PhD Thesis (Yuxiu Liu) Final 1-10-10.doc (Restricted Access)
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Thesis (Ph. D.)--University of Rochester. Dept. of Chemical Engineering, 2010.
The dominant voltage loss in proton exchange membrane (PEM) fuel cells is associated with the cathode and includes oxygen reduction reaction (ORR), proton transport in the ionomer phase, and oxygen transport. The proton transport within the ionomer can result in significant voltage loss, especially under dry conditions and/or at high current densities. The proton resistance in the cathode is determined by AC impedance in a H2/N2 cell, and is analyzed by a 1-D transmission-line model assuming the capacitance and resistance are uniformly distributed through the entire electrode thickness. The electrode proton resistivity is found to be independent of the electrode thickness and the carbon-support platinum %wt, as it strongly depends on the ionomer-to-carbon weight ratio (I/C) and relative humidity (RH). A critical point exists for the ionomer network in the electrode, beyond which point the ionomer tortuosity is around 1, while below the point, the tortuosity increases dramatically. The effects of the electrode ingredients on the proton resistivity and cell performance are investigated by varying the ionomer equivalent weight and the Pt-dispersed carbon supports. The density of the sulfonic acid groups in the cathode determines the proton resistivity and hence the cell performance. The proton resistivity, normalized by the ionomer bulk resistivity depends intrinsically on the volumetric ratio of ionomer-to-carbon (I/C)v, and is also affected by the porosity and roughness of the carbon support as a portion of the ionomer is absorbed by the carbon. The proton resistivity increases with increasing carbon surface area though improving ORR kinetics. The effect of relative humidity (RH) on ORR kinetics is determined to be small. The kinetics of Pt oxidation by water and the effects of potential, temperature, RH, and prior exposure to oxygen environments are studied and characterized by a surface coverage model. The water management in the complete fuel cell is analyzed by a 1-D water transport model to determine the water profile across the cell as a function of current density and RH. The obtained membrane resistances and cell performance are in good agreement with measured values. The predicted water profile allows identifying and predicting critical limitations of dryness at the anode and potential flooding at the cathode.
