7 - MEM 415 FUEL CELL ENGINES Lecture 7-POLARIZATION...

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2/21/2011 1 -- Lecture # 7 -- MEM 415– FUEL CELL ENGINES POLARIZATION: “ ACTIVATION POLARIZATION” (Region 1) Dr. E. Caglan Kumbur Assistant Professor of Mechanical Engineering Office: Curtis 160 Ph: 215.895.5871 e-mail: [email protected] -- Feb 21, 2011 -- MEM 415– Lecture #7 Typical Polarization Curve MEM 415– Lecture #7
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2/21/2011 2 Typical Polarization Curve MEM 415– Lecture #7 Polarization Curve Regions Polarization Curve Regions – Focus on Region I Focus on Region I We will first focus on Region-1 , which is activation polarization .. Region 1: Activation Polarization – Component of interest: Catalyst Layer Reason: Activation overpotential at the electrodes (Kinetic losses) Region 2: Ohmic Polarization – Component of interest: Electrolyte, Catalyst Layer, GDL Reason: Dominated by electrical and ionic conduction losses. Region 3: Concentration Polarization – Component of interest: Catalyst Layer, GDL Reason: Dominated by mass transport limitations. Region 4: Crossover Losses – Component of interest: Electrolyte Reason: Reactant crossover through the electrolyte. Region 5: Departure from maximum thermal voltage – Thermodynamics losses Reason: Departure from the maximum thermal voltage due to entropy change
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2/21/2011 3 Basic Fuel Cell Performance Model…. Let’s design a basic fuel cell model to predict the actual performance of the cell: How would you approach it? E actual = E thermal - Losses (Polarizations) Maximum Theoretical Voltage MEM 415– Lecture #7 Let’s design a basic fuel cell model…. R t t ti l Region IV Region I Region III Region II Region V "# Represents overpotential (polarization or losses) E cell = E(T,P) - " R - c,a -| c,c | - a,a a,c | - x Nernst OCV Ohmic Losses Anode/Cathode Concentration Losses Anode/Cathode Activation Losses Crossover Losses MEM 415– Lecture #7
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2/21/2011 4 Polarization Curve Analysis $% ( ) reactants , ln note its products! AB C D CD o o aa G T RT E T P nF nF a a && & ’( ) * + #, -. /0 1 2 ) 34 56 Pressure effect Temp effect Note that Nernst equation is calculated for pure thermodynamics equilibrium condition , i.e. can only be approached ____________________________ Now we will look a the conditions when we move away from equilibrium , i.e. when we start drawing current MEM 415– Lecture #7 Polarization Curve Regions We will focus on Region I of the polarization curve. It involves _________________ Region 1: Activation Polarization – Component of interest: Catalyst Layer Reason: Activation overpotential at the electrodes (Kinetic losses in catalyst) MEM 415– Lecture #7
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2/21/2011 5 Summary of Last Lecture (Region V) E cathode E anode Current, (i) Equilibrium (no current) • At the anode at equilibrium (no current), the oxidation reaction is balanced with reduction reaction (since there is no reaction when there is no curren ) Anode Electrode – Oxidation Reaction (since there is no reaction when there is no current ).
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7 - MEM 415 FUEL CELL ENGINES Lecture 7-POLARIZATION...

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