In-situ X-ray Spectroscopy and Scattering Diagnostic Studies of - - PowerPoint PPT Presentation

In-situ X-ray Spectroscopy and Scattering Diagnostic Studies of PEFC Cathode Catalysts

• D. Myers, M. Smith, A.J. Kropf,
• M. Ferrandon, and J. Gilbert

Argonne National Laboratory, Argonne, Illinois, United States

• G. Wu, J. Chlistunoff, C. Johnston, and
• P. Zelenay

Los Alamos National Laboratory, Los Alamos, New Mexico, United States DIAGNOSTIC TOOLS FOR FUEL CELL TECHNOLOGIES Trondheim, Norway June 23-24, 2009

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Why don’t we have “two fuel cell cars in every garage”?

 Major hurdles to overcome – Cost

• 50% of cost of PEFC stack is due to Pt catalyst*

– Durability

• Pt and Pt alloy cathode electrocatalysts lose

electrochemically-active surface area with time – Fuel storage, availability, and delivery  How can we get there? – Materials and engineering advances

• better utilization/performance
• lower cost (e.g., PGM alternatives)

– Fundamental studies of materials

• how they work
• what limits their performance

Anode Anode Cathode Cathode

N2 N2 N2 N2 N2 N2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 O2 O2 O2 O2 O2 O2

H+ H+

e- e- e- e-

O2 O2 N2 N2 N2 N2 O2 O2 O2 O2

Electrolyte Electrolyte

*2007 Status, Directed Technologies Incorporated Study, Feb. 2008

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How can we get the necessary information?

 What’s needed for rational design of catalysts: identity of active site; relationship between structure and degradation  Must “see” inside the fuel cell while it’s running with 0.1-10 nm “vision”  Probe must penetrate through flow field, gas diffusion layer, and ionomer to characterize catalyst on the atomic level  X-rays can penetrate through low atomic number materials and have wavelengths on the order of atomic dimensions  Synchrotron X-ray sources (high intensity, tunable wavelength), such as Argonne’s Advanced Photon Source, give us “X-ray vision”

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X-ray Absorption Fine Structure (XAFS)

h

• Oxidation state of absorbing atom
• Distances between atoms
• Number of neighboring atoms
• Identity of neighboring atoms
• Amount of absorbing material in

beam

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Small-Angle X-ray Scattering (SAXS)

Gives information on particles 1 - 100 nm in size

• Shape
• Mean Size
• Size Distribution

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Examples of systems studied with in-situ and ex-situ X-ray techniques

 Pt-based electrocatalyst degradation – Oxidation state and correlation of loss of Pt with voltage

• X-ray absorption in an aqueous environment

– Oxide formation and Pt particle growth as a function of potential cycling

• Small angle X-ray scattering and anomalous small angle X-ray

scattering

• Aqueous environment and MEA

 Non-platinum group metal catalyst composition, structure, oxidation state, and amount of absorbing metal using X-ray absorption – During pyrolysis – Effect of post-pyrolysis acid treatment – As a function of potential in aqueous environment – In MEA during polarization

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Cells for in situ X-ray studies of cathode catalysts

It

X-ray

I0

Fluorescence Detector

If

APS Working Reference Counter

Potentiostat

In Situ Electrochemical Cell

It It

X-ray

I0

Fluorescence Detector

If

APS Working Reference Counter

Potentiostat

In Situ Electrochemical Cell

 300 m thick window machined over three channels of single serpentine flow field* (modified Fuel Cell Technologies Hardware)

*Based on published design: Principi, E.; Di Cicco, A.; Witkowski, A.; Marassi R. J. Synchrotron Rad., 2007, 14, 276.

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Aqueous in-situ XAFS shows potential dependence of Pt loss and Pt oxidation state

10 mV/s 2 XAFS SCANS 10 mV/s

Open Circuit 0.8 V 1.4 V 1.1 V 0.5 V 1.1 V 0.8 V 0.8 V 1.4 V 1.1 V 0.5 V 1.1 V 0.8 V

Potential cycling Š 1st cycle Potential cycling Š 2nd and subsequent cycles

 Height of “white line”  extent of

• xidation of Pt

 Height of Pt L3 absorption edge  amount of Pt in electrode

Absorption edge loss over three cycles

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 11560 11565 11570 11575 11580 11585 11590

Normalized Absorbance Energy (eV) Pt L

3-edge XANES

Pt L

3-edge XANES 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 11560 11565 11570 11575 11580 11585 11590

Normalized Absorbance Energy (eV)

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 11560 11565 11570 11575 11580 11585 11590 11560 11565 11570 11575 11580 11585 11590

Normalized Absorbance Energy (eV) 0.5 V 1.4 V

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Platinum loss occurs during anodic and cathodic potential scans

 Greatest Pt loss observed in anodic step from 1.1 to 1.4 V

0.4 0.6 0.8 1 1.2 1.4

Potential (V) Potential Cycle

2 4 6 8 10 12 14 16

• 2
• 1.5
• 1
• 0.5

0.5

% Loss Edge Step Height  (% Loss)

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XAFS shows platinum loss and oxide formation are linked

 Pt loss is highest during oxide formation  Approximately same extent of oxidation show different Pt loss rates – Evidence against major role of oxide dissolution – Evidence for dissolution of metal – “Time-resolved” experiments are underway  Extent of Pt oxidation decreases with potential cycling - may be indicative of particle growth

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SAXS studies shows Pt particle growth with cycling

20 wt% Pt/C

20 wt% Pt/C 40 wt% Pt/C 2 2.5 3 3.5 4 4.5 2 4 6 8 10 12 14 16 Particle size (nm) Cycle Time (hrs)

= 40 cycles

M.C. Smith et al., J. Am. Chem. Soc., 2008.

10 20 30 40 50 60 70 80 90 100 1 2 3 4 5 6 7 8 Particle Size (nm) Frequency 1 hr 2 hrs 3 hrs 5 hrs 7 hrs 10 hrs 12 hrs 14 hrs 16 hrs

20 40 60 80 100 1 2 3 4 5 6 7 8 9 10

Particle Size (nm)

Frequency

SAXS Analysis TEM Analysis 20 wt% Pt/C 20 wt% Pt/C

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Non-platinum group metal electrocatalysts

 Cobalt or iron either complexed with C-N polymer/molecule or pyrolyzed (J.P. Dodelet, Los Alamos NL, U. South Carolina, 3M, et al.) – Low cost

• (Co ~US$ 3 /oz, abundance 20,000-

30,000 ppb in Earth’s crust vs 3-37 ppb for Pt) – Promising oxygen reduction activity, but lower than platinum group metals – Good durability, but longer testing and cycling tests are needed (>1000 hrs)  Issues: – Identity of the active site is unknown

• Metal center coordinated to pyridinic

nitrogen

• Encapsulated metal catalyzes formation
• f active site

– Metal leaches from catalyst during operation

H C N Co

n

• R. Bashyam and P. Zelenay, Nature, 2006.

Metal particle

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XAFS analysis shows Co-polypyrrole (not pyrolyzed) catalyst changes with time/potential

 Slow break in: possible formation of ORR sites during operation or removal of site- blocking species  Ex-situ XAFS data: as-prepared MEA contained a mixture of cobalt metal and a small oxide fraction  In-situ XAFS data: cobalt metal fraction is removed and/or converted to higher

• xidation state

 Three cobalt species observed in-situ:

H O/N Co

0.0 0.5 1.0 1.5 1 2 3 4 R (Å) Magnitude

0.4 V, 0.3 V 0.2 V, 0.1 V

>0.3V, low RH >0.1 V, high RH

2.83 Å 2.11 Å 1.92 Å 3.10 Å 2.05 Å 2.06-2.08 Å

0.2 V, low RH 0.1 V, low RH

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Los Alamos NL’s pyrolyzed polyaniline-Fe(Co)-C ORR catalysts

Pt/C

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Aqueous cell in-situ data for pyrolyzed polyaniline-Fe-C system

 XAFS shows reversible reduction of Fe3+ catalyst component between 0.64 and 0.44 V  Fe is lost from the electrode with greatest loss

• bserved during this reduction step

0.0 1.0 2.0 3.0

Wt% Fe

0.87 0.64 0.44 0.24 0.44 0.64 0.84 1.04 0.87

Potential (V vs. SHE)

FeS2 Fe2O3 Fe3O4 FeO FeSO4 Fe metal Fe-phthalocyanine

0.0 0.4 0.8 1.2 1.6 2.0 7050 7100 7150 7200 7250 Energy (eV) Absorbance

0.87 V 0.64 V 0.44 V 0.24 V 1 2 3 4 5 6 8

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Pyrolyzed polyaniline-Fe-C catalyst composition

Wt% Fe in indicated coordination environ.

 MEA preparation: – Removes metal – Removes sulfides – Oxidizes Fe2+ to Fe3+

Fe2O3 Fe-pc FeS2 Fe3O4 0.0 0.5 1.0 1.5 MEA, 0.6 V for 200 h Fresh MEA Fe2O3 Fe-pc FeS2 Fe3O4 0.0 0.5 1.0 1.5 MEA, 0.6 V for 200 h Fresh MEA Wt% Fe in indicated coordination environ.

Fe metal FeS FeS2 Fe-pc Fe2O3 Fe3O4 FeO FeSO4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 MEA Acid-Treated Powder

Wt% Fe in indicated coordination environ.

Fe metal FeS FeS2 Fe-pc Fe2O3 Fe3O4 FeO FeSO4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 MEA Acid-Treated Powder

 Fe is lost from MEA during long- term polarization at 0.6 V (approx. 50% loss)  Ratio of Fe2O3 to Fe-pc coordination is approx. unchanged

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Summary

 In-situ X-ray absorption and scattering techniques are powerful for diagnosing the state of PEFC catalysts during operation  New in-situ X-ray fuel cell block design allows XAFS studies in fluorescence mode – Enables study of very low loadings of low Z metals (e.g., Fe and Co) – Eliminates the need to modify flow field design – Allows the study of one electrode of a cell when the opposing electrode contains the same metal (e.g., can study Pt in a Pt cathode with a Pt anode)

X-rays Sample SAXS Detector XAFS Detector X-rays Sample SAXS Detector XAFS Detector

 Future needs/experiments – Combination of scattering and absorption experiments with microsecond time resolution – Simultaneous spatio-temporal resolved (micrometer and microsecond) atomic, electronic, and particle size characterization for a wide range of metals (e.g., Pt and Co in Pt3Co catalyst)

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Acknowledgements

 ANL – Chemical Sciences and Engineering Division – Xiaoping Wang – Nancy Kariuki – Jennifer Mawdsley – Di-Jia Liu – Chris Marshall  ANL – Advanced Photon Source – Mali Balasubramanian – Sector 20 (PNC-CAT) – Nadia Leyarovska – Sönke Seifert – Sector 12 (BESSRC-CAT)  DOE, Office of Science, Basic Energy Sciences  DOE, Office of Energy Efficiency and Renewable Energy, Hydrogen, Fuel Cells & Infrastructure Technologies