Proton exchange membranes: materials, theory and modelling

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Proton exchange membranesmaterials, theory and
modelling

• Andi Hektor, andi_at_ut.ee

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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

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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

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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.
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Different types of fuel cell
Fuel cell type Mobile ion Operating temp. Applications Direct fuel
Alkaline (AFC) OH- 50-200 ºC (low) Space vehicles 10 kW Hydrogen oxygen
Proton exchange membrane (PEMFC) H 50-100 ºC (low) Small and mobile applications 0.01-100 kW Hydrogen, methanol air
Phosphoric acid (PAFC) H 180-240 ºC (medium) Medium applications 100-1000 kW Hydrogen, natural gas - air
Molten carbonate (MCFC) CO32- 650 ºC (high) Medium and large applications 0.1-10 MW Natural gas, oli - air
Solid oxide (SOFC) O2- 500-1000 ºC (high) Wide scale applications 1 kW-10 MW Natural gas, oil - air
Zinc-air Protonic ceramic OH- H 40-100 ºC 600 ºC 0.01-20 kW 10-1000 kW rechargeable natural gas, oil - air
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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.

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High energy-conversion efficiency
Fig 3. Thermodynamic efficency for fuel cells and
Carnot efficiency for heat engines 3.
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Modular design
Fig 4. Fuel cells for different scale
applications 1.
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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.

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Theory and practice

• 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

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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.

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PEMFC Working principle
Fig 5. Schematic of a PEMFC 4.
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PEMFC Anode, polymer electrolyte, cathode
Fig 6. Schematic of the different layers in the
membrane 5.
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Table 1. Proton conductivity (S cm-1) and
activation energy (eV) for some representative
materials at room temperature 6.
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PEMFC Polymer electrolyte

• Nafion

Polysulfone (PS)
Polybezimidazole (PBI)
PolyEtherEtherKetone (PEEK)
Ref. 6
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Fig 8. Conductivity as a function of temperature
for some low temperature proton conductors 6.
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PEMFC Polymer electrolyte and water
Fig 7. Stylized view of polar/non-polar
microphase separation in a hydrated ionomer 7.
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PEMFC Polymer electrolyte and water
Fig 7. Stylised view of water-Nafion morphology
in a hydrated ionomer.
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PEMFC Polymer electrolyte and water
Fig 7. Schematic and hypothetical representation
of the microstructures of Nafion and a sulfonated
PEEKK 8.
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PEMFC Polymer electrolyte and water
Fig 8. A pendant chain of Nafion surrounded by
water molecules.
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Fig 9. Conductivity at 100 C as a function of
relative humidity for Nafion 117, SPEEK 2.48 and
?-Zr sulfophenyl phosphonate (?-ZrP(SPP)) 6.
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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.
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PEMFC Water balance in membrane
e
e
H2 ? ?2H2e
Anode
O24H4e ? ?2H2O
Cathode
H transport
H2
O2
H2O
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.
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PEMFC Water balance in membrane
Fig 11. Relative humidity as a function of
temperature at constant pressure of water vapour
6.
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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.

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DMFC Working principle
e
e
CH3OHH2O? ?CO26H6e
Anode
O24H4e ? ?2H2O
Cathode
H transport
CH3OH
O2
H2O
H2O
fuel crossover
D R Y
H transport
Catalyst poisoning Pt-CO
fuel crossover
H2O
CO2
H transport
Fig 13. Schematic of a DMFC.
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DMFC Problems and possible solutions

• Methanol crossover
• Hybrid membranes, nanocomposites, etc
• Catalyst poisoning (Pt-CO)
• Better complex catalyst (Pt-X), higher
  temperature (gt120C)
• Slow water shift reaction (CH3OHH2O ?
  CO26H6e) below 100 C
• Better complex catalyst, higher temperature
• But the higher the temperature, the worse the
  water balance in membrane
• Water-free membranes?

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Fig 14. Water-free membranes.
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Modelling of Nafion Basic questions

• Morphology of Nafion
• Dynamical behaviour
• Proton conductivity
• Mechanical stability
• Water and fuel diffusion
• Electron conductivity, etc.

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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

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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.
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References

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

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References

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