Glycerol electrooxidation on Pd, Pt and Au nanoparticles supported on carbon ceramic electrode in alkaline media

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 6 8 0 0 e1 6 8 0 9

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Glycerol electrooxidation on Pd, Pt and Au nanoparticles supported on carbon ceramic electrode in alkaline media Esmaeil Habibi*, Habib Razmi Electrochemistry Research Lab., Azarbaijan University of Tarbiat Moallem, P.O. Box: 53714-161, Tabriz, Iran

article info

abstract

Article history:

In the present study, electrooxidation of glycerol was investigated on Au, Pd and Pt

Received 8 July 2012

nanoparticles modified carbon ceramic electrode (CCE) by using different electrochemical

Received in revised form

techniques

19 August 2012

nopotentiometry (CP) and Electrochemical impedance spectroscopy (EIS). Scanning elec-

Accepted 26 August 2012

tron microscopy (SEM) and X-ray diffraction (XRD) were also employed to physicochemical

Available online 16 September 2012

survey of the electrocatalysts. The kinetic parameters of glycerol oxidation, i.e. Tafel slope

such

as:

Cyclic

voltammetry

(CV),

Chronoamperometry

(CA),

Chro-

and activation energy (Ea), were determined on the modified electrodes. The Tafel slopes of Keywords:

166 mV dec
1 on PtjCCE, 177 mV dec
1 on AujCCE and 136 mV dec
1 on PdjCCE were ob-

Alkaline fuel cell

tained. The lowest Ea value of 11.2 kJ mol
1 was calculated on AujCCE. In continuation, the

Glycerol

reaction orders with respect to the glycerol and NaOH concentrations on PdjCCE were

Gold

found to be 0.27 and 0.87, respectively. The CV, CP and CA results showed remarkable

Platinum

electrocatalytic activity and good poisoning tolerance of AujCCE for glycerol oxidation.

Palladium

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

A fuel cell is an electrochemical device, which converts the chemical energy of oxygen and a fuel into electrical energy. In last two decades, direct alcohol fuel cells (DAFCs) based on methanol and ethanol as fuel have attracted a great deal of attention as power sources for portable applications due to the much higher energy density of liquid fuels than gaseous fuels like hydrogen [1]. However, methanol is known as a toxic chemical [2,3]. In contrast, ethanol is nontoxic and has more energy density compared to methanol (8.01 kWh kg
1 versus 6.09 kWh kg
1). But the development of direct ethanol fuel cell is limited because of difficulty of CeC bond breaking for a total oxidation of ethanol at low temperatures [4,5]. Platinum based catalysts are the most studied anodic materials in fuel cells [6]. However, very high cost of platinum prevents its extensive application. Also, Pt can be easily poisoned by intermediate species like COads [7].

A recent trend of investigation is the use of polyols like glycerol and ethylene glycol as fuel in alkaline fuel cells. Glycerol is less toxic than methanol and has relatively high theoretical energy density (5.0 kWh kg
1) [8]. It can be massively produced by microbial fermentation [9] and is a byproduct of biodiesel production [10]. Moreover, partial oxidation of glycerol to mesoxalate, without CeC bond breaking, leads to 10 electrons generation against 14 for the complete oxidation of it to CO
2 3 . This possibility allows achieving 71.5% of the whole existing energy. The main problem of the anodic oxidation of glycerol is the formation of poisonous intermediates on the catalyst surface at low overpotentials. This decreases the total efficiency of the system [11]. The rate of alcohol oxidation processes in an alkaline medium instead of an acidic one significantly increases [12,13]. In alkaline medium Pt-free catalysts based on silver [14], gold [11] and palladium [15,16] can be used for oxidation of alcohols with remarkable electrocatalytic ability.

* Corresponding author. Tel.: þ98 412 4327500; fax: þ98 412 4327541. E-mail address: [email protected] (E. Habibi). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.08.127

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 6 8 0 0 e1 6 8 0 9

carbons [20], multi-walled carbon nanotubes and activated carbon nanofibers [21] have been reported as support for electrocatalyst particles to use in electrocatalysis. In our previous work [22], carbon ceramic electrode (CCE) was introduced as a high performance substrate for deposition of platinum nanoparticles in the electrooxidation of methanol and ethanol. Here, considering the interesting features of CCE such as porosity, high conductivity and stability [23] we used it as substrate for electrochemically preparation of Pd and Au nanoparticles. After preparation of PdjCCE and AujCCE, they were used for the electrooxidation of glycerol in aqueous alkaline solutions and the results were compared with PtjCCE. In continuation, the kinetic parameters of glycerol oxidation, i.e. Tafel slope and activation energy, were determined on the modified electrodes.

Au (111)

Au|CCE

Au (200)

Carbon Ceramic

Au (220)

Pt (111)

Pt|CCE

Pt (200)

Pt (220)

Pd (111)

Pd|CCE Pd (200)

35

42

16801

Pd (220)

49

56

63

70

2 θ / degree Fig. 1 e XRD patterns of PdjCC, PtjCC and AujCC electrodes.

It is largely accepted that in addition to the kind of electrocatalyst particles, the nature of electrode substrate is also an effective factor in dispersion, chemical stability, antipoisoning properties and electrocatalytic activity of metal particles during massive and prolonged electrochemical processes [17]. Different kinds of conductive materials, including Vulcan XC-72 [18], carbon microspheres [19], hollow

2.

Experimental

2.1.

Chemicals

Methyltrimethoxysilane (MTMOS) was from Fluka. Methanol, Glycerol, H2PtCl6, PdCl2, H[AuCl4].xH2O, NaOH and graphite powder of high purity were obtained from Merck. All solutions were freshly prepared with distilled water and purged with nitrogen (99.999%) before each experiment.

2.2.

Instrumentation

The measurements were carried out using an Autolab 100 (Potentiostat/Galvanostat) equipped with a Frequency

Fig. 2 e SEM images of bare carbon ceramic (a), PdjCC (b), PtjCC (c) and AujCC (d) surfaces. Amount of catalyst loading: 0.5 mg cmL2.

16802

2.3.

0.2

25

Pd / CC Pt / CC

-0.2

Au / CC

0

-2

Real

-15

j / mA. cm

j / mA cm-2Geometric

a

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 6 8 0 0 e1 6 8 0 9

-55 -1.1

-0.25

0.6

E / V vs. SCE 1.55

82

3

0.75

j / mA cm 40

1 2

1

-0.05

j / mA cm-2Geometric

-2 Real

b

-0.7

0

0.7

3

Pd / CC Pt / CC Au / CC -2 -1.4

-0.6

0.2

1

Fabrication of modified electrodes

The working electrodes were fabricated in two sections: First, CCE was prepared by using solegel processing method as described in our previous work [22]. Summary, a portion of 0.9 mL MTMOS was mixed with 0.6 mL methanol and 0.6 mL of 0.5 M HCl as catalyst. The mixture was magnetically stirred until a clear and homogeneous solution resulted. Next, 0.3 g graphite powder was added and the mixture was shaken for an additional 1 min. Subsequently, the homogenized mixture was firmly packed into a Teflon tube (with 3.9 mm inner diameter and 10 mm length) and dried for at least 24 h at room temperature. A copper wire was inserted through the other end to set up electric contact. The electrode surface was polished with 1500 emery paper and was rinsed with distilled water. Second, the Pt, Pd and Au nanoparticles were potentiostatically deposited on the surface of CCE from an aqueous sulfuric acid solution (0.2 M) containing 0.005 M H2PtCl6 (for PtjCCE), 0.005 M H[AuCl4] (for AujCCE) and 0.005 M PdCl2 (for PdjCCE). All electrodepositions were made at a constant potential of
0.2 V vs. SCE. Catalyst loading for each electrode was maintained at 0.5 mg cm
2. The amounts of Pt, Pd and Au deposited on the CCE were controlled from the charge consumed during the electrodeposition of each of the electrocatalysts by using Faraday’s law, considering a value of 4, 3 and 2 for n in the deposition of Pt, Au and Pd, respectively (Ptþ4 þ 4e / Pt, Auþ3 þ 3e / Au and Pdþ2 þ 2e / Pd). It was supposed that the contribution of intruder processes, such as hydrogen evaluation at the deposition processes can be disregarded.

E / V vs. SCE

Fig. 3 e Cyclic voltammograms of electrocatalysts in (a) 0.3 M NaOH solution at a scan rate of 50 mV sL1 and in (b) 0.3 M NaOH solution D 0.5 M glycerol at a scan rate of 5 mV sL1. Inset to each figure shows the corresponding cyclic voltammograms that were normalized with real surface area of each electrode. t: 25  C. Response Analyzer (FRA 4.9). This was then interfaced with a personal computer and controlled by GPES 4.9 and FRA 4.9 software. The morphology and structure of modified electrodes were characterized by SEM (LEO 440i Oxford) and XRD (Brucker AXF, D8 Advance) with a Cu Ka radiation source (a ¼ 0.154056 nm) generated at 40 kV and 35 mA. A conventional three electrode cell was used at room temperature. The working electrodes were PdjCCE, PtjCCE and AujCCE with a geometrical area of 0.119 cm2. A saturated calomel electrode (SCE) and a platinum wire were used as reference and auxiliary electrodes, respectively. JULABO thermostat was used to control cell temperature.

3.

Results and discussion

3.1.

Physicochemical characterizations

XRD measurements were performed in order to evaluate the crystallographic structure of the catalysts. The Pt, Pd and Au particles supported on carbon ceramic exhibit an XRD pattern of typical face-centered cubic (fcc) lattice structure as shown in Fig. 1. It can be seen that the diffraction peaks for Pd and Pt (111), (200), and (220) are very well-defined. The diffraction peaks at the Bragg angles of 38.5 , 44.7 and 64.8 correspond to the (111), (200) and (220) facets of Au crystals. The strong peak at 54.8 belongs to carbon ceramic matrix (006). The (111) peaks were used to calculate the crystallite size of catalysts according to the DebyeeScherrer equation [24]. The crystallite sizes of Pt, Pd and Au are 8.9 nm, 10 nm and 19 nm, respectively. The surface morphology of bare CC, PdjCC, PtjCC and AujCC electrodes were investigated by SEM and the

Table 1 e Performance parameters of the glycerol electrooxidation on Pt, Pd and Au modified CCEs. Catalyst type PtjCCE PdjCCE AujCCE

EOn (V vs. SCE)

Ip1 (mA/Geo.cm2)

Ep1 (V vs. SCE)

Ip3 (mA/Geo.cm2)

Ep3 (V vs. SCE)

Ip1/Ip3

Tafel slope (mV dec
1)


0.57
0.45
0.39

46.8 51.8 58.0


0.085
0.085 0.257

19.1 2.60 85.6


0.3
0.4 0.08

2.437 19.90 0.678

166 136 177

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corresponding results are shown in Fig. 2aed respectively. As can be seen in Fig. 2a, the carbon ceramic surface is scaly, dense and has a high porosity. A comparison of Fig. 2bed images reveals that the Pd and Pt particles compared to the Au particles show a better dispersion on the CC substrate. The Pd and Pt particles were accumulated and formed agglomerates with different sizes on the CC surface. As can be seen in Fig. 2d, the size of non-agglomerated gold particles is approximately 50 nm. Probably, the particulate structures of Pd, Pt and Au in Fig. 2 are not the individual metal crystallites. They are clews consisting of crystallite aggregates.

3.2.

Electrochemical characterizations

Cyclic voltammograms of Pd, Au and Pt modified CCEs in 0.3 M NaOH solution at a sweep rate of 50 mV s
1 are shown in Fig. 3a. In the CV of PdjCCE, the broad anodic peak in forward going scan and sharp cathodic peak at
0.49 V vs. SCE are related to the formation and reduction of palladium oxide,

c NaOH / M

20

10

0 -0.8

27

c Gy / M

4 3 1.5 0.9 0.7 0.5 0.4 0.35 0.3 0.25 0.2 0.15 0.08 0.05 0.02 0.01

30

I / mA

b

40

1.2 1 0.7 0.5 0.4 0.2 0.1 0.05 0.02

19

I / mA

a

respectively. The real surface area or electrochemical active surface area (EASA) for PdjCCE was calculated by considering the electric charge associated to the reduction of PdO monolayer formed on the electrode surface, as described by Singh et al. [25]. By assuming a charge value of 4.05 C m
2 for the reduction of PdO monolayer, the EASA was calculated to be equal to 50.37 m2 g
1. For PtjCCE, the EASA was estimated from hydrogen adsorption on Pt surface as described in our previous work [22]. The CV of AujCCE in alkaline medium shows an anodic peak close to 0.4 V vs. SCE during the forward scan and a cathodic peak in the reverse one which are related to the formation and reduction of AuO. The electric charge of 3.86 C m
2 [26] corresponding to the reduction of an AuO monolayer leads to an active surface value of ca. 8.06 m2 g
1. For better describing of the electrochemical behavior of electrocatalysts in the electrolyte, each of the voltammograms in Fig. 3a was divided by the respective EASA. The corresponding results for PtjCC, PdjCC and AujCC electrodes were given in the inset to Fig. 3a.

11

3

-0.1

-5 -0.8

0.6

-0.1

E / V vs. SCE

E / V vs. SCE

c

0.6

d

1.6

1.6

log (I p.f / mA)

log (I / mA)

1.2

Slope Potential 0.75 -0.25 0.79 -0.30 0.87 -0.35 0.91 -0.38 0.94 -0.40 0.97 -0.43 1.00 -0.45

0.1

-1.4

y = 0.2702x + 1.4141 R2 = 0.9778

0.8

0.4 -2

-0.7

log (c OH- / M)

0.6

-2

-1.2

-0.4

0.4

log (c Gy / M)

Fig. 4 e (a) Effect of NaOH concentration on 0.5 M glycerol electrooxidation with PdjCCE. (b) Effect of glycerol concentration in 0.5 M KOH on its electrooxidation with PdjCCE. (c) The plot of log I vs. log cOHL. (d) The plot of log Ip.a vs. log cglycerol. t: 25  C, scan rate: 50 mV sL1.

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3.3.

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Electrooxidation of glycerol

ratio of PdjCCE indicates a good catalytic performance of it to the oxidation of carbonaceous intermediates. The relevant performance parameters of the glycerol electrooxidation on synthesized electrodes have been given in Table 1. In order to have a good comparison between the electrocatalytic activities of the modified electrodes for glycerol oxidation (considering the huge difference between EASAs of electrocatalysts) the CVs in Fig. 3b were divided by the respective EASA and the results were given in the inset to Fig. 3b. As can be seen from it, each real cm2 of the AujCC in comparison with each real cm2 of the PdjCC and PtjCC has remarkable electrocatalytic activity for glycerol oxidation. In order to determine the kinetics of glycerol oxidation on modified electrodes, the Tafel slopes have been calculated from the quasi steady state curves in Fig. 3b. The Tafel slopes were obtained as 166 mV dec
1 on PtjCCE, 177 mV dec
1 on AujCCE and 136 mV dec
1 on PdjCCE. Relatively identical Tafel slopes on the Au and Pt modified electrodes indicate the same reaction mechanism for glycerol oxidation. These results show that the glycerol oxidation on PdjCCE has higher kinetics compared to Pt and Au electrodes.

The cyclic voltammograms of the different modified electrodes in 0.3 M NaOH solution containing 0.5 M glycerol at a scan rate of 5 mV s
1 are shown in Fig. 3b. Onset potential (EOn) of glycerol oxidation on PdjCCE is about
0.45 V vs. SCE, which is approximately 0.12 V higher and 0.06 V lower than those obtained on PtjCCE and AujCCE, respectively. With the increase of electrode potential anodic peak of glycerol oxidation (peak 1) appears at 0.25 V vs. SCE for AujCCE and
0.085 V vs. SCE for PtjCCE and PdjCCE. Afterwards, the currents diminish and reach a plateau. The appearance of another oxidation peak at higher potentials (peak 2) can be attributed to the oxidation of remaining adsorbed OH groups on catalyst surface [27]. However, according to a recent paper reported by Simo˜es and co-workers [11] several glycerol oxidation products remain being formed even at 1.15 V vs. RHE which can cause the formation of peak 2. As can be seen from Fig. 3b palladium compared with platinum and gold is more active surface for glycerol oxidation at higher potentials (from 0.3 to 0.9 V vs. SCE). In the backward scan reactivation of catalyst surface at around 0.08 V vs. SCE for AujCCE,
0.3 V vs. SCE for PtjCCE and
0.4 V vs. SCE for PdjCCE causes the formation of the backward anodic peak (peak 3). This peak is primarily related to the removal of intermediate species not completely oxidized in the forward scan. The ratio of forward anodic peak current (Ip1) to the reverse anodic peak current (Ip3) are found to follow the order PdjCCE > PtjCCE > AujCCE. A higher Ip1/Ip3

- 0.8

0.1

c

39

t / °C 80 65 50 35 20

19

-1 -0.8

-1

1

0.1

e

d 3.7

ln (I / mA)

ln (I / mA)

19

-1.7

f

0.5

-1

3.6

/K

-1

2.6

1

4

E/V

0.15 0.10 0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 -0.35 -0.40

0.45 0.30 0.20 0.10 0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 -0.35 -0.40

0

-4

-2.5

1000 T

0.1

E / V vs. SCE

E/V

0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 -0.35 -0.40 -0.45 -0.50

3.1

t / °C 80 65 50 35 20

-1 -0.8

1

3.5

E/V

2.6

39

E / V vs. SCE

E / V vs. SCE

1

The cyclic voltammograms of PdjCCE were then recorded in 0.5 M glycerol solution in the presence of various concentrations of NaOH ranging from 0.01 to 4 M at a scan rate of 50 mV s
1 (Fig. 4a). One can see that in the concentration range

ln (I / mA)

19

t / °C 80 65 50 35 20

The effect of glycerol and NaOH concentrations

I / mA

b

39

I / mA

I / mA

a

3.4.

3.1

1000 T

-1

3.6

/K

-1

2.6

3.1

1000 T

-1

3.6 -1

/K

Fig. 5 e CVs for glycerol oxidation on (a) PtjCCE, (b) PdjCCE and (c) AujCCE in 0.5 M glycerol D 0.3 M NaOH solution at various temperatures. Arrhenius plots of glycerol oxidation on Pt (d), Pd (e) and Au (f) modified CCE at different potentials. Scan rate: 25 mV sL1.

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from 0.01 to 3 M, the peak current of glycerol oxidation increases and the potential of peak shifts negatively. In concentrations higher than 3 M the anodic peak current falls off. Generally accepted glycerol electrooxidation pathway on Pd in alkaline media can be described as the following steps [27,28]: (1)

Pd þ CH2 ðOHÞ
CHðOHÞ
CH2 ðOHÞ þ 3OH
#Pd (2)

 Pd
ðOHÞads þ Pd
CH2 ðOHÞ
CHðOHÞ
COads /CH2 ðOHÞ
CHðOHÞ
COOH þ 2Pd (3) CH2 ðOHÞ
CHðOHÞ
COOH þ OH
/CH2 ðOHÞ
CHðOHÞ (4)

The present pathway indicates that with the increase of OH
concentration which causes a higher OH
coverage on the Pd surface, the kinetic of glycerol electrooxidation is enhanced. Also, according to the reaction (2) in the availability of too many OH ions the adsorption of glycerol on the Pd active sites facilitated. This causes a higher peak current and negative shift of peak potential. In more alkaline electrolytes (cOH
> 3 M) the adsorption of OH
prevents the glycerol adsorption on the Pd sites and so the reaction rate of glycerol oxidation decreases. Fig. 4c indicates the plots of log I versus log cOH
at various potentials. The reaction order with respect to the OH
concentration is derived from the slopes of straight lines. It was found that the reaction order is dependent on the potential in this region and its amount decreases from 1.00 to 0.75 by potential increasing. The effect of glycerol concentration on the electrooxidation was investigated on PdjCCE at a fixed NaOH concentration of 0.5 M (Fig. 4b). According to the experimental data the anodic peak current increases with the increase of glycerol concentration up to 1 M, after which the current decreases. This can be attributed to the saturation of active sites with glycerol that inhibits the OH
adsorption on Pd sites and causes to a decrease of peak current. The reaction order of 0.27 with respect to the glycerol concentration was obtained from the slope of log Ip.a versus log cglycerol plot (Fig. 4d).

3.5.

The effect of temperature

a

-0.3

Pd|CCE -0.4

E on / V vs. SCE


COO
þ H2 O

Pt|CCE Au|CCE

-0.5

-0.6

-0.7

0

50

100

t / °C

b

-1


½CH2 ðOHÞ
CHðOHÞ
COads þ 3H2 O þ 3e


65

Pd

45

Ea / kJ mol

Pd þ OH
%Pd
ðOHÞads þ e


the glycerol oxidation on the PtjCC, PdjCC and AujCC electrodes at different potentials are given in Fig. 5def, respectively. The values of activation energy (Ea) are calculated from the slopes of these plots and then plotted against the potential in Fig. 6b. A typical feature of the Ea vs. E plots is the appearance of a trough for all electrodes. As can be seen, with increasing the potentials of Pt, Pd and Au modified electrodes up to
0.1, 0.0 and 0.2 V vs. SCE, the Ea values decrease to 16.6, 14.3 and 11.2 kJ mol
1, respectively and then begin to increase with potential. At the potential range of
0.5 to
0.1 V vs. SCE, the apparent activation energies on PtjCCE are lower than those on PdjCCE and AujCCE. On the other hand the Ea on PdjCCE shows higher potential dependence in this potential region. At further positive potentials the oxidation of catalyst active sites and the coverage of its surface with intermediate species inhibit the chemisorption of the reactant molecules on

Au

25

The temperature dependency of glycerol electrooxidation on the modified electrodes was investigated by CV in the range of 20e80  C. The corresponding results for PtjCCE, PdjCCE and AujCCE are given in Fig. 5aec, respectively. Clear enhancement in the oxidation current and the negative shift of the EOn with increasing temperature can be observed for all electrodes. The plot of EOn against the temperature has been displayed in Fig. 6a. A high slope of
3.2 mV/ C for PdjCCE as compared to
2.6 mV/ C for AujCCE and
1.9 mV/ C for PtjCCE suggests that, with a rise in the temperature, the catalytic activity of the first enhances remarkably. Arrhenius plots of

Pt 5 -0.6

-0.2

0.2

0.6

E / V vs. SCE Fig. 6 e The plot of EOn against the temperature (a) and apparent activation energy vs. electrode potential (b) for the glycerol oxidation reaction on the modified electrodes.

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Fig. 7 e Chronoamperograms recorded at different electrode potentials for Pt, Pd and Au modified CCE in 0.5 M glycerol D 0.3 M NaOH solution.

the active sites and therefore lead to the increase in Ea values.

3.6. Chronoamperometric and chronopotentiometric studies In order to evaluate the stability of electrocatalysts for glycerol oxidation in the reaction conditions, CA measurements were performed by applying different potential steps for 1500 s. The corresponding results are shown in Fig. 7. From the figure it is clear that the steady state oxidation current density for AujCCE is the most one and it is much more efficient electrocatalyst compared to two other electrodes. Further performance evaluation tests of the modified electrodes were carried out by chronopotentiometry. CP is a powerful technique to study the poisoning-resistance abilities of electrocatalysts for alcohol oxidation. Fig. 8a compares the chronopotentiometric curves, measured in 0.3 M NaOH solution which contains 0.5 M glycerol at a current density of 3 mA/cm2Geometric (3 mA current was applied to each geometric cm2 of the electrodes). For PtjCC, the electrode potential rapidly increases with polarization time and after 3200 s (deactivation time) steep rises to a higher potential for oxygen evolution while PdjCC and AujCC catalysts are more stable for longer periods of 18600 s and 15600 s, respectively. This indicates a fast poisoning of PtjCC surface with reaction intermediates (glycerate, tartronate .) compared to two other electrodes. Also, it can be seen that through the same scanning time the PdjCCE shows a lower polarization potential than AujCCE. From the standpoint of CP results, the PdjCC and

Fig. 8 e Chronopotentiometric curves of the glycerol oxidation reaction on PtjCCE, PdjCCE and AujCCE in 0.5 M glycerol D 0.3 M NaOH solution under a current density of 3 mA/Geometric cmL2 (a) and 3 mA/Real cmL2 of each electrode.

AujCC catalysts are more suitable than PtjCC for glycerol electrooxidation in alkaline media. Fig. 8b indicates the chronopotentiograms of glycerol oxidation on the electrocatalysts that were obtained by applying the current of 3 mA to each real cm2 of the electrodes. It can be seen from the figure that each real cm2 of the AujCC has striking performance stability for glycerol oxidation.

3.7.

Electrochemical impedance spectroscopy (EIS)

EIS is a valuable and non-destructive technique to study the electrode surface and analyze the kinetics of organic molecules electrooxidation in fuel cells. In the present work EIS was used to investigate the kinetics of glycerol oxidation. EIS studies were performed with amplitude of 5 mV at a frequency range of 100 kHze10 mHz. Fig. 9aec display the Nyquist plots of glycerol electrooxidation on Pd, Pt and Au electrocatalysts at different electrode potentials, respectively. It can be seen from Fig. 9 that with increasing the potential of PtjCCE, PdjCCE and AujCCE up to
0.25,
0.2 and þ0.1 V vs. SCE the diameter of semicircles corresponding to the charge transfer resistance decreases. As a result the reaction rate of glycerol oxidation increases. The increase of semicircle

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 6 8 0 0 e1 6 8 0 9

diameter at higher potentials implies that the charge transfer resistance increases. The EIS results indicate that the glycerol oxidation on these electrocatalysts at various potentials show different

a

16807

impedance behaviors. At potentials
0.55,
0.45,
0.35 and
0.25 V vs. SCE for PtjCCE,
0.2 V vs. SCE for PdjCCE and
0.05, 0.05 and 0.1 V vs. SCE for AujCCE, the impedance exhibits a linear part at lower frequencies which is related to the

1200

900

-Z''/ohm

-Z''/ohm

-0.2 V -0.1 V 0.0 V 500 -0.60 V

400

-0.50 V -0.45 V -0.40 V -0.35 V -0.30 V

-200

-100 0

1500

0

3000

600

Z'/ohm

b

1200

Z'/ohm

360

1200

-0.25 V -0.15 V

-0.70 V

-0.10 V

-Z''/ohm

-Z''/ohm

-0.20 V -0.75 V -0.65 V

170

-0.60 V -0.55 V

0.00 V

400

-0.50 V -0.45 V -0.35 V

-20 0

500

-400 -600

1000

300

Z'/ohm

Z'/ohm 3000

3000

-Z''/ohm

-Z''/ohm

c

1200

1250 -0.30 V

1500 0.10 V

-0.25 V

0.2 V

-0.20 V

0.25 V

-0.10 V -0.05 V 0.05 V

-500 0

3500

Z'/ohm

7000

0 -2000

500

3000

Z'/ohm

Fig. 9 e Nyquist plots measured at different potentials in 0.5 M glycerol D 0.3 M NaOH solution on (a) PdjCC, (b) PtjCC and (c) AujCC electrodes.

16808

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 6 8 0 0 e1 6 8 0 9

10000

have the better performance stability than PtjCCE. On the other hand, EIS showed that PdjCCE has better poisoning tolerance in comparison with two other electrodes.

-0.25 V Pt / CC -0.25 V Au / CC -0.25 V Pd / CC

5000 0

-Z'' / Ω .cm

2

Real

references

0

7000

14000

Z' / Ω .cm

21000

2 Real

Fig. 10 e Nyquist plots of different electrocatalysts in 0.5 M glycerol D 0.3 M NaOH solution at L0.25 V vs. SCE.

diffusion-limited electron-transfer process. Fig. 9b indicates that at potentials
0.15 and
0.1 V vs. SCE, glycerol oxidation on PtjCCE shows negative impedance behavior. This phenomenon was also observed previously in the oxidation of alcohols on Pt-based electrodes, and generally ascribed to the adsorption of reaction intermediates on the catalyst surface [29,30]. When the potential increases, the impedance changes form negative to positive. This implies the removal of intermediates from electrode surface [30]. Meanwhile, this behavior can be seen on AujCCE at potential 0.2 V vs. SCE (Fig. 9c). The absence of negative impedance behavior on PdjCCE (Fig. 9a) shows good poisoning tolerance of it for glycerol oxidation. It should also be noted that the diameters of the impedance arcs of PtjCCE are smaller than those on PdjCCE and AujCCE as indicated in Fig. 10 for potential of
0.25 V vs. SCE, suggesting faster electron-transfer kinetics on Pt than that on the Pd and Au electrocatalysts. The EIS results are in good agreement with those of CV, CA and CP that have been presented above.

4.

Conclusion

In this work, PdjCC and AujCC electrocatalysts were synthesized and characterized for first time. It was found that these electrocatalysts have very different EASA. Next, they were used for the glycerol electrooxidation in alkaline media. The preliminary results from CV indicated that the PtjCC has the lowest onset potential. The CV and CA studies showed that the AujCC has remarkable electrocatalytic activity for glycerol oxidation. However; in terms of Tafel plots analyses, the PdjCC showed a higher kinetics compared to two other electrodes for glycerol oxidation. The effect of NaOH concentration on the PdjCC showed that the glycerol oxidation reaction is highly dependent on the solution pH. The lowest Ea values of 16.6, 14.3 and 11.2 kJ mol
1 were obtained for glycerol oxidation on Pt, Pd and Au modified carbon ceramic electrodes, respectively. Also, CP studies demonstrated that the Au and PdjCCEs

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