1. Proton exchange membranes: materials, theory and modelling
Andi Hektor,

2. Outline 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

3. Outline PEMFC DMFC Modelling of Nafion References • Working principle
• Anode, polymer electrolyte, cathode
• Polymer electrolyte
• Polymer electrolyte and water
• Water balance in membrane
DMFC
• 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
Fuel cell type
Mobile ion
Operating temp.
Applications
Direct fuel
Alkaline
(AFC)
OH- ºC
(low)
Space vehicles: ~10 kW
Hydrogen – oxygen
Proton exchange membrane
(PEMFC) H+ ºC
Small and mobile applications: kW
Hydrogen, methanol – air
Phosphoric acid (PAFC) ºC
(medium)
Medium applications: kW
Hydrogen, natural gas - air
Molten carbonate (MCFC)
CO32-
~650 ºC
(high)
Medium and large applications: MW
Natural gas, oli - air
Solid oxide
(SOFC)
O2- ºC
Wide scale applications:
1 kW-10 MW
Natural gas, oil - air
Zinc-air
Protonic ceramic ºC
~600 ºC kW kW
“rechargeable” natural gas, oil - air

7. Why a fuel cell? 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.

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. Fuel flexibility and pollution
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.

11. Theory and practice Working and future types of fuel cell: Problems:
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

12. Alternatives
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.

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. PEMFC: Polymer electrolyte
Nafion
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 surrounded by 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
Fig 12. Fully optimised (B3LYP/6-31G**) conformations of water clusters of Triflic acid: a) CF3SO3H + H2O; b) CF3SO3H + 2 H2O; b) CF3SO3H + 3 H2O [12].

24. PEMFC: Water balance in membrane
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. PEMFC: Water balance in membrane
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.

27. DMFC: Working principle
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. Schematic of a DMFC.

28. DMFC: Problems and possible solutions
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?

29. Fig 14. “Water-free” membranes.

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

31. Modelling of Nafion: Different methods
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]

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

33. References http://www.fuelcells.org/
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34. References M. Sprik et al., J. Phys. Chem. B, 101 (1997) 2745
S. Walbran, A.A. Komyshev, J. Chem. Phys., 114 (2001) 10039