A B S T R A C TThis paper describes a method to characterize the structure of polytetrafluoroethylene (PTFE) treated gas diffusion layers (GDLs) with and without microporous layers (MPLs) using 3D X-ray micro computed tomographic (mCT) microscopy. In this work, the structure of single and dual layer GDLs is evaluated via mCT for various GDL samples (such as Toray TGP-H-060 and AvCarb EP40) loaded with different MPLs. A new method is presented for separating, or segmenting, the various phases of the GDL, i.e., void space, carbon fiber (including binder and PTFE), and MPL. Through analysis, it was found that the variation in bulk porosity and the average pore diameter of the GDLs depends highly on the GDL series manufacturing and treatment processes. Using advanced image analysis techniques, routines were developed to accurately segment the GDL fibers (including binder/PTFE) and the MPL. The percentage of the intruding MPL material into the carbon fiber paper as a function of the GDL thickness was successfully found for dual layer GDLs, with varying PTFE content and areal weight loading in the MPL. This analysis provides invaluable insight into the physical microstructure of paper-based GDLs, emphasizing the heterogeneous porosity distribution of single layer GDLs and the interaction of the MPL with the carbon fiber paper of dual layer GDLs.

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The impedance characteristics of proton exchange membrane (PEM) fuel cells are studied and analyzed in this thesis. The modeling approaches presented in literature are thoroughly reviewed and categorized as the measurement-modeling and process-modeling approaches. In the former category, a hypothetical equivalent circuit which has the impedance characteristics similar to measured impedances is presented. Since the equivalent circuit is not directly resulted from the physical and chemical properties of the PEM fuel cells, the majority of the measurementmodeling approaches lead to dubious conclusions. In the process-modeling approach, on the other hand, the governing equations of the fuel cell must analytically be solved to determine and the impedance. However, a few process-modeling approaches presented in literature have shown to be indirectly based on the same assumptions as the measurement-modeling approach, and hence, those can also lead to similar conclusions. Therefore, these process-modeling approaches are referred to as the semi-process models here.In this thesis, the first complete process model for PEM fuel cells is presented which is not based on the above-mentioned assumptions. For each source of the losses in the fuel cell (i.e., the ohmic, activation and concentration overpotentials), a process model and equivalent circuit are obtained and compared against the impedance measurements reported in literature. The complete model (obtained by combining the models of the three losses) is then verified against the impedances measured in different operating conditions. Using the verified model, the measured Nyquist plots of the PEM fuel cells reported in literature are categorized. As a result, the dominant physical and chemical parameters controlling various arcs of the Nyquist plot are determined. Finally, the sensitivity analysis of the impedance characteristics of fuel cells is conducted using the verified model. As a result of this analysis, a minimum change in the operating conditions which results in statistically different Nyquist plots are determined.Finally, as an application of the model presented here, the impedance of the cell in the anode and cathode starvation modes are studied. It is shown that the anode starvation cannot be recognized from the impedance measurements, as predicted by the model. iii Preface This research was conducted in the Advanced Thermo-Fluidic Laboratory (ATFL) at University of British Columbia, Okanagan campus, under supervision of Dr. Mina Hoorfar. The results of this research have been published in several peer-reviewed journal articles and conference proceedings. Chapter 1 includes the discussions published as a review journal article (S.M. Rezaei Niya, M. Hoorfar (2013) 'Study of proton exchange membrane fuel cells using electrochemical impedance spectroscopy technique -A review ', Journal of Power Sources, Vol. 240, and also a journal article discussing the details of the process modeling approach (S.M. Rezaei Niya, M.Hoorfar (2015) 'Measurement, semi-process and pr...

The sensitivity of the impedance characteristics to the operating conditions of the proton exchange membrane fuel cell is studied. The impedance of a cell is measured in different potentials (current densities), operating temperatures, and anode and cathode relative humidities to estimate the indifference interval of each of these parameters. The indifference interval of a parameter, defined for a specific point, refers to a range out of which the impedance results are statistically dissimilar from that specific point. The analysis presented evaluates the average indifference intervals of the potential, temperature and relative humidities to be 10 mV, 5 °C and 7%, respectively. In other words, the changes less than the above mentioned limits do not affect the impedance of the cell statistically.

The impedance characteristics of the ohmic overpotential of the proton exchange membrane (PEM) fuel cells are studied analytically using the process modeling approach. The water transport in the membrane, the cathode catalyst layer, and gas diffusion layer are analyzed. The analytical relation of the impedance of the ohmic loss is determined and is converted to an equivalent circuit. Then, the impedance of a PEM fuel cell is measured experimentally in different current densities, operating temperatures and the anode and cathode relative humidities. The measured impedances are compared with the predicted ones from the analytical model. It is shown that the predicted impedance characteristics are in great agreement with the measured ones in all different operating conditions.

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In this study, the leakage current of a proton exchange membrane fuel cell is determined from the polarization curve and impedance measurements. The ohmic loss is specified from the high frequency resistance in the Nyquist plot. The polarization curve is then rotated counter-clockwise to nullify the effects of the ohmic loss. Considering the negligible mass transport loss in low current densities, the activation loss is then determined as a function of the current density. Employing the Tafel assumption, the leakage current can be determined from the best logarithmic fit. The method is then experimentally verified by measuring the polarization curves and Nyquist plots of a cell including the Nafion 212 as the membrane. Tests are conducted using four different gas diffusion layers for all of which the same current leakage is obtained.