# 8 - 2/26/2011 MEM 415 FUEL CELL ENGINES - Lecture # 8...

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2/26/2011 1 -- Lecture # 8 -- MEM 415– FUEL CELL ENGINES POLARIZATION: “OHMIC AND CROSSOVER LOSSES” (Region 2 and 4) Membrane Cathode Anode C H2 Reactant C O2 Reactant H 2 O 2 O 2 H 2 MEM 415– Lecture #8 Dr. E. Caglan Kumbur Assistant Professor of Mechanical Engineering Office: Curtis 160 Ph: 215.895.5871 e-mail: eck32@drexel.edu -- Feb 28, 2011 -- What will be on the What will be on the Test Test #2 ? #2 The test will be closed book, closed notes , with one sheet of 8 1/2 x 11” paper (both side allowed – HAS TO BE HAND-WRITING) as a crib sheet allowed, and will be turned in with the quiz. It will essentially cover all material from class through today. Most of the questions will be from Chapter 3 and 4 of your text book Bring a calculator Chapter 3 and 4 of your text book. Bring a calculator. 1) Everything included in Quiz # 1 (Fair Game !!!) 2) Faraday’s law, how to find tank size, etc. , flow rates of constituents, production rates, cell efficiencies 3) Stoichiometry, fuel utilization efficiency, faradic efficiency 4) Polarization curve – what are the various regions and what dominates losses in them, waste heat, useful power, ohmic, activation, and concentration regions etc. You’ll be responsible for everything , including today’s lecture. 5) How to find E o , E oo and thermodynamic efficiency. 6) Activation polarization (Reg. 1), Butler-Volmer, Tafel, and other forms of loss calculation etc. 7) Ohmic polarization (Reg. 2) and Crossover Polarization (Reg.4) 8) Entropy Losses – Region 5 of the Polarization Curve

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2/26/2011 2 Typical Polarization Curve MEM 415– Lecture #8 Summary Activation Summary Activation Overpotential Overpotential Region I is dominated by activation (kinetic) polarization (loss, aka overvoltage ) - Symbols: ! a,a (a,a = activation at anode) a,c (a,c = activation at cathode) Activation polarization occurs due to the kinetics losses (electrochemical reaction) at the catalyst layers of the fuel cell system. But the polarization losses required to achieve this level of current on each electrode are independent . - Different amounts of activation losses occur at anode and cathode electrodes MEM 415– Lecture #8
2/26/2011 3 Activation Activation Overpotential Overpotential (Polarization) (Polarization) Factors affecting the activation overpotential: 1- Reaction Mechanism: In general, the more complex a reaction mechanism , the greater the overpotential required to break the chemical bonds and generate current. 2- Catalyst Type: A poor choice of catalyst will require a greater polarization to enable the electrochemical reaction at the electrode to proceed. Ex: Pt for PEM fuel cells, Nickel for SOFC 3- Catalyst Morphology: The structure has strong effect. Maximization of TPB area will reduce the activation polarization losses for a given current density. MEM 415– Lecture #8 Activation Activation Overpotential Overpotential (Polarization) (Polarization) Factors affecting the activation overpotential: 4- Operating Parameters: Electrochemical reactions are catalyzed by increased temperature , just like chemical reactions. Better performance with increased temperature.

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## This note was uploaded on 04/26/2011 for the course MEM 415 taught by Professor Kumbur,emin during the Spring '11 term at Drexel.

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8 - 2/26/2011 MEM 415 FUEL CELL ENGINES - Lecture # 8...

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