Stabilization of Small Platinum Nanoparticles on PtâCeO2 Thin Film

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Stabilization of Small Platinum Nanoparticles on Pt-CeO Thin Film Electrocatalysts During Methanol Oxidation

Olaf Brummel, Fabian Waidhas, Firas Faisal, Roman Fiala, Mykhailo Vorokhta, Ivan Khalakhan, Martin Dubau, Alberto Figueroba, Gabor Kovacs, Hristiyan A. Aleksandrov, Georgi N. Vayssilov, Sergey M. Kozlov, Konstantin M. Neyman, Vladimír Matolín, and Jörg Libuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05962 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

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The Journal of Physical Chemistry

Stabilization of Small Platinum Nanoparticles on Pt-CeO2 Thin Film Electrocatalysts During Methanol Oxidation Olaf Brummela, Fabian Waidhasa, Firas Faisala, Roman Fialab, Mykhailo Vorokhtab, Ivan Khalakhanb, Martin Dubaub, Alberto Figuerobac, Gábor Kovácsc, Hristiyan A. Aleksandrovc,d, Georgi N. Vayssilovd, Sergey M. Kozlovc, Konstantin M. Neymanc,e, Vladimir Matolínb, Jörg Libudaa,f* a

Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3,

91058 Erlangen, Germany b

Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V

Holešovičkách 2, 18000 Prague, Czech Republic c

Departament de Ciència de Materials i Química Física and Institut de Química Teòrica i Computacional,

Universitat de Barcelona, C/Martí i Franquès 1, 08028 Barcelona, Spain d

Faculty of Chemistry and Pharmacy, University of Sofia, 1126 Sofia, Bulgaria

e

Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain

f

Erlangen Catalysis Resource Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3,

91058 Erlangen, Germany

Abstract Pt-doped CeOx thin films electrocatalysts have recently been shown to exhibit high activity and stability at the anode of proton exchange membrane fuel cells (PEM-FC). To identify the role of the Pt dopant and the origin of the high stability of Pt-CeOx films, we have applied electrochemical in-situ IR spectroscopy on Pt-CeOx model thin film catalysts during methanol (1 M methanol) oxidation. The model catalysts were prepared by magnetron co-sputtering of Pt (9 to 21 atomic %) and CeO2 onto clean and carbon-coated Au supports. All samples were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) before and after reaction. At pH 1 (0.1 M HClO4) the Pt-CeOx dissolves partially during potential cycling, whereas the films are largely stable at pH 6 (0.1 M phosphate buffer). Electrochemical IR spectroscopy of the adsorbed CO shows that metallic Pt is formed on all Pt-CeOx samples during methanol oxidation. In comparison to Pt(111), Pt aggregates on Pt-CeOx show a CO on-top signal, which is red shifted by at least 25 cm-1 and suppression of the bridging CO signals. Whereas the Pt particles on Pt-CeOx films with high Pt concentration (>20 atomic %) undergo rapid sintering during the potential cycling, small metallic Pt aggregates are stable under the same conditions

Corresponding author, phone: +49 9131 85 27308, FAX: ++49 (0)9131/85 28867, email: [email protected]

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on films with low Pt concentration ( 20 atomic %) we observe the formation of regular Pt crystallites exposing (111) facets and low-coordinated edge and corner sites. For low Pt concentration (< 15 atomic % Pt) we observe the formation of subnano Pt species, which do not expose regular (111) facet sites and show a CO frequencies redshift by > 25 cm-1. DFT calculations show that the on-top CO stretching frequency is closely correlated with the Pt coordination number and, to a lesser extent, with the particle size. Comparison with the experimental data suggests that the subnano Pt species contain around 30 or less Pt atoms and have nominal size below 1 nm. Formation of Pt NPs with regular (111) facet sites is predicted for aggregates containing around 80 and more atoms. At low Pt concentration ( 20 atomic %). This showes that the ceria support efficiently stabilizes even very small Pt particles under the conditions of dynamically changing electrode potentials.


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Acknowledgment The authors acknowledge financial support by the EU (FP7 NMP project ChipCAT Grant No. 310191), by the COST Action CM1104 and by the Deutsche Forschungsgemeinschaft (DFG) within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative. In addition we acknowledge support through cooperation with the Helmholtz-Institute Erlangen-Nürnberg (HI ERN), Forschungszentrum Jülich GmbH. The work has been also supported by the Horizon2020 program of the European Commission (project Materials Networking), by the Spanish MINECO (grants CTQ2012-34969 and CTQ2015-64618-R, co-funded by FEDER), by the Generalitat de Catalunya (grants 2014SGR97 and XRQTC) and by the project LH15277 of the Czech Ministry of Education. Computer resources, technical expertise and assistance were provided by the Red Española de Supercomputación.

Supporting Information Potential dependent in-situ IR spectra during methanol oxidation at pH 1 on different Pt electrocatalysts: (I) Pt(111), (III) Pt(8%)-CeOx/Au, (IV) Pt(15%)-CeOx/C/Au.



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73. Neitzel, A.; Johánek, V.; Lykhach, Y.; Skála, T.; Tsud, N.; Vorokhta, M.; Matolín, V.; Libuda, J., Reduction of Pt2+ Species in Model Pt‒CeO2 Fuel Cell Catalysts Upon Reaction with Methanol. Appl. Surf. Sci. 2016, 387, 674-681. 74. Eyrich, M.; Diemant, T.; Hartmann, H.; Bansmann, J.; Behm, R. J., Interaction of CO with Structurally Well-Defined Monolayer PtAu/Pt(111) Surface Alloys. J. Phys. Chem. C 2012, 116, 11154-11165. 75. Bazin, P.; Saur, O.; Lavalley, J. C.; Daturi, M.; Blanchard, G., FT-IR Study of CO Adsorption on Pt/CeO2: Characterisation and Structural Rearrangement of Small Pt Particles. Phys. Chem. Chem. Phys. 2005, 7, 187-194. 76. Lambert, D. K., Stark Effect of Adsorbate Vibrations. Solid State Commun. 1984, 51, 297-300. 77. Koper, M. T. M.; van Santen, R. A.; Wasileski, S. A.; Weaver, M. J., Field-Dependent Chemisorption of Carbon Monoxide and Nitric Oxide on Platinum-Group (111) Surfaces: Quantum Chemical Calculations Compared with Infrared Spectroscopy at Electrochemical and Vacuum-Based Interfaces. J. Chem. Phys. 2000, 113, 4392-4407. 78. Scheijen, F. J. E.; Beltramo, G. L.; Hoeppener, S.; Housmans, T. H. M.; Koper, M. T. M., The Electrooxidation of Small Organic Molecules on Platinum Nanoparticles Supported on Gold: Influence of Platinum Deposition Procedure. J. Solid State Electrochem. 2008, 12, 483-495. 79. Couto, A.; Rincón, A.; Pérez, M. C.; Gutiérrez, C., Adsorption and Electrooxidation of Carbon Monoxide on Polycrystalline Platinum at pH 0.3–13. Electrochim. Acta 2001, 46, 1285-1296. 80. Hayden, B. E.; Kretzschmar, K.; Bradshaw, A. M.; Greenler, R. G., An Infrared Study of the Adsorption of CO on a Stepped Platinum Surface. Surface Science 1985, 149, 394-406. 81. Hollins, P., The Influence of Surface Defects on the Infrared Spectra of Adsorbed Species. Surf. Sci. Rep. 1992, 16, 51-94. 82. Campbell, C. T.; Peden, C. H. F., Oxygen Vacancies and Catalysis on Ceria Surfaces. Science 2005, 309, 713-714.


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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


Sample number

Sample type

c(Pt) [atomic %]

d(CeOx) [nm]

d(C) [nm]

pH

I

Pt(111)

100

-

-

1

II

Pt(111)

100

-

-

6

III

PtCeOx/Au

8

40

-

1

IV

PtCeOx/C/Au

15

50

20

1

V

PtCeOx/C/Au

21

50

20

6

VI

PtCeOx/C/Au

15

50

20

6

VII

PtCeOx/C/Au

9

50

20

6

Table 1. Overview of the samples used in this study with the atomic Pt concentration c(Pt), the film thickness of CeOx d(CeOx), and the film thickness of the carbon layer d(C). The last column shows pH values in the methanol oxidation experiments



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

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Site (a) 1 2 3 4 5

Nsites (b) 2 2 1 2 1

Nc(Pt) (c) 4 3 6 5 2

Eads [eV] (d) -2.79 -2.43 -2.88 -2.56 -1.95

ν(C-O) [cm-1] (e) 2028 2032 2033 2040 2016

Pt34

1 2 3 4 5 6

8 4 4 4 4 4

6 9 6 6 5 7

Pt79

1 2 3 1 2 3 4

24 12 24 24 24 6 24

6 7 9 6 7 9 9

-2.21 -1.97 -2.23 -2.15 -2.14 -2.09 -2.20 -2.10 -1.99 -2.21 -1.97 -1.95 -1.75

2038 2054 2043 2041 2038 2047 2043 2047 2069 2042 2047 2062 2063

Pt140

1 2 3 4

24 24 24 24

6 7 9 9

-2.27 -1.96 -1.64 -1.69

2043 2052 2072 2067

Pt201

1 2 3 4 5 6

24 24 6 12 48 8

6 7 8 7 9 9

-2.05 -1.98 -1.74 -2.04 -1.70 -1.53

2039 2046 2056 2044 2068 2076

Pt8/CeO2(111)

1 2 3 4 5 6 7 8

1 1 1 1 1 1 1 1

4 4 3 5 2 5 6 3

-2.30 -2.29 -1.52 -2.12 -1.87 -2.07 -0.88 -1.17

2029 2022 2025 2024 2047 2023 2050 2019

Pt34/CeO2(111)

1 2 3 4 5

4 4 4 8 4

6 6 9 6 5

-2.44 -2.38 -1.76 -2.28 -1.75

2039 2036 2057 2028 2025

System Pt8

Pt116

Table 2. Overview of the CO vibrational frequencies and adsorption energies calculated by DFT method for all on-top sites of supported and unsupported Pt particles: (a) label of site according to Figure 6; (b) number of symmetry equivalent sites on the Pt particle Nsites; (c) coordination number with respect to other Pt atoms Nc(Pt); (d) CO binding energy relative to CO in the gas phase; (e) calculated CO stretching frequency values corrected with respect to the gas phase experimental value ν(C-O) = ω(C-O)+14 cm-1 (see text); solely these corrected


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frequency values ν(C-O) are discussed in this work. Calculated electronic states of both Pt8/CeO2(111) and Pt34/CeO2(111) models reveal two electrons donated by the Pt particles to the support, as manifested by the presence of two Ce3+ cations in each case. This number is not modified by CO adsorption in most of the Pt sites. Only when CO is adsorbed in sites 4 and 6 of the supported Pt8 particle the number of Ce3+ cations decreases to one, while in site 4 of the Pt34 particle it increases to three. All our attempts to converge the states with two electrons donated to the ceria support failed in these three special cases.



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. Overview of the electrocatalyst samples used in this work: I, II: Pt(111) reordered and cleaned by flame annealing; III to VII: Pt-CeOx thin film samples on Au or C/Au substrates. Schematic representation (left); SEM images of the sample after preparation (middle); SEM images of the samples after the methanol oxidation


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Figure 2. Schematic illustration of the experimental procedure applied for the in-situ IR spectroscopy experiments in this work: (1) Potential cycling in hanging meniscus configuration with working electrode (WE), counter electrode (CE), and reference electrode (RE), (2) in-situ IR spectroscopy measurement in thin layer configuration, and (3) equilibration in retracted position before the next in-situ IR spectroscopy experiment.



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. Comparison of the electrochemical IR spectra taken during methanol oxidation on different Pt electrocatalysts at pH 1 (1 M CH3OH in 0.1 M HClO4): (a) comparison of the IR spectra at an electrode potential of 0.35 VAg/AgCl (reference: -0.2 VAg/AgCl); (b) peak position of the on-top CO band as a function of the electrode potential.


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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. Potential dependent in-situ IR spectra during methanol oxidation at pH 6 (1 M CH3OH in 0.1 M phosphate buffer) on different Pt electrocatalysts: (II) Pt(111), (V) Pt(21%)CeOx/C/Au, (VI) Pt(15%)-CeOx/C/Au, (VII) Pt(9%)-CeOx/C/Au. All reference spectra were taken at -0.15 VAg/AgCl.



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. Comparison of the electrochemical IR spectra taken during methanol oxidation on Pt-CeOx electrocatalysts with different Pt concentration at pH 6 (1 M CH3OH in 0.1 M HClO4): (a) IR spectra at an electrode potential of 0.4 VAg/AgCl (reference: -0.15 VAg/AgCl); (b) peak position of the on-top CO band as a function of the electrode potential.


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Figure 6. Overview of unsupported and supported model Pt particles used in the DFT studies on CO adsorption. The numbers indicate the symmetrically inequivalent Pt atoms in each particle (see also Table 2). Yellow, brown and red spheres represent Ce4+ cations, Ce3+ cations and O2- anions, respectively. 31 Environment ACS Paragon Plus


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. Summary of the stretching frequencies calculated by DFT on different on-top sites of each Pt model particle (black – unsupported particles; red – supported particles): (a) CO stretching frequencies on all on-top sites of each particle and the corresponding tendency visualized by the dashed lines; (b) average on-top stretching frequencies and their standard deviation taking into account weights of all on-top sites in each particle; (c) correlation between the CO stretching frequency and the Pt coordination number with respect to neighboring Pt atoms. 32 Environment ACS Paragon Plus

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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 8. Stability of the Pt NPs on Pt-CeOx thin film electrocatalysts with different Pt concentration: Electrochemical IR spectra during repeated cycles of methanol oxidation on (a) sample V, Pt(21%)-CeOx/C/Au at 0 VAg/AgCl and at 0.4 VAg/AgCl and (b) on sample VI, Pt(15%)-CeOx/C/Au at 0.4 VAg/AgCl. All reference spectra were taken at a potential of 0.15 VAg/AgCl.



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TOC graphic: For Table of Contents Only


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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


Sample number, pH

Single crystal (reference) Pt(111)

(I) pH 1

(II) pH 6 PtCeOx thin ﬁlms

(III)

Pt

pH 1

SEM (pre reaction)

SEM (post reaction)

Ce

O 2

Pt(8%)CeOx

200 nm

200 nm

Au

(IV) pH 1

Pt

Ce

O 2

Pt(15%)CeOx C Au

(V)

Pt

pH 6

pH 6

Pt

Pt(9%)CeOx C Au

200 nm

200 nm

200 nm

200 nm

200 nm

2

Ce

O 2

Pt(15%)CeOx C Au

(VII)

200 nm

O

Pt(21%)CeOx C Au Pt

200 nm

Ce

pH 6

(VI)

200 nm

Ce

O 2

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

CE

WE

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RE

3

2

1

electrolyte (1-100 m)

Surface cleaning by potential cycling CaF2 hemisphere

IR beam

IR spectroscopy in thin layer conﬁguration ACS Paragon Plus Environment

5 min equilibration in retracted position

Page 37 of 43

0.35 V vs. Ag/AgCl, pH 1

a

(I) Pt(111)

COt

COb

COt

COb

R/R = 0.5%

(III)

2064

3x

Pt(8%)CeOx/Au

COt.lc

(IV)

2024

Pt(15%)CeOx/C/Au 3x

2035 2500

2400

2300

2200

2100

2000

1900

1800

1700

-1

 [cm ]

COt.lc

2070 2060

b

(I) Pt(111)

40 cm-1V-1

2050

 = 29 cm-1 (IV) Pt(15%)CeOx/C/Au

2040 -1

 [cm ]

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


2030

-1

 = 40 cm

(III) Pt(8%)CeOx/Au

2020 2010 2000 1990 -0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

E vs. Ag/AgCl [V]


0.8


(II) Pt(111), pH 6 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

(V) Pt(21%)CeOx, pH 6 -0.15 V vs. Ag/AgCl

-0.15 V vs. Ag/AgCl 2047

-0.1 V

-0.1 V

1823

-0.0 V

-0.0 V

0.1 V

0.1 V

0.2 V

0.2 V

0.3 V

0.3 V

0.4 V

0.4 V

0.5 V

0.5 V

0.6 V

0.6 V 1839

0.7 V 12

CO2

2343

2500

2400

13

CO2

2278

2300

2200

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2000

1900

1800

1700

2500

2400

2300

2200

2100

2000

1900

1800

1700

-1

 [cm ]

 [cm ]

(VII) Pt(9%)CeOx, pH 6

(VI) Pt(15%)CeOx, pH 6 -0.15 V vs. Ag/AgCl

-0.15 V vs. Ag/AgCl

-0.1 V

-0.1 V 2022

-0.0 V

-0.0 V

0.1 V

0.1 V

0.2 V

2029

0.2 V

0.3 V

0.3 V

0.4 V

0.4 V

0.5 V R/R= 0.05%

0.6 V 0.7 V

0.5 V R/R= 0.05%

0.6 V 0.7 V

2039

2400

R/R = 0.1 %

2048

-1

2500

2022

0.7 V

R/R = 0.1 %

2059

2100

2042

2300

2200

2100 -1

 [cm ]

2035

2000

2500 2400 1900 ACS 1800 1700 Plus Environment Paragon

2300

2200

2100 -1

 [cm ]

2000

1900

1800

1700

Page 39 of 43


1 2 3 4 5 6 7 8 9 0.4 V vs. Ag/AgCl, pH 6 10 11 COb COt 12 13 14 15 (II) Pt(111) 16 17 2040 18 19 2017 2057 20 21 -0.1 V x3 22 vs. Ag/AgCl (V) Pt(21%)CeOx 23 24 (VI) Pt(15%)CeOx 2048 x3 25 26 x3 27 (VII) Pt(09%)CeOx 2033 28 29 30 2500 2400 2300 2200 2100 2000 1900 1800 1700 31 -1 32  [cm ] 33 34 2060 35 36 (II) Pt(111) 2055 37 38 39 2050 40 41 2045 (V) Pt(21%)CeOx 42 ( ) 43 -1 2040  = 26 cm 44 45 2035 46 (VI) Pt(15%)CeOx 47 48 2030 49 50 2025 51 52 2020 53 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 54 E vs. Ag/AgCl [V] 55 56 57 58 59 60

a

 [cm-1]

b


COt

COb


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

(i) Pt8

(ii) Pt34

(iii) Pt79

(iv) Pt114

(v) Pt140

(vii) Pt8/CeO2(111)

(vi) Pt201

(viii) Pt34/CeO2(111)


Page 40 of 43

Page 41 of 43

a

Individual CO stretching frequencies for on-top sites 2080

Pt8/CeO2(111) Pt34/CeO2(111)

(C-O) [cm-1]

2070 2060 2050 2040 2030

Pt79

Pt116

Pt201

Pt140

Pt34

2020

Pt8 0

b 2080

50

100 150 Pt atoms per nanoparticle

(C-O) [cm-1]

200

250

Average CO stretching frequencies for on-top sites

2070

Pt79

Pt116

Pt201

Pt140

2060

Pt34

2050

Pt8 2040 2030

Pt34/CeO2(111) Pt8/CeO2(111)

2020

0

c 2080

50

100 150 Pt atoms per nanoparticle

200

250

Individual CO stretching frequencies for on-top sites

2070 (C-O) [cm-1]

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


2060 2050 2040 2030 2020

1

2

3

4 5 6 7 Pt coordination number


8

9

10


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

a

b

(V) Pt(21%)CeOx/C/Au, pH 6 0.4 V vs. Ag/AgCl

0 V vs. Ag/AgCl

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(VI) Pt(15%)CeOx/C/Au, pH 6 0.4 V vs. Ag/AgCl 2025

1. cycle

Pt(111)

Pt(111)

2. cycle

4. cycle

= 0.05 %

2049

3. cycle

R/R = 0.1 %

R/R 2057

1. cycle 5. cycle

2. cycle 1. cycle

potential cycling 3. cycle

2. cycle 3. cycle

6. cycle 7. cycle

4. cycle

8. cycle

4. cycle 5. cycle

9. cycle

5. cycle

2042

2100

2009

2000  [cm-1]

2047

1900 2100

2019

2000

R/R = 0.05 %

2031

1900

2100

 [cm-1]


2000  [cm-1]

1900

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