J. Phys. Chem. C 2007, 111, 11897-11902
11897
Dependence of Onset Potential for Methanol Electrocatalytic Oxidation on Steric Location of Active Center in Multicomponent Electrocatalysts Dongsheng Geng†,‡ and Gongxuan Lu*,† State Key Laboratory for Oxo Synthesis and SelectiVe Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, China ReceiVed: February 3, 2007; In Final Form: April 30, 2007
The dependence of onset potential for methanol oxidation on steric location of the active center in multicomponent electrocatalysts is reported in this paper. In general, the onset potential decreases with the decrease of lattice parameter in the catalysts investigated. By comparison of cyclic voltammetry (CV) and X-ray diffraction (XRD) results, it has been found that the onset potential decreases with decreasing lattice parameter for Bi and Pb series catalysts. The dependence of onset potential and lattice parameter showed a different tendency for Ir and Rh series catalysts, that is, an initial increase and then a decrease. This dependence may be due to the different steric locations of the active center corresponding to the lattice parameters (a). In addition, the current densities of methanol oxidation were also compared. Ir series catalysts displayed higher peak current densities than PtRu/C catalyst. Correlation of the structures and electrocatalytic activity of multicomponent catalysts were discussed.
Introduction In the last three decades, there has been a growing drive to study the direct electrocatalytic oxidation of methanol for potential use as a cell in so-called direct methanol fuel cells (DMFCs), because methanol is a renewable liquid fuel. DMFCs are silent and nonpolluting, and they use an easily distributed, high-energy-density fuel, methanol. However, the oxidation of methanol requires a catalyst to achieve high current densities required for practical application. One of the factors that limits development and utility is that the known anode and cathode electrocatalysts produce satisfied current densities only at high overpotentials.1 The anode catalyst is particularly problematic, because it must perform under kinetically demanding conditions to accomplish the six-electron (6 e-) oxidation of methanol: CH3OH + H2O f CO2 + 6H+ + 6e-. Platinum is known as an excellent catalyst for the dehydrogenation of methanol, but the process also requires the catalytic surface to provide the oxygen source for complete oxidation of methanol into carbon dioxide. Platinum is extremely susceptible to poisons such as CO, which is often an intermediate (typically from dissociative chemisorption of the fuel) during reactions. The CO remains strongly absorbed, especially at bridging and 3-fold hollow sites, and blocks the surface active sites from further involving catalytic reaction, resulting in a dramatic decrease of efficiency. To mitigate poisoning, Pt-based alloys (such as PtSn, PtNi, PtMo, PtOs, PtW, PtRh, PtRuSn, and PtRuIr, particularly PtRu) have been studied.1-20 The performance of these alloy catalysts is significantly enhanced compared with Pt catalyst itself. These alloys are thought to work by a bifunctional or ligand effect mechanism, in which the platinum activates the methanol C-H bond and also binds the CO intermediate, and the other metal * To whom correspondence should be addressed: tel/fax +86-9314968178; e-mail
[email protected]. † Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.
(or metals) activates water to provide the necessary oxygen for complete oxidation of methanol to CO2. Despite these extensive efforts, some factors about structure-property relationships are still not well understood. For example, the relationship between lattice parameter of catalysts and onset potential of methanol electrooxidation has been rarely reported. Although it is widely accepted that alloyed PtMs are the most active electrocatalysts for methanol oxidation, the different M influences the alloy degree, and the alloy degree in turn influences the steric location of active center. From the viewpoint of recovering a hint for designing high-performance anode catalyst, it is essential to study the structural effect of electrocatalysts. In this work, the structural effect of multicomponent electrocatalysts for methanol electrocatalytic oxidation has been investigated. The experiments were focused on the dependence of onset potential upon steric locations of the active center. The results clearly showed that onset potentials of methanol oxidation were gradually decreased with decreasing lattice parameter. 2. Experimental Section 2.1. Synthesis of Multicomponent Catalysts. The synthesis of electrocatalysts was carried out by a modified NaBH4 reduction method. The catalyst having 30 wt % metal and Pt: Ru:Ir weight ratio of 2:1:1 was typically prepared as follows: Carbon black (Vulcan XC-72, Cabot International) was impregnated with H2PtCl6, RuCl3, and H2IrCl6 precursor salts in water. Then the suspension was sonicated for 30 min. Subsequently, excess quantities of 0.1 M NaBH4 solution were added, drop-by-drop, into the suspension with vigorous stirring at 70 °C. After that, the mixtures were stirred for 5 h at this temperature for the complete reduction of Pt, Ru, and Ir from their respective metal salts. Finally, the resulting material was washed with distilled water several times and dried at 100 °C for 3 h. Herein, 17 different catalysts were prepared by this
10.1021/jp0709510 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/25/2007
11898 J. Phys. Chem. C, Vol. 111, No. 32, 2007
Geng and Lu ethanol and placing a drop of the suspension on a copper grid covered with perforated carbon film. X-ray photoelectron spectroscopy (XPS) was carried out on a VG Escalab 210 electron spectrometer with Mg KR radiation. A Philips X’Pert diffractometer equipped with Cu KR radiation was used to obtain the power X-ray diffraction (XRD) patterns. 2.3. Electrochemical Characterization. CHI 660A electrochemistry workstation and a three-compartment cell were employed for the electrochemical measurement. The glassy carbon (GC) electrode with an area of 0.07 cm2 was the working electrode. Pt wire and saturated calomel electrode (SCE) were used as the counterelectrode and reference electrode, respectively. All potentials in this study were reported with respect to this SCE. 2.3.1. Ink Preparation. In order to deposit the electrocatalysts on the working electrode, the ink was prepared. Typically up to 5 mg of the electrocatalysts was dispersed in ethanol (1 mL) and Nafion (20 µL) in an ultrasonic bath during 30 min. Twenty microliters of the ink was deposited onto the electrode and dried in an air oven for 30 min. 2.3.2. Methanol Electrooxidation Experiments. The voltammetry experiments were performed in 0.5 M H2SO4 solution containing 1 M CH3OH at a scan rate of 50 mV s-1. N2 gas was purged for nearly 30 min before the experiment was started. In all the experiments, stable voltammogram curves were recorded after scanning for seven cycles in the potential region from -0.25 to 1.0 V in 0.5 M H2SO4 solution. 3. Results and Discussion The XRD patterns of all electrocatalysts are shown in Figure 1. All of the XRD peaks could be indexed to the face-centered cubic (fcc) structure. The shift of the XRD peaks toward high diffraction angles indicated the alloying of Ru and the other metal into the fcc structure of Pt. NaBH4 is apparently such a strong reducing agent, especially under 70 °C, that it unselectively reduces all metal ions so as to form the required alloy phase. Further, we calculated the lattice parameter (afcc) values by using the equation:21
Figure 1. XRD patterns of all catalysts: (A) Wide-angle profile; (B) detailed measurements near the fcc 220 peak. (a) PtRu/C, (b) PtRu3Pb1/C, (c) PtRuPb/C, (d) PtRu1Pb3/C, (e) PtPb/C, (f) PtRu3Ir1/C, (g) PtRuIr/C, (h) PtRu1Ir3/C, (i) PtIr/C, (j) PtRu3Rh1/C, (k) PtRuRh/C, (l) PtRu1Rh3/C, (m) PtRh/C, (n) PtRu3Bi1/C, (o) PtRuBi/C, (p) PtRu1Bi3/ C, and (q) PtBi/C.
method. The nominal contents, presented by mass and molar percentages and metal loading amount, are summarized in Table 1. 2.2. Physical Characterization. The morphologies and size of all catalysts were characterized by transmission electron microscopy (TEM) (JEM1200EX). Sample preparation for TEM examination involved ultrasonic dispersion of the sample in
afcc )
x2λ sin θ
(1)
where λ is the wavelength of the X-ray (1.540 56 Å), θ is the angle at the maximum of the peak.
TABLE 1: Nominal Composition of the Prepared Catalysts contents, mass % sample
Pt
Ru
PtRu/C
19.8
10.2
PtRu3Pb1/C PtRuPb/C PtRu1Pb3/C PtPb/C
19.8 19.8 19.8 19.8
7.65 5.1 2.55
PtRu3Ir1/C PtRuIr/C PtRu1Ir3/C PtIr/C
19.8 19.8 19.8 19.8
PtRu3Rh1/C PtRuRh/C PtRu1Rh3/C PtRh/C PtRu3Bi1/C PtRuBi/C PtRu1Bi3/C PtBi/C
M
contents, mol %
total metal
carbon
Pt
30.0
70.0
50
50
Ru
M
2.55 5.1 7.65 10.2
30.0 30.0 30.0 30.0
70.0 70.0 70.0 70.0
53.6 57.5 62.0 67.3
39.9 28.6 15.4
6.5 13.9 22.6 32.7
7.65 5.1 2.55
2.55 5.1 7.65 10.2
30.0 30.0 30.0 30.0
70.0 70.0 70.0 70.0
53.3 56.9 61.0 65.7
39.7 28.3 15.1
7.0 14.8 23.9 34.3
19.8 19.8 19.8 19.8
7.65 5.1 2.55
2.55 5.1 7.65 10.2
30.0 30.0 30.0 30.0
70.0 70.0 70.0 70.0
50.3 50.4 50.5 50.6
37.5 25.0 12.5
12.2 24.6 37.0 49.4
19.8 19.8 19.8 19.8
7.65 5.1 2.55
2.55 5.1 7.65 10.2
30.0 30.0 30.0 30.0
70.0 70.0 70.0 70.0
53.6 57.6 62.1 67.5
40.0 28.6 15.4
6.4 13.8 22.5 32.5
Onset Potential in Multicomponent Electrocatalysts
J. Phys. Chem. C, Vol. 111, No. 32, 2007 11899
The fcc lattice parameters a of these catalysts are listed in Table 2 under the assumption that these alloy particles are completely homogeneous. From this table we can find that the lattice parameter a gradually decreased in the order PtRuPb/C > PtRuBi/C > PtRuIr/C > PtRuRh/C. These results indicated that the addition of the third metal changed the alloy degree of catalysts. Table 2 also presents the crystallite size of these catalysts determined by Scherrer’s equation:22
d)
0.9λ B2θ cos θ
(2)
where λ is the wavelength of the X-ray (1.540 56 Å), θ is the angle at the maximum of the peak, and B2θ is the width of the peak at half-height. All the crystallite sizes range were from 3.3 to 5.3 nm except for PtRu1Bi3/C and PtBi/C (7.2 nm). Methanol electrooxidations on all catalysts were measured by cyclic voltammetry in electrolytes of 0.5 M H2SO4/1 M CH3OH at room temperature. The cyclic voltammograms (CVs) of some catalysts (PtRu/C, PtRuIr/C, PtRuRh/C, PtRuPb/C, and PtRuBi/C) in H2SO4 without and with methanol are shown in Figure 2, panels A and B, respectively. Herein, we measured the electrochemical activity area (EAA, in square meters per gram) for all the catalysts by integrating charge in the hydrogen absorption region of the cyclic voltammogram in H2SO4 with the assumption of 210 µC cm-2 as the oxidation charge for one monolayer of H2 on a smooth Pt surface.23 The measured EAA values are given in Table 2. Rh and Ir series catalysts have higher electrochemistry active areas due to the fact that Rh and Ir have Pt-like H2 absorption characteristics. Onset potential of methanol oxidation in a positive scan was a key factor for evaluating the catalyst activity, which was determined by overlapping the CV of all catalysts in H2SO4 with and without methanol (Figure 2).24 The onset potentials of methanol oxidation for all catalysts were listed in Table 2. Herein, we compared the onset potential with lattice parameter. Because the lattice parameter reflects the alloy degree (viz., the compositional homogeneity), the results indicate the different steric locations of the active center. Figures 3 and 4 present the relationship between onset potential and lattice parameter for all catalysts. From Figure 3A,B, we can see that the onset potential decreases with the decrease of lattice parameter for Bi and Pb series catalysts. However, such phenomena cannot be found in Ir and Rh series catalysts (Figure 3C,D). The “volcano” and “N” type
Figure 2. Cyclic voltammetric curves recorded at 50 mV/s scan rate in 0.5 M H2SO4 without (A) and with (B) 1 M methanol.
tendency was obtained for Ir and Rh series catalysts, respectively. The onset potential increases initially and then decreases with the increase of a for Ir and Rh series catalysts. Although the above results were different, a dependence of onset potential on a in multicomponent electrocatalysts was obtained (Figure 4). In general, the onset potential decreases with decreasing lattice parameter in our catalysts investigated. From these results we can conclude that the catalysts with higher alloy degree have lower onset potential. This result is opposite to that of Lamy and co-workers,25 who reported that the alloyed PtRu/C catalyst was not the best catalyst. It is known that CO, an intermediate in methanol oxidation, strongly absorbs on Pt, typically at bridging and 3-fold hollow sites, and blocks the surface’s active site from further methanol
TABLE 2: Lattice Parameters, Crystallite Size,a and Onset Potential of Methanol Oxidation for All Catalysts catalysts
2θ, deg
d, Å
EAA, m2 g-1
crystallite size, nm
onset potential, V
10-3
a, Å
PtRu/C
68.5037
1.369 29
(3.871 ( 2.6) ×
14.5
3.3 ( 0.3
0.15
PtRu3Pb1/C PtRuPb/C PtRu1Pb3/C PtPb/C
67.4131 67.6071 67.4133 67.2549
1.389 23 1.385 71 1.389 22 1.392 11
(3.926 ( 1.7) × 10-3 (3.916 ( 1.8) × 10-3 (3.926 ( 1.7) × 10-3 (3.934 ( 1.5) × 10-3
13.7 4.4 8.8 7.7
4.0 ( 0.4 4.2 ( 0.4 4.1 ( 0.4 3.9 ( 0.3
0.27 0.25 0.3 0.34
PtRu3Ir1/C PtRuIr/C PtRu1Ir3/C PtIr/C
68.3694 68.4679 68.1302 67.8873
1.372 11 1.370 38 1.376 34 1.380 67
(3.878 ( 2.4) × 10-3 (3.873 ( 2.5) × 10-3 (3.890 ( 2.1) × 10-3 (3.902 ( 1.8) × 10-3
10.5 16.8 11.1 15.3
3.5 ( 0.4 3.3 ( 0.3 3.9 ( 0.5 5.3 ( 0.5
0.20 0.16 0.21 0.17
PtRu3Rh1/C PtRuRh/C PtRu1Rh3/C PtRh/C
68.6577 68.8031 68.9980 69.1004
1.367 05 1.364 52 1.361 14 1.359 37
(3.863 ( 2.7) × 10-3 (3.856 ( 3.0) × 10-3 (3.847 ( 3.2) × 10-3 (3.842 ( 3.4) × 10-3
12.3 17.5 18.3 16.5
3.7 ( 0.4 3.4 ( 0.4 3.9 ( 0.5 3.7 ( 0.3
0.14 0.2 0.15 0.12
PtRu3Bi1/C PtRuBi/C PtRu1Bi3/C PtBi/C
68.0956 67.8657 67.5843 67.5377
1.376 96 1.382 14 1.386 12 1.386 97
(3.891 ( 2.2) × 10-3 (3.908 ( 1.8) × 10-3 (3.917 ( 1.5) × 10-3 (3.920 ( 1.5) × 10-3
11.1 9.9 13.4 12.1
4.6 ( 0.4 5.4 ( 0.6 7.2 ( 0.7 7.2 ( 0.8
0.18 0.21 0.24 0.39
a
Determined from the X-ray diffraction peaks
11900 J. Phys. Chem. C, Vol. 111, No. 32, 2007
Geng and Lu
Figure 3. Plot of onset potential vs lattice parameter: (A) Bi, (B) Pb, (C) Ir, and (D) Rh series catalysts.
Figure 5. Models for three theoretical hemispheres PtM (M ) Ru, Ir, Bi, Pb, Rh) crystallites with different steric location of active center (aA > aB > aC).
Figure 4. Plot of onset potential vs lattice parameter for all catalysts.
electrooxidation, resulting in a dramatic decrease in efficiency. In general, the M atomic fraction (XM) in PtM/C (M ) Ru, Ir, Rh, Bi, Pb) alloy catalysts can be calculated by the following equation:4,26 XM ) (loc - afcc)/k, where loc ) 3.9155 Å is the lattice parameter for pure platinum supported on carbon and k ) 0.124 Å is a constant. From the equation it is clear that the atomic fraction of M in the PtM alloy increases with decreasing a value, which can be attributed to the incorporation of more M atoms into the lattice of Pt during PtM alloy formation. Figure 5 describes three hypothetical hemisphere models for PtM crystallite. They have the same diameter but different atom distributions and aA > aB > aC. Model A indicates more aggregation of Pt atoms than models B and C. From these models, we also know that there are more separated Pt atoms and Pt-M neighbors in model C. It can be seen that the steric
locations of active center are different for the different a values and Pt-Pt distance is enlarged because of the decrease of a. Therefore, lower lattice parameter implies higher alloy degree,26 which can enlarge the Pt-Pt distance and maximize the number of Pt-M neighbors and hence provides more nucleation sites and decreases the number of bridging and 3-fold hollow sites for CO adsorption, which increases methanol electrooxidation activity. These results are in accord with report of Wieckowski and co-workers,27 in which Pt-Ru boundary is the active center for methanol oxidation and more Pt-Ru boundaries can enhance the reaction activity. On the other hand, onset potential can be related to Ea (activation energy) for methanol oxidation. The lower onset potential shows a lower Ea value, that is, there is little driving force for methanol oxidation in this case. Table 3 shows some gas-phase bond dissociation energies of M-O diatomic compounds.28 The most oxophilic element is Ru. Although these bond strengths, derived from gas-phase diatomics, can provide only a rough guideline for electrochemical processes occurring at the metal/solution interface, some interesting trends emerge. The strength of the Pt-O bond is less than that of the other M-O bonds, that is, Ru, Ir, and Rh. Combining Pt with these oxophilic elements should give a more
Onset Potential in Multicomponent Electrocatalysts
J. Phys. Chem. C, Vol. 111, No. 32, 2007 11901
Figure 7. Typical TEM image of PtRuIr/C. Figure 6. Current densities for the oxidation of 1 M methanol in 0.5 M H2SO4 for all catalysts: (1) PtRu/C, (2) PtRu3Bi1/C, (3) PtRuBi/C, (4) PtRu1Bi3/C, (5) PtBi/C, (6) PtRu3Ir1/C, (7) PtRuIr/C, (8) PtRu1Ir3/ C, (9) PtIr/C, (10) PtRu3Pb1/C, (11) PtRuPb/C, (12) PtRu1Pb3/C, (13) PtPb/C, (14) PtRu3Rh1/C, (15) PtRuRh/C, (16) PtRu1Rh3/C, and (17) PtRh/C.
TABLE 3: M-O Bond Strengths for Gas-Phase Diatomic Compounds molecule
D°298K (kJ mol-1)
O-Pt O-Ru O-Rh O-Ir O-Pb O-Bi
391.6 ( 41.8 528.4 ( 41.8 405.0 ( 41.8 414.6 ( 42.3 382.0 ( 12.6 337.2 ( 12.6
active anode catalyst. However, the strengths of Pb-O and Bi-O are less than that of Pt-O. Herein, two factors must be taken into consideration for explaining the changed trend of onset potential. One is the change of a, and the other is the content of oxophilic element. For Pb and Bi series catalysts, the dispersion degree of Pt is improved and the content of the most oxophilic element Ru is also increased with decreasing a, which can overcome the barrier for electron transfer easily and then decrease the onset potential and Ea as shown in Figure 3A,B. For Rh and Ir series catalysts, because of the complicated strength order, Ru-O > Ir-O > Rh-O > Pt-O, although the compositional homogeneity is improved, the content of Ru is not increased gradually with decreasing a. So the “volcano” and “N” type changes are obtained in Figure 3C,D. The current densities of methanol electrooxidation on all catalysts also were given. A histogram based on representation was built to allow a better comparison of the results (Figure 6). As shown in Figure 6, Ir series catalysts (6-9) display the highest peak current densities for methanol electrooxidation among all catalysts. The activity of PtRuIr/C (7) is about 3.5 times than that of PtRu/C (1), especially. However, both Bi and Pb series catalysts (2-5 and 10-13, respectively) display lower current densities than that of PtRu/C. For the current densities of Rh series catalysts (14-17), only densities of PtRuRh/C and PtRu1Rh3/C are close to that of PtRu/C. The densities of PtRu3Rh1/C and PtRh/C are less than that of PtRu/C. The above facts indicate that PtRuIr/C is superior to others for methanol electrooxidation. The catalysts also were characterized by TEM and XPS methods. Figure 7 presents a typical TEM image of PtRuIr/C nanocatalyst. It can be seen that well-dispersed PtRuIr nanoparticles were formed on the carbon support and PtRuIr particles size ranges were 3-5 nm. No XPS peaks for chloride indicated that the Cl- ions were completely removed during washing. For PtRu/C, PtRuPb/C, PtRuBi/C, PtRuIr/C, and PtRuRh/C
Figure 8. Pt 4f peaks of X-ray photoelectron spectra.
alloy nanoparticles, typical Pt 4f spectra are shown in Figure 8. The Pt 4f7/2 and 4f5/2 lines appear at ∼71 and ∼74 eV, respectively, with a theoretical ratio of peak areas of 4:3. The comparison of the binding energies indicates that Pt is present in the zerovalent metallic state in all alloy nanoparticles. 4. Conclusion In conclusion, our studies of the onset potential of methanol electrooxidation and the lattice parameter for multicomponent electrocatalysts have provided important insights into the mechanistic correlations between onset potential and the steric location of the active center. The dependence of onset potential for methanol electrocatalytic oxidation on steric location of the active center in multicomponent electrocatalysts is established on the basis of the results analysis. The characterization results showed the onset potential decreases with decreasing lattice parameter in our catalysts investigated. These dependences can be attributed to the change of steric location of the active center (viz., the enlarged Pt-Pt distance) resulting from the decreasing lattice parameter. Detailed delineation of the proposed mechanism is part of our ongoing work. Acknowledgment. The 973 (G20000264) Research Fund is acknowledged for support of the research. References and Notes (1) Reddington, E.; Sapienza, A.; Guau, B.; Viswanathan, R.; Sarangapani, S.; Smotkin, E. S.; Mallouk, T. E. Science 1998, 280, 1735. (2) Antolini, E. Mater. Chem. Phys. 2003, 78, 563. (3) Le´ger, J.-M. Electrochim. Acta 2005, 50, 3123. (4) Guo, J. W.; Zhao, T. S.; Prabhuram, J.; Chen, R.; Wong, C. W.; Electrochim. Acta 2005, 51, 754.
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