Pt Electrode Enhanced by

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J. Phys. Chem. C 2008, 112, 18672–18676

Methanol Electrochemical Oxidation on Au/Pt Electrode Enhanced by Phosphomolybdic Acid Y. Q. Wang,†,‡,§ Z. D. Wei,*,†,‡,§ L. Li,§ M. B. Ji,§ Y. Xu,‡ P. K. Shen,*,| J. Zhang,§ and H. Zhang§ State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing UniVersity, Chongqing, 400044; China, School of Chemistry and Chemical Engineering, Chongqing UniVersity, Chongqing, 400044; China, School of Material Science and Engineering, Chongqing UniVersity, Chongqing, 400044; China, The State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, China ReceiVed: April 10, 2008; ReVised Manuscript ReceiVed: September 10, 2008

Methanol electro-oxidation was investigated on a Au-modified Pt (Au/Pt) electrode together with phosphomolybdic acid (H3PMo12O40, PMo12). Au/Pt electrode was prepared by chemically replacing underpotentially deposited Cu on Pt surface in HAuCl4 solution. The morphology of a Au/Pt electrode was characterized by field emission scanning electron microscope. The results showed that Au particles with a mean size of 10 nm were uniformly dispersed on the Pt surface. The methanol electro-oxidation on the Pt electrode was markedly enhanced not only by the phosphomolybdic acid solution but also by adatom Au. The onset potential of methanol oxidation shifts 400 mV toward the negative direction on the Au/Pt electrode with phosphomolybdic acid in comparison with a Pt electrode with phosphomolybdic acid. It is supposed that adsorbed hydrogen and intermediate CO from the methanol dehydrogenation and oxidation were electrocatalytically oxidized by the oxidant state of phosphomolybdic acid with the aid of Au catalysis. 1. Introduction The direct methanol fuel cell (DMFC), which can convert the chemical energy stored in the methanol directly into electricity, is suited for portable devices and transportation applications due to its high energy density at low operation temperature and ease of handling a liquid fuel.1 However, two problems block the DMFC from commercialization: the high loading of noble metal catalysts and the slow oxidation kinetics of methanol. To date, much effort has been devoted to the development of electrocatalysts by increasing catalytic activity and reducing noble metal loadings. Much work has been done to look for a new and efficient catalyst that can not be poisoned easily by intermediate CO, such as the bicomponent catalysts Pt-X/C(X ) Ru, WOx, Mo, Sn, TiO2),2-8 and the tricomponent catalyst Pt-Ru-X/C (X ) Mo, W, Cu, Au, Sn, Ni, Co, Fe)9-16 based on the so-called “bifuctional mechanism”. Among them, Pt-WOx and Pt-Ru-WOx outclasses Pt, Pt-Sn, and Pt-Ru in electrocatalytic activity, but their stability is in doubt in an acid medium. In recent years, surface-modified electrocatalysts have attracted much attention due to their unique structure and new electronic and electrocatalytic property.17,18 Heteropoly acids consisting of heteropolyanion and balanced cationic ion with cagelike structure have attracted much attention due to their strong proton-conducting ability and reduction/oxidation property. For instance, heteropoly acids with a formula of H4SiW12O40 can enhance the methanol oxidation on Pt-Ru/ Sn/C electrodes via reinforcing OH adsorption on electrode * Corresponding authors. (Wei) Tel: +86 23 60891548. Fax: +86 23 65106253. E-mail: [email protected]. (Shen) E-mail: [email protected]. † State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University. ‡ School of Chemistry and Chemical Engineering, Chongqing University. § School of Material Science and Engineering, Chongqing University. | Sun Yat-Sen University.

Figure 1. Schematics in the preparation of the Au/Pt electrode.

surface.19,20 It has been proved that the methanol electrooxidation on Pt and Pt-Ru electrodes could be enhanced by PMo12, another form of heteropoly acids.21-23 This work is motivated by a recent observation from Won Bae Kim et al. 24,25 that CO(g) can be oxidized by phosphomolybdic acid under catalysis of Au, and the reduced phosphomolybdic acid sequentially serves as fuel for the PEMFC. It is well-known that CO is a harmful intermediate to Pt catalysts in methanol electroxidation.26,27 Therefore, we cannot help wondering whether there is a possibility to effectively remove CO by introduction of phosphomolybdic acid and Au in methanol electrooxidation? On the basis of this point, in the present report, methanol electro-oxidation is investigated on a Au-modified Pt electrode together with phosphomolybdic acid. 2. Experimental 2.1. Chemicals and Instruments. A Pt disk (geometric area 0.071 cm2, diameter 3 mm) and a Pt wire served as the working electrode and the counter electrode, respectively. A saturated potassium chloride silver chloride electrode (SSCE) was used as the reference electrode. A CHI 440A electrochemical station (CH Instruments, Inc.) was employed for the synthesis and electrochemical assessment of the catalysts. All the chemicals were of analytical grade. Ultrapure water (18.2 MΩ) was used throughout. The morphology of the Au/Pt electrode was

10.1021/jp8058646 CCC: $40.75  2008 American Chemical Society Published on Web 11/01/2008

Electrochemical Oxidation on Au/Pt Electrode

J. Phys. Chem. C, Vol. 112, No. 47, 2008 18673

Figure 2. CV of a Pt disk in 0.2 mol L-1 H2SO4 (dashed line) and in 0.2 mol L-1 H2SO4 plus 0.01 mol L-1 CuSO4 (solid line) at a sweep rate of 10 mV s-1.

characterized by field emission scanning electron microscope (FESEM) (FEI Nova 400, Peabody, MA). 2.2. Preparation of the Au/Pt Electrode and Electrochemical Studies. The schematic steps for preparation of the Au/Pt electrode are shown in Figure 1. The first step is to form an underpotential deposition (upd) Cu adlayer on the Pt surface; then the upd-Cu adlayer is replaced by Au in the HAuCl4 solution. The process of replacement takes place via reaction 1.

3upd-Cu ⁄ Pt + 2AuCl4-)2Au ⁄ Pt + 3Cu2+ + 8Cl+ (1) The amount of Au produced by replacement of the upd-Cu adlayer depends on the coverage of the upd-Cu adlayer, which can be adjusted by means of deposition potential and time as well as the concentration of the Cu ions used for the underpotential deposition of Cu. In detail, a Pt disk electrode was first polished on a polishing cloth with 0.05 µm alumina slurry in water and then cleaned by ultrapure water from a Milli-Q water treatment system. Then the electrode was immersed in nitrogen-purged 0.5 M H2SO4 and was voltammetrically scanned from 0 to 1.8 V (vs Ag/ AgCl) at a rate of 100 mV s-1 to clean the surface. The cleanliness of the Pt disk surface was checked by cyclic voltammetry from -0.2 to 1.2 V (vs Ag/AgCl). All potentials in this study are quoted with reference to SSCE unless otherwise stated. The cleaned Pt disk was immersed in a solution of 0.2 M H2SO4 containing 0.01 M CuSO4 and was kept at 0.15 V for a length of time to produce a upd-Cu adlayer on the Pt surface. No sooner was the upd-Cu prepared than the electrode was rinsed with ultrapure water and instantly transferred to the solution containing 0.5 mM HAuCl4, 0.5 M HCl, and 0.25 M H2SO4 for 30 min to accomplish the replacement between the upd-Cu on the Pt surface and the Au ions in the solution. 3. Results and discussion 3.1. Formation of a upd-Cu on a Pt Electrode. Figure 2 shows the cyclic voltammogram (CV) of the Pt disk in a solution of 0.2 M H2SO4 with 0.01 M CuSO4 at a scan rate of 10 mV s-1. For comparison, a voltammogram of the same electrode in a solution of 0.2 M H2SO4 without CuSO4 is also presented. In Figure 2, the dramatic current increase in the negative-going scan after a potential of 0.1 V indicates the bulk copper deposition on the Pt disk surface. In other words, the deposition of upd-Cu happens at a potential above 0.1 V. The peak in region M of Figure 2 is assigned to the deposition of upd-Cu. The four distinct peaks in region N of Figure 2 correspond to four types of states of the upd-Cu on the Pt according to the literature.28 In this work, a potential of 0.15 V was chosen as the upd-Cu formation potential, Eupd. 3.2. Formation of the Au/Pt Electrode by Replacement. The EDX elemental maps of the fresh Au/Pt electrode after replacement are shown in Figure 3. It shows there is still a

Figure 3. EDX elemental maps of a fresh Au/Pt electrode.

Figure 4. CV of electrodes Pt, Auh/Pt and Aul/Pt in 0.5 mol L-1 H2SO4 at a sweep rate of 50mV s-1.

Figure 5. SEM image of a Au/Pt electrode after Cu anodic removal.

certain amount of Cu atoms (Figure 3d) in addition to the specks corresponding to elements Au (Figure 3c) and Pt (Figure 3b). To rule out the possible influence of upd-Cu on the subsequent methanol electroxidation, a fresh Au/Pt electrode was anodically polarized at 0.8 V to remove the residual upd-Cu on the Pt surface. After such anodic polarization treatment, no signal corresponding to Cu was detected in the CV of the treated Pt disk, as illustrated in Figure 4. The morphology of the Au/Pt electrode after copper removal is presented in Figure 5. It shows that the Au particles disperse uniformly on the Pt surface and that the Au particles have an average diameter of about 10 nm. The presence of Au on the Pt surface is confirmed by the CV in Figure 4. The current peak around 0.9 V in the negativegoing scan corresponds to the reduction of Au oxides, which comes from the oxidation of Au after a potential of 1.0 V in

18674 J. Phys. Chem. C, Vol. 112, No. 47, 2008

Figure 6. LSV of electrodes at a sweep rate of 1 mV s-1 in 0.5 mol L-1 H2SO4 + 10-5 mol L-1 PMo12 (9 for Aul/Pt, 0 for Pt, and ! for Au), 0.5 mol L-1 H2SO4 + 1 mol L-1 CH3OH (1 for Aul/Pt, 3 for Pt, and 8 for Au), and 0.5 mol L-1 H2SO4 + 1 mol L-1 CH3OH + 10-5 mol L-1 PMo12 (• for Aul/Pt, O for Pt, and y for Au).

Wang et al. the Pt surface.7,35 Additionally, several authors have suggested that methanol adsorption and subsequent oxidation cannot occur while adsorption sites are blocked by adsorbed hydrogen (Pt-H), which is formed by underpotential deposition during the cathodic scan and not fully desorbed on the anodic scan until ∼400 mV vs SHE.36 For methanol electro-oxidation, adsorbed hydrogen (Pt-H) can also be produced by methanol dehydrogenation via reaction 2. Although a lot of mechanisms about methanol electrooxidation have been proposed, the following three steps are usually included in the essential processes during methanol electro-oxidation,

CH3OH + Pt f Pt-COa + 4Pt-H +

-

4Pt-H f 4Pt + 4H + 4e +

(3) -

Pt-COa+H2Oa f CO2+2H + 2e

Figure 7. Chronoamperograms of the Aul/Pt, Pt, and Au electrodes at a potential of 400 mV vs SHE in 0.5 mol L-1 H2SO4 + 1 mol L-1 CH3OH + 10-5 mol L-1 PMo12.

the positive-going scan.29-31 Thus, the size of the peak around 0.9 V in the negative-going scan was also used to assess the coverage of the Au on the Pt electrode. The superscripts h and l in Auh/Pt and Aul/Pt denote the electrodes with high and low coverages of Au, respectively. Figure 4 also tells that the presence of Au on Pt can suppress hydrogen adsorption/ desorption in a potential range from -0.2 to 0.1 V and the Pt-O electrochemical reduction from 0.6 to 0.2 V. Thus, too high a coverage of Au on Pt surface is not good for methanol electrooxidation, according to the widely accepted perception. 3.3. Methanol Electro-oxidation on a Au/Pt Electrode in PMo12-Containing Solution. Linear sweeping voltammograms (LSV) of Aul/Pt and Pt electrodes at a sweep rate of 1 mV s-1 in three different kinds of electrolytes are shown in Figure 6. Lines investigated in PMo12 electrolyte without methanol in Figure 6 demonstrate that phosphomolybdic acid (PMo12) itself has no electrochemical activity, regardless of whether it is on a Aul/Pt or Pt electrode. On a Aul/Pt electrode, the methanol oxidation starts at a much more negative potential in the electrolyte with PMo12 than in the electrolyte without PMo12. In addition, the current of methanol oxidation also increases much more noticeably in the electrolyte with PMo12 than in the electrolyte without PMo12. The onset potential of methanol oxidation on the Aul/Pt electrode shifts about 0.4 V toward the negative direction in the electrolyte with PMo12 relative to that in the electrolyte without PMo12. On the electrode Pt, although the enhanced catalysis of PMo12 to methanol electroxidation is clear, it is not as noticeable as that on the Aul/Pt electrode, as demonstrated in Figure 6. It discloses that the presence of Au on the Pt electrode also participates in the catalytic process of methanol oxidation. The start of methanol oxidation on a Pt electrode usually occurs at potentials considerably above its thermodynamic reversible potential of 20 mV vs SHE.32 This behavior is seen in the oxidation of other small organic molecules, such as ethanol and formic acid.33,34 As a matter of fact, methanol oxidation did not begin until near 650 mV vs SHE, coinciding with the potential at which adsorbed COad is fully oxidized from

(2)

(4)

where H2Oa denotes oxygen-donating s species. Reaction 2 can start as soon as the Pt is in contact with the methanol, regardless the potential,7 but reactions 3 and 4 cannot occur until 400 and 650 mV vs SHE, respectively, according to previous studies.32-36 Reaction 2 cannot generate any current. Thus, it raises a question, which reaction should be responsible for the current before 400 mV vs SHE in Figure 6? We think reaction 5 is responsible for the current observed in Figure 6. In other words, it is the united function of catalyst Au and oxidant PMo12 that makes reaction 5 starts before 400 mV vs SHE. The chronoamperograms of electrodes Aul/Pt and Pt at potential of 400 mV vs SHE in 0.5 mol L-1 H2SO4 + 1 mol L-1 CH3OH + 10-5 mol L-1 PMo12 shown in Figure 7 confirm that the catalysis of Au and the oxidation of PMo12 are two indispensable factors for starting up reaction 5. Au

Pt-H + PMo12O403- 98 Pt + HPMo12O402- + e- (5) CO formed in reaction 2 as a byproduct of methanol electrooxidation strongly adsorbs on the Pt surface and subsequently suppresses the methanol oxidation reaction.37 CO poison leads to a decrease in electrode performance. Thus, whether the CO is effectively removed from the Pt surface determines the rate of methanol oxidation. The difference in methanol oxidation on the Aul/Pt and Pt electrodes almost certainly lies in that the latter is short of Au catalysis to reaction 6: Au

Pt-CO + PMo12O403- + H2Of Pt + CO2 + H2PMo12O401- + 2e- (6) The enhanced CO removal of phosphomolybdic acid during methanol electro-oxidation on the Au/Pt electrode at a potential of 600 mV vs SHE was also checked by chronoamperometry, as shown in Figure 8. During the whole experiment, the current of methanol oxidation on the Aul/Pt electrode is always larger in the electrolyte with PMo12 than that in the electrolyte without PMo12. It once more confirms that the electro-oxidation of methanol on a Pt electrode can be enhanced by the united function of Au and phosphomolybdic acid. For comparison, a bare Au electrode, that is, without a Pt substrate, was also checked in methanol and PMo12-containing solutions. The results are shown in Figures 6, 7, and 8. It may not be difficult to understand why no methanol electro-oxidation current was observed in these experiments. Au is not a good catalyst in any respect for methanol dehydrogenation. Thus, a reaction like reaction 2 cannot proceed on a Au electrode. In

Electrochemical Oxidation on Au/Pt Electrode

Figure 8. Chronoamperograms of electrodes Aul/Pt and Au at a potential of 600 mV vs SHE in 0.5 mol L-1 H2SO4 + 10-5 mol L-1 PMo12 (O for Aul/Pt and 3 for Au), 0.5 mol L-1 H2SO4 + 1 mol L-1 CH3OH (y for Aul/Pt and ∇ for Au), and 0.5 mol L-1 H2SO4 + 1 mol L-1 CH3OH + 10-5 mol L-1 PMo12 (•for Aul/Pt and 1 for Au), respectively.

J. Phys. Chem. C, Vol. 112, No. 47, 2008 18675 adatom Au. The onset potential of methanol oxidation shifts 400 mV toward the negative direction on a Au/Pt electrode with phosphomolybdic acid in comparison with a Pt electrode with phosphomolybdic acid. It is supposed that adsorbed hydrogen and intermediate CO from the methanol dehydrogenation and oxidation were electrocatalytically oxidized by the oxidanton state of phosphomolybdic acid with the aid of Au catalysis. The excellent hydrogen and CO cleaning capability of phosphomolybdic acid was accomplished via the following processes. Au

Pt-H + PMo12O403-f Pt + HPMo12O402- + eAu

Pt-CO + PMo12O403- + H2Of Pt + CO2 + H2PMo12O401- + 2e-

Figure 9. The 13th cycle of CV of a Aul/Pt electrode in 0.5 mol L-1 H2SO4 + 1 mol L-1 CH3OH (solid line) and 0.5 mol L-1 H2SO4 + 1 mol L-1 CH3OH + 10-5 mol L-1 PMo12 (dashed line) at a sweep rate of 50 mV s-1.

On a Pt electrode, although the enhanced catalysis of PMo12 to methanol electroxidation was observed, it was not as noticeable as that on a Aul/Pt electrode because the Pt electrode is short of Au catalysis for the above reaction. However, the presence of Au on the Pt suppresses hydrogen adsorption/ desorption in a potential range from -0.2 to 0.1 V and the Pt-O electrochemical reduction, from 0.6 to 0.2V. Thus, too high a coverage of Au on a Pt surface is not good for methanol electrooxidation. Acknowledgment. This work was financially supported by the NSFC of China (Grant No. 20676156), by the Chinese Ministry of Education (Grant No. 307021), China National 863 Program (2007AA05Z124), and the Chongqing and Guangdong Sci&Tech Key Project (CSTC2007AB6012 and 2007A010700001). References and Notes

Figure 10. The peak current jp of methanol oxidation on a Aul/Pt electrode at peaks a and b in Figure 9 vs CV cycle number in 0.5 mol L-1 H2SO4 + 1 mol L-1 CH3OH (×) and 0.5 mol L-1 H2SO4 + 1 mol L-1 CH3OH + 10-5 mol L-1 PMo12 (O).

this case, there will be no species such as Au-H and Au-CO. Therefore, the subsequent reactions like reactions 3, 4, 5, and 6 would certainly not take place if the methanol dehydrogenation step did not happen in advance. To further demonstrate the enhanced catalysis of phosphomolybdic acid to methanol electro-oxidation on a Au/Pt electrode, methanol oxidation was investigated by CV on a Aul/ Pt electrode in solutions with and without phosphomolybdic acid. Fifty cycles of CV were conducted in the two systems. Their 13th CV cycle is recorded in Figure 9. The peak current, jp, of methanol oxidation at peaks a and b in the positive-going scan in Figure 9 vs CV cycle number is shown in Figure 10. The peak current jp would fall quickly with CV cycle number if CO accumulated quickly on the electrode surface. Figure 10 clearly shows that phosphomolybdic acid has a good CO cleaning capability, which is accomplished via the process illustrated by reaction 6. 4. Conclusions Methanol electro-oxidation on a Pt electrode was markedly enhanced by the united function of phosphomolybdic acid and

(1) Winter, M.; Brodd, R. J. Chem. ReV. 2004, 140, 4245. (2) Bock, C.; Paquet, C.; Couillard, M.; Botton, G. A.; MacDougall, B. J. Am. Chem. Soc. 2004, 126, 8028. (3) Lu, Q.; Yang, B.; Zhuang, L.; Lu, J. J. Phys. Chem. B 2005, 109, 1715. (4) Mukerjee, S.; Urian, R. C.; Lee, S. J.; Ticianelli, E. A.; McBreen, J. J. Electrochem. Soc. 2004, 151, A1094. (5) Jayaramam, S.; Jaramillo, T. F.; Baeck, S. H.; McFarland, E. W. J. Phys. Chem. B 2005, 109, 22958. (6) Gonzalez, M. J.; Peters, C. H.; Wrighton, M. S. J. Phys. Chem. B 2001, 105, 5470. (7) Wei, Z. D.; Atsushi, M.; Tadayoshi, O.; Masatoshi, O. Phys. Chem. Acta Sin. 2002, 18, 1120. (8) Antolini, E. Mat. Chem. Phys. 2003, 78, 563. (9) Roth, C.; Goetz, M.; Fuess, H. J. Appl. Electrochem. 2001, 31, 793. (10) Lima, A.; Coutanceau, C.; Leger, J. M.; Lamy, C. J. Appl. Electrochem. 2001, 31, 379. (11) Neto, O. A.; Franco, E. G.; Arico, E.; Linardi, M.; Gonzalez, E. R. J. Eur. Ceram. Soc. 2003, 23, 2987. (12) Bauer, A.; Gyenge, E. L.; Oloman, C. W. J. Power Sources 2007, 167, 281. (13) Gotz, M.; Wendt, H. Electrochim. Acta 1998, 43, 3637. (14) Maillard, F.; Lu, G. Q.; Wieckowski, A.; Stimming, U. J. Phys. Chem. B 2005, 109, 16230. (15) Zhang, X.; Zhang, F.; Chan, K. Y. Catal. Commun. 2004, 5, 749. (16) Liu, J. Y.; Cao, J. Y.; Huang, Q. H.; Li, X. W.; Zou, Z. Q.; Yang, H. J. Power Sources 2008, 175, 159. (17) Zhang, J. L.; Vukmirovic, M. B.; Sasaki, K.; Nilekar, A. U.; Mavrikakis, M.; Adzic, R. R. J. Am. Chem. Soc. 2005, 127, 12480. (18) Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Nørskov, J. K. Angew. Chem., Int. Ed. 2006, 45, 2897. (19) Arico, A. S.; Modica, E.; Ferrara, I.; Artonucci, V. J. Appl. Electrochem. 1998, 28, 881.

18676 J. Phys. Chem. C, Vol. 112, No. 47, 2008 (20) Arico, A. S.; Kim, H.; Shukla, A. K.; Ravikumar, M. K.; Antonucci, V.; Giordano, N. Electrochim. Acta 1994, 39, 691. (21) Ferrell, J. R.; Kuo, M. C.; Turner, J. A.; Herring, A. M. Electrochim. Acta 2008, 53, 4927. (22) Su, Y.; Liu, C. P.; Shan, Y. B.; Han, F.; Li, C. Z.; Wu, L. E.; Xing, W.; Lu, T. H. Acta Chim. Sin. 2004, 62, 1645. (23) Wei, J.; Yang, H.; Yang, Y. G.; Zhang, Z. L. Chin. J. Inorg. Chem. 2003, 19, 945. (24) Kim, W. B.; Voitl, T.; Rodriguez-Rivera, G. J.; Dumesic, J. A. Science 2004, 305, 1280. (25) Kim, W. B.; Voitl, T.; Rodriguez-Rivera, G. J.; Evans, S. T.; Dumesic, J. A. Angew. Chem., Int. Ed. 2005, 44, 778. (26) Lizcano-Valbuena, W. H.; Paganin, V. A.; Leite, C. A. P.; Galembeck, F.; Gonzalez, E. R. Electrochim. Acta 2003, 48, 3869. (27) Chen, Y. X.; Miki, A.; Ye, S.; Sakai, H.; Osawa, M. J. Am. Chem. Soc. 2003, 125, 3680. (28) Danilov, A. I.; Molodkina, E. B.; Polukarov, Y. M. J. Electrochem. 1998, 34, 1249.

Wang et al. (29) Park, I. S.; Lee, K. S.; Jung, D. S.; Park, H. Y.; Sung, Y. E. Electrochim. Acta 2007, 52, 5599. (30) Engelsmann, K.; Lorenz, W. J.; Schmidt, E. J. Electroanal. Chem. 1980, 114, 1. (31) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429. (32) Iwasita, T. Electrochim. Acta 2002, 47, 3663. (33) Tripkovic, A. V.; Popovic, K. D.; Grgur, B. N.; Blizanac, B.; Ross, P. N.; Markovic, N. M. Electrochim. Acta 2002, 47, 3707. (34) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020. (35) Spendelow, J. S.; Goodpaster, J. D.; Kenis, P. J. A.; Wieckowski, A. J. Phys. Chem. B 2006, 110, 9545. (36) Hsueh, K. L.; Gonzalez, E. R.; Srinivasan, S. Electrochim. Acta 1983, 28, 691. (37) Chrzanowski, W.; Wieckowski, A. Langmuir 1998, 14, 1967.

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