Operando Time-resolved XAFS Study for Surface Events on a Pt 3 Co/C. Cathode Catalyst in a PEFC during Voltage-Operating Processes

Transcription

1 Operando Time-resolved XAFS Study for Surface Events on a Pt 3 Co/C Cathode Catalyst in a PEFC during Voltage-Operating Processes Nozomu Ishiguro ǁ, Takahiro Saida ǁ, Tomoya Uruga, Shin-ichi Nagamatsu, Oki Sekizawa, Kiyofumi Nitta, Takashi Yamamoto, Shin-ichi Ohkoshi, Yasuhiro Iwasawa, Toshihiko Yokoyama ǁ#, and Mizuki Tada* ǁ# ǁ Institute for Molecular Science, 38 Nishigo-naka, Myodaiji, Okazaki, Aichi , Japan. Department of Chemistry, Graduate School of Science, Th e University of Tokyo, Hongo, Bunkyo-ku, Tokyo , Japan. Japan Synchrotron Radiation Research Institute, SPring-8, Koto, Sayo, Hyogo , Japan. Innovation Research Center for Fuel Cells, The University of Electro-Communications, 1-5-1, Chofugaoka, Chofu, Tokyo , Japan. Department of Mathmatical and Material Sciences, Faculty of Integrated Arts and Sciences, The University of Tokushima, 1-1, Minamijosanjima-cho, Tokushima , Japan. # The Graduate University for Advanced Studies (SOKENDAI), 38 Nishigo-naka, Myodaiji, Okazaki, Aichi , Japan. 1

2 Figure SI 1. (A) Schematics of an in-situ XAFS cell for PEFCs. (B) A photo of the cell at SPring-8 BL40XU beamline. 1. Experimental detail The in-situ XAFS cell has a 13-stacked structure based on the JARI (Japan Automobile Research Institute) standard cell (Figure SI 1). An outer holder was made by SUS 316 equipped with a rubber heater. A current corrector was made by Au-coated Cu, which was connected to a PG stat. An insulator of Teflon was used as a separator of anode and cathode. Windows for X-rays (21 mm 4 mm) were prepared in the outer holders, insulators, and current correctors of anode and cathode. An MEA was sandwiched between two carbon grooved channels for gas flowing (1 mm width), Teflon gaskets, and gas diffusion layers (GDLs). The GDL was made by glassy carbon fiber. Gases were flowed inside the channels and accessed to the MEA through the GDLs. A thermocouple was inserted to the side of the carbon flow channels, and a cell temperature was controlled with heaters connected to the outer holders. All XAFS measurements were performed at a window close to the center of the cell. 2

3 Table SI 1. Structural parameters estimated by the curve-fitting analysis of in situ Pt L III -edge EXAFS Fourier transforms at 0.4 and 1.0 V for the Pt/C and Pt 3 Co/C cathode catalysts and related Pt compounds (see Figure 3) Sample Shell CN R /nm E 0 /ev 2 /10-5 nm 2 (a) Pt foil a (R f = 0.09%) (b) as-received Pt 3 Co/C powder b (R f = 0.04%) (c) Pt/C at 0.4 V c (R f = 0.26%) (d) Pt/C at 1.0 V c (R f = 0.31%) (e) Pt 3 Co/C at 0.4 V d (R f = 0.33%) Pt-Pt 12.0 ± ± ± ± 0.1 Pt-O 1.4 ± ± ± 4 8 ± 4 Pt-Co 1.9 ± ± ± 2 2 ± 1 Pt-Pt 5.3 ± ± ± ± 0.8 Pt-O 0.1 ± Pt-Pt 8.7 ± ± ± ± 0.1 Pt-O 1.0 ± Pt-Pt 6.7 ± ± ± ± 0.2 Pt-O -0.0 ± Pt-Co 1.8 ± ± ± 3 6 Pt-Pt 6.3 ± ± ± 1 6 Pt-O 0.6 ± (f) Pt 3 Co/C at 1.0 V d (R f = 0.09%) Pt-Co 1.8 ± ± ± 1 6 Pt-Pt 5.5 ± ± ± 1 6 a k = nm -1, R = nm. b k = nm -1, R = nm. Measured at 10 K. c k = nm -1, R = nm. 2 of all scattering paths, R and E 0 of Pt-O were fixed at the fitted results of Pt/C at 1.0 V. d k = nm -1, R = nm. 2 of all scattering paths, R and E 0 of Pt-O were fixed at the fitted results of Pt 3 Co/C at 1.0 V. 3

4 Figure SI 2 (A) A schematic of in-situ conventional QXAFS measurements at SPring-8 BL01B1 and (B) a schematic of in-situ time-resolved QXAFS measurements at SPring-8 BL40XU. 4

5 Figure SI 3. XRD patterns of Pt/C and Pt 3 Co/C catalysts. (a) Pt/C, (b) as-received Pt 3 Co/C powder, (c) as-prepared Pt 3 Co/C MEA, (d) used Pt 3 Co/C MEA after in-situ time-resolved XAFS, (e) the calculated pattern of Pt 67, (f) that of Pt 3 Co 69, (g) that of PtCo 69, and (h) that of Co 70. 5

6 Figure SI 4. TEM images (1 and 2) and particle size distributions (3) of the cathode layer sample on as-prepared MEA of Pt 3 Co/C (A) and that on used MEA after in-situ time-resolved XAFS measurements (B). 6

7 Table SI 2. Summary of metal-metal bond lengths Sample Method Bond Bond length /nm Pt foil XRD a Pt-Pt Co foil XRD a Co-Co Pt 3 Co XRD a Pt-Pt, Pt-Co, Co-Co PtCo XRD a Pt-Pt, Pt-Co, Co-Co As-received Pt/C powder XRD a Pt-Pt Pt/C in an as-prepared MEA XRD a Pt-Pt As-received Pt 3 Co/C powder XRD a Pt-Pt, Pt-Co, Co-Co Co K-edge EXAFS b Co-Co Co K-edge EXAFS b Co-Pt Pt L III -EXAFS b Pt-Co Pt L III -EXAFS b Pt-Pt Pt 3 Co/C in an as-prepared MEA XRD a Pt-Pt, Pt-Co, Co-Co TEM c Pt-Pt, Pt-Co, Co-Co Pt 3 Co/C in an MEA at N 2, 0.4 V Pt L III -EXAFS b Pt-Co Pt L III -EXAFS b Pt-Pt Pt 3 Co/C in a used MEA after the XRD a Pt-Pt, Pt-Co, Co-Co in-situ time-resolved XAFS TEM c Pt-Pt, Pt-Co, Co-Co measurements a The results of XRD (Figure SI 3). b The results of Pt L III -edge and Co K-edge EXAFS (Table SI 1 and Table 2). c Averages of bond distances observed in the lattice images of TEM (Figure SI 4). 7

8 Figure SI 5. Series of in situ time-resolved Pt L III -edge QXAFS spectra for Pt 3 Co/C MEA. (A) Series of in-situ time-resolved Pt L III -edge XANES spectra for the voltage-cycling process of 1.0 V 0.4 V. (B) Series of in-situ time-resolved Pt L III -edge k 3 -weighted EXAFS Fourier transforms at k = nm -1 for the voltage-cycling process of 0.4 V 1.0 V. 8

9 Figure SI 6. In situ time-resolved XAFS spectra at 0.4 and 1.0 V for the Pt 3 Co/C MEA (recorded for 500 ms). (A) Pt L III -edge XANES spectra, (B) k 3 -weighted Pt L III -edge EXAFS oscillations, (C) and their Fourier transforms at k = nm -1 : solid line, experimental data; dashed line: fitted data. (a) t = 90 s (at 0.4 V), (b) 140 s (at 1.0 V), (c) t = 180 s (at 1.0 V), and (d) t = 230 s (at 0.4 V). 9

10 Table SI 3. Structural parameters estimated by curve-fitting analysis of in situ time-resolved Pt L III -edge EXAFS Fourier transforms at 0.4 and 1.0 V for the Pt 3 Co/C MEA (see Figure SI 6) Sample Shell CN R /nm E 0 /ev 2 /10-5 nm 2 (a) t = 90 s (0.4 V) (R f = 1.7%) (b) t = 130 s (1.0 V) (R f = 1.7%) (c) t = 180 s (1.0 V) (R f = 1.2%) Pt-O 0.0 ± Pt-Co 1.8 ± ± ± 4 6 Pt-Pt 6.3 ± ± ± 2 6 Pt-O 0.5 ± Pt-Co 2.2 ± ± ± 2 6 Pt-Pt 5.2 ± ± ± 2 6 Pt-O 0.4 ± Pt-Co 1.9 ± ± ± 4 6 Pt-Pt 6.0 ± ± ± 2 6 Pt-O 0.0 ± (d) t = 230 s (0.4 V) (R f = 1.1%) Pt-Co 1.8 ± ± ± 4 6 Pt-Pt 6.4 ± ± ± 2 6 k = nm -1, R = nm. 2 of all scattering paths, and R and E 0 of Pt-O were all fixed at values on Table SI 1. 10

11 Table SI 4. Kinetic parameters for the Pt 3 Co/C MEA in voltage-cycling processes estimated by operando time-resolved XAFS at Pt L III -edge Process Parameter y 0 A Rate constant (k, k ) /s -1 Pt 3 Co/C 0.4 V 1.0 V XANES white-line height a ± 0.02 Pt valence CN (Pt-Pt) a ± 0.03 CN (Pt-Co) a 1.98 no change CN (Pt-O) a ± 0.03 current in the fuel cell b ± A 9.90 ± 0.07 A 2.86 ± ± A 4.34 ± 0.05 A ± Pt 3 Co/C 1.0 V 0.4 V XANES white-line height a ± 0.05 Pt valence 0.29 CN (Pt-Pt) a ± 0.1 CN (Pt-Co) a 1.93 no change CN (Pt-O) a ± 0.2 current in the fuel cell b ± A ± 0.04 A 3.68 ± ± A ± 0.04 A ± a These parameters were fitted with the following functions: y = y 0 + Aexp(-kt) b The rate constants of the charge in the fuel cell were estimated by curve fitting of changes in currents in the fuel cell recorded on the PG stat with the following functions: y = y 0 + A 1 exp(-k 1 t) + A 2 exp(-k 2 t) (electric current in the fuel cell) 11

12 Figure SI 7. Series of in situ time-resolved Pt-L III XAFS spectra for the Pt/C MEA. (1) Series of in situ time-resolved Pt L III -edge XANES spectra, and (2) series of in situ time-resolved k 3 -weighted Pt L III -edge EXAFS Fourier transforms at k = nm -1, for the voltage-operating process of (A) 0.4 V 1.0 V and (B) 1.0 V 0.4 V. 12

13 Figure SI 8. In situ time-resolved Pt L III -edge XAFS spectra at 0.4 and 1.0 V for the Pt/C MEA (recorded for 500 ms). (A) Pt L III -edge XANES spectra, (B) k 3 -weighted Pt L III -edge EXAFS oscillations, and (C) their Fourier transforms at k = nm -1 : solid line, experimental data; dashed line: fitted data. (a) t = 90 s (at 0.4 V), (b) 140 s (at 1.0 V), (c) t = 180 s (at 1.0 V), and (d) t = 230 s (at 0.4 V). 13

14 Table SI 5. Structural parameters estimated by curve-fitting analysis of in situ time-resolved Pt L III -edge EXAFS Fourier transforms at 0.4 and 1.0 V for the Pt/C MEA (see Figure SI 8) Sample Shell CN R /nm E 0 /ev 2 /10-5 nm 2 (a) t = 90 s (0.4 V) (R f = 3.1%) (b) t = 130 s (1.0 V) (R f = 1.7%) (c) t = 180 s (1.0 V) (R f = 0.80%) Pt-O 0.1 ± Pt-Pt 9.4 ± ± ± Pt-O 0.3 ± Pt-Pt 8.2 ± ± ± Pt-O 0.7 ± Pt-Pt 8.0 ± ± ± (d) t = 230 s (0.4 V) Pt-O 0.0 ± (R f = 3.9%) Pt-Pt 9.1 ± ± ± k = nm -1, R = nm. 2 of all scattering paths, and R and E 0 of Pt-O were all fixed at values on Table SI 1. 14

15 Figure SI 9. Time profiles of (1) electric charge and Pt valence on an equivalent scale, (2) CN of Pt-Pt bond, (3) CN of Pt-O bond, and (4) R of Pt-Pt bond for the voltage-cycling processes on Pt/C. (A) 0.4 V 1.0 V and (B) 1.0 V 0.4 V. k = nm -1 ; R = nm. : electrical charge in the cell, : Pt valence, :Pt-Pt, and :Pt-O. Dashed lines on (A1) and (B1) correspond to be the valence state of Pt foil. The intervals of right and left axes in (1) were scaled to be identical (in coulombs). 15

16 Table SI 6. Kinetic parameters for the Pt/C MEA in voltage-cycling processes estimated by operando time-resolved XAFS at Pt L III -edge Process Parameter y 0 A Rate constant (k, k ) /s -1 Pt/C 0.4 V 1.0 V XANES white-line height a ± Pt valence a CN (Pt-Pt) a ± CN (Pt-O) a ± current in the fuel cell b ± A ± 0.03 A 1.84 ± ± A 5.26 ± 0.03 A ± Pt/C 1.0 V 0.4 V XANES white-line height a ± 0.03 Pt valence 0.26 CN (Pt-Pt) a ± CN (Pt-O) a ± 0.02 current in the fuel cell b ± A ± 0.03 A 2.16 ± ± A ± 0.02 A ± a These parameters were fitted with the following functions: y = y 0 + Aexp(-kt) b The rate constants of the charge in the fuel cell were estimated by curve fitting of changes in currents in the fuel cell recorded on the PG stat with the following functions: y = y 0 + A 1 exp(-k 1 t) + A 2 exp(-k 2 t) (electric current in the fuel cell) 16

17 Reference (1) Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, Z.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K.; Iwashita, N. Chem. Rev. 2007, 107,