YSZ (111

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Oxidation of Platinum in the Epitaxial Model System Pt(111)/YSZ(111): Quantitative Analysis of an Electrochemically Driven PtOx Formation H. P€opke, E. Mutoro,† B. Luerssen, and J. Janek* Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 58, 35392 Giessen, Germany ABSTRACT: The oxidation of platinum (Pt) plays a key role in electrochemistry, both in the solid and liquid state, and in surface science. This work provides a comprehensive comparison of current knowledge of Pt oxidation in these three fields. By presenting new data of the solid state epitaxial model-type interface Pt(111)/yttria-stabilized zironia (YSZ (111)), fundamental insights are obtained: (i) analogous features in cyclic voltammograms of the interfaces Pt(111)/acid aqueous electrolyte and Pt(111)/YSZ(111) are correlated and their differences explained. (ii) By comparing the cathodic peak shapes of cyclic voltammograms of Pt/YSZ to temperature-programmed desorption spectra, the peaks can be attributed to an adsorption and an oxide-like state of oxygen. (iii) The linearity between the accumulation of oxygen at the Pt electrode and the logarithm of polarization time for sufficiently high anodic potentials, which is wellknown in aqueous electrochemistry, is also found at the interface Pt/YSZ for the first time, and the mechanism leading to this time law is discussed.

1. INTRODUCTION Whenever oxygen interacts with a platinum (Pt) surface or interface, Pt oxide formation and its influence on the properties of the system under investigation (e.g., an electrode system) are of great importance. Therefore, it has been studied extensively over the past decades in different disciplines such as surface chemistry as well as liquid and solid state electrochemistry, both experimentally and by theoretical modeling. Because of many unresolved questions, investigations are still continuing. In the field of surface science oxidation of Pt is important for mechanistic studies in heterogeneous catalysis1 8 and for oxidation/corrosion processes.9 20 A good understanding of the oxidation mechanism has been obtained by combining results of various investigation techniques, such as XPS,10,14,15,20 LEED,10 12,14,15 EELS,10,11,13,15 AES,11,12 TPD,11 13,15,17 UPS,11 CAICISS,14 XRD,16 STM,17,19 and measuring wire resistance9 or work function11 (see explanation of abbreviations at the end of the article). In liquid state electrochemistry the electrochemically driven oxidation of Pt at the Pt/electrolyte interface plays a key role for electrocatalysis, in corrosion and electrode degradation. Again, information about the formation of Pt oxide has been gained by a number of analytical methods, including electroanalytical techniques,10,21 43 XPS,10,24,28,39,41 LEED,44 STM,29,42 EQCN,33,37,43 XRR,34 and AES.37 In addition to experimental studies, theoretical calculations (mainly based on DFT and a few kinetic Monte Carlo or molecular dynamic simulations) were carried out to support and expand the state of knowledge in surface science and liquid state electrochemistry.3,20,45 56 r 2011 American Chemical Society

In solid state ionics, the electrode system Pt/yttria-stabilized zirconia (YSZ) is applied in sensors,57 62 recently utilized as electrode in μ-SOFCs,63 67 and is the prototype solid state electrode model system for fundamental research.68 75 However, despite extensive research, the formation of platinum oxide and its impact on the electrochemical properties are still under debate. Activation and deactivation of the oxygen exchange reaction (O2 h 2O2 + 4e ) at the three-phase boundary (tpb) electrolyte/electrode/gas phase have been attributed to the decomposition and formation of platinum oxide;72,76 78 however, a detailed mechanistic understanding is missing due to the very limited experimental possibilities to investigate the tpb (in situ), especially under relevant operating conditions of typical application devices (i.e., at high/ambient oxygen partial pressure and elevated temperature). A promising way to get a more detailed view of the role of the tpb is first to understand the simpler two-phase boundary Pt/YSZ under electrochemical operation and then in a second step focus on the tpb by taking also the gas phase into consideration. Of course, the tpb is also of general relevance for gas diffusion electrodes.79 81 Although the Pt oxide formation and decomposition at the Pt/YSZ interface have been assumed to occur in several publications72,82 87 (detailed overviews of literature can be found in refs 71 and 72), no comprehensive study utilizing a chemically and structurally well-defined model system was carried out. Such model-type studies enabled fundamental insights into liquid state Received: October 7, 2011 Revised: December 2, 2011 Published: December 14, 2011 1912

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Figure 1. The three model systems compared in this study and the driving forces leading to an oxidation of platinum: (a) a Pt(111) surface exposed to oxygen, (b) the interface Pt(111)/liquid electrolyte, and (c) the interface Pt(111)/YSZ(111).

electrochemistry and surface science and can also improve our current understanding of the Pt/YSZ solid state interface. There are two reasons for this gap: (i) the preparation of single crystalline model electrodes on YSZ was experimentally more challenging88 compared to the application of commercially available platinum single crystals (surface science) or to produce a single crystalline Pt surface via flame annealing (liquid state electrochemistry). (ii) Only a very limited number of characterization techniques exist providing information from a solid/solid interface, while the gas/solid or liquid/solid interface is much easier accessible. In addition, the technique needs to offer high sensitivity due to the very small amount of Pt oxide formed (typically in the (sub)monolayer range); the sensitivity of many techniques is either not sufficient (e.g., X-ray diffraction (XRD)) or in situ investigations are challenging, if possible at all (e.g., transmission electron microscopy (TEM)). Electrochemical investigation techniques, such as cyclic voltammetry (CV), offer the extremely high sensitivity necessary to monitor (sub)monolayer(s) of product formation and decomposition at any electrode/electrolyte interface. Note that the amount of oxide and the binding strength of oxygen to Pt is determined typically from oxygen evolution (desorption from Pt surface or oxide decomposition) as monitored during temperature-programmed desorption (TPD) in surface science or CV in electrochemistry. Another advantage of electrochemical methods is that the oxygen activity is easy to control and can be set to extremely high or low values according to the Nernst equation; the only limits in the case of the interface Pt/YSZ are morphological changes during anodic polarization89,90 and the reduction of the YSZ at strong cathodic potentials.91 Due to the very small oxidation rate of platinum, a high oxygen activity is essential to reduce the time to form at least a small amount of oxide within a reasonable time. In addition, by using potential scanning methods, kinetic data of Pt oxide formation and decomposition can be obtained. The intrinsic drawback of a nondirect evidence for platinum oxide formation by any electrochemical investigation is overcome in this study by excluding other possibilities of oxygen storage at the interface, on the one hand, and by reproducing the oxidation behavior known from liquid state electrochemistry as well as comparing data to results from the field of surface science, on the other hand. Furthermore, CVs of liquid state electrochemistry are compared to those of the interface Pt/YSZ, the shape of CVs of Pt/YSZ are compared to TPD spectra, and the well-known rate of oxygen accumulation at the interface

Pt/aqueous electrolyte (linearity between the amount of oxygen and log(time))21,25,30 32,38 is reproduced for the interface Pt/ YSZ for the first time. For being able to compare data of the interface Pt/YSZ to published data of model-type studies in surface science and liquid state electrochemistry, the samples need to fulfill the following requirements: (i) the microscopic structure must be comparably simple, at best be represented only by one crystallographic orientation, and (ii) all processes not taking place at the interface Pt/YSZ (e.g., at the tpb or diffusion of oxygen through the Pt film) must be suppressed. These two requirements are fulfilled by macroscopically large, semicoherent single crystalline platinum films in (111) orientation on YSZ(111) single crystals88,92 with a minimum tpb length, as used in this study; the diffusivity of oxygen in Pt films with a low grain boundary density is known to be very small,93 and the gas tightness of these film has been demonstrated by morphological changes upon anodic polarization.89,90 Figure 1 depicts schematics of the three model systems including Pt{111} surface planes in contact with different reactive phases: (a) a gas phase or vacuum, (b) an aqueous liquid electrolyte, and (c) the solid electrolyte YSZ(111). In surface science studies (Figure 1a) the Pt is oxidized upon heating under (near) ambient conditions9,10,16 or by exposing the platinum surface to highly reactive oxygen (from O3 or by decomposition of NO2)11,12,14,15,17,18 to overcome the kinetic hindrance of oxide formation. The driving force for Pt oxide formation in liquid state electrochemistry (Figure 1b) is achieved by anodic polarization.10,21 37,39 43 At the interface Pt/YSZ (Figure 1c, which shows a simplified two-dimensional representation of the model-type interface investigated in this study) oxidation of Pt can take place without an electrochemical driving force at elevated temperature under ambient oxygen pressure, if the temperature lies within the thermodynamic stability range of platinum oxide (maximum ∼910 K)9,16,53—but the oxidation rate is extremely slow, and therefore usually anodic potentials are applied to accelerate the oxide formation.72,82 87 A remarkable difference between the solid/solid interface Pt/YSZ and the two other model systems is created by the different mechanical boundary conditions. The platinum surfaces in contact to vacuum, gas phase, or a liquid electrolyte can change morphologically—it is well-known that a roughening takes places at the initial stage of oxide formation.17,29,34,42,55,56 Comparable processes are not possible at the interface Pt/YSZ without causing mechanical strain, formation of microscopic hollows, cracks, blistering, or even delamination of the thin and 1913

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Figure 2. Schematic of the sample geometry and experimental setup.

deformable Pt film.89,90 Of course, pronounced morphological changes impact the electrochemical properties89 and are therefore minimized in this investigation. The differences in the Pt oxidation mechanism implied by the different properties of the three interfaces will be considered in the discussion. This work aims for a better fundamental understanding of the Pt oxidation at the Pt(111)/YSZ(111) model interface. By comparing and discussing data on Pt oxidation of the three different Pt(111) model systems (Figure 1), significant similarities are found which, on the one hand, demonstrate the oxidation of platinum at the interface Pt(111)/YSZ(111) unequivocally and, on the other hand, allow general mechanistic statements on the oxidation process.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Single crystalline Pt(111) films were grown on one-sided polished YSZ(111) single crystals (9.5 mol % Y2O3; Crystec, Germany) using pulsed laser deposition (PLD), similar as reported previously (Tsubstrate ∼ 650 K; platinated alumina mask to obtain a circular Pt film of about 16 mm2).70 72,88 90 Subsequently, the samples were annealed for 12 h at 1023 K, and the counter electrode (Ag paste sintered at 773 K for 0.5 h) was applied opposite to the Pt working electrode (Figure 2). As all Pt films deposited on YSZ(111) by PLD so far grew in (111) orientation and only their crystallite size, defect concentration, and surface morphology may vary,88,94 only scanning electron microscopy (SEM) images of the surfaces before (Figure 3a) and after annealing (Figure 3b) were taken. 2.2. Electrochemical Measurements. A two-point measurement was used for the electrochemical characterization (Figure 2). No reference electrode was needed due to the very high polarization resistances of the dense working electrode film for potentiostatic polarizations; also, the highest capacitive currents occurring during the CV measurements were sufficiently low to result in a low IR drop. All electrochemical measurements were carried out using a Zahner potentiostat (IM6, Germany). For determination of the polarization resistance electrochemical impedance spectroscopy (EIS) was used (ac amplitude: 10 mV). The EIS data were fitted with ZSimpWin (EChem Software) using the equivalent circuit depicted in Figure 3c. All cyclic voltammograms (CVs; scanning speed ν = 2 mV/s) shown in sections 3.2 and 3.3 represent the steady state. The oxidation rates (section 3.4) have been obtained as follows: (i) potentiostatic polarization (E) for a defined time t, followed by (ii) a negative potential scan with ν = 10 mV/s from the polarization potential E to E = 1.0 V. CView2 (Scribner Associates) was utilized to determine the cathodic peak areas of CVs and negative potential scans; a straight line was drawn for baseline correction. Based on this peak area, the corresponding amount of charge or oxygen was calculated. For the conversion in monolayers (ML), the platinum atom density of a {111} plane (1.50 1015 cm 2 calculated for a lattice constant of 3.92 Å) was used.

Figure 3. Sample characterization: SEM images of the Pt film grown by PLD before (a) and after (b) the annealing treatment (12 h, 1023 K) and (c) Nyquist plot of an EIS measurement of the sample at 673 K in air.

A temperature of 673 K (controlled with a thermocouple) was selected for the electrochemical measurements for several reasons: (i) the temperature is within the stability range of Pt oxide under ambient oxygen pressure,9,16,53,84 (ii) YSZ shows sufficiently high oxygen conductivity, (iii) morphological changes caused by temperature-induced dewetting can be excluded, (iv) impurity segregation (cation diffusion) is extremely slow during the electrochemical studies,95 97 and (v) this temperature is of interest for μ-SOFCs.63 67

3. RESULTS AND DISCUSSION 3.1. Sample Characterization. The PLD process results in a covering film of nanocrystallites (Figure 3a), which develops a smooth surface during annealing (Figure 3b). One of the few crystallized droplets at the top edge of this SEM image is utilized to ensure a correct focusing. The polarization resistances (RW) of the platinum film electrodes, which are inversely proportional to the oxygen exchange rate between the gas phase and the electrolyte via the working electrode, were estimated by EIS. An exemplary Nyquist plot, the used equivalent circuit, and the fitting data can be found in Figure 3c. As expected, the polarization resistance (RW = 83 MΩ) is high due to the very limited tpb length (mainly given by the macroscopic edge of the Pt film). Note that the exact value of RW might differ slightly from the determined value due to an extremely low time constant caused by the high resistance so that only the beginning of the semicircle was obtained. However, independent of the exact RW value, one must conclude that there is only a very low oxygen exchange rate. This means that processes taking place at the tpb are negligible for the following CV measurements, the Pt film electrodes are impermeable for oxygen, and in the CV measurements only interface processes are observed, and thus the samples fulfill the requirements necessary for a direct comparison with model systems in liquid state electrochemistry and surface science. 1914



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Figure 4. Comparison of typical CVs of the interfaces of (a) Pt/acidic aqueous electrolyte and (b) Pt(111)/YSZ(111); the latter was obtained at 673 K and a scan rate of 2 mV/s. Depending on the anodic reversal potential, different cathodic peak areas were found.

3.2. CV Comparison between Pt(111)/Aqueous Electrolyte and Pt(111)/YSZ(111): Similar Features Suggesting Similar Processes. First, we like to refer to an earlier comparison

of Pt electrodes in liquid and solid state electrochemistry.85 In the present study, we realized significant improvements of the sample toward a well-defined model system offering a much better comparability to electrodes in liquid electrolytes, which becomes apparent e.g. in the more similar cathodic peak shape and also allowed the reproduction of the above-mentioned logarithmic time law, which could not be observed in ref 85. Typical CVs of Pt electrodes in acidic (sulfuric) aqueous electrolytes (for appearance and attribution of peaks we like to refer to refs 10, 23, 24, 28 33, 37, 39, 43, and 98 101) and of Pt(111) on YSZ(111) show analogous and different features, as indicated in Figure 4. The potential scales are referred to the reversible hydrogen electrode (RHE) in the aqueous case and to the redox potential of the O2(Pt, air)/O2 (YSZ) electrode. Note that in liquid state electrochemistry often polycrystalline platinum has been used; however, the shapes of CVs of Pt(111) electrodes are very similar as polycrystalline Pt electrodes typically show a preferred (111) orientation.31 The characteristic feature in CV in aqueous solution at lowest potential (Figure 4a, marked in red) is the underpotential deposition region of hydrogen (UPD). Of course, this feature cannot be seen at the interface Pt/YSZ due to the absence of hydrogen. Then we see in both systems a potential range without peaks (marked in orange) where only capacitive currents occur. Looking at more positive potentials, again both systems show peaks resulting from Pt oxide formation and reduction in the anodic and cathodic sweep, respectively. In aqueous electrochemistry, it was shown by different studies based on mass determination that Pt is directly oxidized to PtOx without a transition state of Pt(OH)x.33,37,43 Regarding the anodic double peak (Figure 4a), there is some disagreement in the literature. Although in studies using single crystalline Pt(111) electrodes the double peak was obtained,29,31 this feature was later attributed to the onset of Pt oxide formation at different potentials for different orientated facets of polycrystalline electrodes.37,102 At this point it should be noted that especially for the peaks representing UPD of hydrogen a strong influence of surface disorder on peak shapes in CVs was observed,98 101 which might also occur in the oxygen redox processes. The strong increase of the anodic current due to oxygen evolution (marked in blue) is missing in the system Pt/YSZ. Studying fully covering and

impermeable Pt film electrodes, gaseous oxygen can only be formed in the case of bubble formation underneath the film89,90 and can only be released through cracked bubbles. To avoid irreversible changes of the electrode system, we did not expand the scanned potential range to see this process. As reviewed previously,71 depending on the experimental conditions no, one or two anodic peaks were found for Pt/YSZ. In liquid state electrochemistry the assignment of the cathodic peak is well proven spectroscopically to result from Pt oxide decomposition (the experimental methods were mentioned in the Introduction). The analogous interpretation of these peaks in the system Pt/YSZ was justified by comparison to aqueous electrochemistry82,84 87 or by referring to thermodynamic arguments.83,84 To further strengthen this peak assignment to Pt oxide reduction unequivocally, we discuss and exclude other potential processes in the following, which in principle might cause cathodic peaks: First of all, an influence of the YSZ electrolyte can be excluded. Reduction of Zr4+ is possible but requires a potential below 1.5 V in air. Second, a maximum in the differential capacity of the electrochemical double layer103 105 could lead to a pair of peaks in CV. At 673 K this effect should be very feeble, and in addition it should occur above the cathodic peak potential.103,105 Third, in principle it is possible that the oxidation and reduction of impurities at the interface lead to peaks.71 Especially silicon was often found as main impurity in YSZ and at the Pt/YSZ interface.71,106 109 Despite our previous hypothesis,71 we currently believe that silicon as well as other impurities typically present at the Pt/YSZ interface71,107 are not responsible for this cathodic peak of Pt film electrodes. Due to a high affinity to oxygen, stronger cathodic potentials would be necessary for the reduction of these impurity oxides, as e.g. shown for iron.110,111 Additionally, in analytical transition electron microscopy studies of samples similar to those used in this study no impurity phase was found,92 and also the 0.25 ML of SiO detected by X-PEEM at the interface after polarization71 is also not sufficient to explain larger peaks. Finally, as already mentioned, all processes taking place at the tpb can be excluded due to the chosen sample geometry, resulting in the high polarization resistance and because of a previously reported experiment varying the tpb length of similar Pt film samples.71 3.3. Comparing the Shape of TPD Spectra to CV Measurements: Assignment of Cathodic Features in CV to Oxygen Desorption and Pt Oxide Decomposition. CV and temperature-programmed desorption (TPD) show clear analogies 1915


The Journal of Physical Chemistry C allowing a comparison of results of both techniques. In a first step, the Pt surface or interface is exposed to oxygen (the oxygen loading in the CV can be influenced by the anodic reversal potential or an anodic polarization pretreatment). In the second step, one experimental variable is changed (linearly) with time, i.e., the potential in CV and the temperature in TPD, leading to oxygen evolution. Thus, the amount of oxygen and its (relative) bonding strength to Pt can be monitored. First, we will present CV data and then discuss these results with respect to available TPD data. Figure 5 shows magnifications of the cathodic peaks of the CV set already partly presented in Figure 4b. For small amounts of oxygen (up to ∼0.2 ML) a broad and undefined peak is found, which shifts to less negative potentials with increasing peak areas. A second peak appears clearly above ∼0.2 ML (Figure 5a). The second peak becomes larger with increasing anodic vertex potential and shifts slightly to more negative potentials; no further peaks appear (Figure 5b). The peak shift between the smallest and largest peak of 16 mV is about twice the expected IR drop (ΔI = 19 μA cm 2; resistance of electrolyte and counter electrode ∼480 Ω cm2; IR = 9.1 mV). The observations in TPD spectra are very similar: a broad peak appears which shifts to lower temperature with increasing oxygen coverage for low amounts of oxygen (up to 0.25 ML), and a second sharp peak occurs with increasing coverage, shifting to higher temperature for high amounts of oxygen (>∼1.2 ML).12,15 In a transition region from low to high oxygen coverages more or less pronounced additional peaks have been observed in TPD spectra,11,12,15,17 whereas no analogous features are found in CVs of the of the present epitaxial Pt/YSZ interface. Thus, a closer look at the two Pt(111) systems is necessary to understand this difference. By combining TPD spectra, LEED, STM, and DFT modeling, a detailed insight into the initial oxidation of a Pt(111) surface loaded with oxygen has been obtained.17,55 Up to 0.25 ML, oxygen adsorbs on Pt forming a (2 2) structure on the surface. Within the range from 0 to 0.25 ML, a higher amount of oxygen results in increasing repulsion and therefore a lower desorption temperature (of the broad peak in TPD). Additional oxygen occupies the energetically higher (2 1) state up to 0.5 ML and causes a new and sharper peak at lower desorption temperature. Increasing the oxygen coverage further leads to a buckling of the substrate structure until a (structural) precursor of platinum oxide is formed at 0.75 ML (at this stage the Pt is already highly coordinated by oxygen). The buckling of the surface results from lifting of Pt atoms by oxygen and causes a decrease of the strong repulsive forces between the oxygen atoms at high coverage. Once the oxygen coverage reaches about 1 ML, a structure very similar to α-PtO2 was found. α-PtO2 is the stable Pt oxide on a Pt(111) surface predicted by DFT calculations53 and has experimentally been detected.16 In an experiment with atomic oxygen,15 oxidized platinum was identified by XPS for coverages higher than 1.1 ML. Transferring this detailed knowledge from the field of surface science to the interpretation of CV peaks of our Pt(111)/ YSZ(111) interfaces, we attribute the first broad peak to an adsorption-like state of oxygen, representing Pt undercoordinated by oxygen. Most likely, the Pt structure at the interface is not yet changed (significantly). As there is no transition state visible in the CVs, we attribute the second peak directly to a platinum oxide(-like) structure. Two obvious differences between the Pt(111)/O2 surface and the Pt(111)/YSZ(111)

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Figure 5. Magnifications of the CVs depicted in Figure 4b. (a) Two different peaks (marked with dotted lines) can be distinguished clearly at low anodic vertex potentials. (b) Growth of the second peak with increasing anodic vertex potential (no additional feature appears). The different colors correspond to different anodic vertex potential as detailed in Figure 4b.

interface help to explain the direct transition from adsorbed oxygen to Pt oxide at the solid/solid interface. On the one hand, YSZ is oxygen terminated—although this oxygen “belongs” to the YSZ92,112 (there is no charge transfer from the platinum to this oxygen), there are about 0.55 ML oxygen permanently at the interface which might promote the formation of oxygenrich platinum structures. On the other hand, because of the mechanically rigid interface (Pt/ionic crystal), a buckling of platinum at the interface should lead to strong elastic strain in the interface structure and is therefore energetically not favored. Instead of a buckled structure, the formation of “subsurface” oxygen species, i.e., the incorporation of oxygen into the platinum lattice (compare to refs 42, 47, 48, 50, and 55), is expected to be preferred. Therefore, the Pt would be more oxygen coordinated and thus the structure be more oxide-like in an earlier stage. 3.4. Oxidation Rate and Time Law: A Comparison between Solid and Liquid State Electrochemistry. In liquid state electrochemistry there is a well-known and controversially discussed time law observed for the oxygen accumulation at the Pt electrode: in the potential range of oxide formation, a linearity between the amount of oxygen and the logarithm of the 1916


The Journal of Physical Chemistry C polarization time was observed in several studies.21,25,30 32,38 In the present study this time law is reproduced for the interface Pt/YSZ (Figure 6) for a wide range of times and for different potentials, adding further strong evidence to the formation of Pt oxide. In Figure 6a, a typical set of negative potential scans after polarization with E = 100 mV for different times (tpolarization) is shown. As depicted in Figure 6b, a linearity between log(tpolarization) and the area-normalized charge (Q/A) occurs for the entire set of data. Only for polarization with E = 0 mV the time law appears not to follow the log(t) dependence. Ignoring the data point at the longest polarization time, a better linear fit was possible by plotting the charge versus the square root of time. However, including this last data point, no satisfying linear fit to the square root of time could be obtained. This might be explained by small inaccuracies in the baseline correction71 used for the peak area estimation of the small peaks observed in the 0 mV series of the measurement and should not be stressed too much. A comparison of the rates of oxygen accumulation at the interface Pt/YSZ to similar data of Pt/aqueous electrolytes31 shows that a similar increase of the rate needs a lower increase of the potential in the system Pt/YSZ. More precisely, the rates observed in31 the potential range of 1 V e E e 1.8 V are in the same order of magnitude to those of 0.05 V e E e 0.2 V for the system Pt/YSZ. We explain this difference as follows: the activation energy for the kinetically hindered process of oxygen accumulation at Pt is reached easier at the higher testing temperature in the solid state electrochemical experiment compared to the liquid state system. The strong kinetic hindrance of oxide formation is also demonstrated by comparing the oxidation and reduction behavior of Pt: the formation of very few oxide layers requires a significant electrochemical driving force and several hours of time, while the reduction of the entire amount of oxide is completed within a few seconds during the negative potential scan. Explaining the logarithmic time law (Figure 6b) on the basis of a microscopic mechanism is challenging as during the transition from interfacial oxygen adsorption to oxide formation no change in the accumulation rate of oxygen is observed. However, a number of different microscopic mechanisms may lead to the same macroscopically observed behavior. In liquid state electrochemistry, different models have been suggested to explain the logarithmic time law. Most often a place exchange mechanism is assumed.22,23,26,30,34,36 38,44 Oxygen atoms on the Pt surface change their places with Pt atoms of the electrode in the rate-determining step, resulting in a roughening of the electrode surface. In some studies the place exchange was assumed to be field driven (high field model);22,26,30 a linear decrease of the field strength with increasing oxide thickness results directly in the logarithmic time law. The main problem of the place exchange model is that a structural change of the platinum does not take place until a certain potential is reached or a certain charge was transferred.34,37,42,44 Surface science studies support the assumption that a change of the Pt structure occurs only if the oxygen coverage exceeds certain values.12,14,15,17,55 Another model discussed is the nucleation and growth mechanism,25,27 in which the nucleation rate and/or the nucleation growth of the oxide is rate-determining. As reported in ref 27, this model cannot be applied to self-inhibiting systems which is the case for Pt. The point defect model39,113 assumes a defective oxide layer between metal and electrolyte; defects are generated

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Figure 6. (a) Typical set of negative potential scans after anodic polarization with E = 100 mV in air for different times (T = 673 K, ν = 10 mV/s). (b) Overview of charges vs time for different polarization potentials; the depicted lines are linear fits of the measured values.

and annihilated at the interfaces. This model was developed to understand passive states of metals114 and holds only for an already existing oxide layer. Correspondingly, this model was utilized for studying an already existing oxide layer in steady state (including a simultaneous formation and dissolution of the oxide layer at the Pt/oxide and oxide/electrolyte interface, respectively).39 To find a suitable explanation for the observed time law, we refer to a statement in ref 30: in general, any activation-limited reaction in which the activation energy increases linearly with time (= linearly with ongoing reaction) results in a logarithmic time law. Thus, a property of the sample must be found, which changes linear with ongoing reaction and must hold for the very beginning of oxygen accumulation at the electrode, when nothing happens but a transition of an oxygen ion from the electrolyte to the electrode’s surface, including a charge transfer. Looking for such a behavior, the electronic structure of the Pt(111) surface may play a role: the work function of Pt increases linearly for an oxygen coverage from 0 to 0.75 ML,11 suggesting that a barrier for the charge transfer is built up. Another hint for this direction of interpretation is, when oxidized, platinum gets semiconducting or insulating,51 also showing the strong change in the electronic structure and the buildup of a high barrier for the charge transfer. Accordingly, the charge transfer through very thin passivating platinum oxide layers was described in terms of the tunnel effect.113,115 117 1917



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mechanism occurring at both interfaces. We conclude that the electronic structure of Pt (in form of the building up of a barrier for the charge transfer; see Figure 7) has a pronounced impact on the time law. Detailed insights into the processes taking place at the interface Pt/YSZ will also contribute to a better understanding of the oxygen exchange at the tpb. The formation of a passive layer of Pt oxide under the Pt electrode and extending to the tpb should strongly decrease the electrode performance. A promising approach for further research might be to destabilize the Pt oxide,118 for example, by doping or alloying strategies to prevent an electrode passivation/degradation during operation.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses †

Figure 7. At the solid/solid interface Pt/YSZ adsorbed oxygen (Oad, light blue) and Pt oxide (Oox, darker blue) form at positive potential governed by a logarithmic time law, while during applying negative potential a fast reduction takes place. The inset shows a negative sweep in cyclic voltammetry (CV) highlighting the two oxygen species.

At this point we can only offer the hypothesis discussed above to explain the observed time law, but we also like to note that the range of changing sample properties at the very beginning of oxygen accumulation is small. Further work—involving more advanced in situ techniques—is required to get a more detailed mechanistic view on the ongoing processes and changes of the sample properties during building up the passive layer. Apart from the challenging interpretation of the time law in both systems, the very comparable situation in liquid and on solid state electrolytes is remarkable from the fundamental point of view—despite the large differences in temperature, electrolyte structure, and the interface structure electrode/electrolyte. Obviously comparable processes determine the properties of both systems to a large extent.

4. CONCLUSIONS A comprehensive overview of Pt oxidation at three different interfaces (Pt/solid, Pt/liquid, and Pt/gas phase) is presented by comparing model-type studies from liquid state electrochemistry and surface science to new data of the solid state model electrode Pt(111)/YSZ(111). The detailed experimental investigation of the model system Pt/YSZ revealed a good agreement with results of the other two model systems. The systems Pt/acidic aqueous electrolyte and Pt/YSZ show similar CV features. The higher complexity of the liquid state CVs is attributed to the larger number of reactions, while the cathodic peak of oxide reduction is similar, suggesting that similar processes take place and pointing toward Pt oxide decomposition. By comparing CV data (Pt/YSZ) to TPD spectra (Pt/gas phase), we propose that two different oxygen species are formed at the Pt/YSZ interface upon polarization (Figure 7): (i) oxygen in an adsorbed state at low coverages and (ii) oxygen in platinum oxide (or at least in an oxide-like precursor structure) at high coverages. The well-known oxidation rate of Pt in aqueous electrolytes (linearity between the amount of PtOx and log(tpolarization)) was reproduced, suggesting a comparable or even the same

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’ ACKNOWLEDGMENT We thank the DFG for financial support (LU 1480/1-1). ’ ABBREVIATIONS XPS, X-ray photoelectron spectroscopy; LEED, low-energy electron diffraction; EELS, electron energy lost spectroscopy; AES, Auger electron spectroscopy; TPD, temperature-programmed desorption; UPS, ultraviolet photoelectron spectroscopy; CAICISS, coaxial impact collision ion scattering spectroscopy; XRD, X-ray diffraction; STM, scanning tunneling microscopy; EQCN, electrochemical quartz crystal nanobalance; XRR, X-ray reflectivity; DFT, density functional theory; X-PEEM, X-ray photoelectron emission microscopy. ’ REFERENCES (1) Hendriksen, B. L. M.; Frenken, J. W. M. Phys. Rev. Lett. 2002, 89, 046101. (2) Carlsson, P.; Osterlund, L.; Thormahlen, P.; Palmqvist, A.; Fridell, E.; Jansson, J.; Skoglundh, M. J. Catal. 2004, 226, 422–434. (3) Wang, J. G.; Li, W. X.; Borg, M.; Gustafson, J.; Mikkelsen, A.; Pedersen, T. M.; Lundgren, E.; Weissenrieder, J.; Klikovits, J.; Schmid, M.; Hammer, B.; Andersen, J. N. Phys. Rev. Lett. 2005, 95, 256102. (4) Hendriksen, B. L. M.; Bobaru, S. C.; Frenken, J. W. M. Top. Catal. 2005, 36, 43–54. (5) Ackermann, M. D.; Pedersen, T. M.; Hendriksen, B. L. M.; Robach, O.; Bobaru, S. C.; Popa, I.; Quiros, C.; Kim, H.; Hammer, B.; Ferrer, S.; Frenken, J. W. M. Phys. Rev. Lett. 2005, 95, 255505. (6) Chen, M.; Cai, Y.; Yan, Z.; Gath, K.; Axnanda, S.; Goodman, D. Surf. Sci. 2007, 601, 5326–5331. (7) G€unther, S.; Scheibe, A.; Bluhm, H.; Haevecker, M.; Kleimenov, E.; Knop-Gericke, A.; Schl€ogl, R.; Imbihl, R. J. Phys. Chem. C 2008, 112 15382–15393. (8) Gao, F.; Wang, Y.; Cai, Y.; Goodman, D. W. J. Phys. Chem. C 2009, 113, 174–181. (9) Berry, R. J. Surf. Sci. 1978, 76, 415–442. (10) Peuckert, M.; Bonzel, H. P. Surf. Sci. 1984, 145, 239–259. (11) Parker, D. H.; Bartram, M. E.; Koel, B. E. Surf. Sci. 1989, 217, 489–510. (12) Saliba, N.; Tsai, Y. L.; Panja, C.; Koel, B. E. Surf. Sci. 1999, 419, 79–88. 1918


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YSZ (111

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