Low-Platinum Nanostructured Catalysts for Fuel Cells

1. 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. 14 Source: Transportation Energy Data Book: Edition 19, DOE/ORNL-6958, September 1999, and EIA Annual Energy Outlook 2000, DOE/EIA-0383(2000), December 1999 12 Domestic Oil Production Heavy Trucks 10 GAP 8 Light Trucks Millions of Barrels per Day 6 Passenger Vehicles 4 2 Automobiles 0 1970 1980 1990 2000 2010 2020 U.S. Transportation Oil Demand Strategies needed to close the production/utilization gap Use fuel cells to increase efficiency and decrease fuel consumption

3. Electricity from electrochemistry H2 H2 1 O2 O2 2 H2O H2O H2O H2O H2O 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 Perfluorosulfonic acid (Nafion® polymer) membrane

4. Hydrocarbons as hydrogen source S CH3-(CH2)x-CH3 Gasoline/diesel • 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

6. 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

7. 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?

8. Low Pt catalysts for fuel cells H2O O2 O O O H+ Pt? H+ MOx H+ e- Nafion H+ carbon Pt supported on MOx•H2O supported on carbon O2 + 4 H+ + 4e-  2 H2O • 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

9. Pt-MOx systems • 1. Pt-FePOx•xH2O - 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-SnOx•xH2O - 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

10. 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-FePOx•yH2O + 50% VC 20%Pt-VC

11. FePOx Cyclic Voltammetry • FePOx has no activity for the ORR • 9 wt %Pt-FePOx is highly active for the ORR FePOx/VC 9%Pt-FePOx/VC 0.1 M HClO4 1600 rpm 60 °C

12. 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

13. 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 O P Fe • 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

15. Next steps… • Reduce particle size of oxides to improve electrical properties of FePOx VC impregnated with Pt-FePOx Solution-filtered Pt-FePOx

16. 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

17. 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

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