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Neili Loupea, Jonathan Doana, Ryan Crusea, Nicholas Dimakisb, Khaldoon Abu-Hakmehc, SeungSoon Jangc, William A Goddard IIId, Eugene S. Smotkina
a: Department of Chemistry and Chemical Biology b: Department of PhysicsNortheastern University University of Texas Rio Grande Valley Boston, MA 02115 Edinburg, TX 78539e.smotkin@neu.edu
c: School of Materials Science and Engineering d: Materials and Process Simulation CenterGeorgia Institute of Technology California Institute of TechnologyAtlanta, GA 30332 Pasadena, CA 91125
Operando IR and Raman Spectroscopy of the Catalyst Ionomer Interface
DOE Catalysis Working Group MeetingArgonne National Laboratory, Building 241/D172Wednesday, July 27. 2016
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Overview
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1. Operando spectroscopy fuel cells (IR, XAS, Raman)2. Why operando?
Structural changes during catalysis (XAS case study)3. IR spectroscopy of catalyst ionomer interface
Molecular orbital explanation of COads Stark tuningStark tuning: Probe for interfacial co-adsorption and electrocatalysis
4. IR spectroscopy in proximity to the catalyst/ionomer interfaceShort chain, 3M and Nafion membranes
Band assignments in terms of exchange site local symmetryC1 (sulfonic acid) and C3V (sulfonate) local symmetry modesHydration dependent distribution of per site λ (waters/exchange site).
Molecular dynamics modeling and state of hydration IR spectroscopy5. Fuel cell operando Raman spectroscopy (Case study: Non-PGM catalyst)
Membrane exchange site transition from C1 to C3V symmetry during ORRNo Stark tuning of Fe-O stretching modeCatalysis hypothesized as due to 14 ppm Pt contaminant
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Operando IR and Raman spectroscopy
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Operando direct methanol fuel cell X-ray absorption spectroscopy: A case study exemplifying the need for operando measurements.
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21750 22250 22750 23250Energy, eV
Abs
orpt
ion
Expulsion of CO2 bubble
Formation of CO2 bubble.
CO2 phase-out was mitigated by application of a backpressure to the methanol anode.
S Stoupin et al. Pt and Ru X-ray Absorption Spectroscopy of PtRu Anode Catalysts in Operating Direct Methanol Fuel Cells J Phys Chem B, 110, 9932 -9938, (2006)
0.0
0.5
1.0
22080 22120 22160 22200Energy, eV
Nor
mal
ized
abs
orpt
ion
Ru oxide Ru oxide hydrate
Fresh JM
Ru metal (blue)
Ru in operating PtRuJM electrode (red)
Operando DMFC anode is always metallic
ACS San Francisco – September 14, 2006
Johnson Matthey PtRu (1:1) structural changes during catalysis
As received PtRu (1:1)
• Ru oxidation ~58%• N = 5.6• [Ru]/[Pt] = 0.44• Pt-O bonds present• Ru-O bonds ~2.8 avg
Operando PtRu (1:1)
• Ru oxidation ~15%• N = 8.2• [Ru]/[Pt] = 0.50• No Pt-O bonds• Ru-O bonds ~0.24 avg
S Stoupin et al., J.Phys Chem B, 110, 9932 (2006).
Vibrational Spectroscopy of the Pt/ionomer interface
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I Kendrick et al. Elucidating the Ionomer-Electrified Metal Interface, J.Am. Chem. Soc., 132 (49), 17611–17616 (2010)
Co-adsorbates include sulfonate exchange site and the –CF3 group.
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Stark Tuning: Potential dependent COads stretching frequencies.Higher electrode Fermi levels = Reduced CO stretching frequencies.
B. Hammer, O. H. Nielsen and J. K. Norskov, Catalysis Letters, 1997, 46, 31-35
Stark tuning: From model surfaces to fuel cell Pt/ionomer interfaces
Model surfacesR Liu et al. Potential-Dependent Infrared Absorption Spectroscopy of Adsorbed CO andX-ray Photoelectron Spectroscopy of Arc-Melted Single-Phase Pt, PtRu, PtOs, PtRuOs,and Ru Electrodes, J. Phys. Chem. B, 104 (15), 3518–3531 (2000)V Stamenkovic et al. Vibrational Properties of CO at the Pt(111)−Solution Interface: theAnomalous Stark-Tuning Slope, JPC B, 109, 678-680 (2005)
MEA Catalyst ionomer interfacesI Kendrick et al. Elucidating the Ionomer-Electrified Metal Interface, J. Am. Chem. Soc.,132 (49), 17611–17616 (2010)S E Evarts et al. Ensemble Site Requirements for Oxidative Adsorption of Methanol andEthanol on Pt Membrane Electrode Assemblies, ACS Catalysis, 2, 701 (2012)I Kendrick et al. Operando Infrared Spectroscopy of the Fuel Cell Membrane Electrode Assembly Nafion-Platinum Interface, International Journal of Hydrogen Energy, 39, 6, 2751-2755 (2014)
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Potential (mV) Potential (mV) Potential (mV)
Wav
enum
ber (
cm-1
)
30o C 50o C 70o C
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COads Stark tuning on Pt/Nafion interfaces
Potential (mV) Potential (mV) Potential (mV)
Wav
enum
ber (
cm-1
)
30o C 50o C 70o C
Operando Stark tuning at MEA Pt-COads /Nafion interfaces
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Potential (mV) Potential (mV) Potential (mV)
Wav
enum
ber (
cm-1
)
30o C 50o C 70o C
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Operando Stark tuning at MEA Pt-COads /Nafion interfaces
Potential (mV) Potential (mV) Potential (mV)
Wav
enum
ber (
cm-1
)
30o C 50o C 70o C
Operando Stark tuning at MEA Pt-COads /Nafion interfaces
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Operando COads Stark tuning elucidates site specific (potential dependent) adsorption sites.
Adsorption potential100 mV
Adsorption potential200 mV
Adsorption potential300 mV
Adsorption potential400 mV
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1400 1200 1000
1414 cm-1
SO3H νas, COC νas
905 cm-1
SO3H νs, COC νs
1057 cm-1
COC νas, SO3-1 νs
970 cm-1
SO3-1 νs, COC νas,
1400 1300 1200 1100 1000 900
CF2
CF2CF
CF2CF2
OCF2
CF2
(BB) (BB)
(4)
(3)(2)
(1)
CF2
CF2S
O
O
O
M NX
Short Side Chain
3M
Nafion
CF2
CF2CF
CF2CF2
OCF2
F2C
(BB) (BB)
(2)
(1)
S
O
O
O
M NX
Nafion
CF2
CF2CF
CF2CF2
OCF2
CFOF3C
(BB) (BB)
(B) (4)
(3) (2)(1)
(A)
CF2CF2
M
SO O
O
M NX
1400 1200 1000
910 cm-1
SO3H νs, COC-A νs
1414 cm-1
SO3H νas, COC-A νas
1061 cm-1
COC-A νas, SO3-1 νs
970 cm-1
SO3-1 νs, COC-A νas
1400 1300 1200 1100 1000 900
1400 1200 1000
1060 cm-1
SO3-1 νs, COC vs
1412 cm-1
SO3H νas926 cm-1
SO3H νs, COC νs
991 cm-1
SO3-1 νs, COC vas
1400 1300 1200 1100 1000 900
Wavenumber (cm-1)
Hydration dependent IR spectra: Short chain, Nafion & 3M membranes
C3v local symmetry C1 local symmetry
983* (969) cm-1 : SO3- vs, COC-A vas
1059* (1061) cm-1 : COC-A vas, SO3- vs
786* (910) cm-1 : SO3H vs, COC-A vs
1405* (1414) cm-1 : SO3H vas, COC-A vas
Kendrick, I.; Yakaboski, A.; Kingston, E.; Doan, J.; Dimakis, N.; Smotkin, E. S. J. Polym. Sci. Pt. B-Polym. Phys. 2013, 51, 1329.
C3v,LF
C3v,HF C1,HF
C1,LF
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Nafion exchange-site normal mode animations
Kendrick, I., et al., Operando Raman Micro-Spectroscopy of Polymer Electrolyte Fuel Cells. Journal of The Electrochemical Society, 2016. 163(4): p. H3152-H3159.
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Normal mode animation snapshots (C3V): Dissociated exchange sites.
Kendrick, I., et al., Operando Raman Micro-Spectroscopy of Polymer Electrolyte Fuel Cells. Journal of The Electrochemical Society, 2016. 163(4): p. H3152-H3159.
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Normal mode animation snapshots (C1): Associated exchange sites.
Freq
uenc
y λavg = 3
λlocal
λavg = 10 λavg = 15
1400 1200 1000 8001400 1200 1000 800 1400 1200 1000 800
Dehydrated Hydrated
Wavenumber (cm-1)
1414 cm-1
SO3H νas, COC-A νas
1061 cm-1
COC-A νas, SO3-1 νs
970 cm-1
SO3-1 νs, COC-A νas
910 cm-1
SO3H νs, COC-A νs
EW ∼ 1100 g32 oligomers 10 side chains separated by 14 -CF2- monomers320 exchange sites
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RamanInfrared
Wavenumber (cm-1)
1066 cm-1969 cm-1
C1 band 910 cm-1
C3V bands
1200 1000 800
1100 mV
1000 mV
900 mV
800 mV
700 mV600 mV
500 mV
400 mV
300 mV
200 mV
100 mV
0 mV
1200 1100 1000 900 800
Operando spectroscopy: Transitioning between hydration (C3V) and dehydration (C1).
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C3VC1
Nafion dehydration: C1 modes negatively correlated to C3V modes.
DFT build-up of the exchange site solvation sphere.Transitioning from C1 to C3V local symmetry
Webber et al. Mechanically Coupled Internal Coordinates of Ionomer Vibrational Modes, Macromolecules, 43, 5500-5502 (2010)Kendrick et al. Theoretical and experimental infrared spectra of hydrated and dehydrated Nafion Journal of Polymer Science Part B: Polymer Physics, 51, 18, pp. 1329–1334 (2013)
C1
C3V
• An initial cubic MD box was constructed with side lengths of ∼80 Å (shrank to 75 Å, during preliminary relaxation process)
• (Nafion with ∼20 wt % (H2O, H3O+)• Nafion EWs close to 1100 g/mol
• 32 independent oligomers 10 side chains separated by 14 -CF2- monomers
• 4800 H2O molecules• 320 H3O+ ions
• MD contained total 37,504 atoms. • Effective level of hydration λ= 15.
Karo, J., et al., Molecular Dynamics Modeling of Proton Transport in Nation and HyltonNanostructures. Journal of Physical Chemistry B, 2010. 114(18): p. 6056-6064.
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Nafion Molecular Dynamics
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320 S centered10 Å cubes
λlocal =15
λlocal =4
λlocal =0
λavg =15
λlocal =40
λlocal =10
Nafion molecular dynamics: At any λave a λ distribution exists.
Nafion hydration (C3V)/dehydration (C1) of the exchange site environment
• The C1 and C3V group modes involve the same side-chain functional groups (Triflate/COC group modes (C3V) or triflic acid/COC group modes (C1)).
• The C1 and C3V modes do not coexist at extreme states of hydration.
• C1 and C3V modes do coexist at intermediate state-of-hydration and there is a distribution of per site λvalues.
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Local symmetry
Group Mode Assignment DFT (cm-1) Transmission (cm-1)
HydratedDOW-[H]
C3v,LF SO3-1 νs, COC νas, 923* 970
C3v,HF COC νas, SO3-1 νs 1051* 1057
DehydratedDOW-[H]
C1,LF SO3H νs, COC νs 777* 905C1,MF COC νs , SO3H νas 960* 1001C1,HF SO3H νas, COC νas 1396* 1414
HydratedNafion-[H]
C3ν,LF SO3-1 νs, COC-A νas 983* 969
COC-B νas, CF3 δu, SO3-νs
973* 983
C3v,HF COC-A νas, SO3-1 νs 1059* 1061
DehydratedNafion-[H]
C1,LF SO3H νs, COC-A νs 786* 910CF3 δu, COC-B vas 981* 983
C1,MF CF3 δu, COC-B δs, COC-A ρr, SO3H νas 986* 999sC1,HF SO3H νas, COC-A νas 1405* 1414
Hydrated 3M-[H]
C3v,LF SO3-1 νs, COC vas 975* 991
SO3-1 νs, SC CC v, COC δs 980* 1012
C3v,HF SO3-1 νs, COC vs 1014* 1060
Dehydrated3M-[H]
C1,LF SO3H νs, COC νs 791* 926COC vas 984* 1012
C1,MF COC δs, SO3H νas 1009* 1031C1,HF SO3H νas 1395* 1412
Symmetric stretching, νs; Asymmetric stretching, νas; Wagging, ω; Bending, δs; Umbrella bending, δu; Rocking, ρr;Backbone, BB; Side Chain, SC; Calculated value *
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IR band assignments of short chain, 3M and Nafion in terms of exchange site local symmetry (manuscript in preparation).
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Ionomer vibrational band assignments in terms of exchange site local symmetry, and as group modes, are essential for the interpreting operando Raman spectroscopy of the fuel cell catalytic layer.
Operando Raman Spectroscopy of the fuel cell ionomer/Pt interface of a non-PGM catalysts: A case study.
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Operando confocal Raman micro-spectroscopy depth profiling to select focal point for fuel cell study.
Kendrick et al. Operando Raman Micro-Spectroscopy of Polymer Electrolyte Fuel Cells Journal of The Electrochemical Society, 163 (4) H3152-H3159 (2016)Loupe et al., Twenty years of operando IR, X-ray absorption, and Raman spectroscopy: Direct methanol and hydrogen fuel cells, Catalysis Today, In press, doi:10.1016/j.cattod.2016.06.012
Fuel cell polarization curve
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C1
C3V
C1+C3V
Fe-Nx/C cathode catalyst operating under O2
C (no Fe) cathode catalyst operating under O2
Fe-Nx/C cathode catalyst operating under N2
631 cm-1
600 cm-1
564 cm-1
631 cm-1
600 cm-1
564 cm-1 631 cm-1
600 cm-1
564 cm-1
0 mV
100 mV
200 mV
300 mV
400 mV
500 mV
600 mV700 mV
800 mV
900 mV
1000 mV
1100 mV
Average
Kendrick Raman cell. Potential-dependent Raman spectra (650 to 550 cm-1) of the Fe-Nx/C cathode catalyst operating under O2 (left), under N2 (middle), and under O2 after Fe removal (right). Absence of Stark tuning of the Fe-O peak at 564 cm-1 confirms Fe-O is not an electrocatalytic site. Analysis of as-received catalyst showed 13.5 ppm Pt.
Operando Raman Micro-Spectroscopy of PEM fuel cell.
Conclusions
• This is the first demonstration of operando Raman spectroscopy of a fuel cell with flowing reactant streams over a standard MEA structure and controlled temperature.
• Stark tuning of the Fe-O site in a non-PGM catalyst was not observed in the operando Raman (i.e., Fe-O peaks were invariant with potential). Catalysis was likely due to Pt contaminant assessed at 13.5 ppm.
• Local symmetry based assignments of short chain, 3M and Nafion vibrational group modes (C1 vs. C3V) enables elucidation of shift of the λ per site distribution with state of hydration. This explains the gradual shift of ionomer spectra from C1 to (C1 + C3V) to C3V characteristics representing fully dehydrated, partially hydrated and totally hydrated exchange site environments respectively.
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Thanks
Thanks to funding from:NuVant Systems Inc.US Department of Energy EERE (DE-EE-0000459)Army Research Office (W911NF-12-1-0346)
Neili Loupe (NEU): Vibrational band assignments, and IR spectroscopyJonathan Doan (NEU): Vibrational band assignments and IR spectroscopyRyan Cruse (NEU): Per site λ analysisMatthew Ingargiola (NEU): Per site λ analysis Nicholas Dimakis (UT-RGV) : DFT Normal mode analysisKhaldoon Abu-Hakmeh (Georgia Tech): MD simulations of λ = 3, 10 and 15Seung Soon Jang (Georgia Tech): MD simulations of λ = 3, 10 and 15Max Diem (NEU): Raman spectroscopyIan Kendrick (NEU): IR and Raman spectroscopyWilliam A. Goddard III (Cal Tech): MD simulations and use of his X3LYP functional for DFT build-up of hydration sphere.
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Supplemental slides
The following slides are supplemental slides that were not shown at the CWG.
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C3v local symmetry C1 local symmetry
1051* (1057) cm-1 : COC vas, SO3- vs
777* (905) cm-1 : SO3H vs, COC vs
C3v,LF
C3v,HF C1,HF
C1,LF
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Short Side-Chain exchange-site normal modes
923* (970) cm-1 : SO3- vs, COC vas
1396* (1414) cm-1 : SO3H vas, COC vas
C3v local symmetry
975* (991) cm-1 : SO3- vs, COC vas
1014* (1060) cm-1 : SO3-1 νs, COC vs 1395* (1412) cm-1 : SO3H vas
C3v,LF
C3v,HF C1,HF
C1,LF
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3M PFSA exchange-site normal modesC1 local symmetry
791* (926) cm-1 : SO3H vs, COC vs
The metal-oxygen bond dissociation energy (Do 298 K (M-O)) in diatomic molecules versus group. Periods are determined by symbols.
Face centered cubic (FCC) phase compositions in the PtRuOs system.
Ley diagram
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C3v,HFC1,HF C1,LF C3v,LF
State-of-hydration dependent IR spectra
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CF2
CF2CF
CF2CF2
OCF2
CF2
(BB) (BB)
(4)
(3)(2)
(1)
CF2
CF2S
O
O
O
M NX
Short Side Chain
3M
Nafion
CF2
CF2CF
CF2CF2
OCF2
F2C
(BB) (BB)
(2)
(1)
S
O
O
O
M NX
Nafion
CF2
CF2CF
CF2CF2
OCF2
CFOF3C
(BB) (BB)
(B) (4)
(3) (2)(1)
(A)
CF2CF2
M
SO O
O
M NX