Electrochemistry in membrane fuel cells 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

Electrochemistry in membrane fuel cells 2 ½ O 2 + 2e + 4 H + H 2 O H 2 H + + 2e Charge eg. Engine O 2 /air H2H2 Electron flux in the external circuit 3/2 O 2 + 6e + 6 H + 3 H 2 O CH 3 OH + H 2 O CO 2 + 6H + + 6e Charge eg. Engine O 2 /air Methanol Electron flux in the external circuit PEMFC H 2 + ½ O 2 H 2 O +  H DMFC CH 3 OH + 3/2 O 2 CO H 2 O +  H Anode Membrane Cathode Anode Membrane Cathode Operation principle of membrane fuel cells

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

Electrochemistry in membrane fuel cells 4 Specific features of electrochemical reactions (C’td) Faraday’s law A + n e e - → B Existence of several reactions Current yield Ohm’s law Consequences 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 H2O2H2O2 H2O2H2O2 Outlet Feed External plate Bipolar plate Backing Membrane-electrode assembly PEMFC: Electrolyte = Conducting polymer Reduce the membrane thickness Improve the electrical connections Split view of a polymer electrolyte membrane fuel cell

Electrochemistry in membrane fuel cells 6

7 Active layer Active layer Backing Cathode Anode Membrane = Hydrated Conducting gel Carbon materials - conducting - hydrophobic Carbone 30 nm Platinum 2 nm  m20  m300  m 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 H 2 2H+ + 2e ½ O H+ + 2e H 2 O O 2 transport by convection and diffusion Cathode Anode Membrane Water Feed ? Liquid water Formation? Migration H + Electroosmosis (H 2 O) Diffusion of H 2 O Diffusion to Pt Diffusion to Pt Electrons Water management (excessive) Drying Flooding Heat H 2 transport by convection and diffusion

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

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  cm 2

Electrochemistry in membrane fuel cells 11 Calculation of the membrane resistance Area S Thickness e I I Ohm’s law: 1-D model Demonstrate : Calculate R for Nafion 112, 115 et 117 with S=100 cm 2 and  =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 cm 2 Electrode of PEMFC, S=100 cm 2,  =200

Electrochemistry in membrane fuel cells 13 Thermodynamics and theoretical yields of PEMFC’s U th, thermoneutral voltage U th = -  H / nF U rev, reversible voltage U rev = -  G / nF Theoretical yield  th  th =  G /  H PEMFCDMFC n 2 6  H (kJ/mol)  G (kJ/mol) U th (V) U rev (V)  th 83.0% 96.8%

Electrochemistry in membrane fuel cells 14 Variations with temperature Variations with pressure Present case: Water formation from O 2 and H 2

Electrochemistry in membrane fuel cells 15 FC cell voltage at zero current: the real case E 0, Zero current voltage << Voltage predicted by the thermodynamics. Why ? 1- Oxygen reduction: slow process H 2 O 2 is an inetermediate, with E(H 2 O 2 /H 2 O)=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 H 2 + ½ O 2 H 2 O Internal current density (cross over), i n = proport. Flux of H 2 diffusion Potential variation proport. to Ln(i n ) Usually, E 0 = V

Electrochemistry in membrane fuel cells 16 Kinetics of electrochemical processes Butler-Volmer’s model A + e B 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  = E - E 0 Exchange current density Charge transfer coefficient

Electrochemistry in membrane fuel cells 17 Example  = 0.5, i 0 variable Linear part Exponential part (irreversible) : Tafel Tafel’s law for  large enough  =a+blog(i) Kinetics of electrochemical processes (C’td)

Electrochemistry in membrane fuel cells 18 Electrode reactions: Hydrogen oxidation Platinum : Excellent catalyst « Easy reaction » Volmer-Tafel’s model : 2 Pt + H 2 2 PtH) ads Slow process 2H 2 O + 2PtH) ads 2Pt + 2H 3 O + + 2e Fast process Current density Overpotential  + 30 mV i10 i

Electrochemistry in membrane fuel cells 19 Platinum : One of the less worse catalysts Overall slow reaction Kinetics and mechanism : Pt or PtO 2 ? * Potential < 0.8 V (High cd) Pt + O 2 PtO 2 ) ads Fast process PtO 2 ) ads + H + + e PtO 2 H) ads Slow process PtO 2 H) ads + 3 H + + 3e 2H 2 O + Pt Fast  mVi10 i * Potential > 0.8 V (Low cd) PtO 2  + 60 mVi10 i Electrode reactions: Oxygen reduction

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

Electrochemistry in membrane fuel cells 21 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  d More complex relationship between i and  Kinetics of electrochemical processes (C’td)

Electrochemistry in membrane fuel cells 22  d : depends on mass transfer rate (diffusion and convection) When  tends to infinite, C As = 0 and i tends to i L, limiting current density i L =96 A/m2 Example :  = 0.5 Kinetics of electrochemical processes (C’td)

Electrochemistry in membrane fuel cells 23 Control by mass transfer phenomena in FC’s The involved phenomena GasConvection (bipolar plates, backing) Diffusion (backing, active layers) Knudsen diffusion (active layers) WaterTransport 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 Fuel cells For usual electrochemical reactors

Electrochemistry in membrane fuel cells 25 Available voltage in PEMFC’s Ohmic drop Diffusion control Revrsible voltage U rev = -  G/2F Zone 2 Zone 3 Hydrogen cross-over, PtO 2, H 2 O 2 etc. Electrochemical activation Cell voltage (V) Current density (A/cm2) Zone 1 U rev Zero current voltage

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 , characteristic dimension Thickness of the Nernst’s film Thickness of the electrode? 10 µm D, diffusion coefficient m 2 /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 Fuel cell Current Voltage Current Steady-state techniques Fixed current Low-rate scanning (of potential or current) Transient methods High-rate scanning Impedance spectroscopy Current step Frequency range: 50 kHz – 10 mHz Interpretation Estimation of the ohmic drop In most cases: No reference electrodes

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

Electrochemistry in membrane fuel cells Z'(ohm.cm²) - Z " ( ohm. cm ² ) 5 kHz 10 mHz Tension Q Equivalent electrical circuit Ract Rm Response of the electrodes <1 Hz 100 Hz

Electrochemistry in membrane fuel cells 32 Equivalent electrical circuit: a simple case Tension C Rt Rm  infiniteZ = R m  = 0Z = R p = R m + R t -Z ’’ Z’ Rm Rt Rp  =1/(R t C)

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

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 [funded by Johnson Matthey, worlds largest producer of Platinum, including that used by Mr Grove, producer of catalyst and MEAs]