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Getting started with electrochemistry in polymer electrolyte membrane fuel cells (PEMFC):

Getting started with electrochemistry in polymer electrolyte membrane fuel cells (PEMFC):. Francois Lapicque Laboratoire des Sciences du Génie Chimique, CNRS –ENSIC, Nancy. Background of electrochemical phenomena in FC Features of electrochemical reactions Transport and transfer

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Getting started with electrochemistry in polymer electrolyte membrane fuel cells (PEMFC):

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  1. Getting started with electrochemistry in polymer electrolyte membrane fuel cells (PEMFC): Francois Lapicque Laboratoire des Sciences du Génie Chimique, CNRS –ENSIC, Nancy • Background of electrochemical phenomena in FC • Features of electrochemical reactions • Transport and transfer • Available electrochemical methods for their investigation Presented by: Dr Bradley Ladewig brad.ladewig@gmail.com Electrochemistry in membrane fuel cells

  2. Operation principle of membrane fuel cells H2 PEMFC H2 + ½ O2 H2O + DH DMFC CH3OH + 3/2 O2 CO2 + 2 H2O + DH H2 H+ + 2e Anode Membrane Cathode Charge eg. Engine ½ O2 + 2e + 4 H+ H2O Electron flux in the external circuit O2/air Methanol CH3OH + H2O CO2 + 6H+ + 6e Anode Membrane Cathode Charge eg. Engine 3/2 O2 + 6e + 6 H+ 3 H2O Electron flux in the external circuit O2/air Electrochemistry in membrane fuel cells

  3. Anode: A B + e Cathode: C + e D Specific features of electrochemical reactions Particularités de la réaction électrochimique Heterogeneous process involving the exchange of charges Transfer to the electrode Current : Electrons Current: Ions Adsorption Charge transfer (Chemical Processes) Desorption Transport Electrochemistry in membrane fuel cells

  4. Specific features of electrochemical reactions (C’td) Faraday’s law Existence of several reactions A + ne e - → B Current yield Consequences Ohm’s law • Ohmic drop : linked to Joule effect • Reduce the electrode gap • Improve the electrical conductivity of the medium To be minimised Electrochemistry in membrane fuel cells

  5. H2 O2 Feed H2 O2 Outlet Membrane-electrode assembly Bipolar plate External plate Backing Split view of a polymer electrolyte membrane fuel cell • PEMFC: • Electrolyte = Conducting polymer • Reduce the membrane thickness • Improve the electrical connections Electrochemistry in membrane fuel cells

  6. Electrochemistry in membrane fuel cells

  7. Active layer Active layer Cathode Anode Backing Backing Membrane = Hydrated Conducting gel • Carbon materials • conducting • - hydrophobic Carbone 30 nm Platinum 2 nm 50-150 mm 20 mm 300 mm Electrodes and membrane Pt-Ru catalyst deposited on XC-72 X. Xue et al. Electrochem. Comm. 8 (2006) 1280 Electrochemistry in membrane fuel cells

  8. ½ O2 + 2 H+ + 2e H2O H2 2H+ + 2e Diffusion to Pt Diffusion to Pt Cathode Membrane Anode O2 transport by convection and diffusion Migration H+ Electroosmosis (H2O) H2 transport by convection and diffusion Electrons Electrons Liquid water Formation? Diffusion of H2O Water Feed ? Heat Heat Water management (excessive) Drying Flooding Electrochemistry in membrane fuel cells

  9. DIFFUSION LAYER (backing) Graphite porous structure (e.g. Toray paper) + hydrophobic agent (PTFE) Thin layer of carbon Materials (Vulcan XC-72R + platinum particles (Proton exchanging) e.g. Nafion Electrochemistry in membrane fuel cells

  10. Ohmic behaviour of PEMFC’s • * Membrane resistance • Importance • of hydration • * Other resistance sources : • Electrodes • Backings • Bipolar plates • Electrical connectors • Current leads R < 0.3 W cm2 Electrochemistry in membrane fuel cells

  11. I Area S I Thickness e Calculation of the membrane resistance Ohm’s law: 1-D model Demonstrate : Calculate R for Nafion 112, 115 et 117 with S=100 cm2 and k=0.1 S cm-1 Calculate the ohmic drop for current density at 0.1, 0.3 et 1 A cm-2 Electrochemistry in membrane fuel cells

  12. C R Time constant of a capacitor and a resistor in series Calculation of the equivalent complex impedance Time constant: RC C: double layer capacitance (see above). 30 µF cm-2 Calculation of the time constant in two cases: Flat electrode plane, S=100 cm2 Electrode of PEMFC, S=100 cm2, g=200 Electrochemistry in membrane fuel cells

  13. PEMFC DMFC n 2 6 DH (kJ/mol) -285.83 -726.51 DG (kJ/mol) -237.13 -703.35 Uth (V) 1.481 1.229 Urev (V) 1.229 1.215 hth 83.0% 96.8% Thermodynamics and theoretical yields of PEMFC’s Uth, thermoneutral voltage Uth = -DH / nF Urev, reversible voltage Urev = - DG / nF Theoretical yield hth hth = DG / DH Electrochemistry in membrane fuel cells

  14. Variations with temperature Variations with pressure Present case: Water formation from O2 and H2 Electrochemistry in membrane fuel cells

  15. FC cell voltage at zero current: the real case E0, Zero current voltage << Voltage predicted by the thermodynamics. Why ? Usually, E0 = 0.9 - 1.04 V 1- Oxygen reduction: slow process H2O2 is an inetermediate, with E(H2O2 /H2O)=0.68 V 2- Presence of Pt oxides, shift of the equilibrium potential 3- Existence of an internal current caused by hydrogen diffusion through the membrane followed by combustion at the cathode H2 + ½ O2 H2O Internal current density (cross over), in = proport. Flux of H2 diffusion Potential variation proport. to Ln(in) Electrochemistry in membrane fuel cells

  16. A + e B Kinetics of electrochemical processes Butler-Volmer’s model • Model assumptions: • Reversible reaction • One electron exchanged • Overall process controlled by charge transfer rate Development of the model: theory of the activated complex between A et B Expression for the current density i versus the overpotential h= E - E0 Charge transfer coefficient Exchange current density Electrochemistry in membrane fuel cells

  17. Kinetics of electrochemical processes (C’td) Example a = 0.5, i0 variable Exponential part (irreversible) : Tafel Linear part Tafel’s law for h large enough h=a+blog(i) Electrochemistry in membrane fuel cells

  18. Electrode reactions: Hydrogen oxidation Platinum : Excellent catalyst « Easy reaction » Volmer-Tafel’s model : 2 Pt + H2 2 PtH)ads Slow process 2H2O + 2PtH)ads 2Pt + 2H3O+ + 2e Fast process hh+ 30 mV i 10 i Current density Overpotential Electrochemistry in membrane fuel cells

  19. Electrode reactions: Oxygen reduction Platinum : One of the less worse catalysts Overall slow reaction Kinetics and mechanism : Pt or PtO2 ? * Potential < 0.8 V (High cd) Pt + O2 PtO2)ads Fast process PtO2)ads + H+ + e PtO2H)ads Slow process PtO2H)ads + 3 H+ + 3e 2H2O + Pt Fast hh+ 120 mV i 10 i * Potential > 0.8 V (Low cd) PtO2 hh+ 60 mV i 10 i Electrochemistry in membrane fuel cells

  20. C Ract Charge transfer resistance , Ract T=60°C S=100 cm2 i=0.5 A/cm2 b=17.4 V-1 (56 mV/decade) and Ract = 1.13 mW Calculation of the time constant Ract.C Electrochemistry in membrane fuel cells

  21. Kinetics of electrochemical processes (C’td) Case of high current densities: mass transfer can become rate-controlling Existence of an additional overpotential The overpotential is the sum of the charge transfer overpotential (Butler Volmer) and the concentration overpotential hd More complex relationship between i and h Electrochemistry in membrane fuel cells

  22. Kinetics of electrochemical processes (C’td) hd: depends on mass transfer rate (diffusion and convection) When h tends to infinite, CAs = 0 and i tends to iL, limiting current density iL=96 A/m2 Example : a = 0.5 Electrochemistry in membrane fuel cells

  23. Control by mass transfer phenomena in FC’s The involved phenomena Gas Convection (bipolar plates, backing) Diffusion (backing, active layers) Knudsen diffusion (active layers) Water Transport through the membrane Sharper problems For dilute reacting gases (air, reforming hydrogen) Problems raised by liquid water: Flow hindrance in the various parts: lower transfer rates i(lim) = 0.5 – 2 A cm-2 Electrochemistry in membrane fuel cells

  24. Cell voltage Usual reactors For usual electrochemical reactors Fuel cells Electrochemistry in membrane fuel cells

  25. Cell voltage (V) Revrsible voltage Urev = -DG/2F Urev Hydrogen cross-over, PtO2, H2O2 etc. Zero current voltage Electrochemical activation Ohmic drop Diffusion control Zone 1 Zone 2 Zone 3 Current density (A/cm2) Available voltage in PEMFC’s Electrochemistry in membrane fuel cells

  26. Example of i-E curves Electrochemistry in membrane fuel cells

  27. Dynamics of diffusion processes Transient Fick’s law, 1-D Characteristic time d, characteristic dimension Thickness of the Nernst’s film Thickness of the electrode? 10 µm D, diffusion coefficient 10-10 m2/s (in liquids or in the gel) Electrochemistry in membrane fuel cells

  28. Technology of electrochemical cells Electrical connection with monopolar electrodes Series Parallel Selection of the connection: * Significance of energy losses in the E-converter * Avoid too large currents and low voltages!! Electrochemistry in membrane fuel cells

  29. Electrochemical methods for FC investigations Voltage Current Current Voltage Fuel cell In most cases: No reference electrodes Steady-state techniques Fixed current Low-rate scanning (of potential or current) Interpretation Transient methods High-rate scanning Impedance spectroscopy Current step Frequency range: 50 kHz – 10 mHz Estimation of the ohmic drop Electrochemistry in membrane fuel cells

  30. Complex variable Impedance spectroscopy • Principle Current Voltage • Varying the frequency (10kHz to 10mHz) • Plotting data: Nyquist (-Z’’ vs. Z’), or • Bode (|Z] and F vs. w • Modelling using equivalent circuits or various balances Electrochemistry in membrane fuel cells

  31. 25 Equivalent electrical circuit 20 ) ² cm Rm 15 . Q ohm Ract " ( 10 Tension Z - 5 0 0 5 10 15 20 25 Z ' ( ohm . cm ² ) 10 mHz 5 kHz Response of the electrodes <1 Hz 100 Hz Electrochemistry in membrane fuel cells

  32. Rm C Rt Tension Equivalent electrical circuit: a simple case -Z ’’ w=1/(RtC) w infinite Z = Rm w = 0 Z = Rp = Rm + Rt Z’ Rm Rp Rt Electrochemistry in membrane fuel cells

  33. Electrochemical impedance: equivalent circuit In most cases, only one loop can be observed. Tilted loop in most cases: CPE Rt (charge transfer) Electrochemistry in membrane fuel cells

  34. Some fuel cell references • Larminie, J. and Dicks, A. (2000) Fuel Cell Systems Explained, Wiley, England. • Vielstich W (2003) Handbook of Fuel Cells (4 volumes), Wiley, England. • Grove, W. (1839) On voltaic series and the combination of gases by platinum, Philosophical Magazine Series 3 14:127 – 130. • Fuel Cell Today www.fuelcelltoday.com [funded by Johnson Matthey, worlds largest producer of Platinum, including that used by Mr Grove, producer of catalyst and MEAs] Electrochemistry in membrane fuel cells

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