Ambient Pressure Hard X-ray Photoelectron Spectroscopy for

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Ambient Pressure Hard X‑ray Photoelectron Spectroscopy for Functional Material Systems as Fuel Cells under Working Conditions Yasumasa Takagi,†,⊥ Tomoya Uruga,§,∥ Mizuki Tada,‡ Yasuhiro Iwasawa,§ and Toshihiko Yokoyama*,† †

Department of Materials Molecular Science, Institute for Molecular Science, Myodaiji-cho, Okazaki, Aichi 444-8585, Japan Innovation Research Center for Fuel Cells, The University of Electro-Communications, Chofugaoka, Chofu, Tokyo 182-8585, Japan ∥ Japan Synchrotron Radiation Research Institute, SPring-8, Koto, Sayo, Hyogo 679-5198, Japan ‡ Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan §

CONSPECTUS: Heterogeneous interfaces play important roles in a variety of functional material systems and technologies, such as catalysis, batteries, and devices. A fundamental understanding of efficient functions at interfaces under realistic conditions is crucial for sophisticated designs of useful material systems and novel devices. X-ray photoelectron spectroscopy is one of the most promising and common methods to investigate such material systems. Although X-ray photoelectron spectroscopy is usually conducted under high vacuum because of the requirement of electron detection with the precise measurement of kinetic energies, extensive efforts have been devoted to the measurements in gaseous environments. Very recently, we have succeeded in measuring X-ray photoelectron spectra under real ambient atmosphere (105 Pa), using synchrotron radiation hard X-rays with the photon energy of 8 keV and the windowless electron spectrometer system. In this Account, the novel useful technique of real ambient pressure hard X-ray photoelectron spectroscopy is reviewed. As examples of (near) ambient pressure hard X-ray photoelectron spectroscopy, hydrogen storage of Pd nanoparticles is at first investigated by recording Pd 3d and valence band spectra under hydrogen atmosphere. The Pd 3d and valence band spectra are found to change rather abruptly depending on the hydrogen pressure, demonstrating a behavior like phase transformation. Subsequently, as a main topic in this Account, we describe investigations of the electronic states of platinum nanoparticles on the cathode electrocatalyst in a polymer electrolyte fuel cell (PEFC) under the voltage operating conditions using the near ambient pressure hard X-ray photoelectron spectroscopic system. The Pt 4f and 3d X-ray photoelectron spectra of the cathode Pt/C catalysts clearly show that the oxidized Pt species is at most divalent and the tetravalent Pt species does not exist on the Pt nanoparticles even at the positive cathode−anode voltage of ∼1.4 V. Although the water oxidation reaction may take place at the potential, such a reaction does not lead to a buildup of detectable tetravalent Pt in the PEFC. The voltage-dependent Pt 3d X-ray photoelectron spectra show a clear hysteresis between the voltage increase and decrease processes. The fraction of oxidized Pt species matched the ratio of surface to total Pt atoms in the nanoparticles, which suggests that Pt oxidation occurs as a reaction event at only the first Pt layer of the Pt nanoparticles and the inner Pt atoms do not participate in the reaction practically. The developed technique is a valuable in situ tool for the investigation of the electronic states of PEFCs and other interesting functional material systems and devices under realistic working conditions. Various scientific and engineering research fields, however, have required in situ or operando measurements under realistic working conditions of functional material systems and devices for more essential understanding of the origins of their function mechanisms and development of novel efficient functional material surfaces and interfaces. For instance, the catalytic CO hydrogenation reaction in the Fischer−Tropsch synthesis is usually conducted at >106 Pa, and the ammonia synthesis in the Haber−Bosch process is at 107−108 Pa, which are 14−16 orders of magnitude larger than UHV. Understanding of realistic functions only with the measurements of “dried fish” under high vacuum seem to be no more easily performed without in situ observations of “living fish.”

1. INTRODUCTION Heterogeneous interfaces often play important roles in many scientific, technological, and energy and environmental research fields. A fundamental understanding of efficient functions at interfaces under realistic working conditions is essential for sophisticated designs of useful catalysts, batteries, and other novel devices. In these 50 years, the research field of surface science has developed various observation techniques to investigate clean and well-ordered surfaces at atomic scales with highly elementally, chemically, temporally, and spatially resolved methods.1 These investigations inevitably require ultrahigh vacuum (UHV, ≤10−8 Pa) to keep sample surfaces clean during the whole experimental observation period. Moreover, many spectroscopic methods need UHV operations because of detection of electrons and ions emitted from the samples. © XXXX American Chemical Society

Received: November 11, 2017

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DOI: 10.1021/acs.accounts.7b00563 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Takagi et al.15 eventually succeeded in real ambient pressure (1 × 10 5 Pa) hard X-ray photoelectron spectroscopy (APHAXPES) measurements using the 8 keV hard X-ray source. In the section 2 of this Account, we will describe the methodology of APHAXPES based on our previous work.15 There have been published many fantastic reviews concerning APXPS, and readers are encouraged to access these previous articles.16−18 As a typical example of APHAXPES, the results of hydrogen storage of Pd nanoparticles is described, examined with Pd 3d and valence band HAXPES measurements. In section 3, we will focus our attention on polymer electrolyte fuel cells (PEFCs). The PEFC is one of the most promising next-generation automotive power sources. Generally, platinum nanoparticles on carbon supports are used in PEFCs as the highest efficiency catalyst. Since their durability is a key factor for their practical use, approaches for improving their long-term durability, particularly for the cathode electrocatalysts in PEFCs, have extensively been investigated by many researchers.19,20 It is important to observe directly the oxidation states of the cathode Pt nanoparticles at positive voltages, because severe oxidation, dissolution, and sintering of Pt nanoparticles may occur at the cathode electrode in addition to carbon corrosion when large positive voltages are applied to the PEFC cathode, for instance, during startup/shutdown operations. In situ observations of PEFCs have been made using various sophisticated methods, such as X-ray absorption fine structure (XAFS),21 X-ray emission spectroscopy,22 and transmission electron microscopy.23,24 Recently, in situ XAFS measurements25,26 of PEFC electrodes have revealed restructuring of the surface through observation of the Pt oxidation states and coordination numbers of Pt−O and Pt−Pt bonds in the Pt NPs at the cathode during voltage operating processes. Moreover, NAPXPS has also been conducted under working conditions using soft,27−29 tender,30 and hard31−33 X-rays. Here, the electronic states of platinum nanoparticles on the cathode electrocatalyst in the PEFC using NAPHAXPES are described. The developed APHAXPES technique clearly demonstrates the usefulness for the investigations of the electronic states of PEFC electrocatalysts and other functional material systems and devices under the working conditions. Some prospects for the near future are finally addressed.

X-ray photoelectron spectroscopy (XPS) is one of the most common analysis methods to investigate surfaces and interfaces.2 Although XPS has many advantages to yield elementally and chemically specific quantitative analysis of surfaces, this technique basically requires high vacuum since the kinetic energy of photoelectrons emitted from the samples is precisely measured with high energy resolution. The principal difficulty to overcome in XPS under ambient pressure is the scattering of electrons by gaseous molecules. The inelastic mean free path of electrons with the kinetic energy of 100 eV in atmospheric pressure (105 Pa) of air is only about 3 × 10−7 m.3 The design of the XPS apparatus under gaseous atmosphere therefore aims at minimizing the path length of the electron trajectories in high pressure regions. The detection of high energy electrons is also advantageous because of larger inelastic mean free paths of higher kinetic energy electrons, although the surface sensitivity, which is also an important advantage of XPS, is inevitably reduced. The first XPS instruments under gaseous atmosphere were designed by the Siegbahn group in Uppsala in the early 1970s4 and were followed by several other designs,5 using laboratory X-ray sources. In the late 1990s, the synchrotron radiation based instruments were developed,6,7 and subsequent commercialization of near ambient pressure (NAP) XPS8,9 resulted in a rapid growth in the number of NAPXPS experimental stations. Most of the current NAPXPS instruments consist of a sample chamber that can be filled with gases. Electrons and gaseous molecules enter the differentially pumped electrostatic prelens system of the NAPXPS spectrometer through a small aperture without a window that partitions the sample environment from the spectrometer. The sample is placed as close as possible to the prelens aperture to reduce the path length of the photoelectrons through the gases. Several differentially pumped electrostatic lens stages carry the photoelectrons to the entrance slit of a hemispherical electron energy analyzer and maintain high vacuum around the electron detector located at the end of the analyzer. Typical NAPXPS experiments are performed at working pressures of 3000 Pa or less, although NAPXPS instruments operating under higher pressures have been developed. The key factors for significantly increasing the pressure limit in NAPXPS may be a high flux X-ray source, a small focused high brilliance beam, a small prelens aperture, a short working distance between the aperture and sample, and high energy photoelectrons generated by high energy X-rays. For example, by adopting a diameter of 50 μm for the prelens aperture and a working distance of 200 μm, NAPXPS for the Pt(111) surface at 100 Torr (1.33 × 104 Pa) was successfully achieved by Kaya et al.,10 while NAPXPS for the Au(111) surface at 110 Torr (1.46 × 104 Pa) using excitation X-rays of 4 keV and a prelens aperture diameter of 100 μm was reported by Axnanda et al.11 An excellent review summary concerning NAPXPS was reported by Starr et al.12 In a meanwhile, XPS using hard X-rays of typically 6−10 keV has also been developed owing to highly brilliant thirdgeneration synchrotron radiation light sources to investigate real bulk characters of functional material systems and devices with much less surface sensitivity.13,14 This technique is commonly called hard X-ray photoelectron spectroscopy (HAXPES), which allows us to investigate bulk sensitive electronic structures of materials due to much larger electron mean free paths. The HAXPES technique is apparently advantageous to ambient pressure XPS as well. Very recently,

2. AMBIENT PRESSURE HARD X-RAY PHOTOELECTRON SPECTROSCOPY 2.1. Photoelectron Mean Free Path

It is crucial to avoid attenuation of emitted photoelectrons due to interactions with gases as efficiently as possible and to maintain clean high vacuum in the electron energy analyzer. The electron mean free path is known to increase with the increase in the electron kinetic energy. Figure 1a shows the electron mean free path λ in atmospheric air environment (105 Pa) as a function of the photoelectron kinetic energy, Ep. The mean free path was evaluated from the sum of the inelastic and elastic cross sections, σ, of electrons interacting with nitrogen and oxygen5 by using the equation of λ = kBT/σP, where kB is the Boltzmann constant, T the temperature, and P the pressure. Note here that the mean free path exhibits approximately linear increase with the kinetic energy. The electron transmission I/I0 can be calculated using the equation as I/I0 = exp[−L/λ] = exp[−σPL/kBT], where L is the electron traveling length. Figure 1(b) gives the electron B


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Figure 2. Schematic view of the APHAXPES apparatus (Scienta Omicron Hipp-2). Adapted with permission from ref 9. Copyright 2010 Scienta Omicron GmbH.

Figure 1. (a) Mean free path of electrons in atmospheric air at 105 Pa as a function of the electron kinetic energy. (b) Photoelectron transmission probability in air with the path length of 60 μm as a function of the air pressure P.

transmission I/I0 as a function of P with varying the electron kinetic energies. Here, L = 60 μm is assumed, which is a typical length in the following APHAXPES experiments. The transmissions I/I0 are ∼4 × 10−4 at Ep = 5.0 keV and ∼2 × 10−2 at Ep = 10.0 keV, with which one could perform real ambient pressure (1 × 105 Pa) photoelectron spectroscopic measurements.

Figure 3. (a) Front cone of the electron energy analyzer. (b) Scanning electron microscope image of the aperture shaped at the top of the front cone. (c) Schematic diagram of the cross section around the aperture and the sample. (d) CCD microscope image of the sample and the front cone at a working distance of 60 μm. Reproduced with permission from ref 15. Copyright 2017 IOP Publishing.

2.2. APHAXPES Apparatus and Beamline

To maintain high vacuum in the electron energy analyzer, one may usually employ differential pumping units without windows between the sample and electron spectrometer. Although the APXPS technique employing extremely thin windows34,35 that allow penetration of sufficient amounts of electrons is sometimes useful and convenient depending on the samples and purposes, its applications may be severely limited for wide variety of material systems, and we will not describe the technique in this Account. Figure 2 shows the example of the ambient pressure photoelectron spectrometer constituting of the electron prelens and lens parts and the electron energy analyzer (Scienta Omicron Hipp-2).9 The maximum pressure, Pmax, below which the electron spectrometer can be operated without discharging, is dominated by the orifice size at the front cone of the prelens part [see Figure 3a for the front cone]. When one wants to set Pmax = 5000 and 100000 [Pa], the aperture diameters D are calculated to be less than ∼300 and ∼70 μm, respectively. The other important care to be taken is to keep the sample pressure nearly equivalent to that of the environment. Since the gases flow into the electron spectrometer [see Figure 3c], the pressure at the sample surface may decrease considerably if the

working distance L between the sample surface and the entrance aperture is too short.36,37 In the fluid dynamics estimations,36,37 it is reported that L/D = 1 makes the sample pressure almost equal (>99%) to the environmental one. The experimental work suggests that L/D ≈ 2 is better and safer. By combining these discussion, D = 30 μm and L = 60 μm are eventually chosen for the following APHAXPES experiments, as shown in Figure 3b−d. The experimental setup for APHAXPES thus determined inevitably requires considerably grazing X-ray incidence geometry (incidence angle of ∼1°), as depicted in Figure 3c,d. The APHAXPES spectrometer constructed by the present authors is installed at the undulator beamline BL36XU of SPring-8.38,39 The X-ray source is an in-vacuum type undulator. Since BL36XU is dedicated mainly for XAFS experiments of PEFCs, the undulator can be tapered for scanning the photon energy over wide ranges. In the APHAXPES experiments, the undulator does not have to be tapered, and intense and sharp hard X-rays are provided without tapering. Channel-cut Si(111) C


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Accounts of Chemical Research 2.4. Hydrogen Storage of Pd Nanoparticles

and Si(220) crystal monochromators are tandemly arranged to cover energy ranges from 4.5 to 35 keV. For the APHAXPES measurements, a high-resolution Si(111) monochromator is additionally inserted (total of 4 monochromator crystals) and provides hard X-rays of ∼6, ∼8, and ∼10 keV with Si333, 444, and 555 reflections, respectively.40 The beamline optics are optimized to provide the X-ray beam of 30 μm (V) × 20 μm (H).32

As a simple example of APHAXPES measurements, hydrogen storage of Pd nanoparticles41 is investigated. The Pd nanoparticle sample is so-called Pd black purchased from SigmaAldrich (99.95% pure, surface area 40−60 m2/g corresponding to the average particle size of ∼2 nm). Pd 3d and valence-band HAXPES are measured using the hard X-rays of 7.94 keV in hydrogen atmosphere of 1−104 Pa. Although no cleaning treatments were conducted, the sample was at first exposed to hydrogen, leading to removal of most surface oxide contamination. The carbon contamination should remain on the surface, although the APHAXPES technique is bulk sensitive. The resultant spectra are depicted in Figure 5. The

2.3. Performance of Real Ambient Pressure HAXPES System

In order to demonstrate the performance of the APHAXPES system, the Au 4f XPS from Au(111) single crystal was recorded.15 Figure 4 shows the Au 4f HAXPES from the

Figure 4. (a) Au 4f HAXPES of Au(111) on mica using 7.94 keV Xrays at environmental pressures 1 to 100 kPa. Each spectrum was recorded in an acquisition time of 10 min. (b) Relative intensity of the Au 4f7/2 peaks as a function of pressure. (c) Au 4f spectrum recorded in an acquisition time of 30 min. The Shirley background was subtracted from the spectra, and the plots were fitted with a Voigt function. Reproduced with permission from ref 15. Copyright 2017 IOP Publishing.

Figure 5. (a) Pd 3d5/2 and 3d3/2 HAXPES of Pd nanoparticles using 7.94 keV X-rays at the hydrogen pressures of 1 and 104 Pa at the temperature of 303 K. (b) Valence band HAXPES with the same conditions as in panel a. (c) Pd 3d5/2 HAXPES of Pd nanoparticles in the hydrogen pressure increasing process from 1 to 104 Pa. (d) Pd 3d5/2 HAXPES of Pd nanoparticles in the hydrogen pressure decreasing process from 104 to 1 Pa.

Au(111) surface at room temperature, depending on the environmental air pressure. The X-ray incidence angle was ∼1°, which is close to the critical angle for the total X-ray reflection from Au(111). The total energy resolution in the present HAXPES measurements is ∼400 meV, estimated by the measurements of the Fermi level. Although the photoelectron intensity is found to be reduced rapidly with the pressure rise as in Figure 4a, the photoelectron spectra can be observed even in the real ambient pressure. This was the first observed X-ray photoelectron spectrum at real ambient pressure. Figure 4b shows that the logarithmic plot of the electron transmission looks clearly linear as a function of P, consistent with the previous equation of I/I0 = exp[−σPL/ kBT]. The electron scattering cross sections of air are estimated from the gradient of the linear line in Figure 4b. The results are 3.6 × 10−21 m2 for Au 4f (7.86 keV) and 5.0 × 10−21 m2 for Au 3d5/2 (5.73 keV, spectra not shown), which are in good agreement with the estimated values of 3.49 × 10−21 and 4.77 × 10−21 m2 given in the literature.5 These findings clearly exemplify that the measurement pressure at the sample surface is really ambient (1 × 105 Pa).

Pd 3d5/2 and 3d3/2 peaks are found to shift to a higher binding energy with an increase in the hydrogen pressure, indicating that the Pd atoms are more positively charged upon hydrogen adsorption. The positive chemical shift of Pd is informative since the electronegativities of H and Pd are almost the same (∼2.20). Figure 5b shows the valence band HAXPES, where the Pd 4d valence bandwidth is found to become narrower upon hydrogen adsorption, possibly because the lattice constant is larger. A new feature also appears at ∼7.5 eV in Figure 5b, which could be ascribed to the Pd−H bond formation. As the hydrogen pressure sequentially increases from 1 to 104 Pa [Figure 5c], the Pd5/2 XPS peak is shifted abruptly between 4000 and 6000 Pa, while in the pressure decreasing process, the abrupt shift is found between 1000 and 2000 Pa. This hysteresis behavior concerning the hydrogen storage process of Pd D


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respect to that of the Pt foil, and the line width is broader possibly because of the nanosize effect.42 On increasing the voltage to 1.4 V, the Pt nanoparticles are oxidized and a new 4f5/2 peak appears at 77.55 eV, which is 3.1 eV higher than the main 4f5/2 corresponding to metallic Pt(0). The 4f7/2 peak is also seen as a shoulder overlapped by the metallic Pt(0) 4f5/2. The previous work43 showed that the Pt 4f XPS from a PtCo alloy catalyst on a strongly deteriorated MEA clearly gave the peaks assigned to Pt(IV) at binding energies 4.4 eV higher than the metallic Pt(0). The present NAPHAXPES thus elucidates that the oxidized Pt peak in the Pt 4f XPS at 1.4 V is ascribed not to Pt(IV) but to Pt(II) species. The Pt 3d5/2 spectra in Figure 6 are deconvoluted into three components: Pt1, Pt2, and Pt3. Referring to the Pt 4f spectra, we can straightforwardly assign Pt1 to metallic Pt(0) and Pt3 to Pt(II). The Pt2 component is considered to be ascribed to Pt(I) species such as Pt-OH, although other possibilities such as the edge atoms are also to be expected. The data acquisition time for the Pt 3d HAXPES is much shorter than that for the Pt 4f HAXPES because the former gives two orders of magnitude more intense signals. Such a favorable feature is quite useful for studies of voltage-dependent Pt oxidation because it requires many spectral data sets in a limited beam time. In contrast, the Pt 3d peak is broader than the Pt 4f due to a much shorter lifetime of the core-hole state. When high energy resolution is needed, the Pt 4f measurement with a longer data acquisition time should be more suitable. We have observed voltage dependence of the Pt oxidation state by recording the Pt 3d NAPHAXPES at the applied cathode−anode voltages of 0.2−1.4 V. The results are shown in Figure 7. The component fractions of Pt1, Pt2, and Pt3 as a function of the applied voltage are plotted in Figure 7c,d,e, respectively. The Pt chemical state exhibits a distinct hysteresis on increasing and decreasing the applied voltage. A similar hysteresis was also reported for in situ XAFS analysis, although

nanoparticles is consistent with the previous work on the hydrogen uptake of Pd black depending on the pressure.41 The present simple ambient pressure observation clearly demonstrates the usefulness of APHAXPES measurements for gas storage functional materials.

3. NAPHAXPES OF THE PEFC UNDER WORKING CONDITIONS For the NAPHAXPES measurements of PEFCs, specially designed membrane electrode assembly (MEA) cells are necessarily developed. Details can be found in the previous literature.15 One of the most important issues may be complete separation (vacuum sealing) between the cathode and anode. In working conditions of PEFCs, atmospheric hydrogen gases (105 Pa) flow through the anode, while humid atmospheric air is introduced at the cathode. The electrolyte Nafion film itself can play a role in sufficiently tight vacuum sealing between the cathode and anode. Thus, the MEA works as the active unit of PEFC composed of a Pt/C cathode layer, a Nafion electrolyte membrane, and a Pt/C anode layer in a stack structure. We assume that the main target may be the cathode, where the oxygen reduction reaction (O2 + 4H+ + 4e− → 2H2O) takes place. Pt/C (Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E50E, Pt 50 wt %, average nanoparticle size of 2.6 nm) is used as catalyst at both the cathode and anode. The cathode electrode is always grounded for the NAPHAXPES measurements. In the present electrochemical setup, the anode is used as both the counter and reference electrode since in the Pt/H2 system the electrode potential can be assumed to be constant over a wide range of current densities. All potentials are given with respect to the anode, namely, as V vs RE/CE. Cyclic voltammogram (CV) measurements were successfully conducted in H2 (anode) and N2 (cathode) operating atmospheres.15,31−33 The Pt nanoparticles are oxidized when a large positive voltage is applied between the electrodes. The threshold voltage for Pt oxidation is ∼0.8 V. We have investigated the oxidation states of the Pt nanoparticles in the cathode at positive voltages by in situ NAPHAXPES. The water vapor pressure during the measurements is 4000 Pa, which is sufficiently humid to operate the PEFC efficiently. Figure 6 depicts the Pt 3d5/2 and 4f HAXPES of the cathode Pt/C catalyst at the bias voltages of 0.4 and 1.4 V, which exhibit reduced and oxidized Pt states, respectively. The Pt 4f HAXPES peak is found to shift by 0.4 eV to a higher binding energy with

Figure 7. (a, b) In situ NAP-HAXPES Pt 3d5/2 spectra of the Pt/C catalyst with an average diameter of 2.6 nm (TEC10E50E) at the cathode for various cell voltages. The voltage is (a) increasing or (b) decreasing. (c−e) Voltage dependence of the concentration of the Pt chemical states (c) Pt1, (d) Pt2, and (e) Pt3 (see the text and Figure 3 for descriptions of the components) obtained by curve fitting. Reproduced with permission from ref 32. Copyright 2017 Royal Society of Chemistry.

Figure 6. Pt 3d5/2 and 4f HAXPES of Pt foil and the Pt/C catalyst at the cathode (hν = 7.94 keV). The Pt foil spectrum was recorded under high vacuum, while the other spectra were recorded under a water pressure of 4000 Pa at the cathode. Reproduced with permission from ref 32. Copyright 2017 Royal Society of Chemistry. E


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atoms on the surface alter their valence, the oxidation may not significantly influence the second layer Pt. The upward- and downward-buckled Pt atoms at voltages >1.0 V could be ascribed to Pt(II) and Pt(I), respectively, as proposed above, indicating no contradiction but good accordance between the HAXPES and XAFS results. On the other hand, agreements with previous NAPXPS works based on Pt 4f using soft and tender X-rays27−30 are also in general good, although some different concluding remarks can be found. In the previous NAPXPS works,27−30 the presence of Pt(IV) species has been reported, possibly because of the different sample treatments concerning maximum cathode voltages and gaseous species (humid oxygen in cathode and humid hydrogen in anode). Other dissimilarities are seen in O 1s XPS (not shown in this work); the previous NAPXPS using tender X-rays successfully observed oxygen species adsorbed on the Pt nanoparticles with almost no sulfonic acid species from Nafion, while in our measurements too much adsorbed water, liquid water layer, and Nafion species prevent us from detecting oxygen species adsorbed on the Pt nanoparticles with good reproducibility. This may be a near future issue to be settled for the investigations of solid/liquid interfaces such as working fuel cells and batteries.

the three different oxidation states of Pt were not quantitatively distinguished.21,24−26 We have also investigated voltage dependence of the oxidation states of Pt nanoparticles with a larger particle size (average size 4.8 nm, TEC10E50E-HT) in the cathode Pt/C. A similar tendency was observed in the Pt 3d5/2 HAXPES. The result indicates a particle size effect on the Pt electronic state.42,44 In the case of fcc cubo-octahedral structure, the Pt nanoparticles with diameters of 2.6 and 4.8 nm correspond to 586 (bulk/surface atom ratio of 54%) and 4033 atoms (73%), respectively. These values are in good agreement with the ratios of the metallic Pt1 at 1.4 V, where the Pt nanoparticles are highly oxidized. These findings suggest that only the surface Pt atoms in the nanoparticles are oxidized and that the atoms in the bulk maintain their metallic state, as depicted in Figure 8.

4. FUTURE PROSPECTS In this Account, the APXPS technique using hard X-rays and its application to PEFC are described. The usefulness of this method is exemplified from the viewpoint of the availability of in situ or operando measurements under realistic working conditions of functional material systems and devices such as the cathode catalyst in PEFC MEA. One can immediately expect a wide variety of applications of this method to chemistry, physics, biology, and related technologies, such as catalysis, gas storage, gas sensors, batteries, and biomaterial systems. Operando measurements allow us to get insight into the origins of functionality and degradation mechanisms of material systems and devices. In electrochemistry including batteries and fuel cells, APHAXPES is useful since this technique can identify the electric potentials of all the components at the electrolyte− electrode interfaces without approach of a detecting electrode.47 It is well-known that electric double layers are constructed at the electrolyte−electrode interface, and the features of the electric double layer strongly depend on the bias voltage. Using APHAXPES, one can directly measure the electric potential of each component at the electrolyte− electrode interface depending on the bias voltage. Through quantitative discussion on the electric potentials of the electrolyte−electrode interface, complicated mechanisms of electrochemical reactions, mobilities of electrons and ions, capacitance, and so forth may be better understood, leading to developments of fantastic highly efficient batteries and fuel cells. One important issue to be improved may be time-resolved APXPS measurements, which is essential when one wants to investigate chemical kinetics and dynamics under working conditions. Unfortunately, the present APHAXPES system requires typical measurement periods of ∼15 min for one spectrum. If the event is completely reproducible, one can repeat the measurements based on the pump-and-probe method. In this situation, the time resolution may be determined by the CCD camera rate. One can perform investigations, for instance, on the behaviors of batteries and

Figure 8. Schematic showing the voltage dependent transformation of the Pt nanoparticles at the cathode electrode. (a) Pt nanoparticle model of TEC10E50E (d = 2.6 [nm]) corresponding to the fcc cuboctahedral-structured nanoparticle: Nedge = 4, Ntotal = 586, and Nsurface = 272. (b) Pt nanoparticle of TEC10E50E-HT (d = 4.8 [nm]): Nedge = 7, Ntotal = 4033, and Nsurface = 1082. The dispersions (D = Nsurface/Ntotal) are 0.46 and 0.27, respectively. These values agree well with the experiments, indicating that only the surface atoms change their valence. Reproduced with permission from ref 32. Copyright 2017 Royal Society of Chemistry.

The above HAXPES results concerning the absence of Pt(IV) species and the oxidation event at the surface Pt layer are in good accordance with the in situ XAFS conclusions.21,24−26,45 The in situ XAFS measurements demonstrated that the direct transformation from metallic Pt to mostly Pt(II) at the nanoparticle surfaces occurs at potentials above 1.2 V, exhibiting an isosbestic point in the XANES series, and no further oxidation to Pt(IV) was observed under fuel cell operating conditions. The previous XAFS measurements for similar samples indicated that the oxygen atoms on the Pt nanoparticles are located both on the outermost surfaces and also in the subsurface of Pt nanoparticles when the applied voltage is 1.4 V.25,45 It is known that the surface oxide on Pt(111) exhibits substantial buckling of both O and Pt atoms.46 The upward-buckled Pt atoms have a chemical bond with both surface-chemisorbed and subsurface oxygen, whereas the downward-buckled Pt atoms interact with surface hydroxyls or subsurface oxygen. In this situation, although the buckled Pt F


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2017A7811). This work was supported by the Polymer Electrolyte Fuel Cell Program from the New Energy and Industrial Technology Development Organization (NEDO) and by the Grants-in-Aid for Scientific Research (KAKENHI) Grant Number 15H05489 from the Japan Society for the Promotion of Science. Fabrications of the front cone with the small aperture by the staff at the Equipment Development Center, Institute for Molecular Science, are gratefully acknowledged.

fuel cells upon abrupt changes of bias voltages by using the time-resolved APHAXPES technique. We hope that the APHAXPES systems are spread into thirdgeneration synchrotron facilities all over the world to develop more efficient, long-lived, environmentally friendly, safe, and harmonic material systems for our 21st century and future earth.

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*E-mail: [email protected].

REFERENCES

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ORCID

Yasuhiro Iwasawa: 0000-0002-5222-5418 Toshihiko Yokoyama: 0000-0003-0161-7216 Present Address ⊥ Y.T.: Japan Synchrotron Radiation Research Institute, SPring8, Koto, Sayo, Hyogo 679-5198, Japan.

Notes

The authors declare no competing financial interest. Biographies Yasumasa Takagi received his Ph.D. in 2005 in Physics from the University of Tokyo. He was Assistant Professor at Institute for Molecular Science, 2007−2017, and moved to SPring-8 (Japan Synchrotron Radiation Research Institute) in 2017. His research interests focus on methodological developments of X-ray photoelectron spectroscopy using synchrotron radiation for material science. Tomoya Uruga received his Ph.D. in 1991 in Engineering from Osaka University. He is Chief Scientist of Japan Synchrotron Radiation Research Institute since 2013 and Designated Professor at the University of Electro-Communications since 2010. His research interests focus on methodological and instrumentation developments of X-ray absorption spectroscopy using synchrotron radiation for various research fields. Mizuki Tada received her Ph.D. in 2005 in Chemistry from the University of Tokyo. She is a Professor at Nagoya University since 2013. She is the team leader of the element visualization team at RIKEN SPring-8 (2014−2019). Her research interests are the design of heterogeneous catalyst surfaces using metal complexes and clusters and operando XAFS imaging of solid materials. Yasuhiro Iwasawa received his Ph.D. in 1973 in Chemistry from the University of Tokyo. He is Professor and Director of Innovation Research Center for Fuel Cells at the University of ElectroCommunications since 2009 and Emeritus Professor at the University of Tokyo. His main research interests come under the general terms “catalytic chemistry” and “surface chemistry” but, more specifically, include catalyst surface design, fuel cells, in situ/operando characterization, and time-resolved and spatially resolved XAFS. Toshihiko Yokoyama received his Ph.D. in 1990 in Chemistry from the University of Tokyo. He is Professor at the Institute for Molecular Science since 2000. His research interests focus on methodological developments of X-ray absorption and X-ray photoelectron spectroscopy using synchrotron radiation for material science.

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ACKNOWLEDGMENTS These experiments were performed with the approval of SPring-8 (Nos. 2014A7810, 2014A7811, 2014B7810, 2014B7811, 2015A7810, 2015B7810, 2016A7810, 2016A7811, 2016B7810, 2016B7811, 2017A7810, and G


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