Fabrication of novel porous Pd particles and their electroactivity towards ethanol oxidation in alkaline media

Fuel 90 (2011) 2617–2623

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Fabrication of novel porous Pd particles and their electroactivity towards ethanol oxidation in alkaline media Qingfeng Yi a,b,⇑, Fengjuan Niu a, Lizhi Sun a a b

School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, Hunan, China Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education, Hunan University of Science and Technology, Xiangtan 411201, Hunan, China

a r t i c l e

i n f o

Article history: Received 28 April 2010 Received in revised form 29 March 2011 Accepted 29 March 2011 Available online 9 April 2011 Keywords: Porous Pd catalyst Ethanol electro-oxidation Pd electrode Fuel cell

a b s t r a c t Novel porous Pd particles (nanoPd–PEG, nanoPd–PEG–EDTA, nanoPd–HCHO–EDTA, nanoPd–EG, nanoPd– HCHO and nanoPd–EG–EDTA) were synthesized by a hydrothermal method using different reduction agents in the absence and presence of EDTA and investigated as electrocatalysts for ethanol oxidation in alkaline solutions. Results showed that PdCl2 was hydrothermally reduced to nano-scale palladium particles and a three-dimensional texture was formed for Pd particles. Presence of EDTA was favorable for the formation of Pd nanoparticles with small sizes of ca. 70 nm. Ethanol oxidation on the present Pd catalysts took place at a more negative anodic potential in 1 M NaOH solution. Among the electrocatalysts investigated, the electrocatalytic activity of the nanoPd–HCHO–EDTA was the greatest, which was characterized by the largest anodic peak current density of 151 mA cm2 and lowest onset oxidation potential of 0.788 V (vs. SCE) for the positive scan. Very low charge transfer resistances on the nanoPd–HCHO–EDTA in 1 M NaOH containing various concentrations of ethanol were obtained according to the analysis for electrochemical impedance spectra (EIS). The prepared porous Pd catalysts were promising alternatives to Pt electrodes applied in alkaline direct alcohol fuel cells. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Development of novel catalysts with highly electrocatalytic activity for alcohol (methanol or ethanol) oxidation has been receiving much attention because electroactivity of anodic materials is one of the main factors influencing the practical application of direct alcohol fuel cells (DAFCs) [1,2]. Although platinum electrodes present efficient catalytic activity for alcohol oxidation, the widely practical application of Pt electrodes as anodes in DAFCs is limited by the high cost of the metal platinum; whereupon nonplatinum catalysts have been tested as anode catalysts for alkaline direct alcohol fuel cells [3,4]. Compared to Pt, palladium is a relatively cost-reasonable noble metal. The Pd electrode exhibits no electrocatalytic activity for alcohol oxidation (AO) in acidic solutions (e.g. H2SO4) while it displays high electroactivity for AO in alkaline solutions such as aqueous NaOH and KOH [5,6]. A variety of palladium nanoparticles have also been fabricated in order to examine their electroactivity for AO in alkaline solutions, such as nanocrystalline oxide Pd/C promoted electrocatalysts [7–9], Pd nanoparticles-supported on ⇑ Corresponding author at: School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, Hunan, China. Tel.: +86 731 58290462; fax: +86 731 58290509. E-mail address: [email protected] (Q. Yi). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.03.038

multi-walled carbon nanotubes (MWCNT) [10,11], Pd nanowire arrays [12], Pd nanoparticles-supported on carbonized TiO2 nanotube [13]. In addition, many binary or ternary composite catalysts involved in Pd have been developed to enhance the electroactivity of the Pd catalyst for AO [14], including Pt–Pd/C [15], Pt–Pd/Ru [16], Pd–MWCNT–Ni [17,18], heterobimetallic Ru/Pd complexes [19], Pd–In2O3/CNTs [20], oxide (CeO2, NiO, Co3O4, Mn3O4)-promoted Pd/C [21], Pd–Ag/C [22], Ni–Pd supported by silicon microchannel plates [23], Pd–NiO/C [24], and Pd–Pt/Ti [25] et al. Porous palladium electrode has recently received much attention due to its some distinguished advantages such as significantly large surface areas and high stability. Several methods have been reported on the preparation strategies of porous Pd catalysts, such as hydrothermal deposition process [26], chemical reduction of tetrachloro complexes [27], dealloying of an Al–Pd alloy [28] and Pd0.2Co0.8 alloy [29], and porous anodic aluminum oxides synthesis [30]. In the present paper, we have made a detailed study on the hydrothermal preparation of novel porous Pd catalysts by using different reduction agents (PEG, HCHO and EG) in the absence and presence of EDTA (ethylenediamine tetraacedic acid), and have studied their electrochemical activity for ethanol oxidation in alkaline media. The Pd nanoparticles are directly immobilized on the Ti surface by the hydrothermal deposition, forming titaniumsupported Pd electrocatalysts with three-dimensional network

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Fig. 1. SEM images of the prepared catalysts nanoPd–PEG (a), nanoPd–PEG–EDTA (b), nanoPd–HCHO–EDTA (c), nanoPd–EG (d), nanoPd–HCHO (e) and nanoPd–EG–EDTA (f). (g) is the SEM image of the cross section of the nanoPd–HCHO–EDTA sample.

structures. Their electrocatalytic activity for ethanol oxidation has been evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

2. Experimental All chemicals were of analytical grade and used as received without further purifications. Water used in this work was firstly

treated by ion exchange resins and then doubly distilled. PdCl2 solution was prepared as follows: PdCl2 solid was firstly dissolved by a small amount of concentrated HCl, then the solution was diluted with water to a given PdCl2 concentration. Titanium sheets were of 99.2% purity. Different Ti-supported porous Pd catalysts were prepared as follows: Firstly, pure titanium sheets (5 mm  10 mm  1.0 mm) were washed by pure water, and then etched in an 18% of HCl solution at 85 °C for 10 min to so that the oxide layer on the titanium

Q. Yi et al. / Fuel 90 (2011) 2617–2623

surface was thinned. Then, the etched titanium sheets were transferred into an autoclave containing 10 mL of 5 m mol L1 PdCl2, x mL reduction agent (R) and y mmol EDTA. Finally, the autoclave was heated at 180 °C for 10 h. After cooling to room temperature, the sample was dried in air at 100 °C for 30 min, and washed with pure water. The prepared titanium-supported porous palladium electrocatalysts were denominated by nanoPd–PEG (R: polyethylene glycol 600 (PEG), x = 0.5, y = 0), nanoPd–PEG–EDTA (R: polyethylene glycol 600 (PEG), x = 0.5, y = 0.05), nanoPd–HCHO–EDTA (R: 10% HCHO, x = 1, y = 0.05), nanoPd–EG–EDTA (R: ethylene glycol (EG), x = 0.5, y = 0.05), nanoPd–EG (R: ethylene glycol (EG), x = 0.5, y = 0), and nanoPd–HCHO (R: 10% HCHO, x = 1, y = 0). The morphological texture of the prepared samples was characterized by scanning electron microscopy (SEM) taken on a JSM6380LV scanning electron microscopy. Electrochemical measurements were performed in a conventional three-electrode cell controlled by the AutoLab PGSTAT30/ FRA electrochemical instrument (the Netherlands). The working electrodes were the prepared electrocatalysts. The counter electrode was large Pt foils and the reference electrode was saturated calomel electrode (SCE). Prior to electrochemical measurements, the prepared porous Pd catalysts of the present investigation were subjected to careful pretreatment, i.e. successive cycling potential scanning between 1.0 and +0.50 V (vs. SCE) at a scan rate of 100 mV s1 was applied in 1 M NaOH solution until a stable voltammogram was obtained. Equilibrium time of the electrodes was 5 s at the beginning of the scan. All potentials reported in this paper were against the SCE. The amplitude of modulation potential for the EIS measurements was 10 mV, and the frequency was changed from 40 kHz to 50 mHz. Before experiments, pure nitrogen gas (99.99%) was bubbled through the solution at least 15 min to remove the dissolved oxygen in the solution. All experiments were carried out at ambient temperature (20 ± 2 °C).

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the Pd nanoparticles are connected with each other to form a three-dimensional texture which provides a considerably large real surface area. The deposited Pd film thicknesses of the samples are reckoned to be ca. 1.1–1.8 lm according to the SEM image (Fig. 1g) of the cross section of the nanoPd–HCHO–EDTA sample. The shadow areas in Fig. 1a and b would be attributed to the presence of organic intermediates formed during the decomposition of the deduction agent PEG. The different Pd particles sizes show the crucial role of the reduction agent and ligand (EDTA). The SEM images of nanoPd–PEG (Fig. 1a), nanoPd–EG (Fig. 1d) and nanoPd–HCHO (Fig. 1e) show that among the three reduction agents investigated (PEG, EG and HCHO), EG is the best reduction agent for the formation of small sizes of Pd particles in the absence of EDTA. Addition of EDTA to Pd2+ results in the formation of coordination compound Pd2+–EDTA and reduces the oxidation potential of Pd2+, leading up to the lowering of the reduction rate of Pd2+ to elemental Pd. This would be favorable for the production of less nano-scale sizes of Pd particles. 3.2. Electrochemical voltammetric responses Cyclic voltammetry (CV) measurements were carried out on the prepared catalysts in 1 M NaOH solution (Fig. 2). It is observed from Fig. 2 that the present electrocatalysts show characteristic CV profiles of polycrystalline Pd electrode except that they exhibit significantly high anodic and cathodic current densities. The oxidation peaks at lower anodic potentials during the forward scan in Fig. 2 are ascribed to the formation of the adsorbed hydroxyl OHads while those at high positive potentials are related to the formation of Pd oxides. A large cathodic peak (r) at ca. 0.42 to 0.43 V, ascribed to the reduction of Pd oxides formed during the forward scan, gives an indication that the prepared catalysts have considerably large real surface areas. Corresponding reactions are shown in Eqs. (1)–(3) [5,31]:

3. Results and discussion

Pd þ OH ! Pd  OHads þ e

ð1Þ

3.1. SEM analysis of the catalysts

Pd  OHads þ Pd  OHads $ Pd  OðPd oxidesÞ þ H2 O

ð2Þ

Pd  OðPd oxidesÞ þ H2 O þ 2e $ Pd þ 2OH

ð3Þ

Morphologies of the prepared samples were examined by scanning electron microscopy (SEM) and characterized by porous network structures (Fig. 1a–f). According to the selected regions (circles in Fig. 1a–f), the sizes of the Pd nanoparticles for the nanoPd–PEG, nanoPd–PEG–EDTA, nanoPd–HCHO–EDTA, nanoPd– EG, nanoPd–HCHO and nanoPd–EG–EDTA are around 220, 100, 70, 80, 150 and 200 nm, respectively. Their SEM images show that

10

j/mA cm-2

0 -10

nanoPd-PEG nanoPd-PEG-EDTA

-20

nanoPd-HCHO -30

nanoPd-EG nanoPd-EG-EDTA nanoPd-HCHO-EDTA

-40

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

E/V vs SCE Fig. 2. Cyclic voltammograms of the electrocatalysts of the present investigation in 1 M NaOH solution at a scan rate of 50 mV s1.

According to the coulombs consumed for the reduction of Pd oxides (involved in the cathodic peak r) formed during the forward scan on the smooth polycrystalline Pd electrode (CV not shown) and the prepared catalysts, the real surface areas of the prepared catalysts nanoPd–PEG, nanoPd–PEG–EDTA, nanoPd–HCHO–EDTA, nanoPd–EG, nanoPd–HCHO, and nanoPd–EG–EDTA are reckoned to be 4.6, 6.5, 17.1, 13.8, 7.8, and 15.2 cm2, respectively, showing the largest active surface area of the nanoPd–HCHO–EDTA among the electrocatalysts. Fig. 3 represents the cyclic voltammograms of the present electrocatalysts in 1 M NaOH + 1 M C2H5OH at a sweep rate of 50 mV s1. By comparing the cyclic voltammograms in the absence of ethanol (Fig. 2), a large and wide anodic peak for ethanol oxidation under anodic condition can be clearly observed in the cyclic voltammograms of all the prepared electrocatalysts in 1 M NaOH + 1 M C2H5OH while a well-defined anodic peak with lower current density in the reverse scan arises (Fig. 3). The oxidation peak in the forward scan is corresponding to the oxidation of freshly chemisorbed species coming from ethanol adsorption. During the negative-going sweep, the reduction of Pd oxides shown in Eq. (3) results in the reactivation of the electrocatalysts, which is evidenced by the presence of the oxidation peak in the reverse scan. It is further observed from Fig. 3 that for the present electrocatalysts, the oxidation reaction in the reverse scan commences at the same potential of 0.38 V, which is in accordance with the onset reduction potential of Pd oxides shown in Fig. 2. According to

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160

as indicated in Fig. 4. A monotonic increase of the oxidation current density with the ethanol concentration c(EtOH) in the c(EtOH) range of 0.1–1 M is observed from Fig. 4. However, when c(EtOH) exceeds 1 M, the peak current density displays little change with the increase of c(EtOH). The increase in the current density with c(EtOH) in the range of 0.1–1 M ethanol can be clarified by the adsorption reaction of ethanol and its further oxidation:

f

140

d

120

e

100

j/mA cm-2

c

80 60

Pd þ C2 H5 OH þ 3OH $ Pd  CH3 COads þ 3H2 O þ 3e

b a

40

ð4Þ

Pd  CH3 COads þ Pd  OHads þ OH ! 2Pd þ CH3 COO þ H2 O

20

ð5Þ

0

Eq. (4) shows that the formation of the adsorbed ethanol is accelerated with the increase of c(EtOH), thereby leading to an increase in the reaction rate of Eq. (5). This, subsequently, yields an increase in the oxidation current density of ethanol. At higher ethanol concentrations, adsorption of ethanol on the Pd active sites approaches to the saturated state, leading to the little increase of the ethanol oxidation current. In addition, a wide peak current plateau is found in Fig. 4 when the ethanol concentration is greater than 0.5 M. This reveals that the large surface areas of the prepared catalysts provide considerable numbers of active sites, keeping a fast formation of Pd–C2H5OHads and Pd–OHads and then a relatively stable peak current density, as indicated in Fig. 4. Inset of Fig. 4 presents the dependence of the peak plateau current density (jp) on the scan rate in 1 M NaOH solution containing 5 M ethanol, showing that the peak plateau current density is independent of scan rate. This strengthens the claim that surface sites are saturated with adsorbed ethanol in high ethanol concentrations. At c(C2H5OH) = 1, 3 and 5 M, the peak current plateau starts to steeply

-20 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

E/V vs SCE Fig. 3. Cyclic voltammograms of the prepared catalysts nanoPd–PEG (a), nanoPd– PEG–EDTA (b), nanoPd–HCHO–EDTA (c), nanoPd–EG (d), nanoPd–HCHO (e) and nanoPd–EG–EDTA (f) in 1 M NaOH + 1 M C2H5OH at a scan rate of 50 mV s1.

Fig. 3, the onset potential for ethanol oxidation, peak current (jp) and peak potential (Ep) are listed in Table 1. It is found from Table 1 that all the prepared catalysts present similar potentials of the anodic peaks corresponding to the reverse scan while their anodic peak potentials in the forward scan change greatly. In addition to the much higher anodic current densities for ethanol oxidation, the present Pd catalysts exhibit more negative onset potentials than the electro-deposited Pd nanoparticles [5] (onset potential: ca. 0.60 V vs. SCE), polycrystalline Pd electrode [6] (onset potential: ca. 0.65 V vs. SCE) and NiO supported Pd/C electrocatalysts [24] (onset potential: ca. 0.58 V vs. SCE) under similar experimental conditions. Further, the onset potential of the nanoPd– HCHO–EDTA for the ethanol oxidation shifts to more negative direction compared to the other porous Pd catalysts. Table 1 also shows that nanoPd–HCHO–EDTA catalyst exhibits the largest ethanol oxidation peak current densities both in the forward and reverse scans. Based on the anodic oxidation peak current densities and onset oxidation potentials, the electrocatalysts of the present investigation follow the activity order: nanoPd–HCHO–EDTA > nanoPd–EG–EDTA > nanoPd–EG > nanoPd–PEG–EDTA > nanoPd– HCHO > nanoPd–PEG In addition, Table 1 presents the ratio (jp(fwd)/jp(rev)) of the magnitude of the peak current density on the forward scan to that on the reverse scan. The ratio is an accessorial method used to evaluate the CO tolerance of the catalysts [32,33]. All the prepared electrocatalysts exhibit higher jp(fwd)/jp(rev) values in excess of 1, much larger than those reported in literature [5]. The CO study involved in the poison-tolerance of the prepared catalysts is under way in this laboratory. Electroactivity of the nanoPd–HCHO–EDTA for the ethanol oxidation was further examined at different ethanol concentrations,

200

150

180

jp/mA cm-2

-1.0

c(EtOH)/M

160

120

0.1 M 0.3 M 0.5 M 1M 3M 5M

140

j/mA cm-2

120

90

100 50

100

150

200

250

300

scan rate/mV s-1

60 30 0 -30 -1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

E/V vs SCE Fig. 4. Cyclic voltammograms of the nanoPd–HCHO–EDTA in 1 M NaOH containing different ethanol concentrations at a scan rate of 50 mV s1. Inset is the dependence of the peak plateau current density (jp) on the scan rate in 1 M NaOH solution containing 5 M ethanol.

Table 1 Results of the study of CVs of the prepared catalysts in 1 M NaOH + 1 M C2H5OH. Catalyst

Nanopd–PEG Nanopd–PEG–EDTA NanoPd–HCHO–EDTA NanoPd–EG NanoPd–HCHO NanoPd–EG–EDTA a

Onset potentiala

Anodic peak for forward scan

Anodic peak for reverse scan

(V vs. SCE)

jp(fwd)/mA cm2

Ep(fwd)/V

jp(rev)/mA cm2

Ep(rev)/V

0.720 0.750 0.788 0.750 0.701 0.706

126 142 151 148 140 153

0.211 0.271 0.371 0.295 0.196 0.269

73 122 141 126 91 125

0.395 0.397 0.406 0.395 0.396 0.399

to to to to to to

0.144 0.039 0.028 0.042 0.01 0.031

Onset potential is defined as the potential at which 1 mA cm2 of current density is reached.

jp(fwd)/jp(rev)

1.7 1.2 1.1 1.2 1.5 1.2

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(a)

160

400th 360th 330th 300th 50th 10th 1st

j/mA cm-2

100 80

0.1

2.0

60 40

0.30

1.5 -Zi/ohm cm2

120

II

2.5

-Zi/ohm cm2

140

II

1.0

II 0.5

20

I

0.25 0.20

0.10 0.05

0.3

0

I

0.15

0.0

0.3

0.4

0.5

0.6

0.7

Zr/ohm cm2

0.5

-20 0 -1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

1

2

3

0.6

E/V vs SCE Fig. 5. Repeatedly sweeping CV profiles of the nanoPd–HCHO–EDTA at a rate of 50 mV s1 in 1 M NaOH + 0.5 M C2H5OH. Inset legend indicates the cyclic number.

(b) 0.45

-Zi/ohm cm2

1.6

30

-Zi/ohm cm2

2.0

(a)

1.2 0.8

-Zi/ohm cm2

25

0.4 0.0

20

-0.4 0.2

6

II

I

0.30

I

0.25 0.20

I

0.15

5

0.10 0.3

0.4

0.5

0.6

0.7

0.05

Zr=ohm cm2

15

5

II II

0.40 0.35

35

4

Zr/ohm cm2

0.00 0.2

(b)

10

R2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Zr/ohm cm2

C1 R1

5

3

1

Q

(c)

0 0

1

2

3

4

5

Zr/ohm cm2 Fig. 6. Electrochemical impedance spectroscopy (dots) and the fitted curves (solid lines) of the nanoPd–HCHO–EDTA catalyst in 1.0 M NaOH at a potential of 400 mV. The amplitude of modulation potential was 10 mV. Frequency was changed from 40 kHz to 50 mHz. Inset (a) is the zoomed main panel of the Nyquist plot. Inset (b) is the equivalent electric circuit compatible with the Nyquist diagram, where, R1: electrolyte resistance; R2: charge transfer resistance; Q: constant phase element (CPE) which is characterized by Yo and n; Yo: admittance (unit: X1); n: deviation extent of the capacitance of the electrode double layer from the ideal capacitance and 1 > n > 0; C: electrochemical capacitor.

decrease at the anodic sweep potential of ca. 0.046, 0.124 and 0.158 V, respectively, showing that the electrochemical oxidation of ethanol is a competitive anodic reaction against the formation of Pd oxides (Eq. (2)) at higher anodic potentials. However, the increase of ethanol concentration is favorable for the ethanol oxidation. Electrocatalytic stability of the nanoPd–HCHO–EDTA catalyst was also investigated using cyclic voltammetry (CV). Fig. 5 shows 400 consecutive CVs of 1 M NaOH + 0.5 M C2H5OH on the nanoPd–HCHO–EDTA catalyst that has been placed in air for 6 months without any special protection. It is observed from Fig. 5 that after being subjected to continuous sweeps, the nanoPd–HCHO–EDTA catalyst presents similar CV profiles to the freshly prepared one as shown in Figs. 3 and 4. In addition, the CV profiles are nearly unchanged with the increase of the cyclic number from the 1st to the 400th sweep. It is further found from

Fig. 7. Electrochemical impedance spectra (dots) and the fitted curves (solid lines) of the nanoPd–HCHO–EDTA catalyst in 1.0 M NaOH containing various ethanol concentrations at a potential of 400 mV ((a)-(b)). Numbers (0.1, 0.3, 0.5, 1, 3 and 5) indicate ethanol concentrations (M). Semicircles I and II represent the adsorption of OH and the electro-oxidation of ethanol respectively. The amplitude of modulation potential was 10 mV. Frequency was changed from 40 kHz to 50 mHz. Inset in (a) is the zoomed main panels. (c) is the corresponding equivalent electric circuit, where, R1: electrolyte resistance; R2 and R3: charge transfer resistance; Q: constant phase element (CPE) which is characterized by Yo and n; Yo: admittance (unit: X1); n: deviation extent of the capacitance of the electrode double layer from the ideal capacitance and 1 > n > 0; C1 and C2: electrochemical capacitor.

Fig. 5 that the nanoPd–HCHO–EDTA catalyst still exhibits high current density for ethanol oxidation. The anodic peak current density during the forward scan on the 400th CV is 150 mA cm2 which is very close to that shown in Table 1. Results show that the prepared nanoPd–HCHO–EDTA catalyst possesses high stability of electrocatalytic activity towards ethanol oxidation in alkaline media. 3.3. Electrochemical impedance spectra (EIS) Electrochemical impedance spectra (EIS) of the nanoPd–HCHO– EDTA catalyst in alkaline media were examined in order to further investigate its electroactivity for ethanol oxidation. Fig. 6 presents the Nyquist diagram of the nanoPd–HCHO–EDTA catalyst at

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Table 2 Values of the elements in different equivalent electric circuits (Inset (b) of Fig. 6 and Fig. 7c) fitted in the Nyquist plots (Fig. 6 and Fig. 7a and b) of the nanoPd–HCHO–EDTA catalyst at 400 mV. C/mol L1

R1 (X) C1 (lF) R2 (X) Yo n C2 (mF) R3 (X)

Inset (b) of Fig. 6

Fig. 7c

0

0.1

0.3

0.5

1.0

3.0

5.0

0.461 82.6 0.864 0.019 0.916

0.638 104.1 0.468 0.1382 0.4569 8.26 9.13

0.660 91.4 0.322 0.3808 0.2440 5.77 3.72

0.677 92.9 0.015 0.5415 0.1170 4.96 2.454

0.674 93.4 3.20 0.2087 0.0239 4.36 1.654

0.945 94.4 2.625 0.2134 0.0279 3.18 1.878

1.200 99.5 0.541 0.4910 0.1010 3.109 1.612

400 mV in 1.0 M NaOH solution. In the absence of ethanol a small semicircle at high frequencies (see the inset (a) of Fig. 6) develops while a linear plot arises at the range of lower frequencies. This semicircle can be ascribed to the formation of adsorbed hydroxyl OHad as shown in Eq. (1). Equivalent electric circuit compatible with the Nyquist diagram in the absence of ethanol is presented in the inset (b) of Fig. 6, where R1, R2, C and Q represent solution resistance, charge transfer resistance, capacitor and constant phase element (CPE), respectively. It is seen that the charge transfer resistance R2 is corresponding to the adsorption process of OH. The Nyquist diagrams of the nanoPd–HCHO–EDTA catalyst in the presence of ethanol are shown in Fig. 7, where a significant decrease of the resistance is witnessed in the total range of frequency examined. Fig. 7 shows that the presence of ethanol leads to the development of a new semicircle II at lower frequencies besides the semicircle I at high frequencies. Similarly, the first small semicircle I at high frequencies is attributed to the adsorption of OH on the surface of the nanoPd–HCHO–EDTA catalyst, as shown in Eq. (1). The semicircle II at lower frequencies is ascribed to the electro-oxidation of ethanol. A steady decrease of the diameter of the semicircle II with the increase of ethanol concentration is observed in the range of 0.1–1 M ethanol (Fig. 7a and b). This leads to the lowering of the charge transfer resistance and the increasing of the electro-oxidation rate of ethanol, depending upon the ethanol concentration in the solution. This is consistent with the analysis of CV data in Fig. 4. Equivalent electric circuit corresponding to the Nyquist diagrams in the presence of ethanol (see Fig. 7a and b) is illustrated in Fig. 7c, where R1 represents solution resistance, R2 and R3 present charge transfer resistances involved in OHads formation and ethanol oxidation corresponding to reactions (4) and (5), respectively. Compared to the equivalent circuit in the absence of ethanol (inset (b) of Fig. 6), Fig. 7c displays combination of (R3C2) corresponding to the electrochemical oxidation of ethanol. Consistency of the fitting curves (solid line) with the experimental data (dotted line) shown in Fig. 6 and Fig. 7a and b indicates the goodness of the fit and the reasonableness of the equivalent circuit. The values of the equivalent circuit elements obtained by fitting the experimental data are shown in Table 2. It is seen that the solution resistance R1 remains almost constant in the ethanol concentration range of 0–1 M due to the same supporting electrolyte of 1.0 M NaOH, except that the R1 gets in increase at high ethanol concentrations (3, 5 M). Table 2 shows that the charge transfer resistance R2, related to the electrochemical adsorption of OH (semicircle I), presents a very low value at any ethanol concentration examined in this work. This indicates a rapid electro-adsorption of OH on the active sites of the nanoPd–HCHO–EDTA catalyst. Also, Table 2 reveals very low values of the charge transfer resistance R3 in the presence of ethanol, and the R3 value decreases with the increase of ethanol concentration. The rapid decrease of the R3 value with ethanol concentration in the range of 0.1–0.5 M ethanol is consistent with the increasing trend of ethanol oxidation current shown in Fig. 4. Table 2 also shows that a relatively stable R3 value is ob-

tained at higher ethanol concentrations (1–5 M), corresponding to the peak current plateau of CV curves (Fig. 4). Results confirm that the prepared nanoPd–HCHO–EDTA catalyst exhibits significantly high electroactivity for the ethanol oxidation in alkaline media. 4. Conclusions A nanoporous electrode is usually expected to exhibit much higher surface area than a smooth electrode. The high surface area is vital to many electrochemical applications such as electrocatalysis. In this work we have successfully fabricated three-dimensional porous Pd particles directly immobilized on the surface of Ti substrate by using a one-step hydrothermal method. Titanium is chosen to be the support because of its good corrosion-resistance, good conductivity of electricity and reasonable cost. Addition of EDTA to PdCl2 solution is favorable for the formation of smaller nano-scale Pd particles of ca. 60 nm. Such a system may represent a promising route for the synthesis of other materials with well-defined porous structures. Electrocatalytic activity of the prepared porous Pd catalysts towards ethanol oxidation in 1 M NaOH solution is evaluated by cyclic voltammetry. The porous Pd catalysts of the present investigation exhibit significantly high anodic current densities and lower onset potentials towards ethanol oxidation in alkaline media. In 1 M NaOH in the presence of ethanol, electrochemical impedance spectra of the nanoPd– HCHO–EDTA catalyst display two semicircles with very low electrochemical resistances, corresponding to the formation of adsorbed hydroxyl OHads and electro-oxidation of ethanol, respectively. The highly electroactivity of the prepared porous Pd catalysts towards ethanol oxidation can be ascribed to their considerable numbers of active sites due to their three-dimensional textures. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20876038), Hunan Provincial Natural Science Foundation of China and Xiangtan Natural Science United Foundation of China (No. 10JJ9003), and the Planned Science and Technology Project of Hunan Province, China (No. 2009GK3084). Qingfeng Yi thanks the Project Sponsored by the Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry, China ([2007]1108). References [1] Antolini E, Salgado JRC, Gonzalez ER. The methanol oxidation reaction on platinum alloys with the first row transition metals: the case of Pt–Co and–Ni alloy electrocatalysts for DMFCs: a short review. Appl Catal B: Environ 2006;63:137–49. [2] Xue X, Ge J, Tian T, Liu C, Xing W, Lu T. Enhancement of the electrooxidation of ethanol on Pt–Sn–P/C catalysts prepared by chemical deposition process. J Power Sources 2007;172:560–9.

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