Impact of Ultrasonic Waves in Direct Electrodeposition of

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Impact of Ultrasonic Waves in Direct Electrodeposition of Nanostructured AuPt Alloy Catalyst on Carbon Substrate: Structural Characterizations and Its Superior Electrocatalytic Activity for Methanol Oxidation Reaction Sidhureddy Boopathi, and Shanmugam Senthil Kumar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp509248e • Publication Date (Web): 02 Dec 2014 Downloaded from http://pubs.acs.org on December 3, 2014

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The Journal of Physical Chemistry

Impact of Ultrasonic Waves in Direct Electrodeposition of Nanostructured AuPt Alloy Catalyst on Carbon Substrate: Structural Characterizations and Its Superior Electrocatalytic Activity for Methanol Oxidation Reaction Sidhureddy Boopathi, Shanmugam Senthil Kumar* Electrodics and Electrocatalysis Division CSIR-Central Electrochemical Research Institute Karaikudi-630006, India

ACS Paragon Plus Environment

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ABSTRACT:

Here, we have successfully developed a facile, fast and efficient synthetic methodology for direct fabrication of stable AuPt alloy nanostructure catalyst on Toray® Carbon (TC) as well as glassy carbon substrates without involving any additional stabilizer or surfactant through ultrasonic wave assisted electrodeposition (USA-ED) using simple Au and Pt metals salt precursor in electrolyte solution. To know the real effect of ultrasonic waves on deposited particles, the same experimental methods also followed without ultrasonic waves. The size, morphology, surface composition and surface structure of AuPt alloy nanostructures were investigated by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDAX), X-ray diffraction (XRD), inductively coupled plasma – mass spectrometry (ICP-MS) and cyclic voltammetric techniques. The observed results clearly reveal that USA-ED method yield densely packed Pt-rich AuPt alloy nanostructure which are more active electrocatalyst for methanol oxidation, whereas in normal Electrodeposition(i.e., without ultrasonic wave- ED) method show loosely packed homogeneous alloy AuPt nanostructure which are very poor for methanol oxidation. More significantly, USA-ED of PtrichAuPt alloy nanostructured surface showed profound enhancement of both electrocatalytic activity and long term stability of methanol oxidation, when compared to ED-Pt-, USA-ED-Pt nanoparticles surfaces and also the state-of-art commercial electrocatalysts of 30% wt. loading of Pt/C (E-TEK). These obtained results confirm that our synthetic methodology of USA-ED is effective for preparing Pt rich AuPt alloy nanostructured electrocatalyst with excellet activity and stability in methanol oxidation reaction.

KEYWORDS ( Ultrasonic waves, Electrodepostion, AuPt alloy catalyst, Methanol oxidation).


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INTRODUCTION:

Direct methanol fuel cells (DMFCs) have attracted considerable interest in terms of high energy conversion efficiency, low operating temperature and the simplicity of handling liquid fuel.1-5 Platinum is considered as the most suitable electrocatalyst for methanol oxidation especially used as an anode catalyst, in low temperature DMFCs. However, pure Pt is readily poised by carbon monoxide (CO) which is generated in the intermediate step during methanol oxidation at low temperature and suppress efficiency of DMFCs.3-5 Pt based bimetallic nanoparticles have been proven as a promising materials for methanol oxidation, to improve the electrocatalytic effect, low poisoning of the catalyst, and minimization of platinum content in catalyst.5-13 The key criteria for intrinsic properties of secondary (bi) metal is: (i) must have a greater ability to form surface oxygen species compared to Pt (called as bi-functional effect)14, (ii) lower d-band electron density than Pt (called as ligand effect)15; (iii) must have ability to suppress the formation of poisonous species adsorbed on more than one surface active site (called as ensemble effect).16 Recent days, Pt modified Au bimetallic electrocatalysts have been attracting much attention due to their enhancement in electrocatalytic activity as well as very good stability towards methanol oxidation.13,17-20 Also, Au is an ideal choice of secondary (bi) metal because it is more stable under DMFCs operating conditions. However, enhanced electrooxidation of methanol using alloyed AuPt nanoparticles system especially in acidic electrolyte is still under dispute. For example, some research groups reported AuPt alloy nanoparticles are promote to the methanol oxidation,20-23 whereas some other research groups also proved exactly opposite effect i.e. AuPt alloy nanoparticles inhibits the methanol oxidation (called as methanol tolerant).24-26 Hence, it is necessary to explore appropriate synthetic strategy


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to design the suface structure of AuPt bimetallic nanoparticle and investigate the effect of Au on Pt catalytic activity in methanol oxidation. In this context, various methodologies have been established in a controlled manner for the synthesis of Au-Pt bimetal nanoparticles such as polyol method24,26, sodium borohydride11, electrochemical27-28, sonochemical29, microwave method18 etc. Among the various chemical and physical methods, electrodeposition (ED) method has proven to be a straight forward and powerful approach for loading of different metal nanoparticles on carbon materials (support) because of no requirement of post synthetic transfer of the hybrid materials.30,31 In general, by changing any one of these parameters such as deposition potential, nature of the electrolyte, deposition time and nature of the substrate, one can control the size/shape, composition, electronic nature of the bimetal nanoparticles.17 However, conventional ED methods have some limitation in yielding percentage of size/shape controlled nanoparticles and poor distribution of nanoparticle on substrate, because of limited in mass transport during the ED. To overcome this problem, people attempted to use USA-ED as an alternative approach not only enhancing mass transport phenomenon, but also to access control over the structure and the size/shape in the metal nanoparticle synthesis.32 Using USA-ED method, one can enhance the mode of mass transport by means of decrease the diffusion layer thickness, and increasing the overall reaction rate in electrodeposition, due to cavitation’s phenomenon. Also, ultrasound is known to affect surface morphology through acoustic cavitation jets at the electrode-electrolyte interface. Most of the reported literature suggest that sonoelectrochemical method can be applicable to solution phase synthesis of size/shape controlled mono and bimetallic nanoparticles by applying different cathodic overpotentials on bulk metal surface/sonotrode during ultrasonic waves condition. Because of acoustic cavitation phenomenon ( involving collapse of cavitation bubbles produced,


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equivalent temperatures of roughly 5000 ᵒC, pressures of about 1000 atmospheres and heating and cooling rates above 1010 K s−1) at electrode electrolyte interface, and most of the deposited particle peel off from the conducting substrate into the electrolyte solution (sonic horn operating at 20–40 kHz).

33,34

Hence, it is really challenging task to deposit mono- and -bimetal

nanoparticle on conducting substrate. However, Xiao et. al., reported that using ultrasonicwave in direct electrodeposition of Au-Pt alloy nanoparticles with ionic liquid –chitosan composite film modified on conducting substrate for sensing application.35,36 We have custom-modified the conventional electrochemical cell

suiting the conditions necessary for the generation of

catalytically active sites containing Au nanoparticles on the glassy carbon electrode (GCE) using USA-ED method in presence of PVP for controlling the nanoparticle size.37 Apart from these reports, to the best of our knowledge none of the work is reported in literature for direct electrodeposition of stable metal nanoparticle films on conducting substrate using ultrasonic waves during electrodeposition. In the present work, we have employed a simple and fast direct method of USA-ED to synthesize stable Pt- rich AuPt alloy nanostructure on TC and glassy carbon electrode (GCE) surfaces without involving any other additives to stabilizing the particles. An advantage of using TC as a working electrode for direct deposition of AuPt bimetallic nanoparticle is as follows: (i) most of fuel cells, TC substrate used as gas diffusion layer (GDL)

and provides good

mechanical strength as well as improve the electrical conductivity between catalyst layer and gas diffusion layer; (ii) one can eliminate the additional step involving to coat or deposit catalyst layer on GDL surface by physical methods. We have also compared the formation of AuPt bimetallic nanoparticle on the carbon substrate without sonic waves and correlated the effect of sonic wave in particle growth mechanism and morphology changes of AuPt nanoparticles.


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Finally, deposited AuPt alloy nanostructure surfaces were examined for the electrocatalytic activity of methanol oxidation reaction in acidic electrolyte method. Experimental Section: Reagents: Analytical grade HAuCl4.3H2O, H2PtCl6.6H2O, H2SO4, KNO3 and Methanol purchased from Merck and used as received without further purification and the aqueous solutions were prepared using Milli-Q water (18.2 MΩ.cm). Apparatus and Instruments: For voltammetric studies, a glassy carbon disk (GCE) of area: 0.07 cm2 (BAS Inc.) & Toray carbon (TC) (Alfa aesar, used active area: 1.00 cm2) are used as a working electrodes; a platinum foil is used as an auxiliary electrode; Ag/AgCl (3 M NaCl) used as a reference electrode (BAS Inc.) and the potential values are referred to this reference electrode unless otherwise noted. Cyclic voltammetry and cronoamperometry experiments were carried out using a potentiostat (Model 100 B Bioanalytical Systems Inc) at ambient temperature (25ᵒC). Horntype sonic vibra cell instrument (Sonic & Materials-USA) was used for generating ultrasound in the electrolytic bath (13 mm diameter titanium horn and 20 kHz ultrasonic outputs). The surface morphology of deposited AuPt particle were characterized using Field emission scanning electron microscopy (Quanta 250 FEG) with the beam voltage of 10 kV. Powder XRD characterization was permormed to study crystallographic information of deposited AuPt particles on carbon substrate by Bruker (D8 Advance) using Cu(kα) ratio (λ= 1.5406 Aº) in the 2θ range of 30ᵒ to 90ᵒ with 2ᵒ/min scan speed. Laser Ablation Inductively Coupled Plasma- Mass spectrometry (LA-ICP-MS, Model:-Thermo Scientific Xseries 2) analysis was used to find out the mass of deposited Pt and Au condents on carbon substrate.


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Ultrasound assisted electrodeposition: Prior to the start of any electrochemical measurement, the working electrode was polished using a 5/0 emery paper and washed thoroughly in sonicated water. TC substrate used as such without any further pre-cleaning procedure. The purity of the GCE and TC surfaces were confirmed by checking their background response in 0.5 M H2SO4. We used our previous reported cell setup with some minor modifications37 such as changes in volume of the electrolyte solution (40 ml instead of 20 ml) and diameter of the horn type sonic wave emitter (13 mm dia horn instead of 3 mm dia) to reduce the power density of sonicwaves for direct USA-ED method of AuPt nanoparticles on carbon substrate,. The ultrasonic power was estimated using the formula: P= m C(water)∆T/t, where P is the ultrasound power, m is mass of the water, C(water) is the heat capacity of water, ∆T is temperature change during deposition, t is the sonication time. To calculate the power density of ultrasound, the power is normalized with ultrasound emitting horn area.38 Calculated ultrasound power density is 13 W/cm2. There are two important criteria to be maintained for obtaining reproducible results such as (i) position and distance of the sonic horn and the working electrode (distance 2 cm); (ii) total volume of the solution taken in the electrochemical cell (40 ml) to reduce ultrasound power density value. In this work, we have taken a mixture of 1 mM HAuCl4 + 1 mM H2PtCl6 with 0.1 M KNO3 solution for direct deposition of AuPt particles on TC and GCE surface using USA-ED method at applying constant potential 0 V vs Ag/AgCl for 200 seconds. The same experimental conditions were also followed for without sonic waves method as control experiments to study the effect sonic waves. For comparison, Pt alone deposited on carbon substrate using ED- and USA-ED- experimental conditions, i.e., with and without sonic waves. Deposited Pt and AuPt particles on carbon


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substrates were washed, dried and used for further physical characterizations and electrochemical studies Results and Discussion: Physical characterizations of electrodeposited AuPt nanostructure on TC substrate: Figure-1 exhibits the typical FE-SEM images of electrodeposited at nanostructured particles on the TC substrate under at constant overpotential of 0 V vs Ag/AgCl for 200 seconds using USAED and ED methods in keeping 1:1 molar ratio of HAuCl4 and H2PtCl6 containing 0.1M KNO3 solution.

Figure 1: FE-SEM images of deposited AuPt nanostructured particles on TC substrate under A) ED and B) USA-ED methods. (Insets: higher magnification of corresponding images) As can be seen from figure-1, the normal ED method of depsoisted AuPt nanostructured particles are loosely packed with low coverage of particles over the entire carbon fibers of TC substrate with average particle size ~70 nm (figure 1A). In higher magnification of corresponding image clearly show the AuPt bimetallic nanostructured particles are nearly interconnected hemispherical shape along with some places of agglomerated particles (inset figure-1A).


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However, in the case of USA-ED method of deposited AuPt particles are closely packed (higher particle density) with average particles size is ~100 nm over the entire carbon fibers of TC substrate (figure 1B) and corresponding higher magnification clearly show the formation of nanocauliflower like structure of AuPt (inset figure-1B). Both methods of deposited AuPt nanoparticles were analyzed by Energy dispersive X-ray analysis (EDAX) to find out the surface atomic composition of the deposited AuPt bimetallic system (see supporting information figure S1&S2) . It shows that ED-AuPt and USA-ED-AuPt containing 0.5:0.5 and 0.25:0.75 ratios of Au & Pt respectively. This result clearly reveals that USA-ED method produce Pt-richAuPt nanostructured particles. In general, a conventional ED method for metal nanoparticles deposition on conducting surface follows two kinds of mechanism such as (i) instantaneous growth mechanism, in which slow growth of nuclei on a small number of active sites, all activated at the same time (instantaneous) after imposition of potential and grow at the same rate; (ii) progressive growth mechanism, in which fast growth of nuclei on many active sites takes place, all activated during the course of electro-reduction.39 The standard redox potentials for Au and Pt can be expressed as follows40: − 0 − AuCl− 4 + 3e → Au + 4Cl − 0 − PtCl2− 6 + 4e → Pt + 6Cl

E o = 0.99V vs NHE − − − − − ሺ1ሻ E o = 0.74V vs NHE − − − − − ሺ2ሻ

Indeed the chosen applied deposition potential is to be more sufficient to reduce both Au3+ and Pt4+ metal ions simultaneously to proceeds progressive growth mechanism. Hence, rate of nucleation and growth of reduced Au and Pt particles are rapid and forms almost homogenous compositions of AuPt nanoparticles under normal ED method. This result is also consistent with the reported literature of polyamidoamine dendrimers-assisted electrodeposition of goldplatinum bimetallic nanoflowers.39 However, in the case of USA-ED method,

the same


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mechanism is operative, but at the same time acoustic cavitation phenomenon also occurs at the electrode-electrolyte interface.

Hence, the acoustic cavitation not only reduces the diffusion

layer thickness thereby enhancing the rate of deposition but also creating Pt-rich AuPt bimetallic surface on the carbon substrate. It was also reported that surface composition of Au0.5Pt0.5/C nanoparticle catalysts can be tuned by manipulating and controlling the heat treatment conditions to either selectively enrich the Au0.5Pt0.5/C nanoparticle catalyst surface with Pt or Au.20 Based on these studies, we strongly believe that ultrasound can create high local temperature at the electrode/electrolyte interface37,41 and thus control and induce generation of Pt-richAuPt bimetallic nanoparticles. In addition, for the comparision purpose, FE-SEM images of ED-Pt and USA-ED-Pt nanostructure surface also given in the supporting information figure S3. ED-Pt showed

loosly packed dendrimeric structure, whereas USA-ED-Pt showed densly packed

smaller particles with aggregated hemisphere structure on the TC substrate. It is well known in reported literature, bulk gold and platinum metals do not form a good solid solution (alloy) because of their large miscibility gap in their bulk phase diagram and the same metals can form alloys only at the nanometer scale at relatively low temperature as surface energy could be main driving force for the formation nanoalloys.42 Figure-2 shows the phase reflections of deposited AuPt bimetallic particles on TC substrate with and without sonic wave methods along with TC substrate alone for comparison. As can be seen in figure 2, (#) symbol indicates the phase reflections of deposited materials of Au & Pt which are assigned to (111), (200), (220) and (311) phase of face centered cubic (fcc) structure of metallic Au and Pt and the remaining phase reflections are corresponding to TC substrate. In normal ED method, XRD pattern corresponding to AuPt particles exhibit a single and symmetric diffraction peak at 39.01° for (111) plane which falls in intermediate diffraction peak between (111) plane of pure Au (2θ =


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38.15°) and Pt (2θ = 39.76°) reflections (see enlarged image of inset figure-2) indicates single phase (homogenous) alloy AuPt nanoparticles. But in the case USA-ED method, diffraction pattern split in two unique peaks (see the enlarged image of inset figure-2) which could be assigned to Pt rich AuPt alloyed nanoparticles 42 rather than mixture of simple monometallic Au and Pt or core-shell structure of Au and Pt. While shift in diffraction pattern of Au (111) plane with relatively less intensity peak towards a higher 2θ (38.52°) value and peak of Pt (111) plane shifting towards lower 2θ value (39.27°) with respect to their individual original peak position of pure mono metallic Au and Pt confirm the deposited AuPt particles should be consist of mixed composition of AuPt alloy nanoparticles. This kind of similar XRD reflections were also observed in reported literature work on using Pt-on-Au nanoparticles as precursors, to produce

Inten sity / a.u .

Pt-richPtAu alloy nanostructures by controlled thermal treatments42 and supports our hypothesis.

Pt

Au

36

37

38

39

40

41

42

2θ

USA-ED-AuPt/TC

ED-AuPt/TC

TC(Torray carbon)

40

50

(2 20 )

(2 00 )

30

60

70

(3 11 (2 ) 22 )

Au - (Ref.No -00-004-0784) Pt - (Ref.No -00-004-0802)

(1 11 )

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


80

90

2θ


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Figure 2: A) XRD phase reflections of electrodeposited apt nanostructured particles with and without sonic wave methods. (Inset: expanded image of AuPt (111) phase of both methods. Cyclic voltammetry is one of the powerful techniques to characterize the surface structure of AuPt alloy nanostructured particles. Cyclic voltammogram were recorded in 0.5 M H2SO4 from 0.2 V to 1.45 V (vs Ag/AgCl) using both ED and USA-ED methods of AuPt alloy nanostructured particles on GCE at the scan rate of 0.1 V s-1 as shown in figure-3. Both methods shows visibly Pt surface of hydrogen adsorption/desorption (-0.2 to 0.1 V) with reduction of Pt oxides (0.6 to 0.1 V). However, reduction of Au oxide (1- 0.6 V) were also visibly seen along with reduction of Pt oxide only in ED method proves the formation of equal composition 1:1 ratio of AuPt alloy nanostructured surface (figure-3A). But, in the case of USA-ED method show high PtO reduction peak than ED method of PtO reduction with Au oxide were barely visible and which can be attributed to the Pt rich AuPt nanostructured suface (figure-3B).

Figure 3: Cyclic voltammogram of electrodeposited AuPt alloy nanostructured surface on GCE substrate using A) ED and B) USA-ED conditons in 0.5 M H2SO4 from 0.2 V to 1.45 V at the scan rate 0.1 V s-1.


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Thus, these obtained cyclic voltammetric results also consistent with FE-SEM, XRD and EDAX results and also good agreement with reported literature for Pt-rich AuPt alloy nanostructured surface on carbon substrate.42 Further, we calculated active electrochemical surface area (ECSA) of AuPt bimetallic nanoparticles by integrating charge associated with hydrogen adsorption/desorption region of Pt surface from the obtained cyclic voltammetric response of both methods of AuPt alloy nanoparticles. Then, the charge is calculated by the following equation: QH =

ሺQ total − QDC ሻ − − − − − ሺ3ሻ 2

ECSA =

QH − − − − − ሺ4ሻ 210 × Pt ݉ܽ‫ݏݏ‬

where, Qtotal is the absolute value of the charge corresponding to the anodic and cathodic region of hydrogen adsorption/desorption, and QDC is the double layer capacitive charge in terms of µC. By assuming a charge of 210 µC cm-2 for the adsorption of a monolayer of hydrogen 43, the electrochemical active surface area (ECSA) was calculated using the Eq. 4 and the obtained ECSA of Pt in ED-AuPt and USA-ED-AuPt –nanostructured surface is 16.9 m2 g-1 (Pt - 0.29 µg) and 13.7 m2 g-1 (Pt – 10.47 µg) respectively. Methanol oxidation: In order to evaluate the catalytic activity of electrodeposited stabilizer-free Pt and AuPt alloy nanostructured surfaces of ED and USA-ED methods, we performed the cyclic voltammetry experiment using 0.5 M H2SO4 solution with or without 0.5 M methanol in the switching potential scan start from -0.2 to 1.15 V at the scan rate 0.1 V s-1 as shown in figure-4 (A-D). As can be seen in figure-4A, there is no obvious methanol oxidation peak current appear in the presence of 0.5 M methanol solution for ED method of AuPt alloy nanostructured surface even


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noticeable Pt surface. It is well know that atleast three adjusent Pt sites should be properly placed in crystallographic arrangement of bimetallic structures which are activates the chemisorption of methanol during methanol oxidation.18 Hence, the present case of ED condtion promotes the effective nucleation of AuPt alloy with atomic level mixing during co-depsotion process and may not able to produce the probability of finding three neighbouring Pt atoms on the surface on AuPt alloy nanostructured surface and resulting in poorer methanol oxidation. On the other hand, in the case of USA-ED method of Pt-rich AuPt alloy nanostructure surface show the characteristic peak response of methanol oxidation in forward scan at 0.872 V and reverse scan peak at 0.477 V (figure-4B). The forward scan peak (If) is attributed to oxidative removal of adsorbed/dehydrogenated methanol fragmentation and during this process, CO, CO2, HCOO are formed and the CO molecule might be adsorbed and poisoned electrocatalyst surface and reverse/backward scan peak (Ib) is due to re-oxidation of CO and other adsorbed intermediate species. The obtained methanol oxidation current value is normalized with the real area of Pt. The observed high peak current density (1.88 mA cm-2) of methanol oxidation clearly indicates necessity of enrichment of Pt atoms in AuPt alloy nanostructured surface (PtArea - 1.44 cm2). To evaluate the contribution of individual components of Au and Pt in AuPt alloy nanostructured surface for methanol oxidation, the results were also compared with ED-Pt (figure 4C) and USAED-Pt (figure 4D). ECSA values for ED-Pt and USA-ED-Pt is 58.9 m2 g-1 (Pt – 0.29 µg) and 16.6 m2 g-1 (Pt – 10.4 µg) respectively (by using eq.4). Figure 4E shows the methanol oxidation of specific acitivy (mA cm-2) and mass activity (A g-1) at various conditions of Pt and AuPt nanostructures surfaces. As can be seen from figure 4E, USA-ED condition of Pt-rich AuPt alloy nanostructure surface show higher mass activity as well as specific activity compared to other three conditions of deposited AuPt and Pt catalysts.


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ED-AuPt

A

0.5M H2SO4

0.9

Current density / mA cm-2

1.2

0.5M H2SO4 + 0.5M CH3OH

0.6 0.3 0.0 -0.3 -0.6 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

USA-ED-AuPt

2.0

0.5M H2SO4 + 0.5 M CH3OH

1.5 1.0 0.5 0.0

1.2

-0.2

ED-Pt

0.6

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Potential / V vs Ag/AgCl Current density / mA cm-2


0.9

B

0.5M H2SO4

Potential / V vs Ag/AgCl C

0.5 M H2SO4 0.5 M H2SO4 + 0.5 M CH3OH

0.3 0.0 -0.3 -0.6

0.60

USA-ED-Pt

D

0.5 M H2SO4

0.45

0.5 M H2SO4 + 0.5 M CH3OH

0.30 0.15 0.00 -0.15

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-0.2

Potential / V vs Ag/AgCl

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Potential / V vs Ag/AgCl

200

0.8 100 0.4

(a) (b) (c) (d)

2.0

-2

USA-ED-AuPt USA-ED-Pt ED-AuPt ED-Pt

1.5 a

1.0

F 0.6

b

0.3

c 0.0

d 0

0.5

50

100

Time / s

0.0 0

600

1200

1800

2400

3000

3600

Time / s

U

SA

-E D

-P t

uP t U SA -E D -A

ED -A

ED -P t

0 uP t

0.0

2.5

Current density / mA cm

1.2


Pt area -2

300

1.6

Mass activity / A/g Pt mass

400

E

Current density Mass activity

2.0

j / mA cm

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Figure 4: cyclic voltammogram of A) ED-AuPt, B) USA-ED-AuPt, C) ED-Pt, D) USA-ED-Pt nanostructured surface on GCE substrate in presence/absence of 0.5 M methanol containing 0.5 M H2SO4 solution at the scan rate of 0.1 V s-1. E) Barchart for the comparision of mass- and specific- activitites of deposited Pt and AuPt nanostructrured surface at various conditons . F) i-t curve respone for a) USA-ED-AuPt, b) USA-ED-Pt, c) ED-AuPt and d) ED-Pt in the mixture of 0.5 M H2SO4 + 0.5 M methanol at constant potential of 0.675 V.


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The electrochemical stability of USA-ED condition of Pt- rich AuPt alloy nanostructured surface for methanol oxidation were investigated by using chronoamperometric (CA) experiment. Figure 4F shows the comparison of CA of Pt- rich AuPt alloy nanostructured surface with USA-ED-Pt, ED-AuPt and ED-Pt at keeping constant potential of 0.675 V for 3600 s in the mixture of 0.5 M H2SO4 + 0.5 M Methanol solutions. USA-ED-Pt, ED-AuPt and ED-Pt surface suddenly decreases the steady state current density to < 0.02 mA cm-2 within 1500s because of poisoning of intermediate species, whereas in the case of USA-ED method of Pt-rich AuPt alloy nanostructured surfaces retains its steady state current density up to 0.350 mA cm-2 which is 17 times greater than other three surface structure. This may be because of ensemble effect is operative in Pt- rich AuPt alloy nanostructured surface as well as synergistic effect of AuPt should also play a crucial role. From these CAs experiments the turnover number (TON) also calculated to understand the difference of electrocatalytic activity and stability of electrocatalyst. From the steady state current density, the TON is calculated using this equation- 5 as follows44 TON ൬

molecules i × 6.02 × 1023 ൰= − − − − − ሺ5ሻ sites nF × 1.28 × 1015

Where, i is the steady state current density after 1500s, n is the number of electrons produced by the oxidation of methanol (n=6), F is the Faraday constant and the density of the topmost atoms of ideal Pt (100) surface is about 1.28x1015. The calculated value of TON for methanol oxidation is 0.286, 0.014, 0.003 and 0.004 s-1 for USA-ED-AuPt, USA-ED-Pt, ED-AuPt and EDPt respectively , which also support that Pt-rich AuPt alloy nanostructured surface catalytically more active and stable electrocatalyst than that of other conditions of ED-AuPt, ED-Pt and USA-ED-Pt electrocatalysts. Further, we compared catalytic effect of methano oxidation with commercially available E-TEK sample of 30% wt loading of Pt/C (see supporting information figure S4) electrocatalyst.


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The Pt/C (E-TEK) electrocatalyst is chemically modified on glassy carbon electrode with the known procedure.24 ECSA of Pt/C nanoparticles is calculated (60.1 m2 g-1 / PtArea 1.86 cm2) using in eqn-4 and this value is more higher (due to very smaller particles of size ~ 2 nm ) than that Pt-rich AuPt alloy nanoparticles (13.7 m2 g-1 / PtArea 1.44 cm2). As can be seen from supporting information figure-S4, methanol oxidation of both forward peak current (If = 0.66 mA cm-2) and reverse peak current (Ib = 0.68 mA cm-2) are almost equal for E-TEK sample of Pt/C. It is worthy to mention that calculated current density (specific activity, current value is normalized with real area of Pt/C) of methanol oxidation for Pt/C (E-TEK) catalyst show almost 2.5 times lower value than that of Pt- richAuPt alloyed bimetallic nanostructured. It is known in literature that the ratio of If/Ib value could be used to evaluate the catalyst tolerance to the carbonaceous species accumulation and pure Pt catalyst is readily poisoned by CO like intermediate and suppressed efficiency of electrocatalyst. The calculated If/Ib ratio value of Ptrich AuPt alloy nanostructured surface and Pt/C (E-TEK) nanoparticle surface are 1.79 and 0.96 respectively, and reveal that Pt- rich AuPt alloy nanostructured particle film could be more efficient in reduceing the adsorbed carbonaceous species (less poisoning) with higher catalytic nature for methanol oxidation than Pt/C (E-TEK). Mass activity changes (i-t curve) of USAED-AuPt were compared with Pt/C (E.TEK) during the methanol oxidation at constant potential 0.675 V (see the supporting information figure S5). Initially Pt/C shows higher mass activity because of high Pt-ECSA, but within 200s it loses it mass activity and attains steady state current density of 13 A g-1 at 1500s. However, USA-ED condition of Pt- rich AuPt alloy show almost 4 time higher (Mass activity 51 A g-1) at 1500s. These observed results clearly confirms that the Pt- rich AuPt alloy nanostructured surface has highly catalytic that promote the methonal oxidation reaction.


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Conclusions: We have demonstrated for the first time, densely packed stabilizer/surfactant- free Pt- rich AuPt alloy nanostructures were successfully fabricated on carbon substrate via USA-ED and showed high catalytic activity of methanol oxidation. On the other hand, in normal ED yields simple homogeneous AuPt bimetallic nanosturcure on carbon substrate which is very poor in methanol oxidation. Moreover, the Pt- rich AuPt alloy nanostructure surface showed improved higher current desnity as well as long stability performance for methanol oxidation than commercial Pt/C (E-TEK) catalyst. The obtained results confirmed the synergetic effects play important role in enhancement of higher electroactivity towards methanol oxidation in addition to the electronic and ensemble effect. This kind of unique electrocatalytic effect of Pt- rich AuPt alloy nanostructures on carbon substrate is more appropriate ideal anode electrocatalyst for real applied DMFCs applications.

AUTHOR INFORMATION Corresponding Author * S.Senthil Kumar ([email protected] ACKNOWLEDGMENT SB thanks C.S.I.R., New Delhi for the award of Senior Research Fellowship. SSK thanks the Department of Science & Technology, India for financial assistance through SERC Fast Track Scheme No.SR/FT/CS-041/2008. Authors thank Dr. KLN. Phani, CECRI, Karaikudi for usefull discussion. ASSOCIATED CONTENT


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Supporting Information. Additional information, EDAX (Figure-S1-S2) and FE-SEM (Figure-S3), CV and i-t cures (Figure-S4 to S5) imges and figures. This material is available free of charge via the Internet at http://pubs.acs.org. ABBREVIATIONS TC , Toray carbon ; GCE, Glassy carbon electrode; USA-ED, Ultrasonic wave assisted – Electrodepostion. REFERENCES (1)

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Table of Contents:


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Impact of Ultrasonic Waves in Direct Electrodeposition of

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