Impurities Contributing to Catalysis: Enhanced Electro-Oxidation of

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Impurities Contributing to Catalysis: Enhanced Electro-Oxidation of Formic Acid at Pt/GC Electrodes in the Presence of Vinyl Acetate Mohamed S. El-Deab, Ahmad M. Mohammad, Gumaa A. El-Nagar, and Bahgat E El-Anadouli J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp507240r • Publication Date (Web): 05 Sep 2014 Downloaded from http://pubs.acs.org on September 12, 2014

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The Journal of Physical Chemistry

Impurities Contributing to Catalysis: Enhanced Electro-Oxidation of Formic Acid at Pt/GC Electrodes in the Presence of Vinyl Acetate

Mohamed S. El-Deaba,b,*, Ahmad M. Mohammada,b,*, Gumaa A. El-Nagara, and Bahgat E. El-Anadoulia

a b

Department of Chemistry, Faculty of Science, Cairo University, Cairo 12613, Egypt

Department of Chemical Engineering, Faculty of Engineering, the British University in Egypt, Cairo 11837, Egypt

*Corresponding author E-mail addresses: [email protected] (M. S. El-Deab); [email protected] (A. Mohammad)

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ACS Paragon Plus Environment


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Abstract Here we demonstrate a remarkable enhancement of the direct formic acid electro-oxidation

(FAO) to CO2 (dehydrogenation pathway, Ipd) at Pt nanoparticle modified GC (nano-Pt/GC) electrodes, in the presence of minute amount (∼ ppm) of vinyl acetate (VA), while suppressing the dehydration pathway (producing the poisoning intermediate CO, Ipind). An excellent electrocatalytic activity of the nano-Pt/GC catalyst for FAO was found in the presence of VA (a possible contaminant) as revealed by comparing the intensity of the corresponding two oxidation peaks Ipd and

Ipind observed, respectively, at 0.25 and 0.75 V vs. Ag/AgCl/KCl(sat). The degree of enhancement of Ipd depends on the surface coverage (θ) of VA at Pt nanoparticles. VA is believed to adsorb and consequently interrupt the surface contiguity of the Pt active sites favorable for CO poisoning. XPS measurements revealed a change in the electronic properties of Pt in presence of VA in such a way that favors the charge transfer during the FAO and/or impede/weaken the adsorption of the poisoning CO. Interestingly, VA (in ppm concentration) improves the electrode’s stability during FAO and also its catalytic tolerance against poisoning with chloride. Several indices were developed to measure the catalytic activity of the electrode in absence and presence of VA, and several techniques as FE-SEM, XRD, EDX, and XPS were employed in the revelation of the electrode’s morphology, crystal structure, composition, and binding energy.

Keywords: Organic impurities; CO tolerance; Electrocatalysis; Direct formic acid fuel cell; Pt nanoparticles. 2


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Introduction The expected rapid depletion in fossil fuels and the environmental risks and emissions associated with their extraction, drilling, and processing have renewed interest in renewable energy resources. Of these, fuel cells (FCs) represent clean, noiseless, safe energy source due to their potentialities as a reliable power source to a variety of electrical devices in a wide range of portable, stationary and transport applications1,2. Basically, a FC is a device converting the chemical energy from a fuel (a chemical, e.g., hydrogen gas, small organic molecules) into electricity via a chemical reaction with an oxidizing agent (typically oxygen from air)1,2. The commercialization of FCs attracted the attention of industrial developers to improve the performance of their products and to abide with the global regulations of greenness. For example, Toyota Motor Corp is about to launch a hydrogen-powered car to the market, which emits only water vapor to the atmosphere and can run five times longer than battery electric cars3. However, the complications allied with the hydrogen transportation and storage (hydrogen is highly flammable and can explode easily) has derived the replacement of hydrogen fuel with liquid fuels as formic acid (FA)4. The merits of FA, as a fuel, are not limited to its ease handling, transporting and storing but expanded to its abundance, low price, and eco-friendliness. Two extra advantages for FA when purged as a fuel in what is called “direct formic acid fuel cells (DFAFCs)” is the appreciated power density of the cell and the low crossover of FA through the Nafion-based membrane that separates the anodic and cathodic compartments of FCs2,4. However, the essential defect of DFAFCs originates from the poisoning of the anodic catalyst (typically Pt-based materials) with carbon monoxide (CO) in-situ generated due to the nonfaradaic dissociation of FA at the surface of the catalyst2,5. This eventually blocks a huge number of the available active site for FA electro-oxidation (FAO) leading to a severe deterioration in the 3



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catalytic activity and performance of DFAFCs6-8. The electrocatalytic activity of Pt-based catalysis against the CO poisoning has been successfully improved by a simple modification with gold nanoparticles (AuNPs) and/or transition metal oxide nano-materials (nano-MOx: M = Ni, Mn, Ta)2,8-12. The modification with AuNPs was intended to interrupt the surface contiguity of the Pt surface sites, which is required for the adsorption of CO. On the other hand, nano-MOx assisted in the rapid oxidation of CO at low potential2,8,9,13. Another important issue influencing the catalytic efficiency of FCs, in general, and DFAFCs, in particular, is the catalyst’ poisoning with inevitable contaminations. This includes the contamination with nitrogen- and/or sulfur- containing compounds (entering the cell with air), halides (where most of the high-surface area fuel cell catalysts are often synthesized from halidecontaining educts), and hydrocarbons (released from piping, blowers, pumps and heat exchangers)7,13-17. These contaminants can indeed adsorb at the surface of the catalyst and can further affect its binding energy with key intermediates. This ultimately influences the reaction kinetics18. For instance, the presence of a minute amount of acrylonitrile (AcN) could effectively reduce the catalytic activity of Pt catalyst towards the oxygen reduction reaction (ORR, the typical cathodic reaction for all FCs)16-17. In the same context, a significant loss in the catalytic performance of FCs has been observed in presence of vinyl acetate (VA), which represents a typical example for airborne hydrocarbon contaminants17,19. However, the conventional negative impact (as pollution, deactivation, and poisoning) that always delivered onto brain when just hearing the word “impurity” may not actually apply for all contaminations and reactions. For instance, in presence of AcN an outstanding enhancement in the catalytic activity (3 orders of magnitudes) of FAO at Pt nanoparticles is observed7.

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Herein, we investigate the influence of contaminating the anodic compartment with VA during FA oxidation at Pt nanoparticle modified GC (nano-Pt/GC) electrode. The catalytic activity of the electrode and its tolerance against halides poisoning is addressed in the presence of minute amounts of VA (∼ ppm). The influence of VA concentration on the catalytic activity of FAO is discussed in details with the help of electrochemical and XPS measurements. Experimental Glassy carbon (GC, d = 3.0 mm) and Pt (d = 1.6 mm) electrodes (from ALS-Japan) were used as working electrodes after polishing with emery papers and aqueous slurries of alumina powder on a microcloth. Before measurements, the electrodes were further cleaned electrochemically as described previously8. A spiral Pt wire and Ag/AgCl/KCl (sat) were used as the counter and reference electrodes, respectively. All potentials will next appear in correspondence to this reference. The electrodeposition of nano-Pt on the bare GC electrode was done in 0.1M H2SO4 containing 1.0 mM H2[PtCl6] solution using a potential step electrolysis from 1 to 0.1 V for 300 s. This typically ends with ca. 67 µg Pt loading (0.089 g/cm-2), as estimated from the amount of charge passed during the Pt deposition. The electrochemical measurements were performed at room temperature (25 ± 1oC) in a conventional two-compartment three-electrode glass cell using an EG&G potentiostat (model 273A) operated with Echem 270 software. The electrocatalytic activity of the nano-Pt/GC electrode towards FAO was examined by measuring cyclic voltammograms (CVs) in 0.5 M H2SO4 containing 0.3 M FA. The influence of VA (purchased from Merck) was investigated by adding different amounts to the electrolyte before measurements. 5



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In order to evaluate the catalytic role of VA towards FAO, an experiment is designed to allow the adsorption of the in-situ generated CO (through the non-faradaic dissociation of FA) onto nano-Pt/GC electrode. This was done by immersed the nano-Pt/GC electrode in 0.5 M FA at open circuit potential for 10 min in presence/absence of VA. The adsorption of CO at the nano-Pt/GC electrode was then verified by measuring the oxidative stripping by linear sweep voltammetry (LSV) in 0.5 M H2SO4. The real surface area of the nano-Pt was calculated by monitoring the charge associated with the hydrogen adsorption/desorption peaks obtained in the typical characteristic CV measured in 0.5 M H2SO4. Current densities were calculated on the basis of the geometric surface area of the working electrode (0.07 cm2). A field emission scanning electron microscope (FE-SEM, QUANTA FEG 250) coupled with an energy dispersive X-ray spectrometer (EDX) unit and the X-ray diffraction, XRD, (PANalytical, X’Pert PRO) operated with Cu target (λ=1.54Å) were employed to evaluate, respectively, the electrode’s morphology, composition, and crystal structure. X-ray photoelectron spectroscopy (XPS) operating with Al Kα radiation assisted in revealing the influence of VA on the binding energy of the underlying Pt nanoparticles. The binding energies derived from XPS measurements have been calibrated to the C1s spectrum (at 284.5 eV) of the carbon support. Results and discussion Materials characterization Investigation of the surface morphology, composition and crystal structure of the nanoPt/GC electrode was sought at the beginning right after assembling. The FE-SEM inspection indicated the deposition of a uniform array of Pt nanoparticles of spherical shape with an average 6


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diameter of ca. 25 nm covering homogeneously the entire GC surface (see Fig. 1A). The EDX and XRD analyses have further confirmed the deposition of nano-Pt and revealed its crystallographic orientation. Figure 1B displays EDX spectrum of nano-Pt/GC electrode and reflects a surface composition of ca. 55% Pt. On the other hand, XRD pattern of nano-Pt/GC electrode (Fig. 1C) indicated nano-Pt are found as a Face-Centered Cubic (FCC) lattice structure, as revealed from the appearance of the prominent Pt crystallite planes (111), (200), (220) and (311)8. Electrochemical characterization The nano-Pt/GC electrode was characterized electrochemically by measuring CVs in 0.5 M H2SO4 in absence and presence of different concentrations of VA. In absence of VA, a typical characteristic CV for a polycrystalline Pt substrate appeared with a broad oxidation peak for the Pt surface commenced at ca. 0.65 V and extended up to 1.25 V (Fig. 2). A corresponding reduction peak for PtO was observed at ca. 0.45 V. In addition, the hydrogen adsorption/desorption (Hads/des) peaks appeared in the potential range from −0.2 to 0.2 V. It is worth mentioning that there are two types of electro-adsorbed H species; the under-potential deposited H (HUPD) and the over-potential deposited H (HOPD)20. Typically, the HUPD appears at a higher positive potential (ca. 30 mV) than ‫ܧ‬ு଴ శ/ுమ (reversible potential of hydrogen evolution = 0.00 V) and corresponds to the weakly adsorbed hydrogen, while the HOPD occurs at a lower potential (−150 mV) than ‫ܧ‬ு଴ శ/ுమ and corresponds to the strongly adsorbed hydrogen. In presence of 5 ppm VA, the same characteristic peak couples (Pt/PtO and Hads/des) continued appearance but with lower intensities (Fig. 2). The decrease of peak intensities is expected with blocking of some Pt active sites by VA adsorption, which in turns, decreases the 7



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(A)

Fig. 1-A

8


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600

(B)

Pt

500

400

Intensity

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


300

200

C

O

100

Pt

Pt

Pt

Pt

0 2

4

6

8

10

Kev

Fig. 1-B

9


12

14


1000

C (002)

(C)

XRD Pt/GC

800

600

Counts

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

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Pt (200)

400 Pt (111) Pt (311)

200 Pt (220)

0 20

40

60

Position (2 θo) Fig. 1-C

Figure 1: (A) FE-SEM image, (B) EDX and (C) XRD pattern of nano-Pt/GC electrode.

10


80

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200

I / µA

0

-200

blank 5 ppm 20 ppm 50 ppm 70 ppm

-400

-600 -200

400

-100

HUPD/HOPD Peaks

0

100

200

VA oxidation

E / mV vs. Ag/AgCl/KCl (sat.)

200

0

I / µA

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


-200

-400

-600

-200

Reductive desorption of VA and/or start of poly VA

0

200

blank 5 ppm 20 ppm 50ppm 70ppm

Pt-Oxide reduction

400

600

800

1000

1200

E / mV vs. Ag/AgCl/KCl (sat.) Figure 2: CVs of the nano-Pt/GC electrode in 0.5 M H2SO4 solutions containing various concentration of VA (0, 5, 20, 50 and 70 ppm). N.B. each CV corresponds to the 1st potential scan obtained at a freshly prepared nano-Pt/GC electrode. Potential scan rate: 0.1 V s−1. Inset corresponds to CVs for the hydrogen adsorption-desorption at nano-Pt/GC electrode in 0.5 M H2SO4 solutions containing the same concentrations of VA. Herein, the potential was scanned from 0.2 V toward the cathodic direction

(−0.2 V). 11



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accessible active electrochemical surface area (ECSA) of Pt. Typically, in absence of VA, the ECSA of nano-Pt catalyst was ca. 0.12 cm2 as estimated from the Hdes peak (using a reported value of 210 µC cm−2)8,9,21 . This value of ECSA decreased significantly in presence of VA (see Table 1). The surface coverage (θ) of VA on nano-Pt/GC catalyst has been estimated from the Hdes peaks. To achieve this, the potential was scanned from 0.2 V (where there is no faradaic current) toward the cathodic direction (e.g., −0.2 V). A systematic increase in θ associated with a decrease in ECSA was observed with increasing the VA concentration (see inset of Figure 2 and Table 1). Interestingly, upon increasing the VA concentration (Fig. 2), the peak heights of PtO formation (in the potential region of 0.9−1.2 V) and HOPD (adsorption) were systematically increased while those of PtO reduction, HUPD (adsorption and desorption) and HOPD (desorption) were decreased. Once again, the decrease in peak intensities is expected as a result of VA adsorption. However, the increase in peak currents is associated with other electrochemical reactions that overlapped with the above-assigned reactions to display the overall observed current. The increase of the PtO formation peak can be understood in view of the possible oxidation of the adsorbed VA in the same potential domain of PtO formation17. However, the increase in HOPD adsorption peak is a bit complicated as it was not associated with a similar increase in HOPD desorption peak. One may presume a reductive desorption of VA at this potential, initial steps of electropolymerization of VA and/or induced enhancement of hydrogen spillover by VA. These assumptions require more experiments and characterizations to be disclosed.

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Electrocatalysis of FAO The electrocatalytic activity measurements were pursued in 0.5 M H2SO4 containing 0.3 M FA. At nano-Pt/GC (Fig. 3A− solid line), two oxidation peaks were observed in the forward scan at ca. 0.2 and 0.75 V, respectively. The first peak corresponds to the direct FAO to CO2 with a peak current Ipd (dehydrogenation pathway), while the second peak at ∼0.75 V is assigned to the oxidation of the poisonous adsorbed CO (COads− produced from the non-faradaic dissociation of FA) to CO2 (with a peak current Ipind)2,8,22-25. Actually, at ca. 0.5 V, the Pt surface becomes hydroxylated, which enhances much the oxidative removal of COads. Figure 3A indicates further the lower intensity of Ipd in comparison to Ipind, which infers about the extensive poisoning of the Pt sites by COads under the prevailing conditions. It is worth mentioning here that the Ipd/Ipind ratio can be used to probe the level of Pt surface poisoning by COads and/or the catalytic activity of the electrode. In the reverse “cathodic-going” sweep, most of COads has been oxidized and released from the Pt surface; hence, the FAO can easily proceed via the direct dehydrogenation route. That is why the intensity of the backward peak current (Ib) increased intensively. The Ipd/Ib ratio can further be calculated to monitor the degree of tolerance against poisoning with CO. The ideal case for this ratio is to approach unity, i.e., no poisoning occurred, as in case of FAO on Pd substrates. It is noteworthy that the catalytic activity of the unmodified Pt is controlled by a high surface coverage of COads in the anodic sweep, while it is controlled by a high surface coverage of OHad during the reverse scan10. Surprisingly, in presence of 10 ppm VA (Fig. 3A − dashed line), the FAO at nano-Pt/GC electrode exhibited an interesting enhancement in Ipd at the expense of Ipind, concurrently, with a slight negative shift in the onset potential of the direct FAO. The Ipd/Ipind ratio increased consequently from 0.45 in absence to 5.6 in presence of 10 ppm VA. The Ipd/Ib ratio increased as well from 0.17 to 0.65, which infers a 13



5

Ib

(A)

4

I / mA cm -2

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3

Ipind

2

Ip d

1

0 -200

0

200

400

600

E / mV vs. Ag/AgCl/KCl (sat.)

Fig. 3-A

14


800

1000

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2.5

e

(B)

d

2.0

I / mA cm -2

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f 1.5

c 1.0

b a 0.5

0.0 -200

0

200

400

600

800

E / mV vs. Ag/AgCl/KCl (sat.) Fig. 3-B

Figure 3: (A) CVs for FAO at nano-Pt/GC electrode in 0.5 M H2SO4 solutions containing 0.0 (solid line) and 10 ppm (dashed line) VA. (B) The LSV of the same electrode in 0.5 M H2SO4 solutions containing (a) 0.0, (b) 1.0, (c) 5.0, (d) 20.0, (e) 50.0 and (f) 70 ppm VA. Potential scan rate of 0.1 Vs-1. 15



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better tolerance for CO poisoning. Interestingly, with increasing the VA concentration (or surface coverage, θ) the Ipd continued increasing while Ipind decreased up to 50 ppm VA (θ ≅ 69%), see Fig. 3B (curves a−e) and Fig.4A. The relative Ipd/Ipind and Ipd/Ib ratios increased correspondingly as well (see Table 1) up to the same VA concentration (50 ppm), at which Ipd/Ipind and Ipd/Ib ratios equaled 87.0 and 1.0, respectively. The increase in Ipd/Ipind indicates an improvement in the catalytic activity of the nano-Pt/GC electrode toward FAO, which presumably originates from retarding the CO adsorption and thus favoring the direct oxidation pathway. Moreover, the increase in the Ipd/Ib ratio infers the efficient FAO to CO2 and high tolerance of the electrode against poisoning with accumulated COads. It is really interesting for the Ipd/Ib ratio to equal 1.0 (similarly to the ideal behavior of Pd) at the nano-Pt/GC in presence of a minute amount (ppm) of VA. This actually stimulates a high catalytic activity for FAO in the forward and backward potential sweep, a feature that is always desirable in fuel cells manufacturing. A further increase in VA concentration induced a complete disappearance of the indirect peak of FAO and pertained the Ipd/Ib ratio almost at unity (Table 1), which indicates that FAO was shifted exclusively towards the direct pathway. Nevertheless, a higher concentration of VA would consume a significant part of the Pt active sites on which FA adsorb and consequently, the reaction rate and Ipd would get lowered (see Fig. 3B curve f and Fig. 4A).

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Table 1: Effect of vinyl acetate concentration on FA oxidation at Pt/GC electrode

a

[VA]/ppm

Surface coverage a θ/%

Real surface area (S)b/cm2

Ipd/Ib

Onset potential / mV

Ipd/Ipind

0

0.0

0.1228

0.2

−115

0.5

5

18

0.1007

0.6

−274

3.4

10

26

0.0909

0.7

−284

5.6

20

31

0.0847

0.8

−280

10

40

48

0.0639

0.9

−246

27

50

69

0.0381

1.0

−220

87

70

85

0.0184

1.1

−230

600

90

92

0.0098

1.0

−210

1000

100

98

0.0025

1.0

−200

1050

(θ = 1 – Spresence /Sabsence). Spresence and Sabsence refer to the real surface area of Pt/GC electrodes in

the presence of VA and its absence, respectively. b

As estimated from the charge consumed during hydrogen desorption peaks (inset of Fig. 2)

using a reported value of 210 µC cm−2.

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We have recently developed another important index, “oxidation capacity (OC)”, to probe the catalytic activity, which normalizes the charge consumed in the direct oxidation peak (Qd) to the number of available Pt active sites7;

OC =

Qd number of Pt active sites

(1)

The number of Pt active sites was estimated assuming a surface concentration of Pt atoms of ca. 1.5 ×1015 atoms cm−2. The increase in OC of the electrode is equivalent to an increase of the electrode’s tolerance against CO poisoning and catalytic activity towards FAO. It is worth mentioning that Qd (as well as OC) depends on the number of available (unoccupied neither by CO nor VA) Pt active sites. Unexpectedly, by increasing θ (i.e., the number of free Pt active sites decreased), the OC of electrode towards FAO increased exponentially (see Fig. 4B. This was really surprising as a decrease in the Pt active sites would typically induce a decrease in Qd and the catalytic activity towards FAO. This may attract attention to a surface reconstruction of the Pt surface sites with θ. It is presumed that the adsorption of the poisoning CO at the Pt surface requires the existence of three adjacent Pt surface sites. Any interruption in this arrangement would act against the CO adsorption and the catalyst’s poisoning. It is presumed that the adsorption of VA on nano-Pt/GC electrode hinders the “non-faradaic” dissociation of FA (producing the poisoning CO) and thus favors the direct oxidation of FA to CO2. The increase in θ means a deterioration of the surface contiguity of the Pt sites necessary for the CO poisoning. Therefore, more active sites of Pt are available for direct FAO, thus increasing Qd. This is not applied systematically for very high values of θ, where most of the Pt active sites are occupied by VA and few Pt sites refrained for FAO. 18


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200

Ipd

(A)

Ipind 150

Current Peak/µA

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


100

50

0 0

20

40

60

Surface Coverage of VA (θ %)

Fig. 4-A

19


80

100


8

(B) 7

6

Log (O C)

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

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5

4

3

2 0

20

40

60

Surface Coverage of VA (θ%)

Fig. 4-B

20


80

100

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3.0

(C)

2.5

CO,FA molecules/ Pt atoms

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


FA Molecules

2.0

1.5

1.0

0.5 CO Molecules

0.0 0

10

20

30

40

50

60

Surface Coverage of VA (θ %) Fig. 4-C

Figure 4: The influence of VA surface coverage on (A) direct (Ipd) and indirect (Ipind) peak currents, (B) oxidation capacity, and (C) active Pt sites available for FA and CO adsorption.

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Alternatively, a change in the adsorption mode of FA onto the nano-Pt/GC electrode with θ may assist in understanding the data of Fig. 4B. In order to reveal the adsorption mode of FA onto the Pt surface at different θ, the number of adsorbed FA and CO molecules was estimated from the amount of charge consumed in the direct and indirect peaks, respectively, of FAO assuming 2-electron transfer processes (data are plotted in Fig. 4C. Interestingly, Fig. 4C reveals the existence of two adsorption modes for FA onto the Pt surface. At low θ (≤ 30%), every molecule of FA is bound to a single Pt site (the FA/Pt ratio is ~ 1) because of the availability of plenty active Pt sites for FAO. On the other hand, at high θ (≥ 30%), the steric hindrance of VA and the shortage in the available free Pt sites may stimulate another adsorption mode, in which two molecules of FA are likely adsorbed onto a single Pt site. A similar phenomenon has recently been observed for the adsorption of acrylonitrile onto nano-Pt/GC surfaces17. Note also the continuous reduction of the CO/Pt ratio (from 3 into less than 0.5) withθ, which highlights the remarkable improvement in the catalytic tolerance of the nano-Pt/GC electrode against the CO poisoning.

Origin of electrocatalytic enhancement The origin of the catalytic enhancement of FAO in presence of VA was next investigated. One of the experiments we developed to assist in this evaluation involved the adsorption of CO at the nano-Pt/GC surface at open circuit potential in absence and presence of VA and the amount of CO adsorbed was quantitatively estimated by the technique of linear sweep voltammetry (LSV). Figure 5A (a-d) depicts a reduction in the amount of CO adsorbed at the nano-Pt/GC surface with increasing the VA concentration, which agrees with previous results. The other interesting finding in Fig. 5A is the negative shift in the onset potential of CO 22


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3

a

(A)

b

2

I / mA cm -2

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


c

1 d

0

-200

0

200

400

600

E / mV vs. Ag/AgCl/KCl (sat.) Fig. 5-A

23


800

1000


35000

(B) 30000

25000

Counts (s)

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

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20000

15000

a

10000

b 5000 85

80

75

70

65

Binding Energy (ev) Fig. 5-B

Figure 5: (A) Oxidative CO stripping at the nano-Pt/GC electrode in presence of (a) 0, (b) 5, (c) 10 and (d) 30 ppm VA in 0.5 M H2SO4 at a scan rate of 50 mVs-1. CO was allowed to be adsorbed at the nano-Pt/GC electrode from 0.5 M FA for 10 min. (B) XPS spectra of Pt4f in (a) absence and (b) presence 100 ppm of VA. The vertical solid lines refer to the binding energy of elemental Pt. 24


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oxidation in presence of VA (Fig.5A (b-d)). This means that the CO oxidation is catalytically enhanced at lower potentials, which highlights the electronic effect in the catalytic enhancement of FAO. The electronic influence was likely originated from a modification in the electronic structure of Pt surface, which weakens the Pt–CO bonding. To verify this, XPS measurements was sought, and a positive shift in the binding energy of Pt 4f (0.16 eV) was observed for the nano-Pt/GC electrode in the presence of 100 ppm VA in comparison to that in absence of VA (Fig. 5B). This recommends a partial charge transfer from the Pt atoms to VA, which modifies the electronic structure of Pt surface in such a way that weakens the Pt−CO bonding.

Tolerance against halide’s poisoning One of the important issues challenging the commercialization of FCs is the electrolyte’s contamination with chloride ions, which often contaminate the catalysts during synthesis and water contained in the feed-streams of FCs. The competitive adsorption of Cl− ions can effectively lower the catalytic activity of nano-Pt/GC electrode towards FAO. We have then investigated qualitatively the influence of adding VA to FA on the catalytic tolerance of the nano-Pt/GC electrode against the Cl− poisoning. In a chronoamperometric measurement at 0.3 V the catalytic performance of the nano-Pt/GC electrode was measured in 0.5 M H2SO4 solution containing 0.3 M FA. Figure 6A indicates that at the early stage of the experiment a fast decay in the current, which inherently originate from the continuous CO poisoning. At a certain moment, a dosage of Cl− ions (50 ppm) was added, and a sharp decrease in the current was observed. Actually, Cl− ions can behave similarly as CO in poisoning the Pt surface. Further additions of Cl− dosages can reduce the catalytic activity but with different extents, as we are dealing now with different surface coverage and different concentration of active Pt sites. The same 25



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experiment was repeated in 0.5 M H2SO4 solutions containing 0.3 M FA in presence of 30 ppm VA. Interestingly, the catalytic tolerance of the nano-Pt/GC electrode towards Cl− poisoning was enhanced largely, as indicated from the smaller decay in currents upon adding the consecutive Cl− dosages (Fig. 6B). This was further verified by a third experiment, in which the Cl− dosage was first added followed VA (Fig. 6C). The addition of VA, in this case, could restore significantly the catalytic activity of the electrode and increase the buffering capacity against Cl− poisoning. However, the addition of VA after adsorption of CO resulted in more a pronounced sharp decrease in current (Fig. 6D), which means that VA cannot replace the strongly adsorbed CO and thus cannot restore the catalytic activity of the catalyst. Thus, one might argue that the observed enhancement resulted from addition of VA towards FAO (Fig. 3) does not originate from the oxidative removal of CO at low potential but mainly due to third body effect (interruption of Pt surface) which resulted in resist of CO formation (i.e., inhibit the indirect pathway).

26


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1.0

(A)

0.8

+50 ppm Cl¯ 0.6 I / mA cm -2

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


0.4

0.2

+50 ppm Cl¯ +50 ppm Cl¯

0.0 0.0

0.5

1.0

1.5 time (h)

Fig. 6-A

27


2.0

2.5


4

(B)

3

+50 ppm Cl¯ I /mA cm -2

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

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2


1


0 0.0

0.5

1.0

1.5 time (h)

Fig. 6-B

28


2.0

2.5

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(C)

1.0

+50 ppm Cl¯

0.8

I / m A cm -2

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


0.6

0.4 +30 ppm VA

0.2

0.0 0.0

0.1

0.2 time (h)

Fig. 6-C

29


0.3

0.4


1.0

(D)

0.8

I /mA cm -2

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

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0.6

+30 ppm VA

0.4

0.2

0.0 0.00

0.05

0.10

0.15

0.20

0.25

0.30

time (h) Fig. 6-D

Figure 6: Current transients (i-t) for nano-Pt/GC electrode measured at 0.3 V in (A) 0.5 M H2SO4 containing 0.3 M FA where several dosages of 50 ppm Cl− ions were added successively, (B) 0.5 M H2SO4 containing 0.3 M FA + 30 ppm VA where several dosages of 50 ppm Cl− ions were added successively, (C) 0.5 M H2SO4 containing 0.3 M FA and a dosage of 50 ppm Cl− ions followed by another of 30 ppm VA were added sequentially and (D) 0.5 M H2SO4 containing 0.3 M FA where 30 ppm VA were added after certain time as indicated in the figure.

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Stability issue The stability of the nano-Pt/GC electrode was investigated by monitoring the current transients (i-t) at a constant potential of 0.3 V for about 3.5 h of continuous electrolysis in absence (Fig. 7a) and presence of 50 ppm VA (Fig. 7b). At nano-Pt/GC electrode, in absence of VA (Fig. 7a), the current decreased to half its initial value after 20 min of electrolysis mainly due to the accumulation of the poisoning COads at the electrode surface. On the other hand, in presence of VA (Fig. 7b), the catalytic activity of the nano-Pt/GC electrode is maintained throughout the electrolysis, where the measured current remains effectively unchanged at a reasonably high value (ca. 3 mA cm-2). This again supports the positive role of VA in the improvement of the catalytic enhancement of the nano-Pt/GC electrode towards FAO.

31



b

3.0

2.5

I / mA cm -2

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

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2.0

1.5

1.0

0.5

a 0.0 0

1

2

3

Time (h)

Figure 7: Current transients (i-t) for nano-Pt/GC measured at 0.3 V in 0.5 M H2SO4 containing 0.3 M FA in (a) absence and (b) presence of 50 ppm VA.

32


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Conclusions The adsorption of a minute amount of VA onto the nano-Pt/GC electrode could effectively suppress the CO poisoning that always deteriorates the catalytic activity of FAO. Fortunately, the addition of VA (a little higher than 50 ppm) could exclusively shift the FAO towards the direct pathway. The catalytic enhancement is attributed to the interruption of the contiguity of Pt sites required for the CO adsorption and to the modification of the electronic properties of the Pt surface in a way that facilitate the charge transfer during the direct oxidation of FA. The presence of VA could also improve the catalytic tolerance of the nano-Pt/GC electrode against the Cl− poisoning. Moreover, the nano-Pt/GC electrode exhibited a better stability in presence rather than in absence of VA.

33



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References: (1) Wang, Y. ; Chen, K. S.; Mishler, J.; Cho, S. C.; Adroher, X. C. A Review of Polymer Electrolyte Membrane Fuel Cells: Technology, Applications, and Needs on Fundamental Research. Appl. Energy. 2011, 88, 981-1007. (2) El-Nagar, G. A.; Mohammad, A. M. Enhanced Electrocatalytic Activity and Stability of Platinum, Gold, and Nickel Oxide Nanoparticles-Based Ternary Catalyst for Formic Acid Electro-Oxidation. Int. J. Hydrogen Energy. 2014, 39, 11955-11962. (3) Norihiko, S. P. L.; Yoko K., Insight: In Green Car Race, Toyota Adds Muscle with Fuel-Cell Launch. http://www.reuters.com/article/2014/04/17/us-autos-hydrogen-toyota-motor-insightidUSBREA3F1UN20140417 2014. (4) Yu, X.; Pickup, P. G. Recent Advances in Direct Formic Acid Fuel Cells (DFAFC). J. Power Sources. 2008, 182, 124-132. (5) Ha, S.; Larsen, R.; Zhu, Y.; Masel, R. I. Direct Formic Acid Fuel Cells with 600 mA cm–2 at 0.4 V and 22 °C. Fuel Cells. 2004, 4, 337-343. (6) Mikołajczuk, A.; Borodzinski, A.; Kedzierzawski, P.; Stobinski, L.; Mierzwa, B.; Dziura, R. Deactivation of Carbon Supported Palladium Catalyst in Direct Formic Acid Fuel Cell. Appl. Surf. Sci. 2011, 257, 8211-8214. (7) El-Nagar, G. A.; Mohammad, A. M.; El-Deab, M. S.; Ohsaka, T.; El-Anadouli, B. E. Acrylonitrile-Contamination Induced Enhancement of Formic Acid Electro-Oxidation at Platinum Nanoparticles Modified Glassy Carbon Electrodes. J. Power Sources. 2014, 265, 57-61 (8) El-Nagar, G. A.; Mohammad, A. M.; El-Deab, M. S.; El-Anadouli, B. E. Electrocatalysis by Design: Enhanced Electrooxidation of Formic Acid at Platinum Nanoparticles–Nickel Oxide Nanoparticles Binary Catalysts. Electrochim. Acta. 2013, 94, 62-71. (9) El-Nagar, G. A.; Mohammad, A. M.; El-Deab, M. S.; El-Anadouli, B. E. Facilitated ElectroOxidation of Formic Acid at Nickel Oxide Nanoparticles Modified Pt Electrodes. J. Electrochem. Soc. 2012, 159, F249-F254 (10) El-Deab, M. S.; Kibler, L. A.; Kolb, D. M. Enhanced Electro-Oxidation of Formic Acid at Manganese Oxide Single Crystalline Nanorod-Modified Pt Electrodes. Electrochem. Commun. 2009, 11, 776-778. (11) Guo, Z.; Zhang, X.; Sun, H.; Dai, X.; Yang, Y.; Li, X.; Meng, T. Novel Honeycomb Nanosphere Au@Pt Bimetallic Nanostructure as a High Performance Electrocatalyst for Methanol and Formic Acid Oxidation. Electrochim. Acta. 2014, 134, 411-417 34


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(12) Masud, J.; Alam, M. T.; Miah, Md. R.; Okajima, T.; Ohsaka T. Enhanced Electrooxidation of Formic Acid at Ta2O5-Modified Pt Electrode. Electrochem. Commun. 2011, 13, 86-89 (13) El-Nagar, G. A.; Mohammad, A. M.; El-Deab, M. S.; El-Anadouli, B. E. ElectroOxidation of Formic Acid at Binary Platinum and Gold Nanoparticle-Modified Electrodes: Effect of Chloride Ions. Int. J. Electrochem. Sci. 2014, 9, 4523-4534 (14) Gould, B. D.; Baturina, O. A.; Swider-Lyons, K. E. Deactivation of Pt/VC Proton Exchange Membrane Fuel Cell Cathodes by SO2, H2S and COS. J. Power Sources. 2009, 188, 89-95. (15) Kobayashi, K.; Oono, Y.; Hori, M. Missions and Progressions of Impurities WG under NEDO's PEFC Residential CHP System Project. ECS Meeting Abst. 2012, MA2012-02, 12911291. (16) El-Deab, M. S.; Kitamura, F.; Ohsaka, T. Impact of Acrylonitrile Poisoning on Oxygen Reduction Reaction at Pt/C Catalysts. J. Power Sources. 2013, 229, 65-71. (17) El-Deab, M. S.; Kitamura, F.; Ohsaka, T. Poisoning Effect of Selected Hydrocarbon Impurities on the Catalytic Performance of Pt/C Catalysts Towards the Oxygen Reduction Reaction. J. Electrochem. Soc. 2013, 160, F651-F658. (18) Zhu, W.; Rosen, B. A.; Salehi-Khojin, A.; Masel, R. I. Monolayers of Choline Chloride Can Enhance Desired Electrochemical Reactions and Inhibit Undesirable Ones. Electrochim. Acta. 2013, 96, 18-22. (19) St-Pierre, J.; Angelo, M.; Zhai, Y. Focusing Research by Developing Performance Related Selection Criteria for PEMFC Contaminants Modeling. ECS Transactions. 2011, 41, 279-286. (20) Jerkiewicz, G. Electrochemical Hydrogen Adsorption and Absorption. Part 1: UnderPotential Deposition of Hydrogen. Electrocatal. 2010, 1, 179-199. (21) Trasatti, S.; Petrii, O. A. Real Surface Area Measurements in Electrochemistry. Pure Appl. Chem. 1991, 63, 711-734. (22) Habibi, B.; Delnavaz, N. Carbon–Ceramic Supported Bimetallic Pt–Ni Nanoparticles as an Electrocatalyst for Oxidation of Formic Acid. Int. J. Hydrogen Energy. 2011, 36, 9581-9590. (23) Hong, P.; Luo, F.; Liao, S.; Zeng, J. Effects of Pt/C, Pd/C and PdPt/C Anode Catalysts on the Performance and Stability of Air Breathing Direct Formic Acid Fuel Cells. Int. J. Hydrogen Energy. 2011, 36, 8518-8524.

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(24) Osawa, M.; Komatsu, K.-i.; Samjeske, G.; Uchida, T.; Ikeshoji, T.; Cuesta, A.; Gutiarrez, C. The Role of Bridge-Bonded Adsorbed Formate in the Electrocatalytic Oxidation of Formic Acid. Angew. Chem. Int. Ed. 2011, 50, 1159-1163. (25) Gao, W.; Mueller, J. E.; Jiang, Q.; Jacob, T. The Role of Co-Adsorbed CO and OH in the Electrooxidation of Formic Acid on Pt(111). Angew. Chem. Int. Ed. 2012, 51, 9448-9452.

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Table of Contents (TOC) Image

6

Ib 5

4

I / mA cm-2

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


3

2

Ipind a Ip d

1

b

0 -200

0

200

400

600

800

1000

1200

Ε / mV vs. Ag/AgCl/KCl (sat.) CVs for FAO at nano-Pt/GC electrode in 0.5 M H2SO4 solution containing 0.3 M FA in (a) the absence and (b) the presence of 50 ppm vinyl acetate (VA). Potential scan rate: 0.1 V s-1.

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Impurities Contributing to Catalysis: Enhanced Electro-Oxidation of

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