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2009, 113, 12625–12628 Published on Web 06/26/2009
Active Sites for the Oxygen Reduction Reaction on the Low and High Index Planes of Palladium Shinpei Kondo, Masashi Nakamura, Norihito Maki, and Nagahiro Hoshi* Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba UniVersity, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan ReceiVed: May 7, 2009
Active sites for the oxygen reduction reaction (ORR) have been studied on the low index planes, n(100)(111) and n(100)-(110) series of Pd with the use of hanging meniscus rotating disk electrode (RDE) in 0.1 M HClO4 saturated with O2. The Kotekey-Levich plots give a linear line with a slope of about 4, showing four electron reduction for the ORR on Pd electrodes, as is the case of Pt. Activity of ORR, which is estimated by the reduction current density at 0.90 V (RHE) jORR,0.90V, gives the following order on the low index planes: Pd(110) < Pd(111) < Pd(100). This order is completely opposite to that of Pt in 0.1 M HClO4: Pt(100) < Pt(111) < Pt(110). Specific activity of Pd(100) is nearly three times as high as that of Pt(110) at 0.90 V (RHE). The values of jORR, 0.90 V increase linearly with the increase of the terrace atom density on n(100)(111) and n(100)-(110) series of Pd. This fact supports that (100) structure is the active site for ORR on Pd. ORR activity does not depend on the step structure of n(100)-(111) and n(100)-(110) series. Introduction Development of electrocatalysts that have high activity for the oxygen reduction reaction (ORR) is important in order to decrease of the amount of Pt loading in fuel cells. The activity of an electrochemical reaction strongly depends on the atomic arrangement of the electrode.1-4 ORR has been scrutinized on well-defined single crystal electrodes of Pt with the use of a hanging meniscus rotating ring electrode (RDE). The activity for ORR gives the following order on the low index planes of Pt in 0.1 M HClO4, in which no anion is strongly adsorbed on the surfaces: Pt(100) < Pt(111) < Pt(110).5 The Pt(110) surface has densely packed step lines,6,7 whereas Pt(111) and Pt(100) are composed of flat surfaces in electrochemical environments;8-12 the step is the active site for ORR on Pt electrodes. The study was extended to the high index planes of Pt. The activity for ORR increases with the increase of the step atom density on n(111)-(100), n(100)-(111), n(111)-(111), and n(110)-(111) series of Pt in 0.1 M HClO4,13,14 although the structural effects on n(100)-(111) series is less significant than the other series.13 These results also support that the step is the active site for ORR on Pt electrodes. (In the notation of high index planes, the value of n shows the number of terrace atomic rows. Miller indices after n and hyphen show the structure of terrace and step, respectively.) Pd electrodes have catalytic activity as high as Pt electrodes. The activity for formic acid oxidation on Pd electrodes is much higher than that on Pt electrodes,15,16 because no adsorbed CO is produced during formic acid oxidation on Pd electrodes.17 Azic et al. reported ORR on the single crystal electrode of Pd. The activity for ORR on Pd(111) is lower than that on Pt(111).18 A monolayer of Pt film on Pd(111) enhances the activity * To whom correspondence should be addressed. Phone/Fax: 81-43-2903384. E-mail:
[email protected].
10.1021/jp904278b CCC: $40.75
remarkably, giving the higher activity than Pt(111).18 However, there has been no systematic study on the activity of ORR on single crystal electrodes of Pd. In this paper, ORR has been studied on the low and high index planes of Pd using RDE systematically. The results are compared with those on Pt electrodes. Experimental Section A single crystal bead of Pt (about 3 mm in diameter) was prepared with the Clavilier’s method.19 A single crystals of Pd was prepared according to our previous reports.20,21 The crystal was oriented using the reflection beam of He-Ne laser from (111) and (100) facets on the crystal,22 and then mechanically polished with diamond slurries. The polished surface was annealed in an H2/O2 flame about 1300 °C for removing distortions due to the mechanical polishing and then cooled to room temperature in an Ar atmosphere. The annealed surface was protected with ultrapure water, and transferred to the electrochemical cell. The surfaces examined were the low index planes ((111), (100), and (110)), n(100)-(111) and n(100)-(110) series of Pd (n ) 2, 3, 5, and 9). The hard sphere models of the surfaces are shown in the inserts of Figures 2 and 3. Electrolytic solutions were prepared from ultrapure water treated with Milli Q Advanced (Millipore) and suprapure grade chemicals (Merck). All of the potentials were referred to RHE. Voltammograms of ORR were measured in the hanging meniscus RDE configuration.13,14,23,24 Electrochemical analyzer (ALS 701C) and rotating ring disk electrode apparatus (BAS: RRDE-3) were used for the electrochemical measurements. Potential was scanned from 0.20 V, which is before the onset potential of hydrogen absorption, to the positive direction up to 1.0 V at scanning rate 0.010 V s-1. 2009 American Chemical Society
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Figure 1. (a) Rotating rate dependence of the voltammograms of the oxygen reduction reaction on the Pd(100) electrode in 0.1 M HClO4 saturated with O2. Scanning rate: 0.010 V s-1. (b) Koteckey-Levich plots of the voltammograms of oxygen reduction reaction on Pd(100) at various potentials.
Figure 2. Voltammograms of the oxygen reduction reaction using a rotating disk electrode in 0.1 M HClO4 saturated with O2. Rotation rate: 2000 rpm. Scanning rate: 0.010 V s-1. (a) The low index planes of Pd. The result of Pt(110) is also shown for comparison. The inset shows the hard sphere models of the low index planes. (b) The low index planes of Pt.
Results and Discussion
TABLE 1: Current Density of Oxygen Reduction Reaction jORR,0.90V at 0.90 V (RHE) in 0.1 M HClO4
Pd single crystal electrodes give voltammograms characteristic of their orientation in 0.5 M H2SO4. The voltammograms of the prepared electrodes agreed with those reported previously,20,21 thus we judged that the surfaces were welldefined. Voltammograms during rotation at 2000 rpm were identical with those of stationary conditions in 0.1 M HClO4 saturated with Ar, verifying that the solution is extremely clean. Kouteckey-Levich Plots. Figure 1a shows the rotating rate dependence of the ORR voltammogram on Pd(100) in 0.1 M HClO4 saturated with O2. The current of ORR is observed below 1.0 V. The rate of an electrochemical reaction (current) depends exponentially on the overpotential η, which is defined as η ) E - Er, where E and Er are the applied and equilibrium potentials, respectively. The current does not depend on the rotation rate between 1.0 and 0.9 V because ORR is controlled by the rate of electron transfer. The rotation rate affects the current of ORR at lower potentials, at which the rate of electron transfer is faster than the diffusion rate. The current at this potential range is called the limiting current. The limiting current does not depend on the applied potential, because the current is controlled by the diffusion rate. According to the KoteckeyLevich eq 1,25,26 the inverse of the total current density of ORR j-1 depends linearly on the inverse of the square root of the angular velocity ω-1/2 of the RDE. The slope of j-1 vs ω-1/2 plots (Koteckey-Levich plots) gives the number of electrons for the electrochemical reaction.
1 1 1 + ) j jtrans 0.62nFAD2/3ν-1/6cbω1/2
(1)
where n is the number of electrons for the electrochemical reaction, F is the Faraday constant, A is the area of the electrode, D is the diffusion coefficient, ν is the kinematic viscosity, and cb is the bulk concentration, ω: the rotation rate. Following values are used for the analyses: A ) 0.082 cm2, D ) 1.93 ×
Pd (hkl)
jORR,0.90V/mA cm-2
Pt (hkl)
jORR,0.90V/mA cm-2
(100) (111) (110)
1.67 0.12 0.04
(100) (111) (110)
0.43 0.59 0.60
10-5 cm2 s-1, cb ) 1.26 × 10-3 mol dm-3, and ν ) 1.009 × 10-2 cm2 s-1.27 Figure 1b shows the Koteckey-Levich plots of the ORR on Pd(100) electrode between 0.30 and 0.88 V. The plots give a linear line at each potential. The number of electrons of the ORR is 3.7. The other low index planes (Pd(111) and Pd(110)) gave the similar results. These results show that oxygen is reduced via four electron reaction on Pd electrodes, as is the case of Pt electrodes. Low Index Planes. Figure 2a shows voltammograms of ORR on the low index planes of Pd. The results on Pt electrodes are also shown for comparison in Figure 2b. We measured the first scan after the annealing to study ORR on a well-defined surface. ORR depends on the surface structure strongly at the potential region where the reaction is controlled by electron transfer. The activity for ORR is estimated by the current density at 0.90 V according to previous studies.28,29 The following order is obtained for the activity on the low index planes of Pd: Pd(110) < Pd(111) < Pd(100). Flat Pd(100) has the highest activity for ORR in the low index planes of Pd. On the low index planes of Pt, the rate of ORR increases as Pt(100) < Pt(111) e Pt(110) at 0.90 V, as is the case in the previous reports.5 The activity series on Pd is completely opposite to that on Pt. We do not know the reason at present. Table 1 summarizes the current densites of ORR at 0.90 V j ORR,0.90 V on the low index planes of Pd and Pt. The activity of Pd is lower than that of Pt on (111) and (110) surfaces, whereas the value of jORR,0.90V on Pd(100) is more than four times as high as that on Pt(100). The value of jORR,0.90V on Pd(100), which has the highest activity in the Pd electrodes, is compared with that on Pt(110) that gives the highest current density in the low index planes of Pt. The value of jORR,0.90V
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J. Phys. Chem. C, Vol. 113, No. 29, 2009 12627 ORR. On the high index planes of Pt, the activity for ORR increases with the increase of the step atom density.13,14 The structural effect is the most remarkable on n(111)-(111) series of Pt.14 However, the ratio of the lowest to the highest current density of ORR is only 3 in 0.1 M HClO4. On n(100)-(111) and n(100)-(110) series of Pd, the ratio is over 16. The structural effect on Pd electrodes is more drastic than that on Pt electrodes. Higher coverage of the oxide film deactivates ORR strongly on Pt electrodes.3 On Pd electrodes, however, Pd(100) has a higher activity for ORR than Pt(111), although the coverage of the oxide film on Pd(100) is much higher than that on Pd(111) at 0.90 V. Reaction mechanism of ORR on Pd electrodes may differ from that on Pt electrodes. A study using Tafel plot is now underway in our laboratory. Conclusion
Figure 3. Voltammograms of the oxygen reduction reaction on (a) n(100)-(111) and (b) n(100)-(110) series of Pd electrodes using rotating disk electrode in 0.1 M HClO4 saturated with O2. Rotation rate: 2000 rpm. Scanning rate: 0.010 V s-1. Insets show the hard sphere models of the high index planes.
Specific activity for oxygen reduction reaction (ORR) has the following order on the low index planes of Pd at 0.90 V (RHE) in 0.1 M HClO4: Pd(110) < Pd(111) < Pd(100). This order is completely opposite to that on Pt in 0.1 M HClO4: Pt(100) < Pt(111) e Pt(110). Current density of ORR on Pd(100) is nearly three times as high as that on Pt(110) at 0.90 V (RHE). Current density of ORR at 0.90 V (RHE) depends linearly on the terrace atom density on n(100)-(111) and n(100)(110) series of Pd. The current densities plotted against the terrace atom density are on the same line on both series. These results verify that (100) structure is the active site for ORR on Pd electrodes. Difference of step structure has nothing to do with the activity for ORR. Acknowledgment. This work was supported by New Energy Development Organization. References and Notes
Figure 4. Oxygen reduction current density at 0.90 V (RHE) jORR,0.90V plotted against the density of terrace atoms dT on n(100)-(111) and n(100)-(110) series of Pd.
on Pd(100) is nearly three times as high as that on Pt(110). This fact indicates extremely high activity of Pd(100) for ORR. High Index Planes. Previous studies report that the high index planes of Pd have specifically high activity for formic acid oxidation.15,16 We measured ORR on n(100)-(111) and n(100)-(110) series of Pd for the elucidation of the active sites for ORR. High index planes of n(100)-(111) and n(100)-(110) series are composed of a (100) terrace and a monatomic step. Step atoms of n(100)-(111) series form linear step lines, whereas n(100)-(110) series have protruded kink atoms in the step, as shown in the insets of Figure 3a,b. Figure 3a,b presents voltammograms of ORR on n(100)(111) and n(100)-(110) series of Pd in 0.1 M HClO4 saturated with O2. The current density of ORR increases with the increase of terrace atomic rows n on both series. The values of jORR,0.90V are plotted against the terrace atom density dT in Figure 4. The plots give a linear line on both series. Although the step structure of n(100)-(111) series completely differs from that of n(100)-(110) series, the plots of both series are on the same line. These results clearly support the claim that the (100) structure is the active site for ORR on Pd electrodes. The difference of the step structures has no effect on the
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JP904278B
