, Ir, Mo etc) has been successful in minimizing the activation loses and maximizing the exchange current density of the catalyst surface towards alcohol oxidation.612

J Datta et al. has reported the high performance of plurimetalic non Pt catalyst on carbon

support for the electro-oxidation of ethanol.13-14 With regard to creating active reaction centers in 2


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the catalyst matrix, imparting high surface area and increased durability, choice of support materials has also become a critical issue in developing electrode components in fuel cells.15 The mesoporous graphitic carbon is a typical choice for support materials offering highly porous topology, electrically conducting properties and ensuring affordability. However, several disadvantages are also associated with the use of carbon black as support materials, such as poor resistance to corrosion caused by electrochemical oxidation in acid environment16-18 and high specific surface area which may not be fully accessed by the electrolyte, thereby limiting the specific catalytic performances. Further, carbon substrates are not potential surfaces for conducting protons.19 However, it may be noted that instead of using the mesoporous carbon, etectrocatalysis in fuel cell have been reported on the use of graphitic support in the form of graphene, reduced graphene oxide and CNTs for the PtPd nano structures, particularly for alcohol oxidation. Wang et al. has shown highly efficient electrocatalytic activity of PtPd nanocrystals with different size and shapes, supported on reduced graphene oxide, towards ethanol oxidation as well as oxygen reduction reaction.20,21 Improved electrocatalysis has also been reported specifically for EOR on alloyed PtPd nano particles on graphene nano-sheets, reduced graphene oxide and CNT supports.22-25 In fact, various efforts are being made to develop potential electro-catalysts, substituting the conventional graphite substrate by other support materials which can, not only disperse the catalyst particles but at the same time take proactive role in the catalytic phenomenon.26-29 In this investigation we have taken a novel attempt to introduce transitional metal oxide -polymer metal matrix composites as the catalyst support to eliminate the typical involvement of carbon in the direct ethanol fuel cell (DEFC) in acidic environment. Chemically prepared MOPC composite is

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used as the catalyst support and alloyed PtPd NPs are embedded in the metal oxide polymer matrix via NaBH4 reduction of the respective precursors. The spur to introducing MoO3 in our catalytic component is based on the well known multifaceted property of molybdenum oxides and its composites as potential functional materials in rechargeable batteries, energy storage and catalysis.30,31 Our approach is to ensure stability of the catalyst structure in acid medium by modifying the metal oxide with polypyrrole based conducting polymer which, not only constitutes a stable substrate but also increases the electron transport through the conjugated polymer chain serving as one of the structural component in the catalyst matrix. Although, MoO3 has been reported to play a superior role in oxidizing methanol molecule,32 none of the reports indicate development of MoO3 composite with polypyrrole for the study of ethanol oxidation. Weishan et al.in the study of methanol oxidation reported that the stability of MoO3 in acid medium in presence of polyaniline is influenced by the proton doping and un-doping throughout the polymer chain.33 In a further report, on gas sensing composite of MoO3 with vapor-deposited polypyrrole has been found to be an effective sensor for ethanol even at room temperature.34 MOPC is also found to be a promising material in super-capacitor applications.35 Taking into consideration the spectrum of application of MoO3 and its polymer composites, this investigation attempts to validate the PtPd alloyed NPs embedded in metal oxide-polymer composite (MOPC), PtPd/ MOPC, as the promising catalyst-support combination for ethanol oxidation reaction (EOR) for application in DEFC. The several objectives of this study include reducing Pt loading in the electrode component, identifying the beneficial role of using MoO3 as the support materials and enhancing functional properties of the catalyst associated with the 4


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conducting polymer chain. Presumably, the conducting polypyrrole chain acts as a corridor for tunneling charges produced during the electrochemical oxidation, thereby improving the kinetics of the charge transfer reaction. The structural and morphological parameters of the synthesized catalysts were obtained through different physico-chemical characterizations by employing XRD, TEM, SEM, EDX and XPS techniques. The polarization kinetics of the alcohol oxidation was thoroughly studied with the help of a series of electrochemical techniques like cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). EOR intermediates were estimated by using ion chromatographic technique to gauge the extent of the oxidation reaction. Finally the fuel cell performance testing was carried out in a fuel cell testing station (FCTS) using membrane electrode assembly (MEA) fabricated with synthesized catalysts as anode, Nafion 117 membrane and Pt/C as the cathode.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the catalysts. MoO3-Ppy nano composite was prepared by polymerizing pyrrole monomer in presence of MoO3 powder using ammonium vanadate in H2SO4 medium. The preparation of MoO3-Ppy composite (MOPC) has been described in a previous report by one of the Authors’ work.36 The composite matrix was decorated with Pt and the PtPd NPs (approximate atomic ratio, 1:1) under the NaBH4 reduction scheme from precursor salts (H2PtCl6 and PdCl2) taken in proportionate concentrations. Presumably the catalyst loading of approximately 40% is expected to be maintained onto the composite support, further estimated by thermogravimetric analysis (TGA), as described in the Supporting Information.

2.2. Material characterization. X-ray diffraction (XRD) patterns was carried out with the help of SEIFERT 2000 diffractometer operating under CuKα radiation (λ = 0.1540598 nm) 5



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generated at 35 kV and 30 mA with the scans at 10 min-1 for 2θ values between 20 to 90 degrees. Particle size of the electro-catalysts was determined using Debye Scherrer equation considering (111) peak of the Pt face centered cubic (fcc) structure. The elemental ratio of the catalyst layers were derived from EDX analysis using Link ISIS EDX detector (Oxford Instruments, U.K.) coupled with the scanning electron microscope. In order to obtain the morphology and average particle size, the catalysts were suspended on Cu grid (300) mesh and subjected to TEM analysis using JEOL JEM 2010 operated at an accelerating voltage of 200 kV. The XPS measurements were carried out using an Omicron Nanotechnology instrument (Serial No. 0571). Thermo gravimetric analysis (TGA) was performed in nitrogen environment with the help of Netzsch STA 449C (Germany), from room temperature to 1000 °C at the heating rate 5 °C/min.

2.3. Electrochemical characterization. Electrochemical measurements were conducted using a computer controlled potentiostat / galvanostat with PG STAT 12 and FRA modules (Metrohm, Netherlands). A catalyst ink was prepared using 5wt% Nafion solution and isopropanol to fabricate the electrode component. The geometrical area of the electro-catalysts exposed to the solution containing 1 mol L–1ethanol (EtOH) (AR grade, Merck, Germany) & 0.5 mol L-1 H2SO4 was maintained at 0.65 cm2. The working solutions were purged with nitrogen gas (XL grade, BOC India Ltd.) for 30 mins before starting each of the electrochemical experiments.

2.4. Estimation of oxidation products. Chronoamperometric measurements were performed at a constant potential of 0V for a span of 3600 sec. After continuous electrolysis for 1hour the aliquots were subjected to the estimation of EOR products by the help of Ion Chromatography (Metrohm’s Advanced Modular Ion Chromatography) associated with a L6


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7100 pump (Metrohm Ltd.) and a conductivity detector Metrosep A Supp 4-550 organic acid column.

2.5. Performance testing of DEFC. Full cell testing was carried out using unit cell consisting of MEA with an exposed area 1cm2. MEA of the single cell was fabricated with Nafion 117 membrane (Du Pont) with a Pt/C cathode on the one side and prepared catalysts as anode on the other. Catalyst loading on the both side of MEA was maintained at 1 mg cm-2 and a support to catalyst ratio of 60:40 is maintained for each of the catalysts. Fuel cell testing station (Fuel Cell Technologies, Inc.) was used for the performance study of all the synthesized catalysts at a temperature of 40°C. Oxygen is fed into the cathode chamber at a flow rate of 100 standard cubic centimeters per minute (sccm) while flow of acidic ethanol solution to the anode is maintained at 1.0 ml min-1.

3. RESULT AND DISCUSSION 3.1 Structure, morphology and composition of catalyst matrices. A set of XRD patterns for the carbon supported catalysts Pt/C, PtPd/C and MOPC supported catalysts Pt/ MOPC and PtPd/MOPC is presented in Figure1a. The observed diffraction peaks can be indexed as (111), (200), (220), (311) and (222) reflections of fcc structure.21 The (111) plane for Pt and Pd are located at 39.64° and 40.10°, 2θ angle while for Pt/MOPC and PtPd/MOPC the values lie in between at 39.81° and 39.91° indicating alloy formation.22 The magnified view of (111) plane is shown in Figure 1b.The average crystallite size and lattice parameters for the catalysts were calculated considering (111) diffraction peak using Debye-Scherrer equation37 and listed in Table1. Evidently carbon supported particles show narrow size distribution while the metallites

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grow with the formation of ensemble on MOPC, exhibiting ∼1.5 times the particle size observed on carbon support.38 The percent alloying (X%) in PtPd catalysts was determined with the help of Vegard’s law.39 Alloying in the order of 62.1% is observed on carbon support while the NPs grown on MOPC show alloying to the extent of 47.7% only (Table 1). Presumably the partial de-alloying is due to the interaction of PtPd NPs with Mo/MoO3 in the catalyst matrix structure. In an earlier work, the interlayer spacing in MoO3 structure is reported by Biswas et al. to be 6.91 Å for MOPC and the crystalline structure of MoO3 did not change with the incorporation of polypyrrole moieties.36 In case of Pt and PtPd embedded on MOPC matrix, the diffraction peaks of MoO3 does not appear in the XRD pattern, because the MoO3 phase is converted to an amorphous state during the Pt reduction process as also reported by Ioroi et al.39 The SEM image of MoO3 and MOPC are shown in Figure 2a and 2b. MoO3 matrix has typical micro ‘lath’ morphology while the conjugate MOPC matrix resembles a wrapped structure where the polypyrrole network is smeared on the MoO3 crystalline network as represented by Figure 2b. The EDAX analyses give information about the approximate atomic ratio of 1:1 (Table 1) for the binary catalysts. The loading of PtPd nano-particles with respect to MOPC was found to be ~37 % by the TGA studies. (Figure S1in Supporting Information). The TEM images, Figure 3a and 3b show that the metal particles grow with a spherical geometry with 5-6 nm size (Figure 3a' and 3b') for both the Pt/MOPC and PtPd/MOPC systems. The size parameters are in good agreement with XRD data and summarized in Table.1 Based on the statistics of 100 particles from TEM images, the particle size distributions are shown in Figure 3a and 3b respectively. Figure 3c and 3d represent clear images of the Pt/MOPC and PtPd/MOPC 8


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catalyst planes respectively. Selected area diffraction patterns (SADP) are featured with typical rings indicating fcc crystallite structures of the matrices as shown in Figure 3c' and 3d'. The interplaner distances are derived on the basis of FFT analysis as shown in Figure 3c'' and 3d''.The interplaner distance for the Pt/MOPC and PtPd/MOPC are shifted from 0.2262 (for the Pt (111) plane JCPDS 04-0802) to 0.2243nm (Pt/ MOPC) and 0.2241 nm (PtPd/ MOPC), further giving support to alloy formation.22,38 The surface elemental composition and valence states of the PtPd/MOPC catalysts were investigated through XPS analysis. Figure 4a, 4b and 4c shows the XPS profiles of PtPd/MOPC with the signatures for Pt, Pd and Mo. The peaks at 70.967 eV and 74.391 eV correspond to the Pt (0) state in the Pt spectra and are ascribed to 4f7/2 and 4f5/2 respectively. The additional low intensity Pt 4f peaks at 72.057 eV and 76.420 eV for 4f7/2 and 4f5/2, respectively, are essentially the reflection of small amount of Pt (II) in the matrix. The patterns representing Pd 3d spectra of 3d5/2 and 3d3/2 obtained at 335.23 eV and 340.53 eV respectively are very similar in nature. Low intensity shoulders at 337 eV and 341.8 eV correspond to the 3d5/2 and 3d3/2 of Pd with +2 oxidation state. It is therefore evident that along with Pt (0) and Pd(0), partially oxidized Pt (II) and Pd (II) species are also present in the catalyst structure.20,22,38 De-convolution of the spectrum reveals characteristic peaks appearing at 230.9 eV and 234.1 eV for MoO2 and 232.3 eV and 235.5 eV for MoO3. This indicates the existence of Mo (IV) along with Mo(VI) in the catalyst matrix. Some of the Mo (VI) in MoO3 is converted to lower oxidation state Mo(IV) possibly during the borohydride synthetic procedure.40 Further the low intensity metallic Mo signals at 228.28 eV and 231.44 eV correspond to the formation of small amount of PtMo alloy which is in concurrence with the positive shifting of Pt (111) plane in the XRD pattern.

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3.2 Electrocatalytic studies of PtPd/MOPC. The typical surface features of the respective carbon and MOPC supported catalysts Pt/C, Pt/MoO3, PtPd/MoO3, Pt/MOPC and PtPd/MOPC are obtained from the voltammograms (Figure 5) recorded in H2SO4 solution. Figure 5a, 5b and 5c represent the magnified views of the hydrogen adsorption desorption (HAD) and adsorbed oxide reduction (AOR) regions respectively. It is observed that MOPC supported catalysts generate much higher current in both HAD and AOR region compared to carbon supported catalyst, while using only MoO3 as the substrate, a broad plateau region is observed in the double layer charging zone reflecting ‘hydrogen-molybdenumbronze’41,43 (HxMoO3) formation in acid medium. The hydrogen intercalation/de-intercalation43 with the MoO3 lattice can be expressed as ‫ܱ݋ܯ‬ଷ + ‫ ܪݔ‬ା + ‫ܪ → ି ݁ݔ‬௫ ‫ܱ݋ܯ‬ଷ ↔ ‫ܪ‬௬ ‫ܱ݋ܯ‬ଷ + ሺ‫ ݔ‬− ‫ݕ‬ሻ‫ ܪ‬ା + ሺ‫ ݔ‬− ‫ݕ‬ሻ݁ ି

(1)

where, x lies within 0 to 2 (i.e.0 < x < 2), and x > y The electrochemically active surface area (ECSA m2/g) of the synthesized catalysts was determined from HAD and AOR regions.6 The ECSA values, particularly derived from HAD regions are considerably increased on the MOPC matrices compared to vulcan carbon and the values are summarized in Table.1. The ECSA values corresponding to high charge at the hydrogen region attribute to the formation of extensive reaction sites created on the metal NPs decorated MOPC surface. The voltammograms recorded for the catalyst regime, with the addition of ethanol to the acidic f

solution as shown in Figure 6a, are characterized with two forward peak currents ( i p1 and

i pf 2 )

corresponding to the oxidation of ethanol to the acetaldehyde and subsequently to acidic acid 10


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formation.44 Kinetics of EOR becomes faster on the PtPd NPs using MoO3 as well as MOPC supports due to the spillover of H+ between HyMoO3 and HxMoO3, which urge the composite materials behave as good dehydrogenation catalyst. Likewise the reaction propagates through the following pathway: ି௘

‫ܯ‬ሺ‫ܪܥ‬ଷ ‫ܪܥ‬ଶ ܱ‫ܪ‬ሻ ሱሮ ‫ܯ‬ሺ‫ܪܥ‬ଷ ‫ܪܱܪܥ‬ሻ + ‫ ܪ‬ା

(2)

ି௘

‫ܯ‬ሺ‫ܪܥ‬ଷ ‫ܪܱܪܥ‬ሻ ሱሮ ‫ܯ‬ሺ‫ܪܥ‬ଷ ‫ܱܪܥ‬ሻ + ‫ ܪ‬ା

(3)

ି௘

‫ܯ‬ሺ‫ܪܥ‬ଷ ‫ܱܪܥ‬ሻ ሱሮ ‫ܯ‬ሺ‫ܪܥ‬ଷ ‫ܱܥ‬ሻ + ‫ ܪ‬ା

(4)

One of the critical steps in the EOR sequence is the water activation forming M-OH which may alternately assist in the conversion of aldehyde to acetic acid as shown in eq 5. ି௘

‫ܯ‬ሺ‫ܪܥ‬ଷ ‫ܱܪܥ‬ሻ + ‫ ܯ‬− ܱ‫ ܪ‬ሱሮ ‫ܯ‬ሺ‫ܪܥ‬ଷ ‫ܪܱܱܥ‬ሻ + ‫ ܪ‬ା We suggest a mechanism on the subsequent intervention of the MoO3 network, [MoO3]

(5)

N

in the

EOR catalysis which is illustrated by the interaction of the (CH3CO) ads with the lattice oxygen of the network structure in the near vicinity, whereby the partially dehydrogenated ethanolic species undergoes C-C bond cleavage with the formation of M-CO and the methoxy linkage45 as shown in eq 6. ‫ܯ‬ሺ‫ܪܥ‬ଷ ‫ܱܥ‬ሻ + ܱ − ‫ ݋ܯ‬− ܱ − ‫ܪܥ → ݋ܯ‬ଷ ܱ − ‫ ݋ܯ‬− ܱ − ‫ ݋ܯ‬+ ‫ܱܥܯ‬

(6)

In the present situation, therefore the ethanol oxidation may proceed through multiple pathways; (i) reaction may restrict to aldehyde formation, (ii) favorable water activation in presence of Pd in the matrix lead to acetic acid formation and (iii) PtPd/MOPC enable C-C bond cleavage of the

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adsorbed CH3CO species with the chances of CO adsorption on the surface, followed by CO stripping at the MOPC surface leading to CO2 formation. As evidenced by the XPS analysis (Figure 4c) molybdenum exists in multiple valence states (VI / IV) as MoO3 / HxMoO3 and / MoO2. In both the network structures of MoO3 and MoO2 the molybdenum and oxygen atoms are present alternately forming the –O–Mo–O–Mo– linkage, which take part in the reaction sequences.50 Such a mixed valent structure associated with polypyrrole chain becomes an effective platform for facilitating electron and proton conduction through the conducting polymer network during the EOR kinetics and the charge transfer at the reaction sites gets accelerated generating higher oxidation current as observed in the cyclic voltammograms. Figure 6a demonstrates an outstanding increase, almost 90%, in the oxidation current for PtPd catalyst using MOPC compared to using carbon substrate, at room temperature. This is eventually the outcome of large number of electron release at the reaction sites using the MOPC support. Within the EOR framework the methoxy linkage gets associated (eq 7) with the (OH)

ads

and subsequent deprotonation of the intermediate species restore the network structure

(eq 8) with the formation of M-CO accompanied by further electron release. The reaction tends to completion with further chances of CO oxidation with the help of (OH)ads leading to CO2 formation46 which however, in acid medium, could not be detected during our product analysis by ion chromatography. ‫ܪܥ‬ଷ ܱ − ‫ ݋ܯ‬− ܱ − ‫ ݋ܯ‬+ ‫ ܯ‬− ܱ‫ܪܥ → ܪ‬ଷ ܱ − ‫ ݋ܯ‬− ܱ − ‫ ݋ܯ‬− ܱ‫ ܪ‬+ ‫ܯ‬ ିସ௘

‫ܪܥ‬ଷ ܱ − ‫ ݋ܯ‬− ܱ − ‫ ݋ܯ‬− ܱ‫ ܪ‬ሱۛሮ ‫ܱܥ‬ଶ + ‫ ݋ܯ‬− ܱ − ‫ ݋ܯ‬+ 4‫ ܪ‬ା

(7)

(8)

12


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The catalytic utility of MOPC can be further explained with the electronic transition between the noble metallites and non noble transition metal structure during the charge transfer process6 of EOR. According to the order of ionization energy, Pt> Pd> Mo, as shown in Scheme 1, the Mo atoms of the MoO3 network bear a significant ‘+’ve charge (δ++) as the mode of electronic transfer is oriented towards the Pt and Pd atoms. The charge distribution among the metallites in the heterogeneous metal NPs-metal oxide-polymer moiety is schematically represented by Scheme 1. The negative charge in the outer sphere geometry of the ‘hybrid structure’ is capable of activating the bonding and antibonding oxygen orbital with excess electronic charge leading to the superoxo linkage in the [MoO3]N, which further induces faster ethanol oxidation kinetics at the composite matrix as illustrated above. As reflected in the XRD and XPS analysis, the strong interaction of the Pt / Pd particles with MOPC and the existence of Mo in metallic form may also play a beneficial role through the bifunctional mechanism forwarding the oxidation of adsorbed CO on the surface to CO2 formation. 20,49 On the other hand for Pt/C catalyst, the support materials do not have the potential to refresh the Pt active sites for better electro-catalysis. It is to be noted that both the forward peak currents, are ௙

௙

൫݅௣భ ൯raised almost by a factor of 2 and 3 times and ൫݅௣మ ൯ raised almost 1.5 and 2 times for Pt/ MOPC and PtPd/ MOPC respectively, when compared to Pt/C surface. The onset potentials (arbitrarily taken at current output of 5.00 mA cm-2) for all the catalysts exhibit negative shift compared to Pt/C as discernible in the Figure 6b. Extraction of H+ from the reactant ethanol molecule is accelerated due to molybdenum bronze formation, as already mentioned, which in turn reduces the over potential of the dehydrogenation process. The role of Pd to facilitate the OH adsorption20,38 on the catalyst surface has been reported in our earlier work and for this investigation PtPd/MOPC is able to improve the oxidation rate with tremendous potential by 13



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propagating electron transfer to the ad atoms at the reaction sites. Overall, the active participation of the support material MOPC is achieved by forming the hybrid PtPd/MOPC structure that not only triggers the EOR kinetics but at the same time help promote the CO tolerance of the catalyst materials. In order to put more clarity on the ability of the PtPd/MOPC surface towards mitigating the poisoning effect, periodic voltammetry for EOR was continued for a span of 500 cycles.22 The typical cyclic voltammograms are demonstrated in Figure S2 in the Supporting Information. No significant changes in peak current densities are observed throughout the voltammetric scans, indicating good performance stability of the catalyst surfaces. Surface stability of the prepared catalyst regime towards EOR was studied with the help of chrono-amperometric and chrono-coulometric experiments. The variation of current density was recorded for a span of 3600 seconds at a fixed potential of 0.0V as shown in Figure 7. Figure 7a is a perfect demonstration of suppression of poisoning effect47 of the surface of the binary catalyst by using the MOPC support. Further the merit of using MOPC support over and above the bare metal oxide and also using graphitic support, is well recognized in the coulometric plots6 (modified Cottrell’s law ,Q a t1/2) as shown in Figure 7b. The highest deviation for the binary catalyst supported on MOPC signifies better electrode-kinetics towards EOR in acid medium. The chronoamperometric investigations were further extended over a span of 34000 seconds to evaluate the long term stability of the PtPd/MOPC surface. The detailed observations are shown in Figure S3 (Supporting Information). The poisoning rates (% per second) were estimated to be 0.0054 till 4000 sec, 0.0055 till 14000 sec, 0.0065 till 24000 sec and 0.0065 till 34000 sec,

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reflecting trivial poisoning effect even for long duration. This essentially translates to the sustainability of the catalyst surface towards the oxidation reaction. The electrochemical impedance spectra for the catalyst regime are recorded at 0mV with the frequency range 30 kHz to 30 mHz in 1 (M) acidic ethanol solution is shown in Figure 8. The charge transfer resistance, Rct is significantly reduced with the elimination of carbon in the matrix. It implies that NPs embedded on MOPC support is able to control the activation polarization loss by lowering the overpotential for the oxidation reaction on its surface compared to carbon supported catalyst. This in fact corroborates with the voltammetric results. The EIS parameters are summarized in Table.2 and the corresponding equivalent circuit diagram48 is shown in Figure 8a and 8b where Rs, Rct, CPE corresponds to the solution resistance, a constant phase element indicating the double layer capacitance, the charge transfer resistance associated with EOR, and Ro and L are associated with CO oxidation kinetics. In case of Pd containing catalyst spontaneous surface coverage by OHads promote the dissociative adsorption of ethanol molecule by releasing the protons which eventually take part in the in-situ H intercalation-deintercalation by the MoO3 present in the matrix. The carbonaceous intermediates of EOR ultimately result in CO poisoning which becomes the rate determining step, as in case of Pt/C matrices. In this respect MOPC in the catalyst matrix perhaps serve as a better CO tolerant component as indicated by the appearance of an inductive loop and the lowering of L values for MOPC substrate transpires to successful removal of carbonaceous intermediates form the surface.6 Thus it appear that the hybrid structure offer high porosity, large surface area that expands the 3D space and conduces to more intimate contact between the catalyst and electrolyte leading to favorable charge transfer at the reaction sites.

15



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The yield of CH3COO-/CH3COOH during the course of EOR is obtained through ion chromatographic analysis, taking aliquots of the chronoamperometric solutions. Conversion of ethanol to acetate is pronounced using MOPC support as revealed in the chromatogram Figure 9. The estimated acetate formation is found to follow the order, PtPd/MOPC > PtPd/MoO3 > Pt/ MOPC > Pt/MoO3> PtPd/C > Pt/C as revealed in Figure 9b. Almost three fold increase in acetate concentration is observed on MOPC supported catalyst with respect to carbon support. The ethanol sensing property of both polypyrrole and MoO3 facilitates ethanol anchoring on the surface, thus indirectly influencing the catalytic phenomenon towards the oxidative conversion.34 For both the graphitic and non-graphitic support, it is observed that the incorporation of Pd significantly increases the yield of acetate, best known for the spontaneous OHads coverage of the surface in presence of Pd, as explained in our earlier work.38,51 It may be noted that quantification of acetate conversion to further oxidative product is beyond the scope of this chromatographic analysis. As has already been discussed, the PtPd/MOPC formulation readily promotes electronic transition between the ad atoms, thus creating active reaction centers that triggers the EOR kinetics towards completion and prohibit COads poisoning of the surface.13 The credibility of such metal-metal oxide- polymer conjugate formulation involves reduced usage of Pt on non carbon catalyst support for ethanol oxidation.

3.3 Fuel cell performance study. Figure 10 represents testing of the prepared catalysts in a in-house fabricated PEMDEFC. Among the catalysts, PtPd/MOPC shows superior performance compared to carbon supported catalysts with respect to open circuit voltage (OCV) and power density. Trend of OCV values for different catalyst systems are reflected in the onset potentials. In case of Pt/MoO3 and PtPd/MoO3, due to lack of stability, power density values (17.43 and 21.90 mW cm-2) are not consistent with the OCV values in comparison to MOPC 16


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supported catalysts. PtPd/MOPC catalyst shows highest power density of 27.37 mW cm-2 amongst all the materials which is almost 88% ahead of the Pt/C catalyst.

4. CONCLUSION The present investigation successfully demonstrates the energy efficient role of PtPd NPs decorated MoO3-ppy composite towards electro-catalysis of ethanol in acid media. The hybrid structure is developed by embedding the metallites through borohydride reduction method on the as prepared metal oxide polymer support. XPS confirms the presence of Mo in different oxidation states (0, IV, VI) which are instrumental in propagating the oxidation reaction in association with electron transmission through the conducting polymer network. The EOR sequence is illustrated by the suggested mechanistic pathways. The intervention of (MoO3)N is strongly advocated in the oxidation mechanism, supported by the model electronic charge distribution between the metallites in the composite structure which remarkably contributes to the catalytic performance towards the oxidative conversion of ethanol to the ultimate products. Overall, this study evokes further work with such innovative catalyst structures with reduced Pt loading and non carbon catalyst support for fuel cell reactions.

ASSOCIATED CONTENT Supporting Information Preparation of catalysts; stability studies extended through periodic voltammetry for 500 cycles and chronoamperometry upto 34000 seconds;

AUTHORS INFORMATION Corresponding author E-mail: [email protected]; Fax: +91 33 2668 4564; Tel: +9133 2668 4561. 17



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ACKNOWLEDGEMENT Financial support by Ministry of New and Renewable Energy (MNRE), New Delhi, Govt. of India and Department of Science and Technology- Science and Engineering Research Board (DST-SERB), Govt. of India is gratefully acknowledged.

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GRAPHICAL ABSTRACT

26


Page 27 of 34


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

27



30

30

25

(a')

Pt/ MOPC

(b)

20

15

10

PtPd / MOPC

(b')

25

Relative abundance

(a) Relative abundance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 34

20

15

10

5

5 0 3-4

4-5

5-6

6-7

0

7- 8

3-4

Particle size / nm

4-5

5-6

6-7

7-8

Particle size/ nm

(c')

(c)

(d'')

(d)

(d') (c'')

Figure 3. TEM images and the particle size distribution of Pt/ MOPC (a) & (a'), PtPd/ MOPC (b) & (b') respectively; HRTEM fringe, SADP and FFT patterns for Pt/ MOPC (c), (c') & (c'') and for PtPd/ MOPC (d), (d') & (d'').

28


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Figure 4. XPS survey spectrum of PtPd/MOPC alloy catalyst (a) Pt (b) Pd and (c) Mo regions.

29



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Figure 5.Cyclic voltammograms of Pt/C, PtPd/C, Pt/MoO3, PtPd/MoO3, Pt/MOPC and PtPd/MOPC catalyst in 0.5M H2SO4. Scan rate 50 mV s

–1.

Inset: (a) & (b) magnified view of

hydrogen adsorption-desorption region (c) magnified view of oxide reduction peak region.

Figure 6. (a) Cyclic voltammograms (100th scan) of EOR on Pt/C, PtPd/C, Pt/MoO3, PtPd/MoO3, PtPd/MoO3, Pt/ MOPC and PtPd/ MOPC in 0.5 M H2SO4 solution containing 1.0 M ethanol at room temperature (scan rate : 50 mV s–1); (b) Comparison of onset potential at 5.00 mA cm-2 current output. 30


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Figure 7 (a) Chronoamperograms recorded for 1 h at the potential of 0 V (vs. MMS) in solution containing 1.0 M EtOH and 0.5 M H2SO4 on Pt/C, PtPd/C, Pt/MoO3, PtPd/MoO3, Pt/ MOPC and PtPd/ MOPC catalysts. Inset: (a) Poisoning rate vs. t1/2 plots (b) charge density from the chronoamperometric experiments.

Figure 8. Nyquist plots in solution containing 1.0 M EtOH and 0.5 M H2SO4 on Pt/C, PtPd/C, Pt/MoO3, Pt/MOPC, PtPd/MoO3 and PtPd/ MOPC catalysts at 0 mV (vs. MMS). Inset: equivalent circuit diagram (a) Pt/C, PtPd/C and (b) Pt/MoO3, Pt/MOPC, PtPd/MoO3 and PtPd/ MOPC. 31



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Figure 9. Typical Ion chromatograms for acetate produced by electro-oxidation of ethanol in a mixture of 0. 5 M H2SO4 and 1.0 M ethanol on Pt/C, PtPd/C, Pt/MoO3, PtPd/MoO3, Pt/MOPC and PtPd/MOPC electrodes. Inset: (a) Magnified view of Acetate peak (b) Bar diagram for acetate concentration in ppm level.

Figure 10. Polarization and power density plot of the PEM DEFC with Pt/C, PtPd/C, Pt/MoO3, PtPd/MoO3, Pt/MOPC and PtPd/MOPC catalysts. 32


Page 33 of 34

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Scheme 1. Charge distribution model for PtPd / MOPC catalyst.

33



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Table 1. Physical and electrochemical properties as derived from XRD, SEM, TEM and cyclic voltammetry of the different catalysts

ElectroCatalysts

Pt:Pd EDAX Interplaner (bath composition Distance (Å) ratio) (at %)

Lattice Average parameter crystallite size(nm) (Å) XRD TEM

Pt/C

1:0

Pt 100

2.272

3.935

3.84

3.50

PtPd/C

1:1

Pt 45

2.25

3.89

3.40

4.0

Pt 100

2.258

3.910

5.38

5.53

Pt 48

2.255

3.905

5.119 5.09

PtPdalloy (%)

ECSA (m2/g) HAD AOR

-

33.8

62.10

75.34

45.77 70.97

Pd 55 Pt/MOPC

1:0

PtPd/MOPC 1:1

-

39.58 68.40

47.70

62.19 82.39

Pd 52

Table 2. Electrochemical impedance parameters for the different catalysts

Impedance parameter

Pt/C

PtPd/C

Pt/MoO3

PtPd/MoO3

Pt/MOPC

PtPd/MOPC

Rs / Ω

2.65

2.99

2.034

2.802

2.64

2.743

Rct / Ω

32.61

28.42

32.5

27.49

23.33

11.45

R₀/ Ω

-

-

40.4

47.8

39.3

18.46

CPE x 102 / F 2.45 cm–2

1.929

1.63

1.59

1.49

1.432

Lx 102 (H)

-

1.5

1.1

1.4

0.3

-

34


Catalytic Intervention of MoO3 toward Ethanol Oxidation on PtPd

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