CeO2 Promoted Electro-Oxidation of Formic Acid on

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for formic-acid oxidation is crucial for the development of DFAFC. Recently ... commonly used catalyst in DMFC, because of the decreased poison- ing of Pd.6 ...
Electrochemical and Solid-State Letters, 12 共5兲 B73-B76 共2009兲

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CeO2 Promoted Electro-Oxidation of Formic Acid on PdÕC Nano-Electrocatalysts Yi Wang, Shuangyin Wang, and Xin Wang*,z School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 639798 Pd-CeO2 /C electrocatalyst was successfully prepared by first depositing CeO2 onto carbon black through the microwave-assisted heating process under the presence of ammonium, followed by the reduction of Pd nanoparticles. In order to demonstrate the promotive role of CeO2 in the Pd–CeO2 /C system, various electrochemical characterization methods were performed for the formic acid electro-oxidation. It was found that Pd–CeO2 /C showed more enhanced electrocatalytic activity and improved kinetics than conventional Pd/C. It was proposed that the existence of CeO2 may promote the proceeding of the direct oxidation pathway 共dehydrogenation兲 instead of the dehydration pathway. © 2009 The Electrochemical Society. 关DOI: 10.1149/1.3086263兴 All rights reserved. Manuscript submitted December 15, 2008; revised manuscript received January 13, 2009. Published February 26, 2009.

The advantages of a direct formic-acid fuel cell 共DFAFC兲 have been progressively recognized compared to a direct methanol fuel cell 共DMFC兲.1-3 For instance, formic acid is an avirulent, nonflammable liquid fuel with a low crossover rate through the Nafion membrane; hence, a relatively high power density is possible with aqueous solutions. It is well known that the research on electrocatalysts for formic-acid oxidation is crucial for the development of DFAFC. Recently, considerable effort to develop catalytic materials having high activity toward formic-acid oxidation has been carried out.4-9 Generally, Pd metal has been found to be more active than Pt, the commonly used catalyst in DMFC, because of the decreased poisoning of Pd.6 Various forms of Pd-based catalysts, such as palladiumcoated Pt共110兲,8 Pd/C nanocatalyst,5 and Pd-based bimetallic catalysts,9 were developed and reported to exhibit high electrocatalytic activity toward formic-acid oxidation. Transition metal oxides have been found to exhibit a promotive role for the electro-oxidation of methanol.10,11 For instance, Xu and Shen10 prepared Pt–CeO2 /C catalyst used in alkaline methanol fuel cells, where CeO2 shows evident cocatalytic effect for methanol electro-oxidation. The promoting behavior of CeO2 is attributed to oxygen storage capacity at low temperatures, higher reducibility in the presence of Pt, higher dispersion of Pt over CeO2, and prevention of sintering of Pt metal particles.12 In this paper, we prepared the Pd–CeO2 /C electrocatalyst and studied its activity toward the electro-oxidation of formic acid. To the knowledge of the authors, the catalytic performance of the catalysts containing CeO2 toward formic-acid electro-oxidation has not been reported before.

Pd–CeO2 /C were performed by X-ray diffraction 共XRD, Rigaku D/max-2500兲 and transmission electron microscopy 共TEM, JEOL 3010, 200 kV兲. Cyclic voltammetry 共CV兲 and linear sweep voltammetry 共LSV兲 were collected in 0.5 M H2SO4 + 0.5 M HCOOH solution. The working electrode was prepared by dropping 10 ␮L of the electrocatalyst ink onto glass carbon electrode 共GCE兲. The diameter of the GCE was 5 mm. The ink was prepared by ultrasonically mixing 4 mg of the electrocatalyst sample in 2 mL of ethanol. Then, 1 ␮L of Nafion solution of 0.5% in 2-propanol was added on top to fix the electrocatalysts. Pt wire and a saturated calomel electrode 共SCE兲 were used as the counter and reference electrodes, respectively. All potentials in the present study were given vs SCE. The CV test was conducted at 50 mV/s, with potential ranging from −0.2 to 1.0 V. The LSV was measured at 20 mV/s in different temperatures, and the potential range was the same as that of CV. CO stripping was performed as follows. After purging the solution with N2 for 20 min, gaseous CO was bubbled for 15 min to form CO adlayer on catalysts while maintaining potential at 0.1 V. Excess CO in solution was purged with N2 for 20 min, and CO stripping voltammetry was recorded in 0.5 M H2SO4 at 10 mV/s. Results and Discussion Figure 1 compares XRD patterns for Pd/C and Pd–CeO2 /C. Both samples show peaks characteristic of Pd. In the XRD pattern for Pd–CeO2 /C, the diffraction peaks of CeO2 were observed, indicat-

Experimental CeO2 was deposited onto the carbon black by dropwise adding NH3·H2O into a beaker containing carbon black and Ce共NO3兲3 aqueous solution and then evaporating water by heating under 90°C for 1.5 h, followed by microwave heating for 120 s. After rinsing with water, the composite product was dried under 100°C. The CeO2-deposited carbon black was denoted as CeO2 /C, in which the theoretical weight ratio of CeO2 to C is 60:100. The CeO2 /C was dispersed into 80 mL water, and then the stoichiometric 10 mM K2PdCl4 aqueous solution was added to the aqueous suspension under magnetic stirring. Afterward, NaBH4 aqueous solution was dropwise added. After 2 h deposition of Pd particles on CeO2 /C, the suspension was filtered and dried under 105°C. The prepared catalyst was denoted as Pd–CeO2 /C. The theoretical content of CeO2 and Pd was 30 and 20%, respectively. For comparison, Pd was deposited on carbon black in the same way. The theoretical Pd content is 20%, and the catalyst is denoted as Pd/C. The morphology and microstructure analysis of Pd/C and

* Electrochemical Society Active Member. z

E-mail: [email protected]

Figure 1. 共Color online兲 XRD patterns of Pd/C and Pd–CeO2 /C.

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Electrochemical and Solid-State Letters, 12 共5兲 B73-B76 共2009兲

Figure 2. TEM and EDS of different catalysts: Pd/C 共a and c兲 and Pd–CeO2 /C 共b and d兲.

ing the coexistence of both CeO2 and Pd. The particle crystalline sizes of Pd and CeO2 in Pd–CeO2 /C were calculated to be 7.01 and 11.58 nm, respectively, and the particle crystalline size of Pd in Pd/C was calculated to be 6.13 nm using Scherrer’s formula d = 0.9␭K␣1 /B2␪ cos ␪max. In addition, as compared to the peaks of Pd in Pd/C, there is an obvious shift in the diffraction peaks of Pd in Pd–CeO2 /C, suggesting that the addition of oxide has an effect on the crystalline lattice of palladium. To observe the overall morphology of Pd/C and Pd–CeO2 /C, TEM images were recorded and are shown in Fig. 2. As seen from Fig. 2a, the Pd nanoparticles are homogeneously distributed over the carbon surface in Pd/C. For Fig. 2b, although we cannot distinguish Pd nanoparticles from CeO2 nanoparticles, we can see that the particles are not uniformly dispersed and aggregate to a certain degree. It was reported13 that Pt nanoparticles were located adjacent to CeO2 particles other than on bare carbon nanotubes 共CNTs兲 in Pt/CeO2–CNT composites because Pt nanoparticles readily adsorb onto CeO2–CNTs but not onto CNTs. In our case, we deduce that Pd nanoparticles may prefer surrounding CeO2 nanoparticles, and thus the slight aggregation of the particles was observed. Figures 2c and d show the energy-dispersive spectroscopy 共EDS兲 images of Pd/C and Pd–CeO2 /C catalysts, which confirmed the presence of Pd in Pd/C, together with Pd and Ce in Pd–CeO2 /C, respectively. Additionally, according to the EDS analysis results, the contents of Pd and CeO2 in Pd–CeO2 /C are 24.9 and 27.8 wt %, which deviate a little from the theoretical contents of 20 and 30 wt %, respectively. Figure 3a presents the CVs of formic-acid electro-oxidation on Pd/C and Pd–CeO2 /C electrodes. The strong oxidation peaks belong to the oxidation of formic acid and the corresponding intermediates.14 The peak potential of the formic-acid oxidation locates at 0.18 V for Pd/C, and the peak potential negatively shifts to

0.10 V for Pd–CeO2 /C. In addition, the results show not only that the peak current density is much higher on Pd–CeO2 /C than that on Pd/C but also that the onset potential for formic-acid oxidation shifts to a more negative direction, indicating that formic-acid electro-oxidation is more active on Pd–CeO2 /C than on Pd/C. Therefore, the addition of CeO2 has an obviously promotive effect on the electro-oxidation of formic acid. In order to investigate the kinetics of formic-acid oxidation on Pd–CeO2 /C and Pd/C electrodes, the LSVs at different temperatures on the two electrocatalysts are recorded. Figure 3b shows the relationship of the reciprocal of temperature and the logarithm of peak current 共at 0.2 V兲. It was found that the peak current for Pd–CeO2 /C was always larger than that for Pd/C at all the measuring temperatures, which indicates that Pd–CeO2 /C has higher electrocatalytic activity. However, this difference in the measured peak current may simply be due to the difference in size or size distribution of Pd nanoparticles. To exclude this effect, an apparent activation-energy value was calculated according to the Arrhenius equation15 i = Ae−Ea/RT

关1兴

ln i = const − Ea /RT

关2兴

where i is the peak current, R is the gas constant, T is the temperature in Kelvin, and Ea is the apparent activation energy at the peak potential. By linearly fitting the relationship of ln i and 1/T, it can be found that the Ea 共30.85 kJ/mol兲 for Pd–CeO2 /C is obviously smaller than that 共48.64 kJ/mol兲 for Pd/C. A smaller activation energy for the case of Pd–CeO2 /C indicates that, using Pd–CeO2 /C as electrocatalysts, the charge-transfer process is faster. In short, CeO2 obviously promotes the kinetics of the electro-oxidation of formic acid catalyzed by Pd/C.

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Electrochemical and Solid-State Letters, 12 共5兲 B73-B76 共2009兲 45

0.00015

(a)

40

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Pd/C Pd-CeO2/C

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-2

i/A cm

-2

25 20 15

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st

1 scan nd 2 scan

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i/mA cm

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5 0

-0.00010

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1.0

-0.2

0.0

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E/V (vs.SCE)

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Pd/C 0.00010

st

1 scan nd 2 scan

-2

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i/A cm

ln (i (µΑ))

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10.0 9.8 9.6 9.4 9.2 9.0 8.8 8.6 8.4 8.2 8.0 7.8 7.6 7.4 7.2

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0.00305 0.00310 0.00315 0.00320 0.00325 0.00330 0.00335 -1

1/T (K )

The investigation of the mechanism of formic-acid oxidation has a long history.16-18 The most commonly accepted mechanism of formic-acid oxidation is the so-called parallel or dual pathway mechanism.18 Direct oxidation on Pd catalysts occurs via a dehydrogenation reaction without forming CO as a reaction intermediate Pd + HCOOH → Pd + CO2 + 2H+ + 2e−

关3兴

The second reaction pathway forms adsorbed CO as a reaction intermediate by dehydration Pathway 2

Pd + HCOOH → Pd–CO + H2O

0.0

0.2

0.4

0.6

0.8

1.0

E/V (vs.SCE) Figure 4. 共Color online兲 CO stripping curves on Pd/C and Pd–CeO2 /C.

Figure 3. 共Color online兲 CVs of formic-acid electro-oxidation on Pd/C and Pd–CeO2 /C catalysts; 共b兲 the plot of ln i vs 1/T 共i: current at 0.2 V; T: temperature in Kelvin兲.

Pathway 1

-0.2

关4兴

Pd + H2O → Pd–OH + H+ + e−

关5兴

Pd–CO + Pd–OH → Pd + CO2 + H+ + e−

关6兴

Pathway 2 is similar to the well-known bifunctional catalysis of methanol electro-oxidation on Pt-alloy catalysts, which involves the successive oxidation of the functional group 共–OH group兲 with adsorbed CO on Pd surface. To enhance overall cell efficiency and avoid poisoning of the catalyst, dehydrogenation is the desired reaction pathway, while the formation of Pd–CO species is undesired. It was reported19 that OHad species could also form on the surface of oxide. The formation of OHad species at lower potentials can oxidize CO-like or other carbonaceous species on the surface of Pd to CO2,

releasing the active sites on Pd for the electrochemical oxidation reaction. This might explain the significantly promotive effect of CeO2 in the Pd/C electrocatalysts for the electro-oxidation of formic acid. To verify the above-proposed role of CeO2, CO stripping curves were collected, as shown in Fig. 4. However, it can be found that the peak and onset potential of CO stripping, and the CO coverage on Pd–CeO2 /C are similar to those on Pd/C. This indicates that the promotive effect of CeO2 does not result from the easier electro-oxidation of CO on Pd–CeO2 /C than on Pd/C. We believe that the existence of CeO2 may promote the proceeding of the first reaction pathway 共dehydrogenation兲 instead of the dehydration step, which leads to the enhanced charge transfer of the overall reaction. Further investigations of the mechanism are ongoing at our lab. Conclusions In this study, Pd–CeO2 /C nanocatalyst was prepared. The addition of CeO2 into Pd/C catalysts could significantly improve the catalytic performance for formic-acid oxidation in terms of current density, peak potential, and kinetics. Although OHad species could form on the surface of CeO2 and the formation of OHad species could transform CO-like species on the surface of Pd to CO2, releasing the active sites on Pd for further electrochemical oxidation reaction, this is not the main reason for the promotive effect of CeO2 according to CO stripping curves. It is believed that the existence of CeO2 may promote the proceeding of the first reaction pathway 共dehydrogenation兲.

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Electrochemical and Solid-State Letters, 12 共5兲 B73-B76 共2009兲 Acknowledgments

This work is supported by Academic Research Fund AcRF tier 1 共RG40/05兲 and AcRF tier 2 共ARC11/06兲, Ministry of Education, Singapore. Nanyang Technological University assisted in meeting the publication costs of this article.

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