Karen Swider-Lyons and Peter Bouwman

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Low-Platinum Nanostructured Catalysts for Fuel
Cells

• Karen Swider-Lyons and Peter Bouwman
• Naval Research Laboratory
• Washington, DC
• Wojtek Dmowski
• University of Tennessee
• Knoxville, TN

2
U.S. Transportation Oil Demand
Strategies needed to close the production/utilizat
ion gap Use fuel cells to increase efficiency and
decrease fuel consumption
3
Electricity from electrochemistry
Electrochemical conversion process Oxygen and
hydrogen combined on catalysts to produce water,
electricity and heat
e-
H2 2 H 2 e-
anode
H
load
Pt catalysts
electrolyte
H
O2 2 H 2 e- H2O
cathode
H
Resistive losses due to materials and
inefficient reactions.
e-
Proton exchange membrane fuel cell Perfluorosulfon
ic acid (Nafion polymer) membrane
4
Hydrocarbons as hydrogen source

• Logistics fuel
• Richest source of hydrogen
• Must be reformed to hydrogen
• Sulfur, nitrogen and CO2 may be sequestered

Fuel cell
SO2 CO2
H2
H2
H2
H2
H2
H2
H2
OIL
H2
reformer
refinery
Storage tanks
Hydrogen from water electrolysis is expensive
due to high materials and energy costs of
electrolyzers
5
Environmental Issues with Platinum
Mining Pt creates a lot of waste! In fuel cell
era, attention toward environmentally friendly
mining and recycling

• From The Lonmin Group web site

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EPA Advantages of low Pt catalysts

• Lowering Pt will lower the cost of fuel cells
• ? Introduce fuel cells more broadly to consumer
  market
• ? Lower fuel consumption
• Less Platinum used
• ? Less Pt mined and recycled
• ? Less chemical waste

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A possible solution to the problem
Traditional approach Make and test new platinum
alloys Change catalyst microstructure Our
approach Design phase-segregated, mixed
conducting nanocomposites for RAPID TRANSPORT of
chemical species
Transition metals
?
Alloy compositions
?
nanocomposite phases
Nanocomposite phases are still largely
unexplored -due to difficulty in their
characterization?
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Low Pt catalysts for fuel cells

• Focus on lowering Pt in fuel cell cathode
• Cathode has most Pt because
• slow oxygen reduction kinetics
• poor Pt stability and ripening over time.

Support Pt on a metal oxide and improve
opportunity for proton mobility to Pt sites
chem/phys attraction of O2 metal-support
interactions with Pt
Catalyst development via electrochemical and
structural analysis
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Pt-MOx systems

• 1. Pt-FePOxxH2O - hydrous iron phosphate
• Iron phosphate used as an anti-corrosion additive
  in paint
• FePOx is a partial oxidation catalyst
• Under intense scrutiny as a Li-ion battery
  cathode
• 2. Pt-SnOxxH2O - hydrous tin oxide
• Prior ORR literature shows promise for anhydrous
  Pt-SnOx
• Tin hydrates are corrosion resistant

Open Framework Inorganic Materials A. K.
Cheetham G. Férey, T. Loiseau, Angew. Chem.
1999, v. 38 p. 3286.
Example Microporous AlPO4

• Hydrous oxides are excellent for proton
  conduction
• Selected oxides have other ideal catalytic
  properties (e.g. partial oxidation)
• Materials have open framework structures

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Pt-FePOx/Vulcan C as ORR catalysts

• Use rotating disk electrode experiments to
  compare oxygen reduction activity of new
  catalysts to standard catalysts
• Critical factors
• Electrode preparation
• Testing conditions
• 0.1 M HClO4
• 1600 rpm
• 60 C
• Compare to
• theoretical
• values for Pt

Pt-FePO4 catalysts have higher ORR activity than
Pt/carbon standard
9 Pt-FePOxyH2O 50 VC
20Pt-VC
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FePOx Cyclic Voltammetry
FePOx/VC
9Pt-FePOx/VC

• FePOx has no activity for the ORR
• 9 wt Pt-FePOx is highly active for the ORR

0.1 M HClO4 1600 rpm 60 C
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Structure of Pt-FePOx Electrocatalysts
TEM
Conventional X-ray diffraction
Structure of the active Pt-FeO sample is glassy
in conventional electron microscopy and X-ray
diffraction
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Atomic pair distribution function (PDF)
XRD largely amorphous
Short-range order
PDF analysis
S(Q)
X-ray or neutron scattering is Fourier-transformed
to give distribution of inter-atomic distances
in a real space.
High energy, high intensity sources are essential
to minimize errors and improve statistics.
14
s
Structure of Pt-FePOx with PDF analysis

• PDF analysis of high-energy XRD shows ordered
  medium
• range structure
• Microporous structure facilitates high protonic
  conduction
• Pt serves as a glass-modifier and opens pores for
  access
• Fe2/Fe3 mixed valence states for high catalytic
  activity

• Iron phosphate
• (berlinite) a-quartz structure

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Next steps
VC impregnated with Pt-FePOx
Solution-filtered Pt-FePOx

• Reduce particle size of oxides to improve
  electrical properties of FePOx

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Effect of particle size

• Smaller (nano)particle size leads to
• Higher electronic conductivity (tunneling from
  carbon)
• Higher surface area
• (more sites for catalysis)

Pt-FePOx mixed with Vulcan carbon vs. Pt-FePOx
impregnated on Vulcan carbon
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Summary and Outlook

• Fuel cells are efficient fuel conversion systems
  that may lead to significant fuel savings - less
  pollution
• Lower greenhouse gases at a central fuel
  reforming site and hydrogen stored in fuel tanks
• Nanomaterials may be useful routes to lowering Pt
  content of fuel cells and lowering their cost
• New analytical techniques may be needed for the
  accurate study of new nanomaterials

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Acknowledgements

• Department of Energy, EERE
• Office of Naval Research
• The synchrotron experiments were carried out at
  the NSLS - Brookhaven National Lab