Alcohol Dehydrogenation Catalysts Bound to Fuel Cell Electrodes

1. Alcohol Dehydrogenation Catalysts Bound to Fuel Cell Electrodes Tova Sardot and Dr. Eric Kelson Sigma Xi Symposium 2005 California State University, Northridge

2. Hydrogen Fuel Cells Anode: 2H2 –––> 4H+ + 4e- Cathode: O2 + 4H+ + 4e- –––> 2H2O Net: 2H2 + O2 –––> 2H2O

3. Challenges for Fuel Cells • Pressurized hydrogen gas is hazardous (Flammability, etc.) • On-demand hydrogen production is inefficient. (High cost of Pt reformer catalysts) (Pollutants can still form)

4. Research Rationale • Employ organic alcohol fuels (Avoids flammability and storage issues) • Dr. Kelson’s group has developed Ru catalysts for harvesting hydrogen gas from alcohols. • Techniques needed to immobilize catalysts on fuel cell electrodes.

5. Research Objectives • Develop electrode coatings that bind metal catalysts for fuel cell applications. • Specifically: • Paint on Nafion cation exchange resin. • Electropolymerized vinylpyridines.

6. Main Catalyst

7. Cationic Catalyst Models • More simple than actual catalysts. • RuIII/RuII redox couples better behaved. • Charge useful for binding.

8. Cyclovoltammetry • Voltage applied to electrode varied linearly with time as current is simultaneous measured. • Surge of current with increasing or decreasing voltage represents oxidation or reduction, respectively. • Average of peak voltages represents potential of RuIII/RuII redox couple.

9. Nafion Coatings • Nafion is a polymer with sulfonic (SO3-) groups attached to Teflon chains. • Functions as a strong proton donor. • Cationic compounds could exchange for protons within Nafion. • Nafion can be painted onto electrode and then dipped into catalyst solution. • Bound catalyst can be measured electrochemically through its RuIII/RuII couple.

10. Binding Complexes in Nafion Blank Nafion Complex 2 in Nafion Nafion soaked in 2 or 3 solution exhibits clear RuIII/RuII signal. Indicates that complex bound in Nafion

11. Binding Complexes in Nafion For 2 in Nafion (0.033 M Na2SO4) • Linear relationship indicative of bound complex. • Catalyst 1 did not bind in Nafion in spite of sulfonate groups that should have protonated it.

12. Polyvinylpyridine Layers • 2-Vinylpyridine reported to electropolymerize in pH=4 electrolytes at -1.3 V (Ag/AgCl).

13. Polyvinylpyridine Layers • Resulting polypyridine is partially protonated• Protonated groups can bind anions.• Remaining pyridine groups can bind to Ru.

14. Polyvinylpyridine Layers • 2-Vinylpyridine electropolymerizes at -1.3 V (Ag/AgCl) onto Au electrodes at pH=4. • Coating durations of 1 second optimal. • Tested through electrochemistry of RuIII/RuII couple of 2: • Enough to begin distorting signal. • Signal still clear.

15. 2 in Polyvinylpyridine 200 mV shift in RuIII/RuII potential due to coating Without layer: With layer:

16. 2 in Polyvinylpyridine • RuIII/RuII potential restored when layer physically removed. • 200 mV potential shift also observed when 2-vinylpyridine added to 2 in solution. • Behavior suggests 2 must bind to layer pyridines to transfer electrons.

17. 2 in Polyvinylpyridine • Current versus scan rate behavior indicates 2 binds reversibly and rapidly. • Nevertheless, 2 appears to bind to layer to transfer electrons.

18. Conclusions • Nafion electrode coatings are easily formed by solution application. • Cationic complexes 2 and 3 bind in Nafion but 1 does not. • 2-Vinylpyridine and electropolymerize into electrode coatings. • Complex 2 reversibly binds to 2-polyvinylpyridine for electron transfer to electrode.

19. Future Directions • Explore possible binding of complexes 1 and 3 to polyvinylpyridines. • Survey effects of polyvinylpyridine modifications to encourage binding. • Synthetically incorporate catalysts directly into polymer chain.

20. Acknowledgements JPL-NASA Pair Program Dr. Carol Shubin Dr. Eric Kelson