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In-situ X-ray Spectroscopy and Scattering Diagnostic Studies of
PEFC Cathode Catalysts
D. Myers, M. Smith, A.J. Kropf, M. Ferrandon, and J. GilbertArgonne National Laboratory, Argonne, Illinois, United States
G. Wu, J. Chlistunoff, C. Johnston, and P. ZelenayLos Alamos National Laboratory, Los Alamos, New Mexico, United States
DIAGNOSTIC TOOLS FOR FUEL CELL TECHNOLOGIES
Trondheim, NorwayJune 23-24, 2009
2
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
AnodeAnode CathodeCathode
N2N2
N2N2
N2N2
H2H2
H2H2
H2H2
H2H2
H2H2
H2H2
O2O2
O2O2
O2O2H+H+
e-e- e-e-
O2O2N2N2
N2N2O2O2
O2O2
ElectrolyteElectrolyte
*2007 Status, Directed Technologies Incorporated Study, Feb. 2008
3
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”
4
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
5
Small-Angle X-ray Scattering (SAXS)
Gives information on particles 1 - 100 nm in size
Shape Mean Size Size Distribution
6
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
7
Cells for in situ X-ray studies of cathode catalysts
It
X-ray
I0
FluorescenceDetector If
APS
Wor
king
Ref
eren
ce
Cou
nter
Potentiostat
In SituElectrochemical
Cell
ItIt
X-ray
I0
FluorescenceDetector If
APS
Wor
king
Ref
eren
ce
Cou
nter
Potentiostat
In SituElectrochemical
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.
8
Aqueous in-situ XAFS shows potential dependence of Pt loss and Pt oxidation state
10 mV/s
2 XAFSSCANS
10 mV/s
OpenCircuit
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 oxidation 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
Nor
mal
ized
Abs
orba
nce
Energy (eV)
Pt L3-edge XANESPt L3-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
Nor
mal
ized
Abs
orba
nce
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 1159011560 11565 11570 11575 11580 11585 11590
Nor
mal
ized
Abs
orba
nce
Energy (eV)
0.5 V
1.4 V
9
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
1.4Potential (V)
Potential Cycle
0
2
4
6
8
10
12
14
16 -2
-1.5
-1
-0.5
0
0.5
% L
oss
Edge
Ste
p H
eigh
t
(%
Loss)
10
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
11
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
0 2 4 6 8 10 12 14 16
Par
ticle
siz
e (n
m)
Cycle Time (hrs)= 40 cycles
M.C. Smith et al., J. Am. Chem. Soc., 2008.
0102030405060708090
100
0 1 2 3 4 5 6 7 8Particle Size (nm)
Freq
uenc
y
1 hr2 hrs3 hrs5 hrs7 hrs10 hrs12 hrs14 hrs16 hrs
0
20
40
60
80
100
0 1 2 3 4 5 6 7 8 9 10Particle Size (nm)
Freq
uenc
y
SAXS AnalysisTEM Analysis
20 wt% Pt/C
20 wt% Pt/C
12
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 pyridinicnitrogen
• Encapsulated metal catalyzes formation of active site
– Metal leaches from catalyst during operation
H
CN
Co
n
R. Bashyam and P. Zelenay, Nature, 2006.
Metal particle
13
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 oxidation state
Three cobalt species observed in-situ:
H O/N Co
0.0
0.5
1.0
1.5
0 1 2 3 4
R (Å)
Mag
nitu
de
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 RH0.1 V, low RH
14
Los Alamos NL’s pyrolyzed polyaniline-Fe(Co)-C ORR catalysts
Pt/C
15
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
observed 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
Fe2O3Fe3O4
FeO
FeSO4
Fe metal
Fe-phthalocyanine
0.0
0.4
0.8
1.2
1.6
2.0
7050 7100 7150 7200 7250Energy (eV)
Abs
orba
nce
0.87 V
0.64 V
0.44 V
0.24 V
1
2
3
4
5
6
8
16
Pyrolyzed polyaniline-Fe-C catalyst composition
Wt%
Fe
in
indi
cate
d co
ordi
natio
n en
viro
n.
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 in
dica
ted
coor
dina
tion
envi
ron.
Fe m
etal
FeS
FeS
2
Fe-p
c
Fe2O
3
Fe3O
4
FeO
FeS
O4
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
MEA
Acid-TreatedPowder
Wt%
Fe
in in
dica
ted
coor
dina
tion
envi
ron.
Fe m
etal
FeS
FeS
2
Fe-p
c
Fe2O
3
Fe3O
4
FeO
FeS
O4
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
MEA
Acid-TreatedPowder
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
17
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)
18
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