Assignment 3 - 2020 - MIE 517 PDF

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Download Assignment 3 - 2020 - MIE 517 PDF

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MIE 517 – Fuel Cell Systems Written assignment #3 Assigned: Wednesday, March 4, 2020 Due: Thursday, March 19 at beginning of tutorial 5:10pm 1. Considering that resistance can be expressed as a slope of a voltage versus current curve (or, an area-specific resistance as a slope of a voltage versus current density curve), write an expression for the (area-specific) polarization resistance of a fuel cell considering two cases. In the first case, the kinetics of the fuel cell are approximated by the Tafel expression, and in the second, the full Butler-Volmer (B-V) kinetic expression describes the reaction kinetics. Hint: It may be convenient for the full B-V case to first calculate the inverse (reciprocal) of the polarization resistance by differentiating current density with respect to overpotential (rather than overpotential vs. current density), and then taking the reciprocal of the results to represent Rp. 2. For a PEM fuel cell anode with for = rev = 0.5 and n = 2, what activation overpotential (act) is required to increase ifor by 1 order of magnitude and decrease irev by one order of magnitude at 298 K? If i0 = 10-6 A/cm2, what act is required to increase i to 1 A/cm2? 3. Neglecting interconnect resistance and contact resistance within a fuel cell stack (i.e. assuming that they are equal to zero for a given fuel cell stack), draw an equivalent circuit for the case of a fuel cell with internal electronic short-circuiting across the electrolyte and use it to answer the following questions: a. Write an expression for the high-frequency intercept (HFI) of the fuel cell in terms of the individual electrode and electrolyte properties and magnitude of the short circuit resistance (i.e. consider the case of infinitely high frequency, for which the capacitors in the equivalent circuit would be short-circuited). b. Write an expression for the low-frequency intercept (LFI) of the fuel cell in terms of the same properties as in part (a) (i.e. consider the case of zero frequency in which the capacitors would behave as an open circuit). c. For an electrolyte resistance of 1 ohm and a total true polarization resistance of anode + cathode adding up to 1 ohm, write expressions for the apparent polarization resistance (LFI-HFI) of the cell for a value of short-circuit resistance of 1, 10, 100, and 1000 ohms. d. Comment on the implications of the result in part c regarding the evaluation of electrode performance in the presence of a short-circuit across the electrolyte of the cell. 4. Considering the transmission-line equivalent circuit shown in class, explain why an increase in the ionic resistance of the electrode composites would cause an increase in the electrode polarization resistance but not in the series resistance of the fuel cells? Why does the electronic resistance of the electrodes contribute to the series resistance of a fuel cell when the electrodes are thick but not when they are thin? How do these two results from interpreting the transmission line model correspond to the physical structure of fuel cell electrodes?...