Proton exchange membranes: materials, theory and modelling

1. Proton exchange membranes:materials, theory and modelling Andi Hektor, andi@ut.ee

2. Introduction •What is a fuel cell? •Historical background •Different types of fuel cell •Why a fuel cell? •High energy-conversation efficiency •Modular design •Fuel flexibility and pollution •Theory and practice •Alternatives Outline

3. Outline • PEMFC • •Working principle • •Anode, polymer electrolyte, cathode • •Polymer electrolyte • •Polymer electrolyte and water • •Water balance in membrane • DMFC • •Working principle • •Problems and possible solution • Modelling of Nafion • •Basic questions • •Different methods • •Molecular Dynamics? • References

4. What is a fuel cell? Fig 1. Proton and hydroxyl conducting fuel cells [1].

5. Historical background Fig 2. The first functional fuel cell – 50 years before internal combustion engines [2].

6. Different types of fuel cell

7. high energy-conversion efficiency modular design fuel flexibility low chemical and acoustical pollution cogeneration capability rapid load response theory and practice alternatives: advanced batteries, superconducting technologies, air-powered energy storage, solar cells, etc. Why a fuel cell?

8. High energy-conversion efficiency Fig 3. Thermodynamic efficency for fuel cells and Carnot efficiency for heat engines [3].

9. Modular design Fig 4. Fuel cells for different scale applications [1].

10. Hydrogen– The most efficient fuel for all types of fuel cell, but a lot of storage and transport problems. No pollution. Methanol, ethanol, biogas – Good fuel, but lower efficiency. Low CO2 pollution. Natural oil or gas – Not so good fuel, usually need some kind of preprocessing before fuel cell (e.g. sulphur elimination, etc). CO2 pollution, very low NxOy or SxOy pollution. Construction materials for fuel cells – Some bad components (e.g. fluorine, heavy metals, etc), but many possibilities for reproduction. Fuel flexibility and pollution

11. Working and future types of fuel cell: Phosphoric acid (PAFC) – a lot of working medium systems (0.1-1 MW), but quite difficult to manage (liquid phosphoric acid, etc) Proton exchange membrane (PEMFC) – good prospect for small and mobile systems (from cell phone to car), but expensive today Molten carbonate (MCFC) – some working experimental medium-power plants Solid oxide (SOFC) – some working experimental medium and high power and heat plants Problems: expensive materials companies do not have common standards, etc Theory and practice

12. Advanced batteries– Expensive today, long recharge time, etc. E.g., promising for the fuel cell/battery hybrid system of cars. Superconducting technologies – Theoretically very prospective, but a lot of problems in practice. Air-powered energy storage – Perspective only for cars. Alternatives

13. PEMFC: Working principle Fig 5.Schematic of a PEMFC [4].

14. PEMFC: Anode, polymer electrolyte, cathode Fig 6.Schematic of the different layers in the membrane [5].

15. Table 1. Proton conductivity (S cm-1) and activation energy (eV) for some representative materials at room temperature [6].

16. Nafion PEMFC: Polymer electrolyte Polysulfone (PS) Polybezimidazole (PBI) PolyEtherEtherKetone (PEEK) Ref. [6]

17. Fig 8.Conductivity as a function of temperature for some low temperature proton conductors [6].

18. PEMFC: Polymer electrolyte and water Fig 7.Stylized view of polar/non-polar microphase separation in a hydrated ionomer [7].

19. PEMFC: Polymer electrolyte and water Fig 7. Stylised view of water-Nafion morphology in a hydrated ionomer.

20. PEMFC: Polymer electrolyte and water Fig 7.Schematic and hypothetical representation of the microstructures of Nafion and a sulfonated PEEKK[8].

21. PEMFC: Polymer electrolyte and water Fig 8.A pendant chain of Nafion surroundedby water molecules.

22. Fig 9.Conductivity at 100 °C as a function of relative humidity for Nafion 117, SPEEK 2.48 and γ-Zr sulfophenyl phosphonate (γ-ZrP(SPP)) [6].

23. Fig 12.Fully optimised (B3LYP/6-31G**) conformations ofwater clusters of Triflic acid: a) CF3SO3H + H2O; b) CF3SO3H+ 2 H2O; b) CF3SO3H + 3 H2O [12].

24. PEMFC: Water balance in membrane e e H2 2H++2e Anode O2+4H++4e  2H2O Cathode H+ transport H2O H2 O2 H2O H2O diffusion D R Y W E T Electro-osmotic drag H+(H2O) H2O diffusion H2O H+ transport Fig 10.Water balance in polymer membrane.

25. PEMFC: Water balance in membrane Fig 11.Relative humidity as a function of temperature at constant pressure of water vapour [6].

26. It is very difficult to attain good water balance in a membrane at higher than 100 °C at normal air-fuel pressure (water boiling point)! On the other side - the higher the temperature, the better the proton conductivity. PEMFC: Water balance in membrane

27. DMFC: Working principle e e CH3OH+H2O CO2+6H++6e Anode O2+4H++4e  2H2O Cathode H+ transport H2O CH3OH O2 H2O fuel crossover D R Y H+ transport Catalyst poisoning Pt-CO fuel crossover H2O CO2 H+ transport Fig 13.Schematicof a DMFC.

28. Methanol crossover Hybrid membranes, nanocomposites, etc Catalyst poisoning (Pt-CO) Better complex catalyst (Pt-X), higher temperature (>120°C) Slow “water shift reaction” (CH3OH+H2O  CO2+6H++6e) below ~100 °C Better complex catalyst, higher temperature But the higher the temperature, the worse the water balance in membrane Water-free membranes? DMFC: Problems and possible solutions

29. Fig 14. “Water-free” membranes.

30. Morphology of Nafion Dynamical behaviour Proton conductivity Mechanical stability Water and fuel diffusion Electron conductivity, etc. Modelling of Nafion: Basic questions

31. Phenomenological models based on nonequilibrium thermodynamics [9] Statistical mechanical models based on Nernst-Planc equations [10] Statistical mechanical models based on generalised Stefan-Maxwell equations [11,12] Percolation models [13] MD, QM/MM, ab inito simulations [12,14-17] Modelling of Nafion: Different methods

32. MD system size: ~ 104 atoms Potentials: “non-classic” MD potentials for proton transport (water-water, water-acid group, acid group-acid group) [17] Modelling of Nafion: Molecular Dynamics? Fig 15. “Non-classic” MD proton jump between water molecules.

33. References • http://www.fuelcells.org/ • http://www.protonetics.com/fuel.htm • http://www.visionengineer.com/env/fuelcells.shtml • J.J. Baschuck, X. Li, J. Power Sources, 86 (2000) 181 • P. Costamagna, S. Srinivasan, J. Power Sources, 102 (2001) 242 • G. Alberti, M. Casciola, Solid State Ionics, 145 (2001) 3 • http://www.psrc.usm.edu/mauritz/nafion.html • K.D. Kreuer, J. Membr. Sci., 185 (2001) 29 • R.F. Mann et al., J. Power Sources, 86 (2000) 173 • E.H. Cwirko, R.G. Carbonell, J. Power Sources, 67 (1992) 227 • M. Eikerling et al., J. Phys. Chem. B, 105 (2001) 3646 • S. J. Paddison, J. New Mat. Electrochem. Sys., 4 (2001) 197 • M. Eikerling et al., J. Phys. Chem. B, 101 (1997) 10807 • S.J. Paddison, T.A. Zawodzinski, Solid State Ionics, 113 (1998) 333 • D. B. Holt, B.L. Farmer, Polymer, 40 (1999) 4667

34. References • M. Sprik et al., J. Phys. Chem. B, 101 (1997) 2745 • S. Walbran, A.A. Komyshev, J. Chem. Phys., 114 (2001) 10039