Fundamentals of Fuel Cells


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  • How? Basic electrochemistry – Free energyPerformance characteristics – OCV, losses (then explain the Nernst equation)
  • Talk about electrode potential which drives electrons to cathode Hydrogen is oxidized and
  • Efficiency is around 60% in space applicationTemperature : 23 – 70 (new designs) (100 – 250 range in old designs)Anode: 2H2 + 4OH- - 4H2O + 4e-Cathode: O2 + 4e- + 2H2O - 4OH-
  • Transport application : high power to weight ratio, low operating temperature (quick start up time)Operating temp: about 50 - 90 CAnode: H2 - 2H+ + 2e-Cathode: ½O2 + 2H+ + 2e- H2O Water managementis also a problem
  • Needs expensive Pt catalystOperation is mainly similar to that of PEMFCTemperature : about 200 C Many commercial applications are found of capacity in the range of MW (up to 10 MW)
  • Anode:2H2 + 2CO32- - 2H2O + 2CO2 + 4e-Cathode:O2 + 2CO2 + 4e- - 2CO32-Operating temperature: 600 C Plants of capacity 200kW – 2MW
  • Anode: H2 + O2- - H2O+ 2e-Cathode:O2 + 4e- - 4e-
  • You can light up the bulb without heating up the world Silent operation enables localized power generation (at homes)Combination of alternative energies and higher efficiencies will give a better solution for energy demand problems
  • Toshiba fuel cell phone chargerBoing first fuel cell powered manned aircraft (in 2008)
  • Total 500 billion cubic meters
  • 95% steam reforming of methane
  • Light weight – storage efficiency drops downColour and Odour – Needs special equipment to detect leakagesLow viscosity – very fast at spreading Hydrogen boiling point is 22KTalk about carbon nanofibers to absorb hydrogen According to USDoE Cost is a key barrier to shift from fossil economy to hydrogen economy
  • I will tell you the importance of writing G in this way
  • You clearly see that free energy is less than the enthalpy
  • This is contradictory SOFC – high efficient …..but it is highest in operating temperature
  • These are the key losses on SOFCFuel crossover losses are not available with SOFC
  • Talk about P-N JunctionElectric double layer
  • This is the dominant loss in SOFC
  • Bell & Spigot designBanded cellsTubular SOFC needs higher operating temperatures ( 900 – 1000) PSOFC enables less expensive simple metals (instead of Inconel or less effective ceramics ) as interconnect
  • Main problem with GDC: electronic conductivity at low temperature
  • Ohmic losses will increase
  • Fundamentals of Fuel Cells

    1. 1. Fundamental Aspects of SOFC Manoj Ranaweera PhD Student (Low Carbon Technologies Group)
    2. 2. • What is a fuel cell? • Importance• Different types of fuel cells • Applications What Why How • Fuelling of fuel cells • Basic thermochemistry of free energy • Performance characteristics • Cell stack arrangements and components (SOFC) 2
    3. 3. WhatWhat is a fuel cell? HeatChemical Energy Fuel Cell Electrical Energy 3
    4. 4. WhatWhat is the function of a FC? Source: 4
    5. 5. Alkaline fuel What cell PolymerSolid oxide fuel exchange cell membrane fuel cell Fuel Cell Types Molten Phosphoric acid carbonate fuel fuel cell cell 5
    6. 6. AFC PEMFC PAFC MCFC SOFC What • The first fuel cell technologies developed • Used in NASA space missions (Apollo shuttle) • Electrolyte is an alkali (KOH) • CO2 poisoning is a significant problem 6
    7. 7. AFC PEMFC PAFC MCFC SOFC What • Mainly used in transportation applications • Polymer electrolyte • Requirement of highly pure hydrogen is a problem. – Even a slight amount of CO present in fuel will poison Pt catalyst 7
    8. 8. AFC PEMFC PAFC MCFC SOFC What • Considered as the first commercialized fuel cell type • Mainly used for stationary power generation • This contains an acid electrolyte • CO poisoning is a significant drawback 8
    9. 9. AFC PEMFC PAFC MCFC SOFC What • Electrolyte is an alkali carbonate containing CO32- ions. • No need of Noble materials as catalysts • Internal reforming is possible • CO is a fuel 9
    10. 10. AFC PEMFC PAFC MCFC SOFC What • Highest operating temperature • Use a solid ceramic as the electrolyte • High efficient • Fuel flexibility • Durability problems due to high temperature is an issue 10
    11. 11. High efficient energy conversion Silent in operation Importance of FC technologyLow environmental pollution Effective use of alternative energies 11
    12. 12. Application areas 12
    13. 13. Fundamentals of Fuel Cells How 13
    14. 14. How Fuelling of Fuel cells • Hydrogen is the main fuel for fuel cells • Hydrogen does not exist sufficiently as a gas • Reforming of hydrogen from hydrogen contained substances is the way to obtain itFacts: Sources of hydrogen in year 2000 (worldwide) Natural gas 48% Oil 30% 96% from hydrocarbons Coal 18% Water 04% Data from : US Department of Energy 14
    15. 15. Hydrogen Reforming External reforming Internal reforming Steam Partial PyrolysisReforming oxidation 15
    16. 16. Hydrogen Reforming External reforming Internal reforming Steam Partial Pyrolysis Reforming oxidation• For saturated HC : CnH(2n+2) + nH2O - (2n+1)H2 + nCO• For unsaturated HC: CnH2n + nH2O - 2nH2 + nCO• Gas shift reaction : CO + H2O  H2 + CO2 (t > 5000C with Ni Catalyst)• 95 % of hydrogen production in US is from steam reforming (US Energy 2010) 16
    17. 17. Problems with hydrogen• Hydrogen storage – Lightest substance – Colourless and odourless – Very low viscosity• Some different approaches of storage – As a compressed gas (in small quantities) – Liquefied hydrogen (in large quantities) – Use of carrier compound • Some metal hydrides • Methanol …etc.• Cost of hydrogen production is a key barrier 17
    18. 18. Basic thermochemistry to understand fuel cell performances Maximum theoretical electrical energy obtainable from a fuel cell is the “free energy” of the fuel 18
    19. 19. Free energy Gibbs free energy Helmholtz free energyFree energy available Free energy availableunder constant P and T under constant V and T 19
    20. 20. Gibbs free energy • Consider a constant pressure process From 1st law of thermodynamics Work Change of Change ofHeat supplied supplied to + system = enthalpy of + KE and PE to system the system (non- mechanical) Sign convention : Heat in and Work out is POSITIVE Q W dH (1) 20
    21. 21. From 2nd law of thermodynamics Q TdS (2) Q W dH (1)dW TdS dH (3) If the process is isothermaldW d (TS H ) (4)If a thermodynamic function is written as G ( H TS )dW dG (5) (dW ) MAX dG 21
    22. 22. Any chemical reaction will be spontaneous inthe direction where free energy is minimized G H T S 1 H2 O2 H 2O 2 gf ( g f ) H 2O [ (g f )H2 1 ( g f )O ] 2 2 22
    23. 23. Ideal cell voltage 1 H2 O2 H 2O(Welect)max gf (1) 2If emf of cell E H2 2H 2eWelect 2( Ne) EWelect 2FE (2)(1) (2)2 FE gf gfE 2F 23
    24. 24. Theoretical efficiency and temperature Welect Cell efficiency 100 % Hf GfMaximum theoretical efficiency 100% Hf Free energy Temperature G H T S 24
    25. 25. Losses (polarization)• Activation polarization• Ohmic polarization• Concentration (mass transport) polarization 25
    26. 26. Activation polarization• There is a certain voltage required to drive the chemical reaction in one direction. This voltage is known as activation voltage Tafel equation i Vact A ln ( for i i0 ) [i0 exchange current density] i0 Actual cell voltage with only activation losses i Vcell E A ln i0 26
    27. 27. Ohmic polarization• This is due to electronic and ionic resistances• Major kind of losses in SOFC• These can be reduced by – Using an electrolyte which has highest ionic conductivity – Good design and selecting high conductivity materials for cell interconnects electrodes, bipolar plates, etc. – Making electrolyte as thin as possible. (Practically there are limitations to this as electrolyte has to support certain components of a fuel 27
    28. 28. Concentration polarization• This is caused due to fuel and oxidant utilization. Nernst equation 0 0 RT PH 2 . PO2 1 2 0 gf E E ln E 2F PH 2O 2F 1 2 PH 2 PO2 0 . 0 P P0 gf gf RT ln PH 2O P0 28
    29. 29. SOFC Configuration & Components 29
    30. 30. Cell component characteristics• High chemical stability• Excellent electronic conductivity for electrodes and interconnects• Excellent ionic conductivity and zero electronic conductivity for electrolyte• Matching thermal expansion coefficients for individual components• Excellent mechanical stability• Ability to fabricate less expensively• Large range of temperature stability from room temperature to fabrication temperature• Chemical compatibility with other components• Low cost• Prevention of gas leakage for electrolyte• Sufficient porosity for electrodes 30
    31. 31. Cell ConfigurationSelf Support External Support A Electrolyte support E Porous substrate C A Cathode support C Interconnect A Anode Support C 31
    32. 32. Tubular (TSOFC) Planar (PSOFC) Stack Design Segmented cells in series Monolithic 32
    33. 33. Electrolyte• YSZ is commonly used for high temperature SOFC• Good oxygen ion conductor at elevated temperatures• Gadolinium doped Ceria (GDC) is used for ITSOFC 33
    34. 34. Electrodes• Must be a good electronic conductor• Anode – Must be a good electronic conductor – Made out of a cermet of Ni/YSZ – About 20-40% porous structure – Ni content varies from 30 – 70%• Cathode – lanthanum strontium manganite (LSM) or lanthanum calcium manganite (LCM) – Good chemical & thermal stability with electrolyte material 34
    35. 35. Interconnects• Expensive Inconel types metals are needed (if made out of metals)• Traditional material is Lanthanum Chromite (LaCrO3)• Lanthanum Chromite shows a remarkable electronic conductivity at temperatures around 10000C.• Degradation of cathode performances due to Chromium is a problem. – Formation of Strontium Chromate (SrCrO4) decreases the electronic conductivity 35
    36. 36. Optimizing the performance Running Capital Cost Cost Reliability Power density FC type ThermalFuel source Mechanical stability stability Space Durability constraints Chemical Stability 36
    37. 37. Thank You 37