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An Operando Near-Ambient Pressure XPS (NAP-XPS) Study of the Pt Electrochemical Oxidation in HO and HO/O Ambients 2

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Viktoriia A. Saveleva, Vasiliki Papaefthimiou, Maria K. Daletou, Won Hui Doh, Corinne Ulhaq-Bouillet, Morgane Diebold, Spyridon Zafeiratos, and Elena R. Savinova J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12410 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on March 22, 2016

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The Journal of Physical Chemistry

TITLE: An Operando Near-Ambient Pressure XPS (NAP-XPS) Study of the Pt Electrochemical Oxidation in H2O and H2O/O2 Ambients AUTHOR NAMES. Viktoriia A. Saveleva†, Vasiliki Papaefthimiou†, Maria K. Daletou, ‡, Won H. Doh†%, Corinne Ulhaq-Bouillet§, Morgane Diebold †¬, Spyridon Zafeiratos†, Elena R. Savinova†* AUTHOR ADDRESS. †

Institut de Chimie et Procédés pour l'Energie, l'Environnement et la Santé, UMR 7515 du CNRS-UdS 25 Rue Becquerel, 67087 Strasbourg, France.

‡

Foundation of Research and Technology Hellas, Institute of Chemical Engineering Sciences, FORTH/ICE-HT, Stadiou Str., Platani Rion, Patras 26504, Greece §

Institut de Physique et Chimie des Matériaux de Strasbourg, 23 rue du Loess, BP 43, 67037, Strasbourg, France

ABSTRACT: Oxides on the surface of Pt electrodes are largely responsible for the loss of their electrocatalytic activity in the oxygen reduction and oxygen evolution reactions. In this work we apply near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) to study in operando the electrooxidation of a nanoparticulated Pt electrode integrated in a membrane-electrode assembly of a high temperature proton exchange membrane under water and water/oxygen ambient. Three types of surface oxides/hydroxides gradually develop on the Pt surface depending on the applied potential at +0.9, +2.5, and +3.7 eV relative to the 4f peak of metal Pt and were attributed to the formation of adsorbed O/OH, PtO and PtO2, respectively. Presence of O2 in the gas phase results in the increase of the extent of surface oxidation, and in the growth of the contribution of the PtO2 oxide. Depth profiling studies, in conjunction with quantitative simulations, allowed us to propose a tentative mechanism of the Pt oxidation at high anodic polarization, consisting of adsorption of O/OH followed by nucleation of PtO/PtO2 oxides and their subsequent threedimensional growth.

trooxidation of Pt has been subject of numerous studies by electrochemical methods 10–13, X-ray Photoelectron spectroscopy (XPS) 10,14–18, scanning tunneling microscopy (STM)18–20, surface-enhanced Raman spectroscopy (SERS)21–23, X-ray absorption spectroscopy (XAS)24–27, Auger electron spectroscopy (AES)28, ellipsometry7,29, infrared reflection-absorption spectroscopy (IRAS)30 and density functional theory (DFT) calculations 31–35. Gas-phase oxidation of Pt has also been widely described in the literature21,36. Already the early studies recognized the multistep character of Pt electrooxidation and its irreversibility, which lead Conway to the conjecture of the so-called «place exchange» mechanism11, whereby OH species formed on the surface of a positively polarized electrode enter sub-surface via an «exchange» with the surface Pt atoms. In 1970es Winograd et al.37,38 applied XPS to Pt electrodes emersed from liquid electrolyte solutions, and examined them in an UHV chamber after their transfer through atmosphere. Analysis of the Pt4f spectra of the emersed electrode suggested formation of various surface species, including Pt(II) and Pt(IV) oxides and adsorbed oxygen. Sun et al.39 performed XPS measurements at two different takeoff angles and concluded on the formation of a

INTRODUCTION Pt is state-of-the-art material utilized (usually in the form of carbon-supported Pt nanoparticles) at the electrodes of protonexchange membrane fuel cells (PEM FCs), which currently represent the most efficient means of converting chemical energy of hydrogen into electricity. High temperature (HT) is a sub-category of PEM fuel cells which operate at temperatures above 100°C, having the advantage of a simplified water management, enhanced CO tolerance, and high efficiency of electricity and heat co-generation 1,2. The phosphoric aciddoped membrane 3 and Pt based catalysts 2 have so far been the most successful components for HT PEM FCs. When a PEM fuel cell operates close to the OCV (open circuit voltage), the surface of the Pt cathode is oxidized, the extent of oxidation further increasing upon start-up/shut-down cycles. It has been shown for many decades that the oxide films on metals strongly affect the rates of red-ox reactions at their surfaces 4–7. The surface oxidation of Pt results in a decrease of its catalytic activity in the oxygen reduction reaction. It has also been claimed to be a cause of the fuel cell OCV decrease due to establishment of mixed potential between the oxidation of Pt and the oxygen reduction reaction 8,9. Elec1

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adsorbate (Oads, OHads) coverage in the presence of O2 was detected in Ref. 40 with ex-situ XPS combined with an electrochemical cell. Attractivity of the XPS resides in its high surface sensitivity and its capability to distinguish surface constituents in different oxidation states. Development of the near ambient pressure XPS (NAP-XPS) in last decades made it possible to investigate surfaces under reactive environments44. Recently some of us demonstrated the capability of NAP-XPS to monitor in situ the electrode/electrolyte interface45,46. These studies were performed in a model membrane electrode assembly (MEA) of a HT PEM FC based on a phosphoric acid imbibed polymer membrane. The MEAs and the HT membrane appeared to be stable during the measurements. In this study using in-situ NAP-XPS we report on the in operando measurements of the Pt surface oxidation under water and water/oxygen ambient with the aim to better understand the mechanism of Pt electrooxidation at high anodic potentials, and unveil possible oxygen impact on the Pt electrooxidation. To investigate the electrode/electrolyte interfacial processes we compare XPS spectra of a nanostructured Pt electrode under different polarization and gas ambients. Application of NAP-XPS allows us to clearly distinguish formation of both Pt(II) and Pt(IV) oxides, as well as oxygen adsorbed on Pt, and reveal influence of O2 on the surface state of Pt particles. Furthermore, by tuning the photon energy of the X-rays, we analyze the depth distribution of different types of surface oxides, and conclude on the formation of a mixed PtO/PtO2 layer rather than a bi-layer structure.

bi-layer structure with an inner PtO and an outer PtO2 layer at potentials above 1.3 V vs. RHE. Unfortunately, the analysis of Pt4f spectra does not allow one to distinguish adsorbed O from OH. Hence, Wakisaka et al.10,40 used an electrochemical cell attached to the UHV chamber and analyzed XP O1s spectra of emersed electrodes. The authors were able to detect both Oads and OHads and conclude on the structure sensitivity of their adsorption on Pt single crystal planes. Formation of various oxygen-containing species was confirmed by vibrational spectroscopies21,22,30 and XAS24-27, both having the advantage of being applicable for in situ studies. XAS has been used by various research groups for studying electrooxidation of Pt in situ under potential control with either liquid24,25,27 or polymer electrolytes26. Both nearedge24,25 and extended fine structure26,27 of XAS were analyzed. Imai et al.27 in their very elegant work combined XAS with XRD to investigate the electrooxidation of ca. 2 nm carbon-supported Pt particles in real-time, and proposed the oxidation mechanism involving OH and O adsorbates followed by formation of β-PtO2-oxide. Redmond et al.26 also point to the PtO2 formation, which according to their combined EXAFS and kinetic simulation study, starts at particle edges as early as at 0.7 V vs. RHE. Yet, according to Merte et al.24 PtO2 formation occurs at higher potentials and is preceded by the PtO oxide. Weaver’s group 21,22 compared electrooxidation of Pt with its gas-phase oxidation by O2 using SERS. They noticed that while the same type of oxides are formed under the two different environments, thermal oxidation of Pt in dry O2 required temperature of at least 200°C. Meanwhile, in the presence of water Pt oxidation occurs even at ambient temperature. It was thus concluded that gas-phase oxidation of Pt is accelerated in the presence of water vapor and follows an electrochemical mechanism. Despite considerable progress in the understanding of the Pt electrode/electrolyte interface at positive polarization, the mechanism of the surface oxidation is still largely debated. Electrooxidation of Pt continues attracting attention of specialists in electrochemistry, spectroscopy and theory due to its paramount importance in electrocatalysis. One of the open questions is related to the possible influence of O2 on the surface state of Pt electrodes. Indeed, a fuel cell cathode is exposed to water and oxygen simultaneously. While understanding the surface state of Pt electrodes is critically important for the development of efficient fuel cell cathodes, it is still unclear how electrooxidation of Pt electrodes and their chemical oxidation by oxygen add up or compete for surface sites. Paik et al. and Xu et al. 41,42 reported on the significant dependence of the Pt oxidation on the oxygen exposure using electrochemical methods (cyclic voltammetry, coulometry). However, it was concluded that at high electrode potentials electrochemical surface oxides protect Pt surface from the O2 adsorption41. Liu et al.43 and Kongkanand and Ziegelbauer25 reported on the similar oxide coverage in the presence and in the absence of oxygen. Kongkanand and Ziegelbauer25 applied in situ X-ray absorption spectroscopy and concluded that while Quartz-Crystal Microbalance and coulometric measurements showed the same oxide coverage, XAS suggested an influence of O2 on the “place-exchange” between Pt and oxygen atoms earlier proposed by Conway11. An increase of the

EXPERIMENTAL METHODS Working and counter electrodes comprised a thin layer of Pt nanoparticles and a high loading carbon-supported Pt/C catalyst, correspondingly. For the preparation of the nanostructured working electrode (WE), unsupported Pt nanoparticles were prepared as follows. 0.37 mL of a 0.25M aqueous solution H2PtCl6 (Alfa Aesar, 99.95% metal basis, Pt 37.5% min.) were diluted in 16 mL of triply distilled water and placed in an ice bath. A solution of 0.1 M NaBH4 in 1 M NaOH was added dropwise until pH 12 was reached. The solution color turned gradually from light orange to black and after that the mixture was kept under stirring for another 2 h. The catalyst was then washed with triply distilled water until pH reached 7 and was dried under vacuum at 80oC. The polymer-electrolyte membrane based on aromatic polyethers bearing pyridine groups (Advent Technologies S.A.) was imbibed with phosphoric acid 85 wt% at 120oC to reach an acid doping level of 200 wt%. In order to form the thin Pt WE layer on the electrolyte membrane, a 6 x 6 mm2 piece of an acid doped membrane was placed on a vacuum table at 90°C, while a FEP film was placed above the membrane with a 5 x 5 mm2 window. The ink was prepared from the unsupported Pt electrocatalyst in water/isopropanol and was coated on the electrolyte membrane inside the FEP window. The Pt loading on the WE was estimated as ca. 0.08 mg cm-2. For the counter electrode, 30 wt% Pt/carbon black catalyst (Tanaka Kikinzoku International) was supported on a 5 x 5 mm2 carbon cloth based Gas Diffusion Layer (Pt loading ca 1 mg cm-2) and was placed on the back side of the membrane. The purposely higher loading on the CE side was intended to minimize its overpotential 2


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compared to that of the WE. Finally, the MEA was fabricated by hot pressing between Teflon sheets at 150°C for 15 min. An Au mesh was pressed on top of the WE to act as a current collector during the measurements. Pt nanoparticles were characterized by XRD (see Figure S1 in the ESI), which allowed us to estimate the average size of Pt crystallites as 3.5 – 4.0 nm. Pt nanoparticles scraped from the WE were characterized by conventional and high resolution transmission electron microscopy (HRTEM) using a LaB6- JEOL 2100 microscope operating at 200kV and with a point to point resolution of 0.21 nm. Typical TEM image in Figure 1 shows Pt nanoparticles of ca. 3 to 7 nm size, which are combined in larger aggregates. Agglomeration of unsupported Pt nanoparticles complicates determination of their average particle size from TEM images. The average particle size was estimated from the analysis of multiple TEM images as ca. 5 nm. Scanning electron microscopy (SEM) measurements (SEM image in Figure S2 of the ESI) were performed using Jeol JSM-6700F (Japan) electron microscope with the lattice resolution of 1 nm, at accelerating voltage of 10 kV. The study of the Pt electrode oxidation under different gas ambients was performed by means of the NAP-XPS measurements, at the ISISS beamline (BESSY II synchrotron at the Helmholtz Zentrum Berlin) in a setup previously described 44,47 .The samples were placed on a sample holder (see Figure S3 of the ESI) and were heated by an IR laser (cw, 808 nm). The temperature was maintained constant throughout the experiment at 150°C. The state of the MEA was monitored by measuring cyclic voltammograms (CVs) and the membrane resistance was controlled using high frequency impedance spectroscopy measurements throughout the experiment. A µAutoLab potentiostat from Metrohm was used for the electrochemical studies. The measurements were performed either under 0.1 mbar H2O or 0.11 mbar H2O/0.16 mbar O2. The gas composition was continuously monitored by online mass spectrometry (MS). In-situ NAP-XPS measurements were performed under constant voltage applied between the working and the counter electrodes (the current values were controlled by means of chronoamperometry). If not otherwise stated, Pt4f, Au4f, P2p, C1s and O1s spectra were recorded using selected excitation photon energies so as the photoelectrons were emitted at kinetic energy of ca. 750 eV. The depth profiling measurements were carried out for Pt4f spectra using photon energies in the range from 470 eV to 1350 eV. These values correspond to an approximate analysis depth (estimated as three times the inelastic mean free path) between 2.1 nm and 4.5 nm, respectively. To determine the surface atomic ratios, the spectral intensities were normalized by the energy dependent incident photon flux, which was measured using a gold foil with known quantum efficiency. The binding energy (BE) scale was calibrated with respect to the Fermi edge of the electron analyser. Stability of the MEA under the beam was confirmed in our previous publication 48. For the deconvolution of Pt4f peak the peak obtained at the OCV was used as a reference for metallic Pt while for the other Pt oxidation states literature data were used. In all cases the area ratio between the Pt4f7/2 and Pt4f5/2 components was constrained to the theoretical value of 4:3. Background subtraction was carried out using Shirley method. Curve fitting

was performed based on a mixed Gaussian/Lorentzian function modified using an exponential tale function. The simulation of Pt4f XPS spectra was done by SESSA software (Version 2.0) for the following morphologies: planar, islands, layered spheres. The kinetic energy of the photoelectrons was varied according to the experimental values (430, 640, 960 and 1350eV) with the mean free paths calculated by the software. The sample, analyzer, source orientation as well as the aperture parameters were adjusted to the ones of BESSY II. RESULTS Cyclic voltammetry The CV of the MEA obtained in the vacuum chamber in the presence of 0.1 mbar water and shown in Figure 2 exhibits typical features of a Pt electrode such as surface oxidation/reduction and underpotential deposition of hydrogen (HUPD), thus confirming the electrochemical activity of the MEA. In agreement with the previous studies 49–51 the HUPD region is attenuated due to the high temperature (150°C) conditions, while the shape of the CV in the potential interval of surface oxide formation-reduction is largely similar to the one observed at room temperature in aqueous electrolytes. Slight inclination of the CV may be attributed to the non-uniform pressure distribution in the MEA (for more details the reader is referred to the ESI). CV of Figure 2 was acquired in a two-electrode configuration without a reference electrode. To define the potential of the WE we also measured a CV in H2O/H2 = 1/1 mixture (Figure S4 of the ESI), which allowed us to estimate the OCV of Figure 2 as 0.93 V vs. the dynamic hydrogen electrode (DHE) (for further details the reader is referred to the ESI). CV in a water-oxygen atmosphere (0.11 mbar H2O + 0.16 mbar O2) is presented in the ESI (Figure S5). It is noteworthy that introducing O2 in the NAP-XPS chamber resulted in a marginal shift of the CV along the potential axis, thus justifying the choice of the OCV as a reference point for comparing the state of the Pt surface in H2O and in H2O/O2 ambients. It is worth mentioning that HT PEM FCs are usually operated without any gas humidification. Thus, despite the low (submbar) pressure ambient, measurements performed in this work may be considered relevant to HT PEM FCs, especially in what concerns their operation close to the OCV and under start-up/shut-down conditions. The relevance of the state of the Pt electrode in this work to the one achieved at the cathode of a HT PEM FC is further justified by the high degree of Pt utilization (see the ESI). NAP-XPS in water ambient We start with the analysis of the data obtained in 0.1 mbar H2O. The voltage bias between the WE and the CE was increased from 0 to 1.5 V (after the iR correction this corresponds to the interval from 0.93 to 1.83 V vs. the DHE). It should be noted that while under the PEM FC operation condition the cathode potential is inferior of 1 V, it may increase to much higher values during start-up/shut-down processes 52,53. In the survey spectrum of the sample (not shown) photoelectron peaks due to the elements originating from the polymer membrane (C), the phosphoric acid electrolyte (P, O) and the 3



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PtO, and PtO2 oxide, respectively. The BE for PtO and PtO2 oxides in this work are in good agreement with Refs 15,54 (see Table 1 and S1 of the ESI). Figure 4 shows the evolution of the Pt metal and oxide species obtained by Pt4f peak deconvolution as a function of the E-EOCV. One may see that Pt oxidation becomes noticeable ca. 0.3 V positive of the OCV, which roughly corresponds to the anodic current peak observed in the CV of Figure 2. The area of the Pt4f peaks corresponding to the metal component gradually decreases, while those corresponding to adsorbed O/OH and Pt oxides increase (Figure 4). As the voltage bias is increased, the component corresponding to adsorbed O/OH builds up first and then saturates at the highest applied voltage. It is interesting to note that at more positive potentials PtO and PtO2 oxides appear simultaneously rather than sequentially. At the highest applied voltage (0.9 V vs. the OCV) ca. 35 % of Pt on the electrode surface is oxidized (note that at the photon energy of 640 eV the analysis depth estimated as three times the mean-free path λ is ca. 2.5 nm). On the reverse scan the Pt oxide species are gradually reduced leading to the recovery of metallic Pt. The hysteresis on the reverse scan is in agreement with the shape of the CV and results from the slow kinetics of the Pt oxide reduction. NAP-XPS in water/oxygen ambient One of the main objectives of this study was to analyze the influence of oxygen on the oxidation of the Pt surface. Pt4f spectra at selected electrode potentials under H2O/O2 ambient are shown in Figures 3F, G, H, I and J, while the evolution of the Pt components as a function of E-EOCV under H2O and H2O/O2 atmospheres is compared in Figure 5. It appears that oxidation of the WE in the presence of oxygen is shifted to lower electrode potentials, and, the other conditions being equal, the amount of electrochemically oxidized Pt species is systematically enhanced as compared to the water ambient. The largest changes are observed for the PtO2 component (Figure 5D), with its lower onset potential and saturation plateau of ca. 7% in the presence of O2. This is a direct indication that Pt oxidation is enhanced in the presence of oxygen. We assume that a stronger (compared to previous publications25,43) effect of oxygen on the state of a nanostructured Pt electrode is due to the comparable H2O and O2 partial pressures and probably also to the higher temperature in this work. While previous publications were largely devoted to the Pt oxidation at room temperature in aqueous electrolytes11,13,29,39,55, this study is performed under the conditions relevant to the HT PEM FC operation. NAP-XPS: Depth profiling One of the major advantages of synchrotron-based XPS measurements is the opportunity to tune the photon energy of the X-ray source, changing the depth of the photoelectron emission. Non-destructive depth profiling studies were performed by using various photon energies under water and water-oxygen atmospheres at selected values of the voltage bias (Figure 6). It should be noted that spectra recorded at the beginning and at the end of depth profiling measurements were identical testifying for the stability of the electrode in the course of these measurements. The evolution of different Pt components with the analysis depth reveals an increase of the metallic component and a decrease of the O/OH component with the kinetic energy of photoelectrons either under H2O or

electrode (Pt) were observed. The beam-induced effects during long-lasting experiments were tested by sample shifting to a new analysis spot. These tests did not show any noticeable influence of the beam on the Pt electrode oxidation or reduction processes. No electrolyte dehydration was observed during the experiments as evidenced by the constant P/O atomic ratios (see Figure S6 of the ESI). One of the assets of NAP-XPS is that the BE shifts allow one to probe in situ the interfacial polarization45 exactly at the spot where XP spectra are collected. Indeed, Figure S7 shows a clear-cut correlation between the cell bias and the BE shifts of the C1s, O 1s and P 2p peaks, while the position of the main component of the Pt4f peak is voltage-independent since the WE is grounded. Considering this, in what follows we use the BE shifts of the C1s peak as an in situ measure of the WE polarization relative to the OCV (E-EOCV). Such a calibration allows us to overcome the uncertainty in defining the potential of the WE inherent of the two-electrode system utilized in this work. Indeed, calibrating the potential of the WE vs. the C1s peak shift allows one to correct for the polarization of the CE unavoidable in a two-electrode system, as well as for possible inhomogeneities in the potential distribution within the MEA. For a more detailed discussion of the influence of the electrode polarization on the BE shifts the reader is referred to Ref. 45. As discussed in our previous publications45,48, the O1s signal is dominated by oxygen originating from the phosphoric acid electrolyte, complicating its use for the determination of the oxidation state of the platinum electrode. On the other hand, Pt4f spectra shown in Figure 3 show first signs of the Pt oxidation at 0.3 V vs the OCV (corresponding to ca. 1.23 V vs the DHE). While the main Pt4f peak is independent of the applied bias, new components gradually develop at the high BE side of the peak with the increase of the applied voltage. Selfconsistent deconvolution of Pt4f spectra obtained at different voltages in the presence and in the absence of oxygen in the gas ambient requires at least five peak doublets. The major contribution is due to metallic Pt4f7/2 peak at 71.3 eV, while three additional high BE peak components gradually develop at +0.9, +2.5 and +3.7 eV relative of the main Pt4f7/2 peak. While the intensities of these high BE peaks increase with the applied voltage, their position on the BE scale is voltageindependent, suggesting that the corresponding Pt species are electronically coupled to the nanostructured Pt WE. One may notice an additional, low BE Pt4f doublet (gray line in Figure 3), which shifts systematically to lower BEs with the increase of the applied voltage bias, exhibiting the same behavior as the species present in the electrolyte. This allowed us to attribute the low BE Pt4f doublet to Pt particles “disconnected” from the mass of the Pt electrode46 (see Figure S2 of the Supporting Information). The assignment of the Pt4f components is complicated by the large scatter of the BE values reported in the literature for Pt oxides (see Table S1 of the ESI). This is in part due to different procedures used for preparing platinum oxides (gasphase or electrochemical oxidation), as well as to different structures of Pt (single crystals, polycrystalline Pt or Pt nanoparticles). Based on the BE shifts reported in the literature for electrochemically grown Pt oxides (Table 1) the three high BE Pt4f components at +0.9; +2.5 and +3.7 eV relative to the Pt4f7/2 peak of the Pt metal are attributed to adsorbed O/OH, 4


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The Journal of Physical Chemistry pared to PtO surface oxide on Pt(111). Rinaldo et al. 58, using kinetic modeling predicted a chemical transformation of PtOH and Pt-O into a higher Pt oxide during electrooxidation of Pt(111). Investigation of the gas-phase oxidation of Pt(110) by scanning tunneling microscopy, thermal programmed desorption59, AP-XPS together with DFT calculations60 revealed only the presence of PtO2 on the surface at different oxidation stages. Seriani et al.61 confirmed oxidation of Pt(111) to PtO2, and Pt(100) to PtO using first-principles atomistic thermodynamics calculations and molecular dynamics simulations. Their later DFT studies33 revealed size dependence of the thermal stability of Pt oxides formed on platinum nanoparticles. This observation was not, however, confirmed by the experimental data of Ref.62 for the NPs of different size, whereby the authors have shown higher thermal stability of PtO (up to 450K) in comparison to PtO2 (ca. 350K) electrochemically formed on the supported Pt NPs. Higher thermal stability of PtO compared to PtO2 is supported by Refs. 61,33,62–64. Furthermore, Arisetty et al. 65 have reported a stronger temperature dependence of the PtO formation kinetics compared to the PtO to PtO2 transformation. Simultaneous formation of PtO and PtO2 in this work either under H2O or H2O/O2 atmosphere can be tentatively explained by the (i) proximity of standard potentials of PtO and PtO2 formation; (ii) presence of different crystallographic planes on NP facets which favor formation of either PtO or PtO222, (iii) chemical transformation of PtO into PtO2 (Eq3), (iv) and higher thermal stability of PtO vs. PtO2. 2PtO2 = 2PtO + O2 (Eq3) Depth profiling data presented in Figure 6 suggest that the surface contribution of O/OH is photon energy-dependent at all applied potentials, suggesting that these species indeed correspond to the adsorbed O/OH, which are always located at the outer surface of Pt NPs. However, the slopes of the depth profiles for O/OH change with the applied potential. While at lower potentials (0.6 V in water atmosphere, Figure 5A) the contribution of the adsorbed species changes by a factor of ca. 5 when the photon energy is increased from 470 to 1350 eV, at higher potentials this factor decreases to ca. 2. On the contrary, the contributions of PtO and PtO2 are independent of the photon energy at lower applied potentials, but show similar to O/OH photon energy dependence at higher potentials. To better understand the experimental observations, we simulated XPS spectra using SESSA software66,67. Different models consisting of an adsorbed O/OH layer with a 0.3 nm thickness (approximate thickness of a single layer) and an oxide layer with varying thickness (from 0.3 up to 6 nm) over a metallic Pt substrate were used to simulate the real system behavior. Since comparison of the “planar” and “layered sphere” morphologies revealed similar depth dependence for Pt, adsorbed and PtO species (see Figure S9 in the ESI), in what follows we use “planar” sample morphology for simulations. Considering similar photon energy dependence of the PtO and PtO2, one may assume that the oxide layer consisted of a mixture of the two oxides, whose ratio evolved with the applied potential from PtO2:PtO of ca. 1:1 at low to ca. 2:1 at high applied potentials (Figure 6). For the sake of simplicity this mixed oxide layer was simulated considering only one type of Pt oxide (PtO) (Figure S10). The application of “planar” morphology for the oxide layers of thickness more than

H2O/O2 atmosphere. PtO/PtO2 oxides demonstrate a peculiar behavior: while at lower electrode potentials (0.5 / 0.6 V positive of the OCV) they are largely independent of the photon energy Figure 6A, C), at higher electrode potentials (0.8 / 0.9 V positive of the OCV) their depth evolution is similar to that observed for adsorbed O/OH (Figure 6B, D). The depth profiling curves for the intermediate potentials of 0.7 V (forward scan) and 0 V (backward scan) under H2O ambient are presented in the ESI. DISCUSSION Analysis of the Pt4f spectra allowed us to identify three types of surface oxides/hydroxides characterized by XP peaks located at (i) 0.9, (ii) 2.5 and (iii) 3.7 eV higher BE than the 4f peak of the Pt metal. Based on the literature data these peaks were attributed to (i) adsorbed O/OH, (ii) PtO and (iii) PtO2. Strong BE shifts (up to ca. 1 eV higher BE ) for metal atoms upon chemisorption of electronegative adsorbates are in agreement with the literature data56. Adsorption of O/OH followed by formation of Pt surface oxides conforms with the state-of-the-art understanding of the electrooxidation of Pt. 21,14 As mentioned above the domination of the O1s spectra by the phosphoric acid electrolyte made them inusable for distinguishing adsorbed O and OH. The reduction of the Pt oxide species when the potential is stepped back, follows the reverse trend with PtO and PtO2 reduction occurring before the PtO/OH desorption. It appears that formation of PtO and PtO2 is simultaneous rather than sequential both in water and in water/oxygen ambient. Simultaneous appearance of Pt(4+) and Pt(2+) oxides may be in part explained by the close values of their standard potentials (Eq1 and Eq2), even though one should not forget that the handbook values refer to bulk phases rather than to surface oxides. Pt + 2H2O = Pt(OH)2 +2H+ + 2e- E° =0.980 V vs. SHE (Eq1)57 Pt(OH)2 = PtO2 +2H+ + 2eE° =1.045 V vs. SHE (Eq2)57 Literature data related to electrooxidation of Pt vary depending on the experimental conditions and type of the Pt electrode. Kim et al.37 investigated electrooxidation of a Pt foil in 1 M HCIO4 using ex situ XPS and observed formation of chemisorbed O and PtO oxide at 0.7 and 1.2 V vs. SCE (Saturated Calomel Electrode), while PtO2 was detected at higher potential of 2.2 V vs. SCE. Sun et al.39 have also observed sequential formation of Pt(4+) and Pt(2+) oxides on a Pt foil emersed from a 0.5 M H2SO4 electrolyte using ex situ XPS. Meanwhile, Redmond et al. 26 applied X-ray absorption spectroscopy for elucidating the Pt oxide growth mechanism on a nanoparticulated Pt WE in a PEM fuel cell operating at 60°C, and concluded on the formation of PtO2 oxide close to the onset of the Pt surface oxidation at potentials as low as 0.7 V vs. RHE for the particle edges and 0.85 V for the particle facets. According to the experimental data as well as to theoretical calculations, the type of oxide formed on the surface of Pt depends on the surface crystallography and on the size of Pt NPs. DFT calculations performed by Jacob 32 for Pt electrooxidation suggest higher thermodynamic stability of PtO2 com5



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of Pt is in agreement with the earlier ex situ XPS studies of electrodes oxidized at room temperature in aqueous H2SO4 and HClO4 electrolytes37–39. Hence, even if the oxide formation may be accelerated at 150 compared to 25°C, and the onset of the oxide formation in the presence of phosphoric acid may be shifted towards positive potentials due to the phosphate adsorption, the essential mechanistic features of the surface oxidation are likely to be the same either under the HT or low temperature PEM FC conditions. Note however that the relative contribution of the PtO and PtO2 oxide may change with the temperature due to the higher thermal stability of PtO vs. PtO2. Presence of oxygen in the gas phase leads to a negative shift of the onset of the Pt surface oxidation. We confirm a higher adsorbed oxygen coverage reported by Wakisaka et al39. In addition, at potentials above those explored by Wakisaka et al, we observe higher coverage of Pt oxides, most remarkable for PtO2. The earlier appearance of the PtO2 oxide in the presence of oxygen may be attributed to the shift of the equilibrium of Eq3 in the presence of O2. The saturation plateau of the PtO2 component in water-oxygen atmosphere may be explained by the low thermal stability of this oxide at 423K 62 and hence establishing a steady-state conditions between the PtO2 formation and decomposition processes. The effect of oxygen on the state of the Pt surface is likely to be enhanced in the present work compared to previous publications because of the comparable pressures of water and oxygen. Meanwhile, when electrooxidation of Pt is studied in aqueous electrolytes, the concentration of water is exceeding that of oxygen by orders of magnitude. The conditions employed in this work may be considered more relevant to the fuel cell operation.

0.3 nm (ca. 1ML) (Figure S11) revealed its inability to simulate PtO depth-dependence behavior found in the experiments at high applied potentials. Along with this model the “island” morphology was tested in order to investigate the influence of the oxide coverage on the depth profiles (Figure S12). Due to the limitations of the software the “islands’ morphology was applied to the adsorbate and oxide layers separately. The independence of PtO and PtO2 % area from the photon energy observed at low applied potentials could be reproduced by considering two different morphologies: (i) a continuous oxide layer growing under the O/OH adsorbate (top panel of Figure S10), or (ii) oxide islands covering a small fraction of the Pt surface (top panel of Figure S13). The former model results in an unrealistically low contribution of metal Pt at the onset of the Pt oxidation, which is in disagreement with the experimental data (cf. Figure 6). Growth of the thickness of PtO under the “islands” of O/OH (Figure S12) also does not allow one to reproduce either the experimentally observed depth-dependence of PtO and PtO2 at high applied potentials, or further decrease of the contribution of metal Pt observed in the experiment (cf Figure 6). Thus, to account for the experimentally observed behavior we had to assume that at the onset of the electrode oxidation PtO and/or PtO2 nucleate covering a small fraction of the surface (cf. top panel in Figure S13 of the ESI). Further 2D growth (increase of the oxide coverage without increase in its thickness) did not reveal significant change in the depth dependence prompting us to assume a 3D growth of the oxide islands. The latter allowed us to reproduce the experimentally observed photon energy dependence for Pt oxides (cf. top and bottom panels of Figure S13). For O/OH adsorbates reproducing the decrease of the slope of the depth profiles with the electrode potential required an assumption of their decreasing coverage (see FigureS13 in the ESI). We thus conclude that PtO and PtO2 oxides were formed on the account of the conversion of O/OH adsorbates. Based on the experimental and SESSA modeling data, a tentative mechanism of the Pt electrooxidation is proposed in Figure 7. At the first stages of Pt electrooxidation O/OH adsorbates are formed, their surface coverage increasing with the applied potential (confirmed by the increase in the Pt4f XP peak intensity at 72.2 eV in Figure 3B). As the electrode potential is shifted further positive, the oxide nucleation starts. NAP-XPS data show simultaneous emergence of the Pt4f peaks at 73.8 and 75 eV, which can be unambiguously attributed to PtO and PtO2 oxide, correspondingly. The experimental results do not allow one to conclude if the PtO and PtO2 oxides nucleate simultaneously or one of them is formed electrochemically and is then rapidly converted into the other in a chemical step. From the similar behavior of the depth profiles of PtO and PtO2 it is evident that these oxides are intermixed and do not form a layered PtO2/PtO/Pt structure contrary to what has been earlier proposed by Sun et al. 39. We note in passing that Pt4f spectra do not provide any evidence for the formation of sub-surface O or OH formed either by place-exchange or another mechanism. Further increase of the applied potential leads to a 3D growth of PtO/PtO2 oxide islands which may eventually cover the whole surface of Pt NPs. Our finding related to the formation of adsorbed oxygencontaining species, PtO and PtO2 oxides upon electrooxidation

CONCLUSIONS In this study, nanostructured Pt electrode of a high temperature PEM fuel cell was investigated under polarization conditions in water and water/oxygen ambient using NAP-XPS. The binding energy shift of the membrane component (C1s) was used to in situ probe the interfacial polarization. Analysis of Pt4f spectra allowed us to identify three types of surface oxides/hydroxides, namely (i) adsorbed O/OH, (ii) PtO and PtO2. It appears that PtO and PtO2 species are formed at potentials close to the onset of the Pt electrooxidation, and their nucleation-and-growth is simultaneous rather than sequential both in water and in water/oxygen ambient. Presence of oxygen in the gas ambient leads to a negative shift of the onset of Pt surface oxidation, and to an increase of the contribution of PtO2-type surface oxide. A tentative oxide growth mechanism was proposed by comparison of the experimental NAP-XPS data with SESSA simulations. Comparison of the results of this work with earlier ex situ XPS studies make us believe that the electrooxidation of Pt under HT PEM FC relevant conditions follows a mechanism similar to the one occurring in aqueous acid electrolytes. Considering the influence of Pt surface oxides on the electrocatalytic properties and durability of nanostructured Pt electrodes, this study may help in the development of active and stable fuel cell electrocatalysts.

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ASSOCIATED CONTENT

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Supporting Information. XRD data; SEM image; O/P atomic ratio; cyclic voltammetry – influence of O2; NAP-XPS influence of the interface polarization on the BE shifts; depth profiling – additional potentials; SESSA simulation; binding energies of Pt components (full version table). This material is available free of charge via the Internet at http://pubs.acs.org/.

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Funding Sources

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The work was funded by the International Center for Frontier Research in Chemistry (Strasbourg) and partially supported within the European Union's Seventh Framework Programme (FP7/2007-2013) for Fuel Cell and Hydrogen Joint Technology Initiative under Grant No. 621237 (INSIDE).

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ACKNOWLEDGMENT

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Authors are indebted to T. Dintzer for the SEM characterization of the samples and N. Shroti for her assistance with the MEA preparation. We are grateful to S. Neophytides for valuable advice and useful discussions. We thank the Inorganic Chemistry department of the Fritz-Haber- Institut der MPG and in particular, R. Schloegl and A. Knop-Gericke for the opportunity to use ISISS beamline and HZB for the allocation of synchrotron radiation beamtime, and for providing financial support for S.Z., M.D., W.H.D. and E.R.S.

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AUTHOR INFORMATION Corresponding Author *Corresponding author. Phone: ++33(0)3 68 85 27 39. Fax: ++33(0)3 68 85 27 61. E-mail: [email protected]

Present Addresses ¬

Institut Charles Sadron (UPR22-CNRS) 23 rue du Loess, 67034 Strasbourg, France

%

Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon, 34141, Republic of Korea

Author Contributions M.K.D. prepared the samples. W.H.D., M.D., S.Z. and E.R.S. performed the experiments at BESSY II synchrotron facility. C.U.B. performed the TEM/HRTEM measurements. V.P. and V.A.S. analyzed the data. V.A.S. performed XPS spectra simulation. V.A.S., E.R.S., S.Z. and V.P wrote the manuscript.

ABBREVIATIONS HT PEM FC, high temperature proton exchange membrane fuel cell; OCV, open circuit voltage; XPS, X-ray photoelectron spectroscopy; STM, scanning tunneling microscopy; SERS, surfaceenhanced Raman scattering; XAS, x-ray absorption spectroscopy; AES, Auger electron spectroscopy; IRAS, infrared reflectionabsorption spectroscopy; DFT, density functional theory; NAPXPS, near-ambient pressure x-ray photoelectron spectroscopy; MEA, membrane electrode assemble; WE, working electrode; CE, counter electrode; CV, cyclic voltammogram; XRD, X-ray diffraction; HRTEM, high resolution transmission electron microscopy; PA, phosphoric acid; SEM, scanning electron microscopy; MS, mass spectrometry; NPs, nanoparticles; BE, binding energy; DHE, dynamic hydrogen electrode; SCE, saturated calomel electrode; UPD, underpotential deposition; ECSA, electrochemically active surface area.

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Figure 1. TEM image of Pt nanoparticles scraped from the WE. Figure 2. Cyclic voltammogram recorded in the chamber of the NAP-XP spectrometer at 150°C and 0.1 mbar H2O at the sweep rate of 100 mV s-1. Note that in the two-electrode arrangement used for the measurements the x-coordinate corresponds to the voltage applied between the WE and the CE corrected to the Ohmic drop. The arrow shows the open circuit voltage.

Figure 3. Pt4f spectra in 0.1 mbar H2O (A-E) and 0.11 mbar H2O/0.16 mbar O2 (F-J) ambient at 150°C and 0.0 (A,F); 0.3 (B,G); 0.6 (C,H); 0.7 (D,I); 0.8 (J) and 0.9 (E) V vs. the OCV. Deconvolution was done by using 5 doublets: Pt metal (black), “disconnected” Pt metal (gray), and Pt/oxides/hydroxides with BE-BEmet = +0.9 (red); +2.5 (blue); +3.7 (green) eV. Experimental XPS data are shown as black circles and fitting as a pink line.

Figure 4. Influence of the voltage bias on the fractions of (A) Pt metal and (B) oxidized Pt species: O/OHads (red); PtO (blue); PtO2 (green) in 0.1 mbar H2O at 150°C. Forward (anodic, solid line) and backward (cathodic, dashed line) voltage scans are presented. Photon energy 640 eV, corresponding to an information depth of approximately 2.5 nm.

Figure 5. Influence of O2 in the gas phase on the fractions of (A) metallic Pt, (B) O/OH adsorbate, (C) PtO oxide, (D) PtO2 oxide as a function of E-EOCV under 0.1 mbar H2O (solid line) and 0.11 mbar H2O/0.16 mbar O2 (dashed line). Only the forward scan is shown.

Figure 6. The % area contribution of the various Pt components to the overall Pt4f spectra as a function of the photoelectron kinetic energy in 0.1 mbar H2O (A,B) and 0.11 mbar H2O/0.16 mbar O2 (C,D) at 0.5 (C), 0.6 (A), 0.8 (D), and 0.9 (B) V vs. the OCV. Color codes: metallic Pt (black); O/OHads (red); PtO (blue); PtO2 (green). Figure 7. Tentative mechanism of Pt electrooxidation.


10

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Table 1. Literature values of the binding energy (BE) for electrochemically generated Pt oxides and metallic Pt.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

BE of Pt4f7/2 (BE-BEmet) eV Pt0

Chemisorbed O

PtO

Sample tion

descrip-

Conditions

Method

reference

PtO2

72.2 (+0.9)

73.8 (+2.5)

75.0 (+3.7)

Nanostructured Pt electrode integrated in a MEA

0 ÷ 0.9 V vs. OCV in 0.1 mbar H2O at 150°C

NAP-XPS

This work

71.3

70.7

73.3 (+2.6)

74.1 (+3.4)

Polycrystalline Pt foil

0 ÷ 2.2 vs. RHE in 1M HClO4 at RT

71

71.6 (+0.6)

72.5 (+1.5)

74.5 (+3.5)

Pt NPs film/Nafion (MEA)

72.6 (+1.4)

74.1 (+2.9)

Polycrystalline Pt foil

71.0

73.6 (+2.6)

74.4 (+3.4)

Polycrystalline Pt electrode

0 ÷ 2.2 vs. SCE in HClO4, H2SO4 at RT

71.2

72.2 (+1)

74.2 (+2)

Polycrystalline Pt rod

0÷1.5 vs. SCE in 0.5 M H2SO4 at RT

71.3

73.8 (+2.5)

74.6 (+3.3)

Polycrystalline Pt

0÷1.3 vs. SCE in 0.5 M H2SO4 at RT

NAP-XPS, “dip&pull” method Ex situ XPS, emersed electrode Ex situ XPS, emersed electrode Ex situ XPS, emersed electrode

16

71.2

Cell voltage 2 and 2.5V in water vapor and liquid water at RT 2 and 2.5V vs. SCE 16-20 Torr H2O at RT

Ex situ XPS, emersed electrode NAP-XPS

37

71.6 (+0.9)


68

38

39

15

11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. TEM image of Pt nanoparticles scraped from the WE. 82x82mm (600 x 600 DPI)


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Figure 2. Cyclic voltammogram recorded in the chamber of the NAP-XP spectrometer at 150°C and 0.1 mbar H2O at the sweep rate of 100 mV s-1. Note that in the two-electrode arrangement used for the measurements the x-coordinate corresponds to the voltage applied between the WE and the CE corrected to the Ohmic drop. The arrow shows the open circuit voltage. 57x40mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Pt4f spectra in 0.1 mbar H2O (A-E) and 0.11 mbar H2O/0.16 mbar O2 (F-J) ambient at 150°C and 0.0 (A,F); 0.3 (B,G); 0.6 (C,H); 0.7 (D,I); 0.8 (J) and 0.9 (E) V vs. the OCV. Deconvolution was done by using 5 doublets: Pt metal (black), “disconnected” Pt metal (gray), and Pt/oxides/hydroxides with BE-BEmet = +0.9 (red); +2.5 (blue); +3.7 (green) eV. Experimental XPS data are shown as black circles and fitting as a pink line. 177x240mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Influence of the voltage bias on the fractions of (A) Pt metal and (B) oxidized Pt species: O/OHads (red); PtO (blue); PtO2 (green) in 0.1 mbar H2O at 150°C. Forward (anodic, solid line) and backward (cathodic, dashed line) voltage scans are presented. Photon energy 640 eV, corresponding to an information depth of approximately 2.5 nm. 82x108mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Influence of O2 in the gas phase on the fractions of (A) metallic Pt, (B) O/OH adsorbate, (C) PtO oxide, (D) PtO2 oxide as a function of E-EOCV under 0.1 mbar H2O (solid line) and 0.11 mbar H2O/0.16 mbar O2 (dashed line). Only the forward scan is shown. 177x124mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Tentative mechanism of Pt electrooxidation. 82x81mm (300 x 300 DPI)


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Table of Content Graphic 61x47mm (300 x 300 DPI)


Near Ambient Pressure XPS (NAP-XPS) - ACS Publications

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