Presentation on theme: "Proton exchange membranes: materials, theory and modelling"— Presentation transcript:
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 membraneDMFC• Problems and possible solutionModelling of Nafion• Basic questions• Different methods• Molecular Dynamics?References
4 What is a fuel cell?Fig 1. Proton and hydroxyl conducting fuel cells .
5 Historical background Fig 2. The first functional fuel cell – 50 years before internal combustion engines .
6 Different types of fuel cell Fuel cell typeMobile ionOperating temp.ApplicationsDirect fuelAlkaline(AFC)OH-ºC(low)Space vehicles: ~10 kWHydrogen – oxygenProton exchange membrane(PEMFC)H+ºCSmall and mobile applications: kWHydrogen, methanol – airPhosphoric acid (PAFC)ºC(medium)Medium applications:kWHydrogen, natural gas - airMolten carbonate (MCFC)CO32-~650 ºC(high)Medium and large applications: MWNatural gas, oli - airSolid oxide(SOFC)O2-ºCWide scale applications:1 kW-10 MWNatural gas, oil - airZinc-airProtonic ceramicºC~600 ºCkWkW“rechargeable” natural gas, oil - air
7 Why a fuel cell? high energy-conversion efficiency modular design fuel flexibilitylow chemical and acoustical pollutioncogeneration capabilityrapid load responsetheory and practicealternatives: 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 .
9 Modular designFig 4. Fuel cells for different scale applications .
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 todayMolten carbonate (MCFC) – some working experimental medium-power plantsSolid oxide (SOFC) – some working experimental medium and high power and heat plantsProblems:expensive materialscompanies do not have common standards, etc
12 AlternativesAdvanced 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 .
14 PEMFC: Anode, polymer electrolyte, cathode Fig 6. Schematic of the different layers in the membrane .
15 Table 1. Proton conductivity (S cm-1) and activation energy (eV) for some representative materials at room temperature .
17 Fig 8. Conductivity as a function of temperature for some low temperature proton conductors .
18 PEMFC: Polymer electrolyte and water Fig 7. Stylized view of polar/non-polar microphase separation in a hydrated ionomer .
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 .
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)) .
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 .
24 PEMFC: Water balance in membrane H2 2H++2eAnodeO2+4H++4e 2H2OCathodeH+ transportH2OH2O2H2OH2O diffusionDRYWETElectro-osmotic dragH+(H2O)H2O diffusionH2OH+ transportFig 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 .
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+H2OCO2+6H++6eAnodeO2+4H++4e 2H2OCathodeH+ transportH2OCH3OHO2H2Ofuel crossoverDRYH+ transportCatalyst poisoningPt-COfuel crossoverH2OCO2H+ transportFig 13. Schematic of a DMFC.
28 DMFC: Problems and possible solutions Methanol crossoverHybrid membranes, nanocomposites, etcCatalyst poisoning (Pt-CO)Better complex catalyst (Pt-X), higher temperature (>120°C)Slow “water shift reaction” (CH3OH+H2O CO2+6H++6e) below ~100 °CBetter complex catalyst, higher temperatureBut the higher the temperature, the worse the water balance in membraneWater-free membranes?
30 Modelling of Nafion: Basic questions Morphology of NafionDynamical behaviourProton conductivityMechanical stabilityWater and fuel diffusionElectron conductivity, etc.
31 Modelling of Nafion: Different methods Phenomenological models based on nonequilibrium thermodynamics Statistical mechanical models based on Nernst-Planc equations Statistical mechanical models based on generalised Stefan-Maxwell equations [11,12]Percolation models MD, QM/MM, ab inito simulations [12,14-17]
32 Modelling of Nafion: Molecular Dynamics? MD system size: ~ 104 atomsPotentials: “non-classic” MD potentials for proton transport (water-water, water-acid group, acid group-acid group) Fig 15. “Non-classic” MD proton jump between water molecules.
33 References http://www.fuelcells.org/ J.J. Baschuck, X. Li, J. Power Sources, 86 (2000) 181P. Costamagna, S. Srinivasan, J. Power Sources, 102 (2001) 242G. Alberti, M. Casciola, Solid State Ionics, 145 (2001) 3K.D. Kreuer, J. Membr. Sci., 185 (2001) 29R.F. Mann et al., J. Power Sources, 86 (2000) 173E.H. Cwirko, R.G. Carbonell, J. Power Sources, 67 (1992) 227M. Eikerling et al., J. Phys. Chem. B, 105 (2001) 3646S. J. Paddison, J. New Mat. Electrochem. Sys., 4 (2001) 197M. Eikerling et al., J. Phys. Chem. B, 101 (1997) 10807S.J. Paddison, T.A. Zawodzinski, Solid State Ionics, 113 (1998) 333D. 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