Modeling of a polymer electrolyte membrane fuel cell cathode

Two models have been presented in this dissertation for an air cathode of a polymer electrolyte membrane fuel cell: a steady state impedance model and a steady state polarization model.; Two impedance loops have been predicted on an air cathode by our steady state impedance model at a high steady state current density. The high frequency impedance loop is associated with the effective charge transfer resistance and double layer charging, and the low frequency impedance loop is associated with gas phase transport limitations. They are in agreement with the predictions by a similar impedance model in the literature. Several problems in the latter model have been addressed in our impedance model.; By using the steady state polarization model presented in this dissertation, five parameters for an air cathode (the volume fraction of gas pores in the gas diffusion layer, the volume fraction of gas pores in the catalyst layer, the exchange current density for the O2 reduction reaction, the effective ionic conductivity in the catalyst layer, and the ratio of the effective diffusion coefficient of dissolved O2 in a spherical agglomerate particle to the square of the particle radius) have been determined by least square fitting of polarization curves of an air cathode. A decoupling method for calculating the model equations and the sensitivity equations has first been proposed to optimize the numerical efficiency in the estimation of parameters. The polarization curves of an air cathode have been obtained by correcting the experimental polarization curves of an air/H2 polymer electrolyte membrane fuel cell for the voltage drop across the membrane. The validity of the correction method for the voltage drop across the membrane has been justified. A more robust correction method has also been presented. The possible influence on the parameter estimation accuracy of the assumption that a H 2 anode was always at its equilibrium in our experiments has been discussed. The parameter estimates obtained in this dissertation indicate that ionic conduction in the catalyst layer and the gas phase transport in the gas diffusion layer are two processes influencing the polarization performance of an air cathode significantly. The goodness of our steady state polarization model has been demonstrated by comparing our model to a polarization model in the literature. An equation used in our polarization model to describe the O 2 reaction current density in the catalyst layer, which is capable of predicting a change of Tafel slope, has been noticed to be superior to a simple Tafel equation used in the literature.