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

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

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

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

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

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 Small and mobile applications: 0.01-100 kW Hydrogen, methanol – air Phosphoric acid (PAFC) 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 Wide scale applications: 1 kW-10 MW Natural gas, oil - air Zinc-air Protonic ceramic 40-100 ºC ~600 ºC 0.01-20 kW 10-1000 kW “rechargeable” natural gas, oil - air

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.

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

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

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.

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

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.

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

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

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

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

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

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

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

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

PEMFC: Polymer electrolyte and water Fig 8. A pendant chain of Nafion surrounded by water molecules.

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].

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].

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.

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

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.

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.

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?

Fig 14. “Water-free” membranes.

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

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]

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.

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

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