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Progress Towards New Heterocycle-Containing Proton Exchange Membranes

Progress Towards New Heterocycle-Containing Proton Exchange Membranes. February 7, 2008 Corinne Lipscomb Mahanthappa Group. Hydrogen Fuel Cells. The H-Racer. Have been used in: NASA missions since Gemini Concept vehicles Toys. Are being researched for: Automobiles

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Progress Towards New Heterocycle-Containing Proton Exchange Membranes

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  1. Progress Towards New Heterocycle-Containing Proton Exchange Membranes February 7, 2008 Corinne Lipscomb Mahanthappa Group

  2. Hydrogen Fuel Cells The H-Racer • Have been used in: • NASA missions since Gemini • Concept vehicles • Toys • Are being researched for: • Automobiles • Cell phones • Portable electronics • NASA’s continued use Completely Hydrogen-Powered Car http://www.inhabitat.com/2007/05/25/shanghai-to-build-hydrogen-fuel-cell-infrastructure/ Hoffman, P. Tomorrow's Energy. The MIT Press: Cambridge, Massachusetts, 2001.

  3. The Hydrogen Fuel Cell • External circuit for electrons • Oxygen ions and protons form H2O • Palladium Catalyst • Proton Exchange (Polymer Electrolyte) Membranes http://blog.wired.com/cars/2007/05/index.html Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells 2001, 1, 5-39.

  4. Ideal Material • High proton conductivity • Low electron conductivity • Low permeability to fuel and oxidant • Low water permeability • Chemically, thermally, and mechanically stable • Inexpensive • Ability to be fashioned into Membrane Electrode Assemblies (MEAs)

  5. The Industry Standard • Nafion - 1964 • E. I. du Pont de Nemours and Co. Connolly, D. J.; Gresham, W. F. (E.I. Du Pont de Nemours and Company). USA. U. S. Pat. 3,282,875, 1966.

  6. Synthesis of Nafion • Free radical initiated • Pressure to control • gaseous monomer Connolly, D. J.; Gresham, W. F. (E.I. Du Pont de Nemours and Company). USA. U. S. Pat. 3,282,875, 1966.

  7. Advantages of Nafion • Stable material • Selective ion permeability • Compatible with current fuel cell technology • High proton conductivity under aqueous conditions ~ 0.1 S/cm Conductivity Typically measured in Seimens/cm (S/cm) 1 S = 1 -1 Schmidt-Rohr, K.; Chen, Q. Nat. Mater. 2008, 7, 75-83. Deluca, N. W.; Elabd, Y. A. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 2201-2225.

  8. Disadvantages of Nafion • Low conductivity at low water content • Permeable to MeOH (Direct Methanol Fuel Cell) • Poor mechanical strength at high temperatures • Processability and fabrication issues DOE goal - 0.1 S/cm at 120C and 50% relative humidity Nafion cannot meet this goal. Deluca, N. W.; Elabd, Y. A. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 2201-2225. Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Chem. Rev. 2004, 104, 4587-4611.

  9. Proton Conduction Mechanisms • Vehicular diffusion • Proton carried by one molecule • Diffusion • Protons transferred slowly • Structural diffusion or proton “hopping” • Grotthus mechanism in H2O • Rearrangement of hydrogen bonds simultaneously • Protons transferred quickly Kreuer, K. D. Solid State Ionics 1997, 94, 55-62.

  10. Water Mimics • Mineral acids - H2SO4, H3PO4 • Heterocycles • High boiling • Immobilization possible

  11. Tg - Glass Transition Temperature • Amorphous solids - glasses, polymers, etc. • Below Tg - ‘solid-like’ behavior • material becomes rigid upon cooling • Above Tg - ‘liquid-like’ behavior • material softens upon heating • Some variability - depends on the heating/cooling rate • Above Tg segmental mobility increases significantly • important in conductivity

  12. Polymer Molecular Weights Mn - Number Average Molecular Weight PDI - Polydispersity Index the breadth of the distribution GPC - Gel Permeation Chromatography

  13. Nomenclature Small Molecules Polymer Heterocycle-Spacer length Heterocycle-Spacer length-Polymer Im5Si Tz6

  14. Conducting Heterocycle Solvents EW = 740 g/mol Kreuer, K. D.; Fuchs, A.; Ise, M.; Spaeth, M.; Maier, J. Electrochim. Acta 1998, 43, 1281-1288.

  15. Impedance Spectroscopy • Impedance • Real term - Resistance • Change in amplitude • Imaginary term - Capacitance • Phase shift • At high freq. goes to zero Conductivity Experimental • Apply sinusoidal potential • Measure current response • Usually done at high frequencies

  16. Proton Conductivity Imidazole doped Pyrazole doped Heterocycles can conduct protons like water. Kreuer, K. D.; Fuchs, A.; Ise, M.; Spaeth, M.; Maier, J. Electrochim. Acta 1998, 43, 1281-1288.

  17. Imidazole Immobilization ex. Herz, H. G.; Kreuer, K. D.; Maier, J.; Scharfenberger, G.; et al. Electrochim. Acta 2003, 48, 2165-2171.

  18. Free Radical Polymerization

  19. Proton Conductivities Both polymers stable to >200°C 10a (Tg = 51°C) 6 atom spacer 10b (Tg = 32°C) 12 atom spacer Immobilized imidazoles can conduct protons. Herz, H. G.; Kreuer, K. D.; Maier, J.; Scharfenberger, G.; et al. Electrochim. Acta 2003, 48, 2165-2171.

  20. Poly(siloxane) Backbones V4 D3 Persson, J. C.; Jannasch, P. Macromolecules 2005, 38, 3283-3289.

  21. Anionic ROP Mechanism THF V4 D3

  22. Benzimidazole Poly(siloxanes) Persson, J. C.; Jannasch, P. Macromolecules 2005, 38, 3283-3289.

  23. Tg and Heterocycle Content Thermally stable to ~200ºC Tg rises with heterocycle content for poly(siloxanes). Persson, J. C.; Jannasch, P. Macromolecules 2005, 38, 3283-3289.

  24. H+ Conductivity vs. BzIm Content • At lower temperatures - conductivity depends on Tg • At higher temperatures - conductivity depends on heterocycle content For conductivity to be unaffected by segmental mobility: T > Tg + 50ºC Persson, J. C.; Jannasch, P. Macromolecules 2005, 38, 3283-3289.

  25. PEO Backbones Persson, J. C.; Jannasch, P. Chem. Mater. 2006, 18, 3096-3102.

  26. Stability and Physical Properties Tg rises with heterocycle content for multiple polymers. Persson, J. C.; Jannasch, P. Chem. Mater. 2006, 18, 3096-3102.

  27. Conductivity and Mass Fraction Increasing Benzimidazole Content Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Bz8PEO Absolute Proton Conductivity Intrinsic Proton Conductivity Persson, J. C.; Jannasch, P. Chem. Mater. 2006, 18, 3096-3102.

  28. Triazole Tethered Polyacrylates Tz6 HMTz6 Martwiset, S.; Woudenberg, R. C.; Granados-Focil, S.; et. al. Solid State Ionics 2007, 178, 1398-1403.

  29. Triazole Tethered Polyacrylates Tz6 HMTz6 PEG n = 6-8 • X, Y, and Z controlled with feed ratios Martwiset, S.; Woudenberg, R. C.; Granados-Focil, S.; et. al. Solid State Ionics 2007, 178, 1398-1403.

  30. Conductivity with Acid Doping Sample 2 Sample 3 Sample 1 Sample 4 Sample 5 Sample 6 Samples had 28 mol% PEG and 30 mol% HMTz6 compared to Tz6 Doping with TFA increases conductivity significantly Martwiset, S.; Woudenberg, R. C.; Granados-Focil, S.; et. al. Solid State Ionics 2007, 178, 1398-1403.

  31. Proton Conductivity & Tg Sample A Sample B Sample C Sample D Sample E Samples had the same Mol % HMTz6 compared to Tz6 As Tg goes down -  goes up Martwiset, S.; Woudenberg, R. C.; Granados-Focil, S.; et. al. Solid State Ionics 2007, 178, 1398-1403.

  32. Intrinsic Conductivity & Tg As more PEG added, mass fraction of heterocycle goes down • Decreasing mass fraction of • the heterocycle: • decreases Tg • decreases conductivity Sample A Sample B Sample C Sample D Sample E • Decreasing Tg: • increases conductivity Mass fraction of heterocycle and Tg are interconnected. Martwiset, S.; Woudenberg, R. C.; Granados-Focil, S.; et. al. Solid State Ionics 2007, 178, 1398-1403.

  33. Imidazole Polysiloxanes Im5 Im8 Scharfenberger, G.; et. al. J. Fuel Cell 2006, 6, 237-250.

  34. Imidazole Polysiloxanes Im5 Partial Hydrolysis 70% cyclic trimers Same polymerization carried out with Im8 Scharfenberger, G.; et. al. J. Fuel Cell 2006, 6, 237-250.

  35. Triazole Tethered Polysiloxanes Granados-Focil, S.; Woudenberg, R. C.; Yavuzcetin, O.; et. al. Macromolecules 2007, 40, 8708-8713.

  36. Triazole Tethered Polysiloxanes Tz8Si FBz2Si Tz2Si Granados-Focil, S.; Woudenberg, R. C.; Yavuzcetin, O.; et. al. Macromolecules 2007, 40, 8708-8713.

  37. Conductivity & Tether Length Im8Si Tg = 7°C 28% Het. Im5Si Tg = 41°C 36% Het. Tz8Si Tg = -5°C 28% Het. 1000/K Tz2Si Tg = 19°C 43% Het. Different heterocycles need different tether lengths. Granados-Focil, S.; Woudenberg, R. C.; Yavuzcetin, O.; et. al. Macromolecules 2007, 40, 8708-8713. Scharfenberger, G.; et. al. J. Fuel Cell 2006, 6, 237-250.

  38. Conductivity and pKa pKa ~ 14 • Tz2Si higher  than FBz2Si • Despite having: • the same pKa • the same tether length • Tg factored out T-Tg Same pKas different conductivities Granados-Focil, S.; Woudenberg, R. C.; Yavuzcetin, O.; et. al. Macromolecules 2007, 40, 8708-8713.

  39. Conductivity and pKa pKa = 13.6 pKa = 18.6 Tz8Si Im8Si • Despite having: • Tg factored out • the same tether length • the same mass fraction • a higher pKa Higher pKa has a higher conductivity Granados-Focil, S.; Woudenberg, R. C.; Yavuzcetin, O.; et. al. Macromolecules 2007, 40, 8708-8713. Scharfenberger, G.; et. al. J. Fuel Cell 2006, 6, 237-250.

  40. Conductivity and pKa • Tg masks pKa effects • Mass fraction more dominant than pKa • Different heterocycles form the aggregates • necessary for proton conduction under • different conditions. Each polymer system should be optimized separately.

  41. Conclusions • Tg is the most dominant effect • Mass fraction of the heterocycle also dominant • Tether length and pKa are concerns • Immobilization decreases vehicular diffusion allowing for structural diffusion • Polymers shown -  ~ 10-6 - 10-3 S/cm • Nafion -  ~ 10-1 S/cm Heterocycle-containing polymers present a new route towards non-aqueous proton conduction at high temperatures

  42. Future Directions • Optimization of tether length, mass fraction,Tg, and acid doping • Block copolymers - systematic control over morphology and mechanical properties • Living polymerization techniques - systematic control over PDI and molecular weight • Protocols for evaluation of PEMs • TGA for thermal stability, DSC for Tg, & GPC for polydispersity index and molecular weight • EIS for proton conductivity • Protocols for hydrogen fuel cells • Rheology for mechanical stability • Membrane Electrode Assemblies

  43. Acknowledgements Professor Mahesh Mahanthappa • The Mahanthappa Group • Erin Henninger • Joan Widin • Chris Bates • David Bunck • Practice Talk Antendees • Mary Beth Anzovino • Matt Bierman • Maren Buck • Matt Christianson • Jeremy Higgins • Beth Landis • Katie Partridge • Dr. Dino Ress • Helpful Conversations • Beth Landis

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