Abstract
Proton exchange membrane (PEM) fuel cell is regarded as one of the most promising power systems for the future vehicles. When supplied with air and hydrogen, PEM fuel cells show various advantages, such as quick start-up, high efficiency and pollution-free. It is well known that the current distribution in a PEM fuel cell is not uniform. Although both experimental and modeling studies on current distributions have been conducted for parallel and serpentine flow fields, there are no experimental measurements of lateral current density distribution for the interdigitated flow field. The study of lateral current density variation in this dissertation is essential to PEM fuel cells with interdigitated flow field design optimizations. Firstly, separate in situ measurements of current densities under the inlet channel, the land and the outlet channel in a PEM fuel cell with a single interdigitated flow field are conducted. Partially-catalyzed membrane electrolyte assembly (MEA) method is used to measure the local current density separately. The experimental results show that the local current density varies drastically in the lateral direction. In the lower current density region, the current density under the land is higher than that under the inlet channel. However, in the higher current density region, the current density under the inlet channel is higher than that under the land. The results have both similarities and differences with those for parallel flow field and serpentine flow field [1, 2]. One significant difference is the large difference between the inlet and outlet channels. The local current densities under different cathode operation conditions are measured to study if the operation conditions affect the local current densities variations significantly. The experimental results show that different cathode flow rates and cathode back pressures have negligible effect on the pattern of relative magnitudes of local current densities among the three different areas, and the cathode humidification temperature has a drastic effect on the inlet channel current density. The local electrochemical areas (ECA) are also measured by cyclic voltammetry (CV) technique. The experimental results show that ECA under the outlet channel is lower than that under the inlet channel, and ECA under the land is the highest. Furthermore, the lateral current distribution of PEM fuel cells with a single interdigitated flow field is studied with higher resolution. In this study, partially-catalyzed MEA method is also used measure the lateral current under seven locations separately. Different flow rates are applied and similar lateral current distribution tendency is obtained. At most typical PEM fuel cells operation voltages, the current densities under the land are the highest and the current densities under the inlet channel are much higher than that under the outlet channel. Then, CV and electrochemical impedance spectroscopy (EIS) tests are conducted to investigate the underlying reasons for the non-uniform lateral current density distribution. CV and EIS results are consistent with each other provide the mechanisms of the lateral current density variations. Moreover, a two-dimensional model is developed to illustrate the mechanism of the variations in lateral current densities in PEM fuel cells with interdigitated flow fields. Comparing with the previous modeling work, the lateral ECA distribution from the experimental study is integrated into the model, which makes this model more realistic. The modeling results agree with the experimental results very well. The modeling results show that the reactant concentration is decreasing along the lateral direction. Besides, it also shows that the lateral current density is not uniform. The current under the inlet channel is much higher than that under the outlet channel, and the current under the land is the highest in typical cell operation voltages. In addition, the dimensions of the land with, the outlet channel width and the under land gas diffusion layer (GDL) thickness are studied. The modeling results show that for interdigitated flow fields, wider lands and narrower outlet channels with thinner GDL are preferred.