C and Pt&l

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Operando Time-Resolved XAFS Study for Pt Oxidation Kinetics on Pt/C and PtCo/C Cathode Catalysts by PEFC Voltage Operation Synchronized with Rapid O Exposure 3

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Saki Ozawa, Hirosuke Matsui, Nozomu Ishiguro, Yuanyuan Tan, Naoyuki Maejima, Masahiro Taguchi, Tomoya Uruga, Oki Sekizawa, Tomohiro Sakata, Kensaku Nagasawa, Kotaro Higashi, and Mizuki Tada J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02541 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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The Journal of Physical Chemistry

Operando Time-Resolved XAFS Study for Pt Oxidation Kinetics on Pt/C and Pt3Co/C Cathode Catalysts by PEFC Voltage Operation Synchronized with Rapid O2 Exposure Saki Ozawa,† Hirosuke Matsui,*†,‡ Nozomu Ishiguro,‡ Yuanyuan Tan,† Naoyuki Maejima,† Masahiro Taguchi,† Tomoya Uruga,§,¶ Oki Sekizawa,§,¶ Tomohiro Sakata,§ Kensaku Nagasawa,§ Kotaro Higashi,§ and Mizuki Tada*†,‡ † Department of Chemistry, Graduate School of Science & Research Center for Materials Science (RCMS) & Integrated Research Consortium on Chemical Science (IRCCS), Nagoya University, Furo, Chikusa, Nagoya, Aichi 464-8602 (Japan) ‡ RIKEN SPring-8 Center, Koto, Sayo, Hyogo 679-5148 (Japan)

§ Innovation Research Center for Fuel Cells, The University of Electro-Communications, Chofu, Tokyo 182-8585 (Japan) ¶ Japan Synchrotron Radiation Research Institute (JASRI)/SPring-8, Koto, Sayo, Hyogo 6795198 (Japan)

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ABSTRACT: The oxidation of cathode catalyst surface is one of the major factors for cathode catalyst degradation under polymer electrolyte fuel cell (PEFC) operating conditions, however, the oxidation kinetics of Pt cathode catalysts in a membrane electrode assembly (MEA) with O2 has not been investigated. We have investigated operando Pt LIII-edge time-resolved X-ray absorption fine structure (XAFS) analysis for the Pt oxidation processes on Pt/C and Pt3Co/C cathode catalysts controlled by transient PEFC voltage operation (0.4 V → 0.7 – 1.0 V) synchronized with the rapid exchange of cathode gas (N2 → 10% O2 in N2). We plotted Pt valence and the coordination numbers (CNs) of Pt-Pt and Pt-O bonds every 20 ms, and the rate constants of the structural parameters for the Pt oxidation were successfully estimated. The structural kinetics of the Pt oxidation at the cathode suggested differences in the Pt oxidation kinetics between the Pt/C and Pt3Co/C cathode catalysts, showing the kinetic control of the Pt oxidation rate by the alloying with Co to Pt.

1. Introduction Polymer electrolyte fuel cells (PEFCs) are a practical candidate for next-generation energy devices that combine a clean power source with high conversion efficiency.1,2 To realize a hydrogen economy in which PEFCs are commercialized and widespread, the activity of the oxygen reduction reaction (ORR) at the cathode and the durability of a cathode catalyst remain major challenges in terms of membrane electrode assembly (MEA) properties. The actual running modes of fuel-cell vehicles, consisting of repeated changes in cell voltage and other conditions, cause severe degradation of the cathode catalyst, such as dissolution and aggregation of Pt catalyst particles.3–5


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There have been several efforts to improve the performance of cathode catalysts in PEFCs such as alloying of transition metals to Pt. It is efficient to modify the noble-metal catalyst activity by the addition of transition metals such as Co 6, 7 and Pt-alloy catalysts that provide better cathode catalyst performance, such as Pt3Co catalysts, have been developed for PEFC.8–13 The high durability and ORR activity of Pt3Co/C catalysts have been extensively studied by several

different

approaches

including

electrochemical

analysis,8,

9,

11–14

theoretical

calculation,15,16 electron microscopy,17–19 and spectroscopic methods.20–24 STEM-EDS analysis of Pt3Co/C has suggested the formation of Pt-rich surface and Pt-Co alloy core in MEA after acid treatment and annealing process in PEFC.10, 13, 19, 25, 26 The binding of oxygenated species on the Pt-rich surface was suggested to be weakened on the Pt3Co/C catalyst to enhance ORR activity. The analysis revealed the structures and performances of the Pt3Co/C catalysts but their dynamic structural changes and Pt oxidation kinetics with O2 are not clear at moment. X-ray absorption fine structure (XAFS) analysis is a powerful tool for examining the chemical state and local coordination of each element. The high transmittance of hard X-rays enables operando time-resolved quick-XAFS (QXAFS) measurements27-29 of electrocatalysts in MEAs under practical PEFC operating conditions, showing the structural kinetics of cathode electrocatalysts under humidified N2 atmosphere.30–35 For examples, we have reported the structural kinetics of a Pt/C,30, 32 Pt3Co/C, and Pt3Ni/C 31, 33, 34 cathode catalysts for the voltage operation between 0.4 – 1.0 V, and the transitional states of a Pt/C cathode catalyst during PEFC loading with transient voltage.35 The operando time-resolved QXAFS analysis of the Pt3Co/C catalyst under the cathode atmosphere of humidified N2 revealed the positive rate enhancement of the Pt-Pt bond reforming from 1.0 V to 0.4 V, which is suggested to be a reason of high durability of the Pt3Co/C catalyst.31-35 However, the reported operando time-resolved XAFS


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measurements were performed under humidified N2 atmosphere without O2 at the cathode, and the observed dynamic events at the cathode are caused by the redox processes of the Pt catalysts with H2O. There have been no reports on time-resolved XAFS study for Pt oxidation kinetics at the cathode with O2. In this work, we performed the operando time-resolved XAFS analysis of Pt/C and Pt3Co/C cathode catalysts in MEAs after the rapid exchange of cathode gas (N2 → O2) synchronized with the transient voltage operations from 0.4 V to 0.7 – 1.0 V. The synchronization of the exposure to O2 and the voltage operations enables to find the Pt oxidation kinetics with O2 on reduced Pt surface formed at 0.4 V in PEFC. We systematically investigated the rate constants of the Pt oxidation on the Pt/C and Pt3Co/C cathode catalysts with O2, and the effects of Co-alloying to the Pt oxidation kinetics at the cathode are comprehensively discussed. 2. Experimental Section A commercial MEA with a surface area of 3×3 cm2 (Eiwa Co., Ltd.) was used. A polymer electrolyte membrane containing Nafion NR-212 (Sigma-Aldrich; 51 µm thickness) was coated with Pt/C (Pt 46.1 wt%, TEC10E50E, Tanaka Kikinzoku Kogyo (TKK) K.K.) or Pt3Co/C (Pt 46.7 wt%, Co 5.4 wt% (Pt/Co = 2.6), TEC36E52, TKK K.K.) cathode catalysts, and Pd/C anode catalyst (Pd 46.1 wt%, TECPd(ONLY)E50, TKK K.K.) at practical catalyst loadings of 0.5 mg cm-2-Pt, Pt Co, Pd. The prepared MEA sandwiched between gas diffusion layers (TGP-H-030, Toray 3

Ind., Inc.) was inserted in a PEFC cell. The PEFC cell was designed to have a window and a channel in the ±80° directions for in situ XAFS measurements.36 H2 (99.99999%, 150 mL min-1 at the anode), and N2 or O2/N2 mixed gas (99.99999%, 1000 mL min-1 at the cathode) were supplied by mass-flow controllers and bubbled through humidifiers set to a relative humidity of


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93% with a commercial gas supply kit (CNF52742, NF Co., Ltd., TCU-004, ACE Inc.). The fraction of O2/N2 in the mixed gas was determined by the electrochemical current generation at 0.4 V, which resulted in a maximum efficiency of oxygen use of 10% under the reaction conditions. The temperature of the PEFC cell was kept at 353 K using a pair of cartridge heaters. The cell voltage between the anode and cathode was controlled by a potentiostat/galvanostat (herein after, P/G stat; VSP, BioLogic Science Instruments Co., Ltd.) combined with a current amplifier (VMP 3B-20, BioLogic Science Instruments Co., Ltd.). An as-prepared MEA was conditioned by the 150 cycles of aging in 23 fixed current steps (0, 0.01, 0.04, 0.12, 0.16, 0.22, 0.27, 0.33, 0.48, 0.66, 1.31, 1.95, 2.60, 3.25, 3.82, 4.56, 5.20, 5.85, 6.50, 7.15, 7.80, 8.44, and 9.20 A for 6 s per step). Electrochemical surface areas (ECSAs) estimated from the charges of H2 absorption and desorption in the regions of 0.1 to 0.35 V of cyclic voltammograms (CVs) were 60.6 m2 g-1-Pt for Pt/C and 34.8 m2 g-1-Pt3Co for Pt3Co/C, respectively (Figure S1). The average particle sizes of the cathode catalysts were estimated to be 2.4 nm for Pt/C and 5.8 nm for Pt3Co/C, respectively (Figure S2). A rapid gas exchange at the cathode was synchronized with transient voltage operation. A set of solenoid gas valves attached to humidifiers for N2 and 10% O2/N2 mixed gas was installed on the small volume cathode gas inlet and was controlled by the gate signal from the P/G stat (Figures 1(a) and S3). Delay time (Δt), which was time lag of gas diffusion from the valves to the cathode in the XAFS cell, was estimated from the time courses of electrochemical current at 0.4 V after the cathode gas exchange (Figure S4). Δt was calculated by the inflection point of the electrochemical current over 5 of standard deviation and was found to be 20 – 40 ms (Figure S4 and Table S1).


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Operando Pt LIII-edge time-resolved QXAFS measurements were conducted at the BL36XU undulator beamline at SPring-8, Japan (8 GeV, 100 mA). X-rays emitted from an in-vacuo tapered undulator were monochromatized by a Si (111) channel-cut compact monochromator. Higher harmonics were rejected by two vertical mirrors placed between the monochromator and I0 ion chamber. Incident (I0) and transmitted (I1 and I2) X-rays were detected by ion chambers filled with N2 and N2/Ar (85/15), respectively. For the present QXAFS setup, the compact monochromator was oscillated with sine wave function, and Pt LIII-edge QXAFS spectra were recorded by both low-to-high and high-to-low energy scans (11.28‒12.78 keV, 0.3 eV step; 5,000 pts). The PEFC cell with a conditioned MEA was placed between the I0 and I1 ion chambers rotated 80° to the optical path. The Pt LIII-edge jump of the measured MEA was 0.4, achieving the QXAFS analysis every 20 ms. Pt foil was placed between the I1 and I2 ion chambers for the Pt LIII-edge energy calibration. For operando Pt LIII-edge time-resolved QXAFS measurements, within 10 s for voltage operation from 0.4 to 1.0 V, a gate signal was sent from the P/G stat to start continuous QXAFS measurements recorded every 20 ms for 60 s (Figure 1(a)). The decrease in the ECSA value was smaller than 15% for both Pt/C and Pt3Co/C catalysts during the series of QXAFS measurements, suggesting that the degradation of MEA due to repeated X-ray irradiation was negligible. The operando time-resolved X-ray absorption near edge structure (XANES) and extended Xray absorption fine structure (EXAFS) spectra were simultaneously analyzed by using the previously prepared custom program in Igor Pro 6.3.34 The continuous QXAFS data was separated, replotted, and calibrated by the energy of Pt foil spectrum. Then each obtained QXAFS spectrum was processed on a PC parallelized in the thread number of CPU within 30


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min. A series of time-resolved XANES spectra were fitted by the following equation (Eq. 1) to estimate the edge energies and white-line heights. ߤ‫ݐ‬ሺEሻ =

௔భ ଶ

ாି௔మ

ቂ + ܽ‫ ݊ܽݐܿݎ‬ቀ

గ గ

௔య

ቁቃ +

௕భ ಶష್૛ మ ቀଵା ቁ ್య

(Eq. 1)

For the analysis of Pt/C, k3-weighted Pt LIII-edge EXAFS oscillations (k = 30 – 130 nm-1) were Fourier transformed into R-space, and curve fittings were performed in the R-space of 0.13 – 0.33 nm, while for Pt3Co/C, k1-weighted EXAFS oscillations (k = 33 – 133 nm-1) were Fourier transformed into R-space, and curve fittings were performed in the R-space of 0.13 – 0.33 nm. The fitting parameters for the EXAFS Fourier transforms (FTs) were set at the crystalline parameters of Pt, PtO2, and PtCo.37-39 20-merged EXAFS FTs were analyzed to determine some fitting parameters (Tables S2 and S3, Figures S5, S6, and S7). Then the 3000 of time-resolved QXAFS spectra (20 ms time resolution) were processed by curve fitting using the determined fitting parameters. The time (t) profiles of Pt-Pt, Pt-O, and Pt-Co coordination numbers (CNs) estimated by the EXAFS curve fittings were plotted, and their rate constants were estimated by using an exponential function (Eq. 2), CN(t) = α0 + α(1 – exp(–kt))

(Eq.2)

where k (kPt-Pt and kPt-O) represents the rate constants of Pt-Pt bond breaking (decrease in the CN of Pt-Pt) and Pt-O bond formation (increase in the CN of Pt-O). α0, which was the CN of Pt-Pt or Pt-O at 0.4 V in N2, was determined as averaged CN over the period of -10 to 0 s before the cell voltage was changed. The time profiles of Pt LIII-edge XANES white-line height and the CNs included the gradual slope leading up to the fast reaction process after the cell voltage was


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changed, and the rate constant k and coefficient α for the initial period of 0 to 5 s were estimated by the fitting analysis. Total changes in Pt LIII-edge white-line height and the CNs of Pt-Pt and Pt-O bonds (ΔPt-Pt, ΔPt-O) were defined as differences between averaged CNs for 40 – 50 s (final state) and α0 (initial state; average CNs for -10 – 0 s) in each experiment.

3. Results and Discussion During PEFC operation, cathode catalyst repeats reduction and oxidation processes by cell voltage operation, which corresponds to the power on/off processes of PEFC. The repetition of the cell voltage operation causes serious dissolution and degradation of cathode catalysts. However, the kinetics and rate constants of the oxidation of Pt cathode catalysts were investigated under N2 atmosphere in the absence of O2

31-35

and those with O2 are not reported

yet. We prepared reduced Pt cathode catalyst surfaces (Pt/C and Pt3Co/C) at 0.4 V under humidified N2 without O2, and then the cathode catalysts were oxidized by transient cell-voltage operation from 0.4 V to 0.7, 0.8, 0.9, or 1.0 V synchronized with the rapid exchange of the cathode gas from N2 to the mixture of N2 and O2 (10%) (Figure 1(a)). The synchronization of the rapid cathode gas exchange from N2 to the mixture of O2 enables to investigate the dynamic oxidation behaviors and the oxidation kinetics of the Pt catalysts at the cathode by operando time-resolved XAFS analysis. We recorded operando Pt LIII-edge time-resolved QXAFS spectra every 20 ms after the synchronized operation, and plotted changes in the structural parameters (Pt valence estimated by Pt LIII-edge XANES, Pt-O, Pt-Pt, and Pt-Co CNs estimated by Pt LIIIedge EXAFS), which are related to the Pt oxidation at the cathode.


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Series of operando time-resolved XANES spectra and EXAFS FTs for the Pt/C and Pt3Co/C cathode catalysts are presented in Figures 1(b) and S8. After the operation in the cell voltage at 0 s, the Pt LIII-edge white-line height of Pt/C increased immediately in the series of XANES spectra (Figure 1(b1)), showing the oxidation of Pt surface. By the curve-fitting analysis of the EXAFS FTs, the CNs of Pt-O and Pt-Pt were obtained, suggesting the simultaneous Pt-O bond formation and Pt-Pt bond breaking by the oxidation of Pt (Figure S8(b)). Similar but fewer changes were observed in the Pt LIII-edge XANES spectra and EXAFS FTs of Pt3Co/C (Figures 1(b2) and S8(a)). The fitting analysis of the 3000 of XANES spectra and EXAFS FTs showed the time dependency of the structural parameters on the Pt/C and Pt3Co/C catalysts. Figures 2 and S9 summarize the Pt LIII-edge white-line height, CNPt-Pt, CNPt-O, and CNPt-Co versus t. These parameters are related to the valence state of Pt, the average particle sizes of the Pt catalysts, the surface oxidation on the Pt catalysts, and the fraction of Pt-Co alloying in Pt3Co/C, respectively. In the absence of O2 (N2 atmosphere), the oxidation of Pt proceeds with H2O in the cell. On the other hand, the oxidation of Pt proceeds with the competition of O2 and H2O under the 10% O2/N2 conditions. In the cases without the rapid exchange of the cathode gas (N2 atmosphere), Pt valence and local coordination remained constant at 0.4 V for both Pt/C and Pt3Co/C (Figures 2(a, c) and S9(a, c)). CNPt-O of Pt/C and Pt3Co/C were negligible (Tables S2 and S3), suggesting that the surfaces of the Pt catalysts were not oxidized at 0.4 V. CNPt-Pt was observed to be 9.3 ± 1.4 for Pt/C, and CNPt-Pt and CNPt-Co were 7.9 ± 0.6 and 1.7 ± 0.2 for Pt3Co/C, respectively.


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After the transient cell voltage operation, the white-line height and CNPt-O immediately increased, while decrease in CNPt-Pt was clearly observed as shown in Figures 2(a, c) and S9(a, c). The variation ranges of the XAFS structural parameters increased accompanied with the increases in the final cell voltage with a maximum value at 1.0 V. The saturated values (average of CNs from 40 to 50 s) of CNPt-O and CNPt-Pt for Pt/C were 0.17 and 9.23 at 0.7 V, 0.25 and 9.24 at 0.8 V, 0.60 and 8.59 at 0.9 V, 0.91 and 7.68 at 1.0 V, respectively. Considering the average particle sizes of Pt/C (2.4 nm; Figure S2) and the changes in CNPt-O and CNPt-Pt, the surface of Pt was fully oxidized with H2O at 0.8 V, and the decrease in CNPt-Pt suggested that the oxidation of the subsurface of the Pt catalyst proceeded accompanied with surface Pt-Pt bond breaking at 1.0 V. In the case of Pt3Co/C, change in CNPt-Pt was negligible under 0.8 V and the subsurface oxidation was observed at higher voltage than 0.8 V (Figure 2(c)). These results agreed with the in situ XAFS analysis of Pt/C reported before.40 For the reactions in the presence of O2, we similarly analyzed the QXAFS spectra after the exchange of the cathode gas (Figures 2(b, d) and S9(b, d)). The exposure to 10% O2 in N2 at 0.4 V caused slight increases in CNPt-O for Pt/C (0.2 ± 0.3) and Pt3Co/C (0.2 ± 0.1) (Tables S2 and S3). At 0.4 V in the presence of O2, ORR steadily proceeds on the Pt catalysts, and the observed CNPt-O suggested the Pt catalysts slightly oxidized under the steady state conditions with ORR. In contrast, changes in CNPt-O and CNPt-Pt were clearly observed at 1.0 V, showing the oxidation of the Pt catalyst surfaces at 1.0 V. There were negligible changes in CNPt-Co, suggesting that Co localized in the Pt/Co core did not oxidize under the identical conditions (Figure 2(d)). The Co K-edge XANES and EXAFS spectra of Pt3Co/C 33, 34 showed negligible responses to the applied cell voltage between 0.4 V and 1.0 V, and the kinetic parameters of the Co side cannot be estimated.


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Then, we fitted the time profiles of the structural parameters in Figures 2 and S9 by using Eq. 2 and estimated the rate constants of the white-line height, CNPt-O and CNPt-Pt variations (kwlhgt, kPtO

and kPt-Pt, respectively). Figure 3 shows the plots of Δwlhgt, ΔPt-O, and ΔPt-Pt (variations of the

white-line height, CNPt-O and CNPt-Pt) and their rate constants (kwlhgt, kPt-O, and kPt-Pt) versus final cell voltage. As shown in Figure 3, the slopes of Δwlhgt, ΔPt-O, and ΔPt-Pt gradually changed for both Pt/C and Pt3Co/C at around 0.8 V, and the oxidation of Pt significantly proceeded when cell voltage was higher than 0.8 V. Changes in Δwlhgt and ΔPt-O were similar to each other (Figure 3 (a, c)). Simultaneous decrease in Δ Pt-Pt indicated that adsorbed oxygen penetrated into the subsurface layers of the Pt catalysts at the cell voltage higher than 0.8 V.42, 43 Large differences in ΔPt-O and ΔPt-Pt between the Pt/C and Pt3Co/C catalysts clearly showed the effects of Co to suppress Pt oxidation (Figure 3(a, c)). It is to be noted that the operando time-resolved QXAFS analysis revealed the rate constants of the Pt oxidation, which show the oxidation kinetics of the Pt/C and Pt3Co/C catalysts with O2. Interestingly, we found clear differences in the rate constants of the local structure changes of the Pt catalysts (kPt-O and kPt-Pt) between the Pt/C and Pt3Co/C catalysts as shown in Figure 3(d, f): kPt-O and kPt-Pt of Pt/C were much larger than those of Pt3Co/C. These results suggested that the alloyed Co species not only inhibited the oxidation of Pt but also kinetically controlled the reaction rates of the Pt oxidation in the Pt3Co/C cathode catalyst. The rate of the Pt-O bond formation (kPt-O) accelerated when the cell voltage was applied above 0.8 V, while that of the Pt-Pt bond dissociation (kPt-Pt) required a higher potential of 0.9 V (Figure 3 (d, f)). Judging from the fact that the absolute values of kPt-O were larger than those of kPt-Pt, oxygen penetration to the subsurface of the Pt catalysts sequentially proceeded after the


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oxidation of the outer surfaces of the Pt catalysts forming surface Pt-O bonds. In the case of Pt3Co/C, the oxygen penetration was kinetically suppressed compared to Pt/C. We also found different dependency to the cathode atmosphere (in the presence/absence of O2) between the estimated rate constants (kwlhgt, kPt-O, and kPt-Pt). The rate constants of the changes in the white-line height (kwlhgt) were varied in the presence/absence of O2 (Figure 3 (b)), suggesting that the rates of Pt charging with H2O and O2 were different. In the presence of O2, several adsorbed oxygen species such as O, OH, and OOH are formed on various surface sites with different Pt-O bond lengths, and the increase in kwlhgt would be observed in the presence of O2. In contrast, the rate constants of the coordination numbers (kPt-O and kPt-Pt) were similar to each other in the presence/absence of O2 (Figure 3 (d, f)): the rates of the Pt-O bond formation and the Pt-Pt bond breaking were mainly determined by the cell voltages not by the cathode atmosphere with/without O2. The present study evidenced that the reported Pt oxidation kinetics investigated under N2 atmosphere without O2

31-34

were valid for the oxidation kinetics of local coordination

change under PEFC operating conditions with O2.

4. Conclusion Operando Pt LIII-edge time-resolved QXAFS of Pt/C and Pt3Co/C cathode catalysts in PEFCs were successfully measured every 20 ms for the transient cell voltage operation synchronized with the rapid exchange of cathode gas to O2. The systematic analysis of the series of the timeresolved QXAFS spectra showed the rate constants of the oxidation of the Pt cathode catalysts in the absence/presence of O2. The facts that the rate constants of the Pt oxidation widely decreased by the addition of Co clearly suggested the kinetical control of the Pt oxidation rates by the


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alloying with Co to Pt. In addition, the rate constants of Pt charging depended on the cathode atmosphere, while it was found that the rate constants of the local coordination changes by the Pt oxidation were similar in the presence/absence of O2.

Supporting Information. Cyclic voltammograms, TEM images, experimental setup, series of time-resolved XANES spectra and EXAFS FTs, curve-fitting analysis of the EXAFS FTs, and the time profiles of the obtained XAFS structural parameters. This material is available free of charge via the website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H. Matsui). *E-mail: [email protected] (M. Tada). Funding Sources This work was supported by the New Energy and Industrial Technology Development Organization of the Ministry of Economy, Trade and Industry, Japan. Notes The authors declare no competing financial interest. ACKNOWLEDGEMEMT


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This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of the Ministry of Economy, Trade and Industry, Japan. XAFS measurements were performed at SPring-8 (Nos. 2014B7820, 2015A7820, 2015B7820, 2016A7820, and 2016B7822). STEM-EDS was measured at High Voltage Electron Microscope Laboratory, Institute of Materials and Systems for Sustainability, Nagoya University, supported by “Advanced Characterization Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology, Japan. REFERENCES (1)

Stephens, I. E. L.; Rossmeisl, J.; Chorkendorff, I. Toward Sustainable Fuel Cells. Science 2016, 354, 1378–1379.

(2)

Kongkanand, A.; Mathias, M. F. The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells. J. Phys. Chem. Lett. 2016, 7, 1127–1137.

(3)

Schulze, M.; Wagner, N.; Kaz, T.; Friedrich, K. A. Combined Electrochemical and Surface Analysis Investigation of Degradation Processes in Polymer Electrolyte Membrane Fuel Cells. Electrochim. Acta 2007, 52, 2328–2336.

(4)

Ous, T.; Arcoumanis, C. Degradation Aspects of Water Formation and Transport in Proton Exchange Membrane Fuel Cell: A Review. J. Power Sources 2013, 240, 558–582.

(5)

Gilbert, J. A.; Kariuki, N. N.; Subbaraman, R.; Kropf, A. J.; Smith, M. C.; Holby, E. F.; Morgan, D.; Myers, D. J. In Situ Anomalous Small-Angle X-Ray Scattering Studies of Platinum Nanoparticle Fuel Cell Electrocatalyst Degradation. J. Am. Chem. Soc. 2012, 134, 14823–14833.


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Page 15 of 28 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


(6)

Reina, T. R.; Moreno, A. A.; Ivanova, S.; Odriozola, J. A.; Centeno, M. A. Influence of Vanadium or Cobalt Oxides on the CO Oxidation Behavior of Au/MOx/CeO2–Al2O3 Systems. ChemCatChem 2012, 4, 512–520.

(7)

Manzoli, M.; Vindigni, F.; Tabakova, T.; Lamberti, C.; Dimitrov, D.; Ivanov, K.; Agostini, G. J. Mater. Chem. A 2017, 5, 2083-2094.

(8)

Koh, S.; Toney, M. F.; Strasser, P. Activity-Stability Relationships of Ordered and Disordered Alloy Phases of Pt3Co Electrocatalysts for the Oxygen Reduction Reaction (ORR). Electrochim. Acta 2007, 52, 2765–2774.

(9)

Dubau, L.; Maillard, F.; Chatenet, M.; Guetaz, L.; André, J.; Rossinot, E. Durability of Pt3Co/C Cathodes in a 16 Cell PEMFC Stack: Macro/Microstructural Changes and Degradation Mechanisms. J. Electrochem. Soc. 2010, 157, B1887–1895.

(10)

Chen, S.; Sheng, W.; Yabuuchi, N.; Ferreira, P. J.; Allard, L. F.; Shao-Horn, Y. Origin of Oxygen Reduction Reaction Activity on “Pt3Co” Nanoparticles: Atomically Resolved Chemical Compositions and Structures. J. Phys. Chem. C 2009, 113, 1109–1125

(11)

Paulus, U. A; Wokaun, A.; Scherer, G. G.; Schmidt, T. J.; Stamenkovic, V.; Radmilovic, V.; Markovic, N. M.; Ross, P. N. Oxygen Reduction on Carbon-Supported Pt-Ni and PtCo Alloy Catalysts. J. Phys. Chem. B 2002, 106, 4181–4191.

(12)

Colón-Mercado, H. R.; Popov, B. N. Stability of Platinum Based Alloy Cathode Catalysts in PEM Fuel Cells. J. Power Sources 2006, 155, 253–263.

(13)

Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Trends in Electrocatalysis on Extended and Nanoscale PtBimetallic Alloy Surfaces. Nat. Mater. 2007, 6, 241–247.


15


(14)

Page 16 of 28

Dai, S.; You, Y.; Zhang, S.; Cai, W.; Xu, M.; Xie, L.; Wu, R.; Graham, G. W.; Pan, X. In Situ Atomic-Scale Observation of Oxygen-Driven Core-Shell Formation in Pt3Co Nanoparticles. Nat. Commun. 2017, 8, 1–8.

(15)

Anderson, A. B.; Roques, J.; Mukerjee, S.; Murthi, V. S.; Markovic, N. M.; Stamenkovic, V. Activation Energies for Oxygen Reduction on Platinum Alloys: Theory and Experiment. J. Phys. Chem. B 2005, 109, 1198–1203.

(16)

Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Nørskov, J. K. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chem., Int. Ed. 2006, 45, 2897–2901.

(17)

Zhao, X.; Hayashi, A.; Noda, Z.; Kimijima, K.; Yagi, I.; Sasaki, K. Evaluation of Change in Nanostructure through the Heat Treatment of Carbon Materials and Their Durability for the Start/Stop Operation of Polymer Electrolyte Fuel Cells. Electrochim. Acta 2013, 97, 33–41.

(18)

Takao, S.; Sekizawa, O.; Samjeské, G.; Nagamatsu, S.; Kaneko, T.; Yamamoto, T.; Higashi, K.; Nagasawa, K.; Uruga, T.; Iwasawa, Y. Same-View Nano-XAFS/STEM-EDS Imagings of Pt Chemical Species in Pt/C Cathode Catalyst Layers of a Polymer Electrolyte Fuel Cell. J. Phys. Chem. Lett. 2015, 6, 2121–2126.

(19)

Wang, D.; Xin, H. L.; Hovden, R.; Wang, H.; Yu, Y.; Muller, D. A.; DiSalvo, F. J.; Abruña, H. D. Structurally Ordered Intermetallic Platinum−Cobalt Core−Shell Nanoparticles with Enhanced Activity and Stability as Oxygen Reduction Electrocatalysts. Nat. Mater. 2013, 12, 81–87.


16

Page 17 of 28 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


(20)

Hanawa, H.; Kunimatsu, K.; Watanabe, M.; Uchida, H. In Situ ATR-FTIR Analysis of the Structure of Nafion−Pt/C and Nafion−Pt3Co/C Interfaces in Fuel Cell. J. Phys. Chem. C 2012, 116, 21401−21406.

(21)

Takagi, Y.; Nakamura, T.; Yu, L.; Chaveanghong, S.; Sekizawa, O.; Sakata, T.; Uruga, T.; Tada, M.; Iwasawa, Y.; Yokoyama, T. X-Ray Photoelectron Spectroscopy under Real Ambient Pressure Conditions. Appl. Phys. Express 2017, 10, 076603.

(22)

Cui, Y.; Harada, Y.; Ikenaga, E.; Li, R.; Nakamura, N.; Hatanaka, T.; Ando, M.; Yoshida, T.; Li, G.-L.; Oshima, M. In Situ Hard X-Ray Photoelectron Study of O2 and H2O Adsorption on Pt Nanoparticles. J. Phys. Chem. C 2016, 120, 10936–10940.

(23)

Kaneko, T.; Samjeské, G.; Nagamatsu, S.; Higashi, K.; Sekizawa, O.; Takao, S.; Yamamoto, T.; Zhao, X.; Sakata, T.; Uruga, T.; et al. Key Structural Kinetics for Carbon Effects on the Performance and Durability of Pt/Carbon Cathode Catalysts in Polymer Electrolyte Fuel Cells Characterized by In Situ Time-Resolved X-Ray Absorption Fine Structure. J. Phys. Chem. C 2016, 120, 24250–24264.

(24)

Wanjala, B. N.; Fang, B.; Luo, J.; Chen, Y.; Yin, J.; Engelhard, M. H.; Loukrakpam, R.; Zhong, C. Correlation between Atomic Coordination Structure and Enhanced Electrocatalytic Activity for Trimetallic Alloy Catalysts. J. Am. Chem. Soc. 2011, 133, 12714–12727.

(25)

Stamenković, V.; Schmidt, T. J.; Ross, P. N.; Marković, N. M. Surface Composition Effects in Electrocatalysis: Kinetics of Oxygen Reduction on Well-Defined Pt3Ni and Pt3Co Alloy Surfaces. J. Phys. Chem. B 2002, 106, 11970–11979.


17


(26)

Page 18 of 28

Hwang, S. J.; Kim, S.; Lee, J.; Lee, S.; Jang, J.; Kim, P.; Lim, T; Sung, Y.; Yoo, S. J. The Role of Electronic Perturbation in Stability and Activity of Pt-Based Alloy Nanocatalysts for Oxygen Reduction. J. Am. Chem. Soc. 2012, 134, 19508–19511.

(27)

Bordiga, E.; Groppo, G, Agostini.; J, A, van Bokhoven.; C, Lamberti. Reactivity of surface species in heterogeneous catalysts probed by in situ x-ray absorption techniques. Chem. Rev. 2013, 113, 1736–1850.

(28)

Nachtegaal, M.; Muller, O.; Konig, C.; frahm, R. X-Ray Absorption and X-ray Emission Spectroscopy: Theory and Applications, van Bokhoven, J. A.; Lamberti, C., Eds.; John Wiley & Sons, Chichester (UK), 2016; Vol. 2, Chapter 13, p. 155-183.

(29)

Uruga, T. In XAFS Techniques for Catalysts, Nanomaterials, and Surfaces; Iwasawa, Y., Asakura, K., Tada, M., Eds.; Springer: Berlin, 2017; p. 93-108.

(30)

Tada, M.; Murata, S.; Asakoka, T.; Hiroshima, K.; Okumura, K.; Tanida, H.; Uruga, T.; Nakanishi, H.; Matsumoto, S.; Inada, Y.; et al. In Situ Time-Resolved Dynamic Surface Events on the Pt/C Cathode in a Fuel Cell under Operando Conditions. Angew. Chem., Int. Ed. 2007, 46, 4310–4315.

(31)

Ishiguro, N.; Saida, T.; Uruga, T.; Nagamatsu, S.; Sekizawa, O.; Nitta, K.; Yamamoto, T.; Ohkoshi, S.; Iwasawa, Y.; Yokoyama, T.; et al. Operando Time-Resolved X-Ray Absorption Fine Structure Study for Surface Events on a Pt3Co/C Cathode Catalyst in a Polymer Electrolyte Fuel Cell during Voltage-Operating Processes. ACS Catal. 2012, 2, 1319–1330.

(32)

Ishiguro, N.; Saida, T.; Uruga, T.; Sekizawa, O.; Nagasawa, K.; Nitta, K.; Yamamoto, T.; Ohkoshi, S.; Yokoyama, T.; Tada, M. Structural Kinetics of a Pt/C Cathode Catalyst with


18

Page 19 of 28 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


Practical Catalyst Loading in an MEA for PEFC Operating Conditions Studied by in Situ Time-Resolved XAFS. Phys. Chem. Chem. Phys. 2013, 15, 18827–18834. (33)

Ishiguro, N.; Kityakarn, S.; Sekizawa, O.; Uruga, T.; Sasabe, T.; Nagasawa, K.; Yokoyama, T.; Tada, M. Rate Enhancements in Structural Transformations of Pt−Co and Pt− Ni Bimetallic Cathode Catalysts in Polymer Electrolyte Fuel Cells Studied by In Situ Time-Resolved X‑ray Absorption Fine Structure. J. Phys. Chem. C 2014, 118, 15874– 15883.

(34)

Ishiguro, N.; Kityakarn, S.; Sekizawa, O.; Uruga, T.; Matsui, H.; Taguchi, M.; Nagasawa, K.; Yokoyama, T.; Tada, M. Kinetics and Mechanism of Redox Processes of Pt/C and Pt3Co/C Cathode Electrocatalysts in a Polymer Electrolyte Fuel Cell during an Accelerated Durability Test. J. Phys. Chem. C 2016, 120, 19642–19651.

(35)

Kityakarn, S.; Saida, T.; Sode, A.; Ishiguro, N.; Sekizawa, O.; Uruga, T.; Nagasawa, K.; Yamamoto, T.; Yokoyama, T.; Tada, M. In Situ Time-Resolved XAFS of Transitional States of Pt/C Cathode Electrocatalyst in an MEA during PEFC Loading with Transient Voltages. Topics Catal. 2014, 57, 903–910.

(36)

Matsui, H.; Ishiguro, N.; Uruga, T.; Sekizawa, O.; Higashi, K.; Maejima, N.; Tada, M. Operando 3D Visualization of Migration and Degradation of a Platinum Cathode Catalyst in a Polymer Electrolyte Fuel Cell. Angew. Chem., Int. Ed. 2017, 53, 9371–9375.

(37)

Davey, W. P. Precision Measurements of the Lattice Constants of Twelve Common Metals. Phys. Rev. 1925, 25, 753–761.

(38)

Muller, O.; Roy, R. Formation and Stability of the Platinum and Rhodium Oxides at High Oxygen Pressures and the Structures of Pt3O4, β-PtO2 and RhO2. J. Less Common Met. 1968, 16, 129–146.


19


(39)

Page 20 of 28

Buschow, K. H. J.; van Engen, P. G.; Jongebreur, R. Magneto-Optical Properties of Metallic Ferromagnetic Materials. J. Magn. Magn. Mater. 1983, 38, 1–22.

(40)

Nagamatsu, S.; Arai, T.; Yamamoto, M.; Ohkura, T.; Oyanagi, H.; Ishizaka, T.; Kawanami, H.; Uruga, T.; Tada, M.; Iwasawa, Y. Potential-Dependent Restructuring and Hysteresis in the Structural and Electronic Transformations of Pt/C, Au(Core)-Pt(Shell)/C, and Pd(Core)-Pt(Shell)/C Cathode Catalysts in Polymer Electrolyte Fuel Cells Characterized by In Situ X-Ray Absorption Fine Structure. J. Phys. Chem. C 2013, 117, 13094–13107.

(41)

Durst, J.; Lopez-Haro, M.; Dubau, L.; Chatenet, M.; Soldo-Olivier, Y.; Guétaz, L.; BayleGuillemaud, P.; Maillard, F. Reversibility of Pt-Skin and Pt-Skeleton Nanostructures in Acidic Media. J. Phys. Chem. Lett. 2014, 5 , 434–439.

(42)

Angerstein-Kozlowska, H.; Conway, B. E.; Sharp, W. B. A. The Real Condition of Electrochemically Oxidized Platinum Surfaces. Part I. Resolution of Component Processes. J. Electroanal. Chem. 1973, 43, 9–36.

(43)

Gómez-Marín, A. M.; Feliu, J. M. Pt(111) Surface Disorder Kinetics in Perchloric Acid Solutions and the Influence of Specific Anion Adsorption. Electrochim. Acta 2012, 82, 558–569.


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Figure 1. (a) Schematic diagram of operando time-resolved XAFS measurements. (b1) Series of operando Pt LIII-edge time-resolved XANES spectra for Pt/C (0.4 V in N2 → 1.0 V in 10% O2/N2). (b2) Series of operando Pt LIII-edge time-resolved k1-weighted EXAFS Fourier transforms (k = 33 ‒ 133 nm-1) for Pt3Co/C (0.4 V in N2 → 1.0 V in 10% O2/N2).


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Figure 2. Time profiles of (1) Pt LIII-edge XANES white-line height, (2) CN of Pt-O bond, and (3) CNs of Pt-M (M: Pt, Co) bonds for transient voltage operation (0.4 V constant or 0.4 V → 0.8/1.0 V) without/with the rapid exchange of the cathode gas (N2 constant or N2 → 10% O2/N2). (a) Pt/C in N2, (b) Pt/C in N2 → 10% O2/N2, (c) Pt3Co/C in N2, and (d) Pt3Co/C in N2 → 10% O2/N2. The error ranges of the EXAFS curve-fitting analysis were estimated by the definition of the IFEFFIT code. The standard deviations of the steady-state before t = 0 (always 0.4 V in N2) were estimated to be 0.4 ± 0.0 (the XANES white-line height), 9.2 ± 1.1 (CNPt-Pt), and 0.1 ± 0.2 (CNPt-O) for Pt/C, while for Pt3Co/C, 0.4 ± 0.0 (the XANES white-line height), 8.1 ± 1.1 (CNPt-Pt), 1.6 ± 0.5 (CNPt-Co), and 0.1 ± 0.2 (CNPt-O), respectively. For the analysis of typical EXAFS fitting (20-mergeged spectra), the error ranges were 9.3 ± 1.4 (CNPt-Pt) and 0.0 ± 0.3 (CNPt-O) for


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Pt/C, while for Pt3Co/C, 7.9 ± 0.6 (CNPt-Pt), 1.7 ± 0.2 (CNPt-Co), and 0.1 ± 0.1 (CNPt-O) (listed in Table S2, S3). Judging from these values, the error range for each parameter was estimated as the larger value in the comparison of two data. Red line represents data fitted using Eq. 2.


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Figure 3. (a, c, e) Differences in the XANES white-line height and CNs of Pt-O and Pt-Pt bonds (Δwlhgt, ΔPt-O, andΔPt-Pt) and (b, d, f) their rate constants (kwlhgt, kPt-O, and kPt-Pt) during the transient voltage operation with the exchange of the cathode gas. (■: Pt/C in N2, □: Pt/C in N2 → 10% O2/N2, ▲: Pt3Co/C in N2, △: Pt3Co/C in N2 → 10% O2/N2). The similar errors in Figs. 2 and S9 were existed in the plots.


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TOC graphic


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Figure1 223x172mm (150 x 150 DPI)


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Figure2 285x200mm (150 x 150 DPI)



Figure3 210x249mm (150 x 150 DPI)


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C and Pt&l

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