Detection of Hydrogen Peroxide Produced during Electrochemical

The use of Pt-based electrodes is the most commonly employed strategy to avoid .... using a reversible redox couple [Fe(CN)6]4-/3- and to find the sig...
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Anal. Chem. 2008, 80, 750-759

Detection of Hydrogen Peroxide Produced during Electrochemical Oxygen Reduction Using Scanning Electrochemical Microscopy Yan Shen, Markus Tra 1 uble, and Gunther Wittstock*

University of Oldenburg, School of Mathematics and Natural Sciences, Center of Interface Science (CIS), Institute of Pure and Applied Chemistry and Institute of Chemistry and Biology of the Marine Environment, D-26111 Oldenburg, Germany

The substrate-generation/tip-collection mode of scanning electrochemical microscopy was used to detect hydrogen peroxide formed as an intermediate during oxygen reduction at various electrodes. The experiment is conceptually similar to rotating ring-disk experiments but does not require the production of a ring-disk assembly for the specific electrode material in question. In order to limit the extension of the diffusion layer above the sample, the sample electrode potential is pulsed while the Pt ultramicroelectrode probe (UME) is held at a constant potential for oxidative amperometric detection of hydrogen peroxide. The signal at UME is influenced by the sample region within the diffusion length of hydrogen peroxide during the pulse of 2.5 s. The method is tested with three model electrodes showing different behavior with respect to the oxygen reduction reaction (ORR) in acidic solution. Simple analytical models were used to extract effective rate constants for the most important reaction paths of ORR at gold and palladium-cobalt samples from the chronoamperometric response of the UME to a reduction pulse at the sample electrode. The electrochemical oxygen reduction reaction (ORR) is a very important reaction in several applications including fuel cells and electrochemical oxygen sensors.1,2 While the reaction is broadly used, the ORR is a rather complex multistep process.3,4 As shown in Table 1,5 in acidic or neutral solution, the main product of the ORR is H2O2 or H2O, or both, which depends on electrode material, electrode potential, and solution composition. The use of Pt-based electrodes is the most commonly employed strategy to avoid the formation of hydrogen peroxide, because Pt is a good electrocatalyst for the reduction of O2 and of H2O2 to water.6 The * To whom correspondence should be addressed. Fax: (+49-441) 7983979. E-mail: [email protected]. (1) Adler, S. B. Chem. Rev. 2004, 104, 4791-4843. (2) Kobayashi, N.; Nevin, W. A. Appl. Organomet. Chem. 1996, 10, 579-590. (3) Damjanovic, A. In Modern Aspects of Electrochemistry; Bockris, J. O. M., Conway, B. E., Eds.; Plenum Publishing Corp.: New York, 1969; Vol. 5, pp 369-483. (4) Maurice, L. H. In Inorganic Electrochemistry; Scholz, F., Pickett, J., Eds.; Wiley-VCH Verlag GmbH&Co: Weinheim, 2006; Vol. 7a, pp 117-142. (5) Yeager, E. Electrochim. Acta 1984, 29, 1527-1537. (6) Markovic, N. M.; Schmidt, T. J.; Stamenkovic, V.; Ross, P. N. Fuel Cells 2001, 1, 105-116.

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Table 1. Pathways of Electrochemical Oxygen Reduction in Different Solution According to Ref 5 reactions Acidic Media O2 + 2 H+ + 2 e- f H2O2 O2 + 4 H+ + 4 e- f 2 H2O H2O2 + 2 H+ + 2 e- f 2 H2O 2 H2O2 f 2 H2O + O2 (decomposition reaction)

(1) (2) (3) (4)

Basic Media O2 + 2 H2O + 4 e- f 4 OHO2 + H2O + 2 e- f HO2- + OHHO2- + H2O + 2 e- f 3 OH2 HO2- f 2 OH- + O2 (decomposition reaction)

(5) (6) (7) (8)

use of Pt-based electrocatalysts is problematic for mass products due to the limited availability of platinum and the severe environmental impact associated with the refinery of Pt from Ni ores. Furthermore, an increase of the efficiency of fuel cells and improved sensitivities of amperometric oxygen sensors could be reached by a decrease of the overpotential of the ORR by using new catalyst materials. This perspective has triggered an intensive search for alternative electrode materials for oxygen reduction. Among the proposed materials, metal alloys are most promising, including elements such as V,7 Cr,7,8 Ti,9,10 Mo,10 Co,7,10-14 Ni,12,13 Fe,13 Rh,15 Pd,9,10,15 Ir,16 Pt,7,8,13-15 Ru,17 Au,10-14 and Ag.10-14 Metal porphyrins and phthalocyanins have received much atten(7) Lima, F. H. B.; Giz, M. J.; Ticianelli, E. A. J. Brazil. Chem. Soc. 2005, 13, 328-336. (8) Koffi, R. C.; Coutanceau, C.; Garnier, E.; Leger, J.-M.; Lamy, C. Electrochim. Acta 2005, 50, 4117-4127. (9) Fernandez, J. L.; Raghuveer, V.; Manthiram, A.; Bard, A. J. J. Am. Chem. Soc. 2005, 127, 13100-13101. (10) Raghuveer, V.; Manthiram, A.; Bard, A. J. J. Phys. Chem. B 2005, 109, 22909-22912. (11) Pharkya, P.; Alfantazi, A.; Farhat, Z. J. Fuel Sci. Technol. 2005, 2, 171178. (12) Antolini, E.; Salgado, J. R. C.; Gonzalez, E. R. J. Electroanal. Chem. 2005, 580, 145-154. (13) Xiong, L.; Manthiram, A. J. Electrochem. Soc. 2005, 152, A697-A703. (14) Fernandez, J. L.; Walsh, D. A.; Bard, A. J. J. Am. Chem. Soc. 2005, 127, 357-365. (15) Lukaszewski, M.; Grden, M.; Czerwinski, A. J. Solid State Electrochem. 2005, 9, 1-9. (16) Ioroi, T.; Yasuda, K. J. Electrochem. Soc. 2005, 152, A1917-A1924. (17) Zhang, J.; Vukmirovic, M. B.; Xu, Y.; Mavrikakis, M.; Adzic, R. R. Angew. Chem., Int. Ed. 2005, 44, 2132-2135. 10.1021/ac0711889 CCC: $40.75

© 2008 American Chemical Society Published on Web 01/08/2008

tion for sensor application.18-20 In the field of bioelectrochemistry, enzymes such as laccases,21 cytochrome c oxidase,22 bilirubin oxidase,23 and peroxidases24 are intensively investigated. Despite this well-established and important new application and the long history of investigations in the mechanisms of this reaction, there is no method available that allows a rational design of new electrocatalysts that lead to a smooth four-electron reduction of dissolved O2 to H2O, avoiding the formation of aggressive oxygen species. The complexity of the materials and the variation of efficiency with preparation conditions suggest the use of combinatorial approaches for the search of new catalysts, catalyst combinations, and optimization of preparation procedures. Efficient combinatorial approaches depend on combinatorial preparation of materials and efficient screening methods.25-27 Such approaches are increasingly applied for the search for oxygen reduction catalysts.28,29 Recently, scanning electrochemical microscopy (SECM) has been applied to study the electrocatalytic and electrode processes of fuel cells, due to the capability to probe interfacial processes and catalytic activity of the substrates with spatial resolution. SECM and combinations of SECM with other analytical methods have been proposed by Hillier and Bard as techniques for fuel cell electrode screening, concerning hydrogen oxidation, methane oxidation, or O2 reduction.30-32 Bard et al.14 suggested the use of SECM in the tip-generation/substrate-collection mode to test the efficiency of metal catalysts for O2 reduction. In this mode, the ultramicroelectrode (UME, “tip”) produces O2 under galvanostatic condition that is consumed at the sample. A plot of the sample current versus the lateral UME position provided a qualitative mapping of the catalyst efficiency.31 Samples were prepared by spotting metal salts in different ratios, evaporation of the solvent, and thermal decomposition of metal carbonates and metal nitrates. The resulting metal spots showed distinct and systematic differences in the O2 reduction efficiency.14 The method has also been applied to sputtered metal thin films.33 Eckhard et al. introduced the redox competition mode. In this mode, both the sample and the UME consume O2 in a potentiostatic mode.34 At sample (18) Shi, C.; Anson, F. C. Inorg. Chem. 1998, 37, 1037-1043. (19) Shi, C.; Anson, F. C. Inorg. Chem. 1995, 34, 4554-4561. (20) Shen, Y.; Liu, J.; Jiang, J.; Liu, B.; Dong, S. Electroanalysis 2002, 14, 15571563. (21) Farneth, W. E.; Diner, B. A.; Gierke, T. D.; D’Amore, M. B. J. Electroanal. Chem. 2005, 581, 190-196. (22) Collman, J. P.; Fudickar, W.; Shiryaeva, J. Inorg. Chem. 2003, 42, 33843386. (23) Tsujimura, S.; Tatsumi, H.; Ogawa, J.; Shimizu, S.; Kano, K.; Ikeda, T. J. Electroanal. Chem. 2001, 496, 69-75. (24) Wang, M.; Zhao, F.; Liu, Y.; Dong, S. Biosens. Bioelectron. 2005, 21, 159166. (25) Reddington, A. S. E.; Gurau, B.; Viswanathan, R.; Sarangapani, S.; Smotkin, E. S.; Mallouk, T. E. Science 1998, 280, 1735-1737. (26) Guerin, S.; Hayden, B. E. J. Comb. Chem. 2006, 8, 66-73. (27) Brace, K. M.; Hayden, B. E.; Russell, A. E.; Owen, J. R. Adv. Mater. 2006, 18, 3253-3257. (28) Guerin, S.; Hayden, B. E.; Lee, C. E.; Mormiche, C.; Russell, A. E. J. Phys. Chem. B 2006, 110, 14355-14362. (29) Guerin, S.; Hayden, B. E.; Pletcher, D.; Rendall, M. E.; Suchsland, J.-P. J. Comb. Chem. 2006, 8, 679-686. (30) Jayaraman, S.; Hillier, A. C. Langmuir 2001, 17, 7857-7864. (31) Fernandez, J. L.; Bard, A. J. Anal. Chem. 2003, 75, 2967-2974. (32) Shah, B. C.; Hillier, A. C. J. Electrochem. Soc. 2000, 147, 3043-3048. (33) Lu, G.; Cooper, J. S.; McGinn, P. J. Electrochim. Acta 2007, 52, 51725181. (34) Eckhard, K.; Chen, X.; Turcu, F.; Schuhmann, W. Phys. Chem. Chem. Phys. 2006, 8, 5359-5365.

regions with high O2 reduction efficiency, the O2 concentration close to the sample is reduced and this is reflected by reduced reduction currents at the UME. In order to avoid a depletion of O2 in an extended diffusion layer above the sample, the UME potential ET is pulsed. Before the measurement of the reduction current, the interelectrode volume is periodically enriched in O2 by water electrolysis at the UME. Alternatively, the SECM feedback mode has been used to measure the kinetic constant of the ORR at the Pt electrode, but this method is limited to pH 9-12.35 Also optical imaging techniques have been used to obtain information about inhomogeneous diffusion layers above electrodes that allow conclusions about localized ORR. Some examples used fluorescence imaging of a dye that changes fluorescence spectra with pH.36 Since proton transfer in aqueous solution is typically very fast, the image of such a dye represents local proton concentration. Because the imaging is based on the protonation equilibrium of the dye, the applicable pH range extends around the pKa value of the dye. Rudd et al. performed confocal laser imaging of fluorescine in unbuffered KCl solution pH 4.38 above an electrode reducing O2.37 During ORR, protons are consumed and the pH rises in the vicinity of the electrode. Concentration profiles of OH- ions could be directly visualized, and the size of the diffusion layer could be compared to theory that took into account the extent to which ORR yields either H2O or H2O2. None of the above-mentioned methods provides information about the amount of generated H2O2. H2O2 formation not only decreases the efficiency of fuel cell but may also contribute to membrane degradation and corrosion processes of metals, polymer fittings, and carbon materials. Among other characteristics, avoidance of H2O2 is therefore an important parameter for the optimization of fuel cell catalysts. Pletcher and Sotiropoulos investigated the ORR at Pt in neutral and alkaline solution considering the “number napp of electrons that appear to be involved in the reduction of oxygen if it is assumed that the reduction is mass transport-controlled”.38 They found a decrease of napp obtained from plateau currents of Pt UME and Pt rotating disk electrodes with increasing mass transport coefficients indicative of increasing amounts of produced H2O2. H2O2 was, however, not directly detected. The conventional method to test the formation of H2O2 consists in rotating ring-disk electrode (RRDE) experiments.3 Oxygen is reduced at the disk electrode and generated H2O2 is transported under defined hydrodynamic conditions to the surrounding Pt ring electrode where it is amperometrically detected by oxidation. This procedure requires the preparation of a ring-disk electrode assembly for each new catalyst to be investigated. Since the demands for mechanical smoothness and precision are high, this method is not particular suitable for combinatorial tests where one may want to test a number of similar catalysts on a chip-type support. In addition, when an electrocatalyst for oxygen reduction is deposited on the disk electrode by dip coating, metal evaporation, or multilayer assembly, the ring electrode may be contaminated at the same time. Postlethwaite et al. suggested the use of (35) Liu, B.; Bard, A. J. J. Phys. Chem. B 2002, 106, 12801-12806. (36) Bowyer, W. J.; Xie, J.; Engstrom, R. C. Anal. Chem. 1996, 68, 2005-2009. (37) Rudd, N. C.; Cannan, S.; Bitziou, E.; Ciani, I.; Whitworth, A. L.; Unwin, P. R. Anal. Chem. 2005, 77, 6205-6217. (38) Pletcher, D.; Sotiropoulos, S. J. Electroanal. Chem. 1993, 356, 109-119.

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interdigitated array electrodes (IDAs) as an alternative to RRDE experiments for ORR.39 They provided the mathematical treatment and could clearly demonstrate that a much smaller amount of H2O2 could be detected than at RRDE because the collection efficiency is higher and an electrochemical feedback could be exploited. This method depends on the availability of IDAs of the material to be investigated. Although modification was shown, it is difficult to conceive how, for instance, powders of carbon-supported catalysts can be investigated. Therefore, methods are needed that allow the investigation of H2O2 formation with a probe that is not mechanically attached to the sample electrode. In this work, a new application of substrate-generation/tipcollection (SG/TC) mode of SECM is proposed and used to measure H2O2 produced during the ORR. In the SG/TC mode, the substrate generates a species that diffuses into the bulk. A disk-shaped UME with radius rT is used as the probe and positioned in a defined distance d above the substrate. It collects the generated H2O2. The UME is at least 1 order of magnitude smaller than the substrate and has a much thinner diffusion layer than the substrate. Historically, SG/TC experiments with an amperometric UME were pioneered by Engstrom et al. using small carbon UMEs.40 They addressed the transient response of the UME after a potential step at the sample. Martin and Unwin used the SG/TC transients to extract the ratio of diffusion coefficients of redox couples.41 The theory considers shielding effects of the probe and electrochemical feedback between sample and UME.41 Amatore et al. used confocal resonance Raman microscopy,42 potentiometric43 and amperometric UME44 to map the concentration profiles in the vicinity of electrodes with the aim of resolving the interplay of diffusion and natural convection45 using a reversible redox couple [Fe(CN)6]4-/3- and to find the signature of coupled homogeneous reactions.46,47 The method was refined by applying a potential pulse to the substrate electrode, and after a variable delay, the probe electrode was pulsed as well.44 Transient local concentrations could then be reconstructed from series of such experiments. Furthermore, the improvement that can be obtained by using nanometer-sized probe electrodes was experimentally demonstrated.48 SG/TC transients have also be used to investigate the transport of Tl+ through phospholipid monolayers.49 Here we use SG/TC experiments to measure H2O2 produced by the ORR during a potential pulse to the substrate electrode. In order to provide quantitative data (39) Postlethwaite, T. A.; Hutchison, J. E.; Murray, R.; Fosset, B.; Amatore, C. Anal. Chem. 1996, 68, 2951-2958. (40) Engstrom, R. C.; Meaney, T.; Tople, R.; Wightman, R. M. Anal. Chem. 1987, 59, 2005-2010. (41) Martin, R. D.; Unwin, P. R. Anal. Chem. 1998, 70, 276-284. (42) Amatore, C.; Bonhomme, F.; Bruneel, J.-L.; Servant, L.; Thouin, L. Electrochem. Commun. 2000, 2, 235-239. (43) Amatore, C.; Szunerits, S.; Thouin, L. Electrochem. Commun. 2000, 2, 248253. (44) Amatore, C.; Szunerits, S.; Thouin, L.; Warkocz, J.-S. Electrochem. Commun. 2000, 2, 353-358. (45) Amatore, C.; Szunerits, S.; Thouin, L.; Warkocz, J.-S. Electroanalysis 2001, 13, 646-652. (46) Amatore, C.; Bonhomme, F.; Bruneel, J. L.; Servant, L.; Thouin, L. J. Electroanal. Chem. 2000, 484, 1-17. (47) Amatore, C.; Pebay, C.; Scialdone, O.; Szunerits, S.; Thouin, L. Chem. Eur. J. 2001, 7, 2933-2939. (48) Baltes, N.; Thouin, L.; Amatore, C.; Heinze, J. Angew. Chem., Int. Ed. 2004, 43, 1431-1435. (49) Mauzeroll, J.; Buda, M.; Bard, A. J.; Prieto, F.; Rueda, M. Langmuir 2002, 18, 9453-9461.

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equivalent to the results of simple RRDE experiments, the experimental data were fitted to analytical models that contain effective rate constants for the different reaction paths as adjustable parameters. The approach is illustrated with three electrocatalysts of oxygen reduction showing qualitatively different behavior in acidic solution. EXPERIMENTAL SECTION Instruments. SECM measurements were performed on a home-built instrument using a stepper-motor positioning system (Ma¨rzha¨user, Wetzlar, Germany) and a bipotentiostat model CHI 7001 B (CH Instruments, Austin, TX). The software of the CHI 7001B was used to apply the potential pulses and to record the transients at fixed UME position. During scanning experiments, e.g., UME approach, a computer equipped with a 16-bit data acquisition board PCI-DAS1602/16 (Plug-In Electronic, Reichenau, Germany) was used to read the output voltage from the CHI 7001B at each motor position using the program SECMx developed inhouse. The bipotentiostat controlled a four-electrode cell with the working electrode 1 (WE1) being the SECM sample and the working electrode 2 (WE2) being the UME. It allowed for setting the UME potential ET at a constant value while the potential of the macroscopic sample ES was pulsed. An Ag|AgCl|3 M KCl reference electrode and a Pt wire (diameter 0.5 mm) were used as reference and auxiliary electrodes, respectively. All potentials are quoted with respect to the Ag|AgCl|3 M KCl. Solution. Fresh solutions of hydrogen peroxide were made for each experiment by diluting of a concentrated commercial aqueous solution (30% (v/v), Sigma-Aldrich GmbH, Steinheim, Germany). Ammonium sulfate and sulfuric acid were purchased from ABCR (ABCR GmbH & Co. KG, Karlsruhe, Germany). transDiamminedichloropalladium(II) chloride, cobalt(II) sulfate, and ammonium hydroxide solution (50% (v/v)) were purchased from Alfa Aesar (Karlsruhe, Germany). All compounds were used as received. Aqueous solutions were prepared using deionized water. Electrodes. A 25-µm-diameter Pt wire (Goodfellow, Cambridge, U.K.) was sealed under vacuum into a Pyrex glass capillary (inner diameter 0.87 mm, outer diameter 1.5 mm). The UME was polished and shaped conically by a wheel with 180-grid Carbimet paper disks and micropolishing cloth with 1.0-, 0.3-, and 0.05-µm alumina. The UME was sharpened to a RG ≈ 10, where RG is the ratio between the diameters of the glass sheath (rglass) and the radius rT of the active electrode surface. Before each experiment, the UME was polished with 0.3- and 0.05-µm alumina powder and rinsed with water. The Pt UME were then electrochemically cleaned by cycling between -0.2 and 1.5 V at 100 mV s-1 in 0.1 M sulfuric acid. Platinum, gold, and glassy carbon (GC) electrodes (1.5 mm and 3 mm diameter) were purchased from CH Instrument Inc. These electrodes were polished with 1.0- and 0.3-µm R-Al2O3 powders successively, sonicated in water for ∼5 min after each polishing step, and rinsed with water. The metal electrodes were then electrochemically cleaned by cycling between -0.2 and 1.5 V at 100 mV s-1 in 0.1 M sulfuric acid before use. PdCo alloy was deposited onto the cleaned GC electrode in a three-electrode setup at -1.0 V controlled by the CHI 7001 B potentiostat from a stirred aqueous solution containing 0.67 mM Pd(NH3)2Cl2 + 0.2 mM CoSO4 in 0.4 M (NH4)2SO4. The pH was adjusted to 9.42 by NH4OH solution. The deposition time was 120

Figure 2. Calibration curve of H2O2 oxidation currents at a Pt UME at ET ) +1.1 V. Electrolyte is 0.1 M H2SO4.

Data Treatment. In order to extract the rate constants from the measured chronoamperograms, an analytical model (eq 5) was fitted to the experimental data using a least-squares routine and a simplex algorithm taken from ref 50. The complement error function occurring in eq 5 was calculated with the help of the incomplete γ function,50 and for greater arguments, an iterative approximation was used (Supporting Information 5). The program was implemented in Turbo Pascal 6 and runs under DOS and Windows environments. For fitting curves with 2500 data points, ∼150 iteration steps of the simplex algorithm were required, until the mean squared deviation at the vertices of the simplex is less than 10-6 nA. This procedure took 6-7 min on a 1.6-GHz Pentium under Windows 2000 SP4 and occupies 17 MB memory. Figure 1. (a) Schematic illustration the SECM operation in the pulsed SG/TC mode; (b) potential wave form applied to the Au electrode.

s. Energy-dispersive X-ray (EDX) characterization confirmed that the PdCo alloy contained 8.75 ato -% Co. (Figure SI 1, in Supporting Information). Procedures. All experiments were performed sequentially in the same SECM cell made of perfluorinated polymer that accommodated four electrodes. The Au, Pt, or modified GC electrodes were cleaned or modified and then inserted as the bottom of the cell as SECM substrate electrode such that the surfaces of the substrate and UME were parallel. The reference electrode and auxiliary electrode were attached to the inner perimeter of the electrochemical cell. The UME was brought in mechanical contact with the substrate by recording an approach curve while monitoring the oxygen reduction current at the Pt UME (negative feedback). When mechanical contact with the support occurred, the approach was interrupted, and the electrode was then retracted a preset distance d from the surface. A new approach curve was recorded frequently to ensure that d was conserved during the experiment. Nevertheless, because of the shape of the UME, the exact mounting geometry, and the tilt of the sample, there is an uncertainty in d of ∼2 µm, which is negligible compared to the typical working distances used (d > 40 µm). The basic principle of the transient SG/TC mode used in this work is shown in Figure 1a. Chronoamperometric experiments were carried out by stepping the ES from a value where no faradic process occurs to a value, well into the limiting-current region. The appropriate ES values for the ORR were selected from voltammograms of air-saturated solutions. The UME potential (ET) was set to +1.1 V and kept constant.

RESULTS AND DISCUSSION Pt UME as Amperometric Sensor of H2O2 Oxidation. The amperometric detection of H2O2 over a wide concentration range and at different pH is difficult because the H2O2 oxidation (2H2O2 f 2H2O + O2) is a catalytic reaction.51,52 It was reported that the limiting current at Pt electrodes was not proportional to the concentration of H2O2 above 1 mM.53 Because H2O2 oxidation is under mixed kinetic and diffusion control, a lack of available Pt surface sites will become the limiting factor. Recently, new approaches have been proposed using different electrode materials,54 nanostructured metals,55 or immobilized enzymes,56 extending the useful working range. In our experiment, H2O2 was produced from an air-saturated aqueous solution. The concentration of O2 in such solutions at room temperatures (296 K) is about 0.27 mM posing an upper limit on the concentration of H2O2 that can be formed during ORR in air-saturated solution.4,57,58 In this concentration range a linear relation between the H2O2 oxidation current and the H2O2 concentration is obtained provided that a sufficiently positive potential is applied.55 Figure 2 shows a calibration plot for the amperometric H2O2 detection at Pt UME at ET ) +1.1 V in air-saturated solution. A linear dependence of (50) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numerical Recipes in Pascal; Cambridge University Press: Cambridge, UK, 1991. (51) Hall, S. B.; Khudaish, E. A.; Hart, A. L. Electrochim. Acta 1998, 43, 579588. (52) Hall, S. B.; Khudaish, E. A.; Hart, A. L. Electrochim. Acta 1998, 43, 20152024. (53) Zhang, Y.; Wilson, G. S. J. Electroanal. Chem. 1993, 345, 253-271. (54) Westbroek, P.; Van Haute, B.; Temmerman, E. Fresenius J. Anal. Chem. 1996, 354, 405. (55) Evans, S. A. G.; Elliott, J. M.; Andrews, L. M.; Bartlett, P. N.; Doyle, P. J.; Denuault, G. Anal. Chem. 2002, 74, 1322-1326. (56) Horrocks, B. R.; Schmidtke, D.; Heller, A.; Bard, A. J. Anal. Chem. 1993, 65, 3605-3614.

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Figure 3. UME transient currents at ET ) 1.1 V for the collection of H2O2 produced during ORR at an Au electrode at d ) 40 µm. The potential of Au electrode substrate was stepped from +0.4 V to (1) +0.1, (2) +0.05, (3) 0, (4) -0.05, and (5) -0.1 V.

the current on the H2O2 concentration is found up to 0.6 mM. The sensitivity S of the Pt UME was 12.38 nA mM-1. The diffusion coefficient of H2O2 was calculated from the sensitivity with D(H2O2) ) 1.6 × 10-5 cm2 s-1 and falls within the range (0.662.20) × 10-5 cm2 s-1 of previously quoted values,55 indicating that the detection occurs under diffusion-controlled conditions. The steady-state response is found to be very stable. Therefore, such Pt UME can be used as an amperometric sensor to measure H2O2 produced during ORR in air-saturated solutions. Oxygen Reduction at Au Electrode. The ORR at gold electrodes has been studied extensively both in acidic and in alkaline solutions.59-61 In acidic solutions, a 2e- pathway leads to H2O2 as the dominant final product, independent of the surface structure of the electrode.61 In order to detect the H2O2 produced during ORR at a macroscopic Au electrode, a chronoamperometric experiment was performed as shown in Figure 1b. The Pt UME was positioned at d ) 40 µm above the sample and held at ET ) +1.1 V for which the H2O2 oxidation is diffusion-controlled below 0.6 mM (Figure 2). The potential of the Au substrate was stepped from ES ) +0.4 V to different values, which were chosen from the cyclic voltammogram for oxygen reduction at the Au electrode (Figure SI 2 in Supporting Information). The UME transient current was recorded during the potential pulse to the Au electrode (Figure 3). An increasing iT is observed with decreasing ES due to the increased H2O2 production at the Au substrate. More negative ES than -0.2 V caused abnormal UME currents attributed to an overlay of H2O2 and the H2 oxidation reactions at the UME. Hydrogen is formed at the Au electrode below -0.2 V, as can be concluded from the voltammogram in deaerated solution (Figure SI 2 in Supporting Information). The influence of the working distance d was explored by stepping ES from +0.4 to -0.1 V and recording transients at different d. They showed the spatiotemporal development of the H2O2 diffusion layer (Figure 4). The time to reach a steady-state current increases with d. When d is larger than 80 µm, no steadystate situation is reached within the pulse time of 2.5 s. This can be rationalized by considering how far H2O2 may diffuse within the pulse time δ ) (2Dt)1/2,62 where D ) 1.6 × 10-5 cm2 s-1 is the diffusion coefficient of H2O2, t is the time after the pulse onset, (57) Gubbins, K. E.; Walker, R. D. J. J. Electrochem. Soc. 1965, 112, 469-471. (58) Carano, M.; Holt, K. B.; Bard, A. J. Anal. Chem. 2003, 75, 5071-5079. (59) Shao, M. H.; Adzic, R. R. J. Phys. Chem. B 2005, 109, 16563-16566. (60) Adzic, R. R.; Strbac, S.; Anastasijevi, N. Mater. Chem. Phys. 1989, 22, 349375. (61) Strbac, S.; Adzic, R. R. Electrochim. Acta 1996, 41, 2903-2908.

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Figure 4. UME transient currents for H2O2 oxidation at ET ) 1.1 V during ORR at an Au substrate in 0.1 M H2SO4 solution at different distances d. ES was stepped from +0.4 V to -0.1 V; d in part a are (1) 8, (2) 10, (3) 15, (4) 20, and (5) 25 µm and in part b are (6) 40, (7) 50, (8) 60, and (9) 80 µm. Solid lines in (b) were calculated using eqs 4 and 5 using the rate constants in Table 3.

Table 2. Calculated Time for the Diffusion of H2O2 between the Sample and the UME Positioned at Different Distances d (t ) d2/2D), D ) 1.6 × 10-5 cm2 s-1 d/µm

t/s

40 50 60 80 100

0.5 0.78 1.13 2 3.13

and δ is the thickness of the diffusion layer (Table 2). The obtained time corresponds well to the observed transients in Figure 4. For separations shorter than 40 µm, the current transient passes through a maximum and then falls well below the steady-state current observed at d ) 40 µm (Figure 4a). This is due to the hindered diffusion of O2 to the regions of the substrate electrode underneath the SECM probe (including shielding). With decreasing d, this maximum occurs at earlier time and the current decreases to lower values. The results for d > 40 µm (Figure 4b) show that the generated H2O2 is a stable reaction product at the Au electrode that cannot be further reduced in agreement with known RRDE studies of this system. Oxygen Reduction at Pt Electrode. Details of the oxygen reduction on Pt electrodes have been intensively examined for several decades; however, there remains considerable uncertainty regarding the exact mechanism because of its complex kinetics.63 Despite this fact, the behavior of Pt is contrary to that of Au (62) Mauzeroll, J.; Hueske, E. A.; Bard, A. J. Anal. Chem. 2003, 75, 38803889. (63) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jo´nsson, H. J. Phys. Chem. B 2004, 108, 17886.

Figure 6. CVs of PdCo alloy on a GC electrode in (1) deaerated 0.1 M H2SO4 solution without H2O2, (2) air-saturated 0.1 M H2SO4, and (3) deaerated 0.1 M H2SO4 solution with 0.33 mM H2O2. Scan rate 0.02 V s-1.

Figure 5. UME transient currents for H2O2 oxidation at ET ) 1.1 V during an ORR at a Pt substrate in 0.1 M H2SO4 solution at different distances d. ES was stepped from +0.9 to +0.3 V; d in part a are (1) 4, (2) 6, (3) 8, (4) 10, and (5) 12 µm and in part b are (6) 40, (7) 50, (8) 60, and (9) 80 µm.

because Pt is also a good catalyst for the H2O2 reduction. This is reflected in SECM pulse experiments. In this work, the Pt electrode was pulsed from +0.9 to +0.3 V, which was selected from the voltammogram of ORR at a Pt electrode (Figure SI 3 in Supporting Information). As shown in Figure 5, the transient H2O2 oxidation current at the UME can be markedly observed only at small d (curves 1-5 in Figure 5a). The UME current decreases very rapidly when d increases from 4 to 12 µm. The peak current shifts to longer time in agreement with the expectation for a species diffusing from the sample to the UME. When d > 30 µm, only a small H2O2 oxidation current could be detected at the UME with a more sensitive setting of the potentiostat (Figure 5b, note the different scaling compared to Figure 4b). In contrast to Au substrates, no steady-state UME current is obtained above the Pt substrate. Instead, a broad peak appears in the transients (curves (6-8 in Figure 5b). The current transient shows that H2O2 forms only a very thin diffusion layer above the Pt substrate electrode surface because it is further reduced to water at the same potential as the ORR occurs. The result indicates that H2O2 can be treated as an intermediate, but not as the final product of the ORR at Pt electrode. Recently, Shao et al. have confirmed that two 2-electron reduction steps are operative for ORR on Pt electrodes in alkaline solutions by surface-enhanced infrared reflection absorption spectroscopy with attenuated total reflection.64 In addition, the sharp decay of the transients in Figure 5a indicates that H2O2 is predominately formed during the onset of the ORR at the Pt electrode and that much less H2O2 is produced during continuous oxygen reduction at Pt. This can be a consequence of the pulse profile applied to the Pt electrode. ES is stepped from the platinum oxide region into the double layer region. During the pulse, the Pt surface oxide is reduced, and therefore, the oxygen reduction (64) Shao, M.; Liu, P.; Radoslav, R. A. J. Am. Chem. Soc. 2006, 128, 74087409.

may initially follow a different mechanism at a partially oxidecovered surface than at an oxide-free Pt surface at a later time. This finding may have some implication with respect to the longterm stability of fuel cell components under conditions of rapidly changing power demands. Another point of concern is a possible electronic interference between the sample and the UME. Shortly after the potential pulse, the structure of the double layer of the macroscopic sample will change. During this time, the UME will be placed in an electric field that will also cause a change of the effective UME potential even if its potential is controlled by a bipotentiostat. Consequently, a charging current will flow at the UME during that time. In fact, the very high UME currents at t < 25 ms may originate from the electrical interference of the sample and the UME (curves 1-5 in Figure 5a). These currents were therefore not further considerered here. At longer times, the UME currents should not be affected by this interference. Oxygen Reduction at PdCo Alloy-Modified GC Electrode. Recently, the electrodeposition of highly dispersed Pd nanoparticles and its alloys on a substrate has attracted growing interest because of their extraordinarily high catalytic activity in many reactions, especially for ORR.10,15 As a model sample, a PdCo alloy was deposited on a GC electrode as electrocatalyst for ORR and was characterized similar to Au and Pt electrodes (vide supra). Figure 6 shows CVs of the prepared PdCo alloy in 0.1 M H2SO4 after purging with air, Ar or after addition of H2O2. Figure 6, curve 2 (air-saturated solution) shows that the ORR occurs at ES < +0.5 V, i.e., at much more positive values than at the GC electrodes onto which the PdCo particles were deposited.18-20 The curve contains a slope at ES < 0.3 V for which we do not have an detailed explanation at this point. The buildup of the O2 diffusion layer in quiescent solution and slow changes of the surface state of the PdCo alloy during the potential sweep may lead to this observation. The addition of H2O2 to a deaerated 0.1 M H2SO4 solution proves that H2O2 can be reduced only at ES < +0.2 V (Figure 6, curve 3). This model electrode offers the possibility to carry out the ORR under clearly different overall mechanisms because the rate of O2 and H2O2 reduction changes very differently with potential. In the region +0.2 V < ES < +0.5 V, H2O2 cannot be further reduced while at ES < 0.2 V H2O2 it undergoes further reduction. Depending on the ES, water, hydrogen peroxide, or both are the primary products of the ORR at this alloy electrode. Figure 7a shows the SG/TC transients, which were obtained by stepping ES from +0.8 to +0.2 V. The shape of the curves is Analytical Chemistry, Vol. 80, No. 3, February 1, 2008

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Figure 7. UME transient response for H2O2 oxidation at ET ) 1.1 V during ORR at a PdCo alloy substrate electrode in air-saturated 0.1 M H2SO4 solution at different distances d. ES was stepped from (a) and (b) from +0.8 to +0.2 V, and (c) from +0.8 to 0 V; d was in (a) and (b) (1) 40, 2() 50, (3) 60, (4) 80, (5) 100, and (6) 130 µm; in (c) d was (7) 40 and (8) 50 µm. Part a and b contain the same experimental data points. Solid lines in (a) were calculated using eqs 4 and (5 using the rate constants in Table 4. The solid lines in (b) were calculated using k1 ) 0.020 cm s-1, k2 ) 0.012 cm s-1, and k3 ) 0.0078 cm s-1.

different from the transients recorded at Au and Pt electrodes. When d < 80 µm, a broad maximum of the H2O2 oxidation current at the UME is observed. The maximum is shifted to longer times as d is increased from 40 to 60 µm (Figure 7a, curves (1)-(3)). The UME currents in Figure 7a are smaller than the UME currents above the gold electrode (Figure 4b) and bigger than those above at the Pt electrode (Figure 5b) in comparable experiments. The behavior changes qualitatively if ES is stepped from +0.8 to 0 V, i.e. into a range in which H2O2 reduction occurs at PdCo electrodes with a much faster rate than at +0.2 V while the rate of O2 reduction is similar at both potentials (Figure 7c). The UME currents are much lower in Figure 7c than in Figure 7a for the same d, because most of the generated H2O2 was further reduced at the PdCo substrate electrode. The qualitative behavior in this potential region is similar to the Pt electrode (Figure 5b) although the currents at PdCo are larger. When applying even more negative potential pulses to the alloy electrode, the properties with respect to H2O2 and O2 reduction became even more similar to that of the Pt electrode (not shown). Reconstruction of H2O2 Concentration Profiles. Avoiding H2O2 generation during ORR is important for efficiency reasons 756 Analytical Chemistry, Vol. 80, No. 3, February 1, 2008

Figure 8. Concentration profile of H2O2 produced during oxygen reduction at different electrodes calculated from Figures 4, 5. and 7 (open symbols); (a) Au; (b) Pt; (c) PdCo. The time for which the local concentration was calculated is (1) 0.5, (2) 1, (3) 1.5, (4) 2, and (5) 2.5 s. The solid lines are guides for the eye.

but also in order to prolong the live time of membranes, fittings, and metal pieces. In order to relate the finding from electrochemical studies to stability tests of materials and components, it is important to know the transient H2O2 concentrations observed in Figures 4, 5, and 7. Here the local transient concentration of generated H2O2 in the diffusion layer above the different electrode materials can be approximately determined using the calibration curve (S ) 12.38 nA mM-1) obtained for the used Pt UME (Figure 2). From Figure 4b, Figure 5b, and Figure 7a, the UME currents at different d at 0.5 s, 1.0 s, 1.5, 2.0, and 2.5 s were selected. The local concentration was obtained by c(H2O2; d, t) ) i(d, t)/S. The results are summarized in Figure 8. With increasing time, the differences between substrate electrodes become more evident. For the Pt substrate, c(H2O2) reaches more than 10 µM 1 s after the onset of ORR but is too low to be measured at longer t and larger d as shown in Figure 8b (note the different scale of the ordinate in Figure 8b compared to Figure 8a and c). At the Au electrode, almost all O2 is converted to H2O2. The H2O2 concentration at the electrode surface reaches a steady-state value of 0.27 mM, the concentration of O2 in air-saturated 0.1 M sulfuric acid solution.57,58 From these results, it could be concluded that SECM SG/TC mode can be used to quantitatively measure H2O2 produced during ORR at different electrocatalysts.

Table 3. Heterogeneous Rate Constants of Reactions 1-3 at Au Electrodes Obtained by Fitting Expressions 4-7 to the Experimental Curves in Figure 4b d/µm

k1/(cm s-1)

k2/(cm s-1)

k3/(cm s-1)

40 50 60 80

0.0443 0.0445 0.0422 0.0422

0.000 018 0.000 482 0.003 02 0.002 42

0.002 83 0.002 62 0.001 95 0.002 32

Modeling the UME Transient Currents for Au and PdCo Substrate Electrodes. Compared to RRDE, the SECM has the advantage that different substrates can be examined easily, i.e., without the need to construct a RRDE, which might be difficult. Higher interelectrode fluxes are available without the need to rotate the electrode under defined hydrodynamic conditions. In RRDE experiments, the convection and disk electrode size determine the residence time of H2O2 above the disk electrode (and therefore the time for further reactions). These factors have to be considered for an exact quantification of H2O2 generation at the disk by recording the ring current.3 Simple calibration of a collection efficiency using a reversible redox couple does not take into account the possibility that H2O2 enters into a second reduction step while convectively transported parallel to the disk electrode surface. RRDE experiments provide direct evidence for the overall ORR efficiency (via the disk current) and the H2O2 production (via the ring current) and are often used with quite simple models65 although the complexity of the molecular mechanism is well-known.6 In order to obtain equivalent quantitative information from SECM SG/TC transients, we were interested in a fitable analytical model whose adjustable parameters allow some conclusion about the significance of the most important reaction paths of the ORR at the material under investigation. Martin and Unwin provided theory and experimental examples to extract the ratio of diffusion coefficients from SG/TC transients.41 For the SECM SG/TC pulse experiments using a reversible couple and UME of different geometries, Mauzeroll et al. had derived a numerical description that allowed one to extract diffusion coefficients from transients recorded at different distances.62 Here we derive a quantitative analytical formula for the spatial and temporal development of the diffusion layer taking into account the most important reaction of the ORR and a diffusive transport of H2O2 and O2 from and to the substrate electrode in acidic media:

Table 4. Heterogeneous Rate Constants of Reactions 1-3 at PdCo Electrodes Obtained by Fitting Expressions 4-7 to the Experimental Curves in Figure 8b d/µm

k1/(cm s-1)

k2/(cm s-1)

k3/(cm s-1)

40 50 60 80 100 130

0.0199 0.0198 0.0199 0.0161 0.0095 0.0049

0.0122 0.0094 0.0103 0.0125 0.0094 0.0125

0.0089 0.0087 0.0074 0.0042 0.0025 0.0102

that yield some quantitative information about the significance of the three reaction paths at different materials under investigation in a screening process. No information about rate-limiting elementary steps can be derived from them. We calculate the transient currents from the experimentally determined slope S of the calibration function iT ) f (c(H2O2)) at ET and the calculated local concentrations c(H2O2; d, t).

iT ) S c(H2O2;d,t) + iT,off

(4)

A small offset current iT,off had to be considered for unspecified parasitic currents and instrumental offsets. The local H2O2 concentration is obtained from the analytical expression in eq 5.

c(H2O2;d,t) ) k1c(O2)* D(H2O2)(h′ - h)

(x (x

erfc

d

2 D(O2)t

erfc

[ (x erfc

) (x )

d

- erfc

2 D(H2O2)t

)

d

+

2 D(O2)t

+ hxD(O2)t exp(hd + D(O2)h2t) -

d

2 D(H2O2)t

)

]

+ h′xD(H2O2)t exp(h′d + D(H2O2)h′2t)

(5) where

h)

k 1 + k2 D(O2)

(6)

and k1

O2 + 2H+ + 2e 98 H2O2 k2

O2 + 4H+ + 4e 98 2H2O k3

H2O2 + 2H+ + 2e 98 2H2O

(1) (2) (3)

The constants k1, k2, and k3 are effective rate constants for the most important reaction paths 1-3 that each consists of several elementary steps. The constants serve as adjustable parameters (65) Schmidt, T. J.; Paulus, U. A.; Gasteiger, H. A.; Alonso-Vante, N.; Behm, R. J. J. Electrochem. Soc. 2000, 147, 2620-2624.

h′ ) k3/D(H2O2)

(7)

The time passed after the start of the potential pulse to the substrate electrode is denoted by t. c(O2)* ) 0.27 mM is the bulk concentration of oxygen in air-saturated aqueous solution.57,58 and D(O2) ) 2.0 × 10-5 cm2 s-158and D(H2O2) ) 1.6 × 10-5 cm2 s-1 (vide supra) are the diffusion coefficient of O2 and H2O2, respectively. The derivation of eq 5 is detailed in the Supporting Information 4. With the known or experimentally determined values of c(O2)*, and D(O2), D(H2O2), and S (from Figure 2), iT(d,t) can be fitted to the experimental data by varying k1, k2, and k3. Analytical Chemistry, Vol. 80, No. 3, February 1, 2008

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Figure 9. Calculated x(H2O2) values for the ORR at Au (1) and PdCo (2) electrodes. Curve 1 was obtained from the transients in Figure 4b, curve 6; curve 2 was obtained from Figure 7a, curve 1.

This procedure gave excellent agreement of calculated versus measured data for the Au electrode (solid lines in Figure 4b) for distances of 40-80 µm. The values of k1, k2, and k3 that produced this fit are given in Table 3. The independently obtained effective rate constants k1, k2, and k3 for different d are very consistent. The values of k1 are at least 13 times larger than k2, which is in agreement with the experimental observation and with previous reports that H2O2 is the dominant product of the ORR at Au electrodes at -0.1 V.61 The rate constant k3 is more than 1 order of magnitude smaller than k1, indicating that H2O2 cannot be reduced further to water in significant amounts at Au electrodes at the potential used for O2 reduction. The transients recorded at Pt could not be fitted to this model, probably because significant amounts of H2O2 are generated only immediately after the potential pulse (vide supra). For the transients recorded above the PdCo alloy substrate electrode, a satisfactory agreement could be achieved using the rate constants in Table 4. There are systematic deviations between experiment and fit for the broad maximum in the transients for 40 µm e d e 60 µm (Figure 7a). A possible reason might be that, similar to the observation at Pt, the mechanisms changes shortly after the application of the pulse, for instance, because the reaction on the surface proceeds differently at a partially oxide-covered surface. An indication for this can be seen in the responses to more negative potentials (Figure 7c) that are more reminiscent of the transients obtained for Pt substrates (Figure 5b). In contrast to the results on Au, the constants obtained from the fitting show a trend toward smaller values with increasing d. We therefore fitted the entire set of six transients also with one set of constants k1 ) 0.020 cm s-1, k2 ) 0.012 cm s-1, and k3 ) 0.0078 cm s-1 (solid lines in Figure 7b). The transients are qualitatively reproduced correctly; however, the model overestimates the UME currents for large distances and times and underestimates the response at short times. That would be in agreement with the notion that more H2O2 is generated immediately after the application of the potential pulse. The importance of the model is that it allows from the detection of H2O2 at the UME derivation of some quantitative parameters about the overall ORR efficiency at the sample although only H2O2 is directly measured at the UME. The experimental transients reflect the buildup of the O2 diffusion layer (caused by the overall O2 consumption at the substrate) in the slow decrease of the UME current for longer times. The accuracy of such an estimation 758

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depends of course very much on the uncertainty of H2O2 detection at the UME. The simplistic model of course cannot be a good description for mechanistic work and also fails in those cases where the relative importance of the reaction path 1-3 shifts either because of a change in the surface structure of the substrate electrode during the potential pulse (e.g., reduction of surface oxides) or because ORR mechanisms change if the O2 flux toward the substrates changes with the buildup of an O2 diffusion layer. The effective rate constants allow estimates of empirical parameters (e.g., the fraction x(H2O2) of O2 molecules that are reduced to H2O2)65 that are often used to characterize the efficiency of catalyst preparations with respect to reduction of O2 to H2O. x(H2O2) can be approximated from the three effective rate constants and from the local concentration of O2 and H2O2 that are intermediately calculated during the fitting procedure according to eq 8.

x(H2O2;t) )

k1c(O2;0,t) - k3c(H2O2;0,t) (k1 + k2)c(O2;0,t)

(8)

The derivation of the equation is given in Supporting Information 4. The results obtained from the currents transients measured at d ) 40 µm are shown in Figure 9. For the Au electrode (curve 1), the x(H2O2) values decrease from 0.85 (0.06 s) to 0.61 (2.5 s), indicating that H2O2 is the dominant reduction product of ORR at this electrode in acidic solution. At PdCo electrode, at ES ) +0.2 V (curve 2) the x(H2O2) values decrease from 0.3 (0.05 s) to 0.21 (2.5 s) and are thus significantly lower than at the Au electrode. In an attempt to ease even further the treatment of experimental curves in screening processes, two even more simplifying models were also tested with the same boundary conditions regarding mass transport. In model II, oxygen is reduced at diffusion-controlled rate either to water or to H2O2. In model III, oxygen is reduced with finite rate to H2O2 or with finite rate to water. For both mechanisms, analytical expressions were obtained. They are documented in Supporting Information 4 because they might be of interest for testing hypotheses on similar mechanisms. These models were not able to describe the experimentally obtained current transients. This further indicates the importance of path 3 for the overall reaction mechanism. CONCLUSION Transient SG/TC measurements were used to measure the H2O2 production during the ORR. The method is illustrated with three electrode materials showing different behavior as oxygen reduction catalysts. Both, O2 and H2O2 are reduced at Pt, while at Au, only the reduction of O2 to H2O2 occurs. The PdCo alloy containing 8.75 atom % Co is known to produce both H2O and H2O2 as the main reaction product depending on the applied potential. The SG/TC response was described by an analytical expression obtained by solving the partial differential equations. A fit of this expression with three effective rate constants as the only adjustable parameters provided an excellent fit to the experimental data for Au and a satisfactory agreement for the PdCo electrode. The behavior of Pt electrodes is qualitatively different. The amount of H2O2 formed is much lower but H2O2 is formed mainly immediately after the potential step from the platinum oxide region into the region of O2 reduction.

The transient SG/TC detection of H2O2 during ORR has some analogy to classical RRDE experiments but has some advantages: (i) the net transport of H2O2 occurs perpendicular to the substrate electrode by diffusion only. Diffusion is a well-understood phenomenon that can be quantitatively described by simple partial differential equations. (ii) Higher interelectrode fluxes are available without the need to rotate the electrode or otherwise cause convection in the solution. (iii) Only regions in the vicinity of the UME attribute to the detected H2O2. The size of the region contributing to the signal is determined by the diffusion length of the H2O2 during the potential pulse t ) 2.5. Using D(H2O2) ) 1.6 × 10-5 cm2 s-1, this length is l ) (2Dt)1/2 ) 89 µm. With a typical working distance of d ) 40 µm, the radius of the contributing sample region rS amounts to (l2 - d2)1/2 ) 80 µm. Therefore, the detection scheme has the potential for local testing of materials, e.g., after different operation conditions, for arrays of electrocatalysts or gradient materials. ACKNOWLEDGMENT Y.S. thanks the Alexander von Humboldt Foundation for research fellowships. The project was partially supported by

Deutsche Forschungsgemeinschaft (Wi 1617/7). We thank Ms. Sophie Martyna for recording the EDX spectra in the Supporting Information. SUPPORTING INFORMATION AVAILABLE Figures available are as follows: SI 1, EDX spectra of the PdCo alloy; SI 2, voltammograms of the gold electrode in deaerated and air-saturated 0.1 M H2SO4; SI 3, voltammograms the Pt electrode in deaerated and air-saturated 0.1 M H2SO4; SI 4, detailed deviation of eqs 5 and 8 and deviation and discussion of two models that contain further simplifying assumption; SI 5, details on the approximative calculation of the error function complement. This material is available free of charge via the Internet at http:// pubs.acs.org.

Received for review June 5, 2007. Accepted October 26, 2007. AC0711889

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