Detection of Hydrogen Peroxide Produced during Electrochemical Oxygen Reduction Using Scanning Electrochemical Microscopy

Detection of Hydrogen Peroxide Produced duringElectrochemical Oxygen Reduction UsingScanning Electrochemical Microscopy

Yan Shen, Markus Tra1uble, 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 scanningelectrochemical microscopy was used to detect hydrogenperoxide formed as an intermediate during oxygen reduc-tion at various electrodes. The experiment is conceptuallysimilar to rotating ring-disk experiments but does notrequire the production of a ring-disk assembly for thespecific electrode material in question. In order to limitthe extension of the diffusion layer above the sample, thesample electrode potential is pulsed while the Pt ultra-microelectrode probe (UME) is held at a constant poten-tial for oxidative amperometric detection of hydrogenperoxide. The signal at UME is influenced by the sampleregion within the diffusion length of hydrogen peroxideduring the pulse of 2.5 s. The method is tested with threemodel electrodes showing different behavior with respectto the oxygen reduction reaction (ORR) in acidic solution.Simple analytical models were used to extract effectiverate constants for the most important reaction paths ofORR at gold and palladium-cobalt samples from thechronoamperometric response of the UME to a reductionpulse at the sample electrode.

The electrochemical oxygen reduction reaction (ORR) is a veryimportant reaction in several applications including fuel cells andelectrochemical oxygen sensors.1,2 While the reaction is broadlyused, the ORR is a rather complex multistep process.3,4 As shownin Table 1,5 in acidic or neutral solution, the main product of theORR is H2O2 or H2O, or both, which depends on electrodematerial, electrode potential, and solution composition. The useof Pt-based electrodes is the most commonly employed strategyto avoid the formation of hydrogen peroxide, because Pt is a goodelectrocatalyst for the reduction of O2 and of H2O2 to water.6 The

use of Pt-based electrocatalysts is problematic for mass productsdue to the limited availability of platinum and the severeenvironmental impact associated with the refinery of Pt from Niores. Furthermore, an increase of the efficiency of fuel cells andimproved sensitivities of amperometric oxygen sensors could bereached by a decrease of the overpotential of the ORR by usingnew catalyst materials. This perspective has triggered an intensivesearch for alternative electrode materials for oxygen reduction.

Among the proposed materials, metal alloys are most promis-ing, 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-

* 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, pp369-483.

(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.

(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, 171-178.

(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.

Table 1. Pathways of Electrochemical OxygenReduction in Different Solution According to Ref 5

reactions

Acidic MediaO2 + 2 H+ + 2 e- f H2O2 (1)O2 + 4 H+ + 4 e- f 2 H2O (2)H2O2 + 2 H+ + 2 e- f 2 H2O (3)2 H2O2 f 2 H2O + O2 (decomposition reaction) (4)

Basic MediaO2 + 2 H2O + 4 e- f 4 OH- (5)O2 + H2O + 2 e- f HO2

- + OH- (6)HO2

- + H2O + 2 e- f 3 OH- (7)2 HO2

- f 2 OH- + O2 (decomposition reaction) (8)

Anal. Chem. 2008, 80, 750-759

750 Analytical Chemistry, Vol. 80, No. 3, February 1, 2008 10.1021/ac0711889 CCC: $40.75 © 2008 American Chemical SocietyPublished 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 bilirubinoxidase,23 and peroxidases24 are intensively investigated. Despitethis well-established and important new application and the longhistory of investigations in the mechanisms of this reaction, thereis no method available that allows a rational design of newelectrocatalysts that lead to a smooth four-electron reduction ofdissolved O2 to H2O, avoiding the formation of aggressive oxygenspecies. The complexity of the materials and the variation ofefficiency with preparation conditions suggest the use of combi-natorial approaches for the search of new catalysts, catalystcombinations, and optimization of preparation procedures. Efficientcombinatorial approaches depend on combinatorial preparationof materials and efficient screening methods.25-27 Such approachesare increasingly applied for the search for oxygen reductioncatalysts.28,29

Recently, scanning electrochemical microscopy (SECM) hasbeen applied to study the electrocatalytic and electrode processesof fuel cells, due to the capability to probe interfacial processesand catalytic activity of the substrates with spatial resolution.SECM and combinations of SECM with other analytical methodshave been proposed by Hillier and Bard as techniques for fuelcell electrode screening, concerning hydrogen oxidation, methaneoxidation, or O2 reduction.30-32 Bard et al.14 suggested the use ofSECM in the tip-generation/substrate-collection mode to test theefficiency of metal catalysts for O2 reduction. In this mode, theultramicroelectrode (UME, “tip”) produces O2 under galvanostaticcondition that is consumed at the sample. A plot of the samplecurrent versus the lateral UME position provided a qualitativemapping of the catalyst efficiency.31 Samples were prepared byspotting 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 differ-ences in the O2 reduction efficiency.14 The method has also beenapplied to sputtered metal thin films.33 Eckhard et al. introducedthe redox competition mode. In this mode, both the sample andthe UME consume O2 in a potentiostatic mode.34 At sample

regions with high O2 reduction efficiency, the O2 concentrationclose to the sample is reduced and this is reflected by reducedreduction currents at the UME. In order to avoid a depletion ofO2 in an extended diffusion layer above the sample, the UMEpotential ET is pulsed. Before the measurement of the reductioncurrent, the interelectrode volume is periodically enriched in O2

by water electrolysis at the UME. Alternatively, the SECMfeedback mode has been used to measure the kinetic constant ofthe ORR at the Pt electrode, but this method is limited to pH9-12.35

Also optical imaging techniques have been used to obtaininformation about inhomogeneous diffusion layers above elec-trodes that allow conclusions about localized ORR. Some examplesused fluorescence imaging of a dye that changes fluorescencespectra with pH.36 Since proton transfer in aqueous solution istypically very fast, the image of such a dye represents local protonconcentration. Because the imaging is based on the protonationequilibrium of the dye, the applicable pH range extends aroundthe pKa value of the dye. Rudd et al. performed confocal laserimaging of fluorescine in unbuffered KCl solution pH 4.38 abovean electrode reducing O2.37 During ORR, protons are consumedand the pH rises in the vicinity of the electrode. Concentrationprofiles of OH- ions could be directly visualized, and the size ofthe diffusion layer could be compared to theory that took intoaccount the extent to which ORR yields either H2O or H2O2.

None of the above-mentioned methods provides informationabout the amount of generated H2O2. H2O2 formation not onlydecreases the efficiency of fuel cell but may also contribute tomembrane degradation and corrosion processes of metals, poly-mer fittings, and carbon materials. Among other characteristics,avoidance of H2O2 is therefore an important parameter for theoptimization of fuel cell catalysts. Pletcher and Sotiropoulosinvestigated the ORR at Pt in neutral and alkaline solutionconsidering the “number napp of electrons that appear to beinvolved in the reduction of oxygen if it is assumed that thereduction is mass transport-controlled”.38 They found a decreaseof napp obtained from plateau currents of Pt UME and Pt rotatingdisk electrodes with increasing mass transport coefficients indica-tive of increasing amounts of produced H2O2. H2O2 was, however,not directly detected.

The conventional method to test the formation of H2O2 consistsin rotating ring-disk electrode (RRDE) experiments.3 Oxygen isreduced at the disk electrode and generated H2O2 is transportedunder defined hydrodynamic conditions to the surrounding Pt ringelectrode where it is amperometrically detected by oxidation. Thisprocedure requires the preparation of a ring-disk electrodeassembly for each new catalyst to be investigated. Since thedemands for mechanical smoothness and precision are high, thismethod is not particular suitable for combinatorial tests whereone may want to test a number of similar catalysts on a chip-typesupport. In addition, when an electrocatalyst for oxygen reductionis deposited on the disk electrode by dip coating, metal evapora-tion, or multilayer assembly, the ring electrode may be contami-nated at the same time. Postlethwaite et al. suggested the use of

(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, 1557-

1563.(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, 3384-

3386.(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, 159-

166.(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, 5172-

5181.(34) Eckhard, K.; Chen, X.; Turcu, F.; Schuhmann, W. Phys. Chem. Chem. Phys.

2006, 8, 5359-5365.

(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 RRDEexperiments for ORR.39 They provided the mathematical treatmentand could clearly demonstrate that a much smaller amount of H2O2

could be detected than at RRDE because the collection efficiencyis higher and an electrochemical feedback could be exploited. Thismethod depends on the availability of IDAs of the material to beinvestigated. Although modification was shown, it is difficult toconceive how, for instance, powders of carbon-supported catalystscan be investigated. Therefore, methods are needed that allowthe investigation of H2O2 formation with a probe that is notmechanically attached to the sample electrode.

In this work, a new application of substrate-generation/tip-collection (SG/TC) mode of SECM is proposed and used tomeasure H2O2 produced during the ORR. In the SG/TC mode,the substrate generates a species that diffuses into the bulk. Adisk-shaped UME with radius rT is used as the probe andpositioned in a defined distance d above the substrate. It collectsthe generated H2O2. The UME is at least 1 order of magnitudesmaller than the substrate and has a much thinner diffusion layerthan the substrate. Historically, SG/TC experiments with anamperometric UME were pioneered by Engstrom et al. usingsmall carbon UMEs.40 They addressed the transient response ofthe UME after a potential step at the sample. Martin and Unwinused the SG/TC transients to extract the ratio of diffusioncoefficients of redox couples.41 The theory considers shieldingeffects of the probe and electrochemical feedback between sampleand UME.41 Amatore et al. used confocal resonance Ramanmicroscopy,42 potentiometric43 and amperometric UME44 to mapthe concentration profiles in the vicinity of electrodes with theaim of resolving the interplay of diffusion and natural convection45

using a reversible redox couple [Fe(CN)6]4-/3- and to find thesignature of coupled homogeneous reactions.46,47 The method wasrefined 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 fromseries of such experiments. Furthermore, the improvement thatcan be obtained by using nanometer-sized probe electrodes wasexperimentally demonstrated.48 SG/TC transients have also beused to investigate the transport of Tl+ through phospholipidmonolayers.49 Here we use SG/TC experiments to measure H2O2

produced by the ORR during a potential pulse to the substrateelectrode. In order to provide quantitative data

equivalent to the results of simple RRDE experiments, theexperimental data were fitted to analytical models that containeffective rate constants for the different reaction paths as adjust-able parameters. The approach is illustrated with three electro-catalysts of oxygen reduction showing qualitatively differentbehavior in acidic solution.

EXPERIMENTAL SECTIONInstruments. SECM measurements were performed on a

home-built instrument using a stepper-motor positioning system(Marzhauser, Wetzlar, Germany) and a bipotentiostat model CHI7001 B (CH Instruments, Austin, TX). The software of the CHI7001B was used to apply the potential pulses and to record thetransients at fixed UME position. During scanning experiments,e.g., UME approach, a computer equipped with a 16-bit dataacquisition board PCI-DAS1602/16 (Plug-In Electronic, Reichenau,Germany) was used to read the output voltage from the CHI 7001Bat each motor position using the program SECMx developed in-house. The bipotentiostat controlled a four-electrode cell with theworking electrode 1 (WE1) being the SECM sample and theworking electrode 2 (WE2) being the UME. It allowed for settingthe UME potential ET at a constant value while the potential ofthe macroscopic sample ES was pulsed. An Ag|AgCl|3 M KClreference electrode and a Pt wire (diameter 0.5 mm) were usedas reference and auxiliary electrodes, respectively. All potentialsare quoted with respect to the Ag|AgCl|3 M KCl.

Solution. Fresh solutions of hydrogen peroxide were madefor each experiment by diluting of a concentrated commercialaqueous solution (30% (v/v), Sigma-Aldrich GmbH, Steinheim,Germany). Ammonium sulfate and sulfuric acid were purchasedfrom ABCR (ABCR GmbH & Co. KG, Karlsruhe, Germany). trans-Diamminedichloropalladium(II) chloride, cobalt(II) sulfate, andammonium hydroxide solution (50% (v/v)) were purchased fromAlfa Aesar (Karlsruhe, Germany). All compounds were used asreceived. Aqueous solutions were prepared using deionized water.

Electrodes. A 25-µm-diameter Pt wire (Goodfellow, Cam-bridge, U.K.) was sealed under vacuum into a Pyrex glass capillary(inner diameter 0.87 mm, outer diameter 1.5 mm). The UME waspolished and shaped conically by a wheel with 180-grid Carbimetpaper disks and micropolishing cloth with 1.0-, 0.3-, and 0.05-µmalumina. The UME was sharpened to a RG ≈ 10, where RG isthe ratio between the diameters of the glass sheath (rglass) andthe radius rT of the active electrode surface. Before each experi-ment, the UME was polished with 0.3- and 0.05-µm aluminapowder and rinsed with water. The Pt UME were then electro-chemically cleaned by cycling between -0.2 and 1.5 V at 100 mVs-1 in 0.1 M sulfuric acid. Platinum, gold, and glassy carbon (GC)electrodes (1.5 mm and 3 mm diameter) were purchased fromCH Instrument Inc. These electrodes were polished with 1.0- and0.3-µm R-Al2O3 powders successively, sonicated in water for ∼5min after each polishing step, and rinsed with water. The metalelectrodes were then electrochemically cleaned by cycling be-tween -0.2 and 1.5 V at 100 mV s-1 in 0.1 M sulfuric acid beforeuse.

PdCo alloy was deposited onto the cleaned GC electrode in athree-electrode setup at -1.0 V controlled by the CHI 7001 Bpotentiostat from a stirred aqueous solution containing 0.67 mMPd(NH3)2Cl2 + 0.2 mM CoSO4 in 0.4 M (NH4)2SO4. The pH wasadjusted to 9.42 by NH4OH solution. The deposition time was 120

(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. Electro-

chem. Commun. 2000, 2, 235-239.(43) Amatore, C.; Szunerits, S.; Thouin, L. Electrochem. Commun. 2000, 2, 248-

253.(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.

752 Analytical Chemistry, Vol. 80, No. 3, February 1, 2008

s. Energy-dispersive X-ray (EDX) characterization confirmed thatthe PdCo alloy contained 8.75 ato -% Co. (Figure SI 1, in SupportingInformation).

Procedures. All experiments were performed sequentially inthe same SECM cell made of perfluorinated polymer that accom-modated four electrodes. The Au, Pt, or modified GC electrodeswere cleaned or modified and then inserted as the bottom of thecell as SECM substrate electrode such that the surfaces of thesubstrate and UME were parallel. The reference electrode andauxiliary electrode were attached to the inner perimeter of theelectrochemical cell. The UME was brought in mechanical contactwith the substrate by recording an approach curve while monitor-ing the oxygen reduction current at the Pt UME (negativefeedback). When mechanical contact with the support occurred,the approach was interrupted, and the electrode was then retracteda preset distance d from the surface. A new approach curve wasrecorded frequently to ensure that d was conserved during theexperiment. Nevertheless, because of the shape of the UME, theexact mounting geometry, and the tilt of the sample, there is anuncertainty in d of ∼2 µm, which is negligible compared to thetypical working distances used (d > 40 µm).

The basic principle of the transient SG/TC mode used in thiswork is shown in Figure 1a. Chronoamperometric experimentswere carried out by stepping the ES from a value where no faradicprocess occurs to a value, well into the limiting-current region.The appropriate ES values for the ORR were selected fromvoltammograms of air-saturated solutions. The UME potential (ET)was set to +1.1 V and kept constant.

Data Treatment. In order to extract the rate constants fromthe measured chronoamperograms, an analytical model (eq 5)was fitted to the experimental data using a least-squares routineand a simplex algorithm taken from ref 50. The complement errorfunction occurring in eq 5 was calculated with the help of theincomplete γ function,50 and for greater arguments, an iterativeapproximation was used (Supporting Information 5). The programwas implemented in Turbo Pascal 6 and runs under DOS andWindows environments. For fitting curves with 2500 data points,∼150 iteration steps of the simplex algorithm were required, untilthe mean squared deviation at the vertices of the simplex is lessthan 10-6 nA. This procedure took 6-7 min on a 1.6-GHz Pentiumunder Windows 2000 SP4 and occupies 17 MB memory.

RESULTS AND DISCUSSIONPt UME as Amperometric Sensor of H2O2 Oxidation. The

amperometric detection of H2O2 over a wide concentration rangeand at different pH is difficult because the H2O2 oxidation (2H2O2

f 2H2O + O2) is a catalytic reaction.51,52 It was reported that thelimiting current at Pt electrodes was not proportional to theconcentration of H2O2 above 1 mM.53 Because H2O2 oxidation isunder mixed kinetic and diffusion control, a lack of available Ptsurface sites will become the limiting factor. Recently, newapproaches have been proposed using different electrode materi-als,54 nanostructured metals,55 or immobilized enzymes,56 extend-ing the useful working range. In our experiment, H2O2 wasproduced from an air-saturated aqueous solution. The concentra-tion of O2 in such solutions at room temperatures (296 K) is about0.27 mM posing an upper limit on the concentration of H2O2 thatcan be formed during ORR in air-saturated solution.4,57,58 In thisconcentration range a linear relation between the H2O2 oxidationcurrent and the H2O2 concentration is obtained provided that asufficiently positive potential is applied.55 Figure 2 shows acalibration plot for the amperometric H2O2 detection at Pt UMEat 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. NumericalRecipes in Pascal; Cambridge University Press: Cambridge, UK, 1991.

(51) Hall, S. B.; Khudaish, E. A.; Hart, A. L. Electrochim. Acta 1998, 43, 579-588.

(52) Hall, S. B.; Khudaish, E. A.; Hart, A. L. Electrochim. Acta 1998, 43, 2015-2024.

(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.

Figure 1. (a) Schematic illustration the SECM operation in thepulsed SG/TC mode; (b) potential wave form applied to the Auelectrode.

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


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 diffusioncoefficient of H2O2 was calculated from the sensitivity withD(H2O2) ) 1.6 × 10-5 cm2 s-1 and falls within the range (0.66-2.20) × 10-5 cm2 s-1 of previously quoted values,55 indicating thatthe detection occurs under diffusion-controlled conditions. Thesteady-state response is found to be very stable. Therefore, suchPt 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 inalkaline solutions.59-61 In acidic solutions, a 2e- pathway leads toH2O2 as the dominant final product, independent of the surfacestructure of the electrode.61 In order to detect the H2O2 producedduring ORR at a macroscopic Au electrode, a chronoamperometricexperiment was performed as shown in Figure 1b. The Pt UMEwas positioned at d ) 40 µm above the sample and held at ET )+1.1 V for which the H2O2 oxidation is diffusion-controlled below0.6 mM (Figure 2). The potential of the Au substrate was steppedfrom ES ) +0.4 V to different values, which were chosen fromthe cyclic voltammogram for oxygen reduction at the Au electrode(Figure SI 2 in Supporting Information). The UME transientcurrent was recorded during the potential pulse to the Auelectrode (Figure 3). An increasing iT is observed with decreasingES due to the increased H2O2 production at the Au substrate. Morenegative ES than -0.2 V caused abnormal UME currents attributedto 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 beconcluded from the voltammogram in deaerated solution (FigureSI 2 in Supporting Information).

The influence of the working distance d was explored bystepping ES from +0.4 to -0.1 V and recording transients atdifferent d. They showed the spatiotemporal development of theH2O2 diffusion layer (Figure 4). The time to reach a steady-statecurrent increases with d. When d is larger than 80 µm, no steady-state situation is reached within the pulse time of 2.5 s. This canbe rationalized by considering how far H2O2 may diffuse withinthe pulse time δ ) (2Dt)1/2,62 where D ) 1.6 × 10-5 cm2 s-1 isthe diffusion coefficient of H2O2, t is the time after the pulse onset,

and δ is the thickness of the diffusion layer (Table 2). The obtainedtime corresponds well to the observed transients in Figure 4. Forseparations shorter than 40 µm, the current transient passesthrough a maximum and then falls well below the steady-statecurrent observed at d ) 40 µm (Figure 4a). This is due to thehindered diffusion of O2 to the regions of the substrate electrodeunderneath the SECM probe (including shielding). With decreas-ing d, this maximum occurs at earlier time and the currentdecreases to lower values. The results for d > 40 µm (Figure 4b)show that the generated H2O2 is a stable reaction product at theAu electrode that cannot be further reduced in agreement withknown RRDE studies of this system.

Oxygen Reduction at Pt Electrode. Details of the oxygenreduction on Pt electrodes have been intensively examined forseveral decades; however, there remains considerable uncertaintyregarding the exact mechanism because of its complex kinetics.63

Despite this fact, the behavior of Pt is contrary to that of Au(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, 349-

375.(61) Strbac, S.; Adzic, R. R. Electrochim. Acta 1996, 41, 2903-2908.

(62) Mauzeroll, J.; Hueske, E. A.; Bard, A. J. Anal. Chem. 2003, 75, 3880-3889.

(63) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.;Bligaard, T.; Jonsson, H. J. Phys. Chem. B 2004, 108, 17886.

Figure 3. UME transient currents at ET ) 1.1 V for the collection ofH2O2 produced during ORR at an Au electrode at d ) 40 µm. Thepotential 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.

Figure 4. UME transient currents for H2O2 oxidation at ET ) 1.1 Vduring ORR at an Au substrate in 0.1 M H2SO4 solution at differentdistances 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 usingeqs 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 atDifferent Distances d (t ) d2/2D), D ) 1.6 × 10-5 cm2

s-1

d/µm t/s

40 0.550 0.7860 1.1380 2

100 3.13


because Pt is also a good catalyst for the H2O2 reduction. This isreflected in SECM pulse experiments. In this work, the Ptelectrode was pulsed from +0.9 to +0.3 V, which was selectedfrom the voltammogram of ORR at a Pt electrode (Figure SI 3 inSupporting Information). As shown in Figure 5, the transient H2O2

oxidation current at the UME can be markedly observed only atsmall d (curves 1-5 in Figure 5a). The UME current decreasesvery rapidly when d increases from 4 to 12 µm. The peak currentshifts to longer time in agreement with the expectation for aspecies diffusing from the sample to the UME. When d > 30 µm,only a small H2O2 oxidation current could be detected at the UMEwith a more sensitive setting of the potentiostat (Figure 5b, notethe different scaling compared to Figure 4b). In contrast to Ausubstrates, no steady-state UME current is obtained above the Ptsubstrate. Instead, a broad peak appears in the transients (curves(6-8 in Figure 5b). The current transient shows that H2O2 formsonly a very thin diffusion layer above the Pt substrate electrodesurface because it is further reduced to water at the same potentialas the ORR occurs. The result indicates that H2O2 can be treatedas an intermediate, but not as the final product of the ORR at Ptelectrode. Recently, Shao et al. have confirmed that two 2-electronreduction steps are operative for ORR on Pt electrodes in alkalinesolutions by surface-enhanced infrared reflection absorptionspectroscopy with attenuated total reflection.64 In addition, thesharp decay of the transients in Figure 5a indicates that H2O2 ispredominately formed during the onset of the ORR at the Ptelectrode and that much less H2O2 is produced during continuousoxygen reduction at Pt. This can be a consequence of the pulseprofile applied to the Pt electrode. ES is stepped from the platinumoxide region into the double layer region. During the pulse, thePt surface oxide is reduced, and therefore, the oxygen reduction

may initially follow a different mechanism at a partially oxide-covered surface than at an oxide-free Pt surface at a later time.This finding may have some implication with respect to the long-term stability of fuel cell components under conditions of rapidlychanging power demands.

Another point of concern is a possible electronic interferencebetween the sample and the UME. Shortly after the potentialpulse, the structure of the double layer of the macroscopic samplewill change. During this time, the UME will be placed in an electricfield that will also cause a change of the effective UME potentialeven 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 theelectrical interference of the sample and the UME (curves 1-5in Figure 5a). These currents were therefore not further consid-erered here. At longer times, the UME currents should not beaffected by this interference.

Oxygen Reduction at PdCo Alloy-Modified GC Electrode.Recently, the electrodeposition of highly dispersed Pd nanopar-ticles and its alloys on a substrate has attracted growing interestbecause of their extraordinarily high catalytic activity in manyreactions, especially for ORR.10,15 As a model sample, a PdCo alloywas deposited on a GC electrode as electrocatalyst for ORR andwas 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, curve2 (air-saturated solution) shows that the ORR occurs at ES < +0.5V, i.e., at much more positive values than at the GC electrodesonto which the PdCo particles were deposited.18-20 The curvecontains a slope at ES < 0.3 V for which we do not have an detailedexplanation at this point. The buildup of the O2 diffusion layer inquiescent solution and slow changes of the surface state of thePdCo alloy during the potential sweep may lead to this observa-tion. The addition of H2O2 to a deaerated 0.1 M H2SO4 solutionproves that H2O2 can be reduced only at ES < +0.2 V (Figure 6,curve 3). This model electrode offers the possibility to carry outthe ORR under clearly different overall mechanisms because therate of O2 and H2O2 reduction changes very differently withpotential. In the region +0.2 V < ES < +0.5 V, H2O2 cannot befurther reduced while at ES < 0.2 V H2O2 it undergoes furtherreduction. Depending on the ES, water, hydrogen peroxide, orboth are the primary products of the ORR at this alloy electrode.Figure 7a shows the SG/TC transients, which were obtained bystepping ES from +0.8 to +0.2 V. The shape of the curves is

(64) Shao, M.; Liu, P.; Radoslav, R. A. J. Am. Chem. Soc. 2006, 128, 7408-7409.

Figure 5. UME transient currents for H2O2 oxidation at ET ) 1.1 Vduring an ORR at a Pt substrate in 0.1 M H2SO4 solution at differentdistances 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.

Figure 6. CVs of PdCo alloy on a GC electrode in (1) deaerated0.1 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. Scanrate 0.02 V s-1.


different from the transients recorded at Au and Pt electrodes.When d < 80 µm, a broad maximum of the H2O2 oxidation currentat the UME is observed. The maximum is shifted to longer timesas d is increased from 40 to 60 µm (Figure 7a, curves (1)-(3)).The UME currents in Figure 7a are smaller than the UMEcurrents above the gold electrode (Figure 4b) and bigger thanthose above at the Pt electrode (Figure 5b) in comparableexperiments. The behavior changes qualitatively if ES is steppedfrom +0.8 to 0 V, i.e. into a range in which H2O2 reduction occursat PdCo electrodes with a much faster rate than at +0.2 V whilethe rate of O2 reduction is similar at both potentials (Figure 7c).The UME currents are much lower in Figure 7c than in Figure7a for the same d, because most of the generated H2O2 was furtherreduced at the PdCo substrate electrode. The qualitative behaviorin this potential region is similar to the Pt electrode (Figure 5b)although the currents at PdCo are larger. When applying evenmore negative potential pulses to the alloy electrode, the propertieswith respect to H2O2 and O2 reduction became even more similarto that of the Pt electrode (not shown).

Reconstruction of H2O2 Concentration Profiles. AvoidingH2O2 generation during ORR is important for efficiency reasons

but also in order to prolong the live time of membranes, fittings,and metal pieces. In order to relate the finding from electrochemi-cal studies to stability tests of materials and components, it isimportant to know the transient H2O2 concentrations observed inFigures 4, 5, and 7. Here the local transient concentration ofgenerated H2O2 in the diffusion layer above the different electrodematerials can be approximately determined using the calibrationcurve (S ) 12.38 nA mM-1) obtained for the used Pt UME (Figure2). From Figure 4b, Figure 5b, and Figure 7a, the UME currentsat different d at 0.5 s, 1.0 s, 1.5, 2.0, and 2.5 s were selected. Thelocal concentration was obtained by c(H2O2; d, t) ) i(d, t)/S. Theresults are summarized in Figure 8. With increasing time, thedifferences between substrate electrodes become more evident.For the Pt substrate, c(H2O2) reaches more than 10 µM 1 s afterthe onset of ORR but is too low to be measured at longer t andlarger d as shown in Figure 8b (note the different scale of theordinate in Figure 8b compared to Figure 8a and c). At the Auelectrode, almost all O2 is converted to H2O2. The H2O2 concentra-tion at the electrode surface reaches a steady-state value of 0.27mM, the concentration of O2 in air-saturated 0.1 M sulfuric acidsolution.57,58 From these results, it could be concluded that SECMSG/TC mode can be used to quantitatively measure H2O2

produced during ORR at different electrocatalysts.

Figure 7. UME transient response for H2O2 oxidation at ET ) 1.1V during ORR at a PdCo alloy substrate electrode in air-saturated0.1 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 sameexperimental data points. Solid lines in (a) were calculated using eqs4 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.

Figure 8. Concentration profile of H2O2 produced during oxygenreduction at different electrodes calculated from Figures 4, 5. and 7(open symbols); (a) Au; (b) Pt; (c) PdCo. The time for which the localconcentration 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.


Modeling the UME Transient Currents for Au and PdCoSubstrate Electrodes. Compared to RRDE, the SECM has theadvantage 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 torotate the electrode under defined hydrodynamic conditions. InRRDE experiments, the convection and disk electrode sizedetermine the residence time of H2O2 above the disk electrode(and therefore the time for further reactions). These factors haveto be considered for an exact quantification of H2O2 generationat the disk by recording the ring current.3 Simple calibration of acollection efficiency using a reversible redox couple does not takeinto account the possibility that H2O2 enters into a secondreduction step while convectively transported parallel to the diskelectrode surface. RRDE experiments provide direct evidence forthe overall ORR efficiency (via the disk current) and the H2O2

production (via the ring current) and are often used with quitesimple models65 although the complexity of the molecular mech-anism is well-known.6 In order to obtain equivalent quantitativeinformation from SECM SG/TC transients, we were interestedin a fitable analytical model whose adjustable parameters allowsome conclusion about the significance of the most importantreaction paths of the ORR at the material under investigation.Martin and Unwin provided theory and experimental examplesto extract the ratio of diffusion coefficients from SG/TC tran-sients.41 For the SECM SG/TC pulse experiments using areversible couple and UME of different geometries, Mauzeroll etal. had derived a numerical description that allowed one to extractdiffusion coefficients from transients recorded at different dis-tances.62 Here we derive a quantitative analytical formula for thespatial and temporal development of the diffusion layer taking intoaccount the most important reaction of the ORR and a diffusivetransport of H2O2 and O2 from and to the substrate electrode inacidic media:

The constants k1, k2, and k3 are effective rate constants for themost important reaction paths 1-3 that each consists of severalelementary steps. The constants serve as adjustable parameters

that yield some quantitative information about the significance ofthe three reaction paths at different materials under investigationin a screening process. No information about rate-limiting elemen-tary steps can be derived from them.

We calculate the transient currents from the experimentallydetermined slope S of the calibration function iT ) f (c(H2O2)) atET and the calculated local concentrations c(H2O2; d, t).

A small offset current iT,off had to be considered for unspecifiedparasitic currents and instrumental offsets. The local H2O2

concentration is obtained from the analytical expression in eq 5.

where

and

The time passed after the start of the potential pulse to thesubstrate electrode is denoted by t. c(O2)* ) 0.27 mM is the bulkconcentration of oxygen in air-saturated aqueous solution.57,58 andD(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 SupportingInformation 4. With the known or experimentally determinedvalues 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, andk3.

(65) Schmidt, T. J.; Paulus, U. A.; Gasteiger, H. A.; Alonso-Vante, N.; Behm, R.J. J. Electrochem. Soc. 2000, 147, 2620-2624.

Table 3. Heterogeneous Rate Constants of Reactions1-3 at Au Electrodes Obtained by Fitting Expressions4-7 to the Experimental Curves in Figure 4b

d/µm k1/(cm s-1) k2/(cm s-1) k3/(cm s-1)

40 0.0443 0.000 018 0.002 8350 0.0445 0.000 482 0.002 6260 0.0422 0.003 02 0.001 9580 0.0422 0.002 42 0.002 32

O2 + 2H+ + 2e 98k1

H2O2 (1)

O2 + 4H+ + 4e 98k2

2H2O (2)

H2O2 + 2H+ + 2e 98k3

2H2O (3)

Table 4. Heterogeneous Rate Constants of Reactions1-3 at PdCo Electrodes Obtained by FittingExpressions 4-7 to the Experimental Curves in Figure8b

d/µm k1/(cm s-1) k2/(cm s-1) k3/(cm s-1)

40 0.0199 0.0122 0.008950 0.0198 0.0094 0.008760 0.0199 0.0103 0.007480 0.0161 0.0125 0.0042

100 0.0095 0.0094 0.0025130 0.0049 0.0125 0.0102

iT ) S c(H2O2;d,t) + iT,off (4)

c(H2O2;d,t) )

k1c(O2)*D(H2O2)(h′ - h) [erfc( d

2xD(H2O2)t) - erfc( d

2xD(O2)t) +

erfc( d

2xD(O2)t+ hxD(O2)t) exp(hd + D(O2)h

2t) -

erfc( d

2xD(H2O2)t+ h′xD(H2O2)t) exp(h′d + D(H2O2)h′2t)]

(5)

h )k1 + k2

D(O2)(6)

h′ ) k3/D(H2O2) (7)


This procedure gave excellent agreement of calculated versusmeasured data for the Au electrode (solid lines in Figure 4b) fordistances of 40-80 µm. The values of k1, k2, and k3 that producedthis fit are given in Table 3. The independently obtained effectiverate 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 inagreement with the experimental observation and with previousreports that H2O2 is the dominant product of the ORR at Auelectrodes at -0.1 V.61 The rate constant k3 is more than 1 orderof magnitude smaller than k1, indicating that H2O2 cannot bereduced further to water in significant amounts at Au electrodesat 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 onlyimmediately after the potential pulse (vide supra).

For the transients recorded above the PdCo alloy substrateelectrode, a satisfactory agreement could be achieved using therate constants in Table 4. There are systematic deviations betweenexperiment and fit for the broad maximum in the transients for40 µm e d e 60 µm (Figure 7a). A possible reason might be that,similar to the observation at Pt, the mechanisms changes shortlyafter the application of the pulse, for instance, because the reactionon the surface proceeds differently at a partially oxide-coveredsurface. An indication for this can be seen in the responses tomore negative potentials (Figure 7c) that are more reminiscentof the transients obtained for Pt substrates (Figure 5b). In contrastto the results on Au, the constants obtained from the fitting showa trend toward smaller values with increasing d. We thereforefitted the entire set of six transients also with one set of constantsk1 ) 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 repro-duced correctly; however, the model overestimates the UMEcurrents for large distances and times and underestimates theresponse at short times. That would be in agreement with thenotion that more H2O2 is generated immediately after the applica-tion of the potential pulse.

The importance of the model is that it allows from the detectionof H2O2 at the UME derivation of some quantitative parametersabout the overall ORR efficiency at the sample although only H2O2

is directly measured at the UME. The experimental transientsreflect the buildup of the O2 diffusion layer (caused by the overallO2 consumption at the substrate) in the slow decrease of the UMEcurrent for longer times. The accuracy of such an estimation

depends of course very much on the uncertainty of H2O2 detectionat the UME. The simplistic model of course cannot be a gooddescription for mechanistic work and also fails in those caseswhere the relative importance of the reaction path 1-3 shifts eitherbecause of a change in the surface structure of the substrateelectrode during the potential pulse (e.g., reduction of surfaceoxides) or because ORR mechanisms change if the O2 flux towardthe substrates changes with the buildup of an O2 diffusion layer.The effective rate constants allow estimates of empirical param-eters (e.g., the fraction x(H2O2) of O2 molecules that are reducedto H2O2)65 that are often used to characterize the efficiency ofcatalyst preparations with respect to reduction of O2 to H2O.x(H2O2) can be approximated from the three effective rateconstants and from the local concentration of O2 and H2O2 thatare intermediately calculated during the fitting procedure accord-ing to eq 8.

The derivation of the equation is given in Supporting Information4. The results obtained from the currents transients measured atd ) 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 atthis 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) to0.21 (2.5 s) and are thus significantly lower than at the Auelectrode.

In an attempt to ease even further the treatment of experi-mental curves in screening processes, two even more simplifyingmodels were also tested with the same boundary conditionsregarding mass transport. In model II, oxygen is reduced atdiffusion-controlled rate either to water or to H2O2. In model III,oxygen is reduced with finite rate to H2O2 or with finite rate towater. For both mechanisms, analytical expressions were obtained.They are documented in Supporting Information 4 because theymight be of interest for testing hypotheses on similar mechanisms.These models were not able to describe the experimentallyobtained current transients. This further indicates the importanceof path 3 for the overall reaction mechanism.

CONCLUSIONTransient SG/TC measurements were used to measure the

H2O2 production during the ORR. The method is illustrated withthree electrode materials showing different behavior as oxygenreduction catalysts. Both, O2 and H2O2 are reduced at Pt, whileat Au, only the reduction of O2 to H2O2 occurs. The PdCo alloycontaining 8.75 atom % Co is known to produce both H2O andH2O2 as the main reaction product depending on the appliedpotential. The SG/TC response was described by an analyticalexpression obtained by solving the partial differential equations.A fit of this expression with three effective rate constants as theonly adjustable parameters provided an excellent fit to theexperimental data for Au and a satisfactory agreement for thePdCo electrode. The behavior of Pt electrodes is qualitativelydifferent. The amount of H2O2 formed is much lower but H2O2 isformed mainly immediately after the potential step from theplatinum oxide region into the region of O2 reduction.

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

x(H2O2;t) )k1c(O2;0,t) - k3c(H2O2;0,t)

(k1 + k2)c(O2;0,t)(8)


The transient SG/TC detection of H2O2 during ORR has someanalogy to classical RRDE experiments but has some advan-tages: (i) the net transport of H2O2 occurs perpendicular to thesubstrate electrode by diffusion only. Diffusion is a well-understoodphenomenon that can be quantitatively described by simple partialdifferential equations. (ii) Higher interelectrode fluxes are availablewithout the need to rotate the electrode or otherwise causeconvection in the solution. (iii) Only regions in the vicinity of theUME attribute to the detected H2O2. The size of the regioncontributing to the signal is determined by the diffusion lengthof 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 atypical working distance of d ) 40 µm, the radius of thecontributing sample region rS amounts to (l2 - d2)1/2 ) 80 µm.Therefore, the detection scheme has the potential for local testingof materials, e.g., after different operation conditions, for arraysof electrocatalysts or gradient materials.

ACKNOWLEDGMENTY.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 SupportingInformation.

SUPPORTING INFORMATION AVAILABLE

Figures available are as follows: SI 1, EDX spectra of the PdCoalloy; SI 2, voltammograms of the gold electrode in deaerated andair-saturated 0.1 M H2SO4; SI 3, voltammograms the Pt electrodein deaerated and air-saturated 0.1 M H2SO4; SI 4, detailed deviationof eqs 5 and 8 and deviation and discussion of two models thatcontain further simplifying assumption; SI 5, details on theapproximative calculation of the error function complement. Thismaterial 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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