Abstract
In a polymer electrolyte membrane (PEM) fuel cell, local current density distributions are critical to the overall fuel cell performance. Local current density can vary drastically in the lateral direction across land and channel areas. Non-uniform distribution of current density can lead to serious problems at certain locations, which may accelerate membrane and catalyst degradations. It is essential to know the lateral current density distribution in order to optimize the flow field design and operation conditions. Thus the objectives of this study are (a) to directly measure the current density distribution in the lateral direction across land and channel areas in millimeter resolution, (b) to analyze the fundamental mechanisms of the current density variations in the lateral direction by electrochemical methods, (c) to develop a method to enhance fuel cell performance based on experimental results, and (d) to use a 3-dimensional numerical model to simulate the current density distribution. Partially-catalyzed membrane electrode assemblies (MEAs) are used to measure the lateral current density distribution. Results show that in the high cell voltage region, the current density decreases drastically from the center of the land to the center of the channel; while in the low cell voltage region, the current density under the channel becomes higher than that under the land. Flow rates and oxygen concentrations at the cathode side also affect the current density distribution between land and channel. Further studies are conducted to investigate the fundamental mechanisms of such significant lateral current density variations. Results from cyclic voltammetry (CV) show that electro-chemical area (ECA) is a major factor that leads to different local current densities in the lateral direction. Further study by electrochemical impedance spectroscopy (EIS) shows that charge transfer resistance under the land is lower than that under the channel, but mass transport resistance under the land is higher in the low cell voltage region. Inspired by the results that the current density under the land is higher than the channel, a novel method, namely the cold pre-compression treatment is developed to enhance the performance under channel areas. The results show that the overall performance of a PEM fuel cell indeed increases with the cold pre-compression treatment. However, over-compression may have a significantly detrimental effect on the fuel cell performance. In order to simulate the current density distribution in a PEM fuel cell, a 3-dimensional numerical model with a serpentine flow field is developed using FORTRAN. ECA variations in the lateral direction under land and channel are incorporated in the model. The model results agree well with experimental data.