Local current density and water management in PEMFCs

摘要

We have used computational fluid dynamics analysis to investigate the local current density distribution at the membrane-gas diffusion layer (GDL) interface at average current densities ranging from 0.1 to 2.4 A/cm~2. A three-dimensional, non-isothermal model was used with a single straight channel geometry. Both anode and cathode humidification were included in the model. In addition, phase transportation was included in the model to predict the distributions of water vapor and liquid water and the related water management for systems operating at different current densities. The dependency of local current density on total water and thermal management of the fuel cell and its other related linkage with physical parameters were investigated. The simulation results showed that at low average current density, the local current density does not vary along the width but gradually decreases along the cell length. However, the opposite trend starts to emerge as the average current density is increased. The anode water activity was found to play a significant role in determining the membrane conductivity and the local current density variation in the cell. Moreover, at high average current density, the local current density in the downstream end of the channel is dominated by the cathode water rather than the membrane conductivity. Specifically, the cathode water accumulates in the shoulder area and congests the pores of the GDL, thereby blocking the passage of oxygen to the reacting area. The resulting scarcity of oxygen in the shoulder area causes a dramatic reduction in the local current density in this region. Simulations using different cathode stoichiometric rates showed that increasing the cathode stoichiometric rate led to better oxygen transportation to the GDL at the shoulder area, and hence improved to smooth the local current density distribution. The model was validated by comparison with the polarization curve (Ⅰ-Ⅴ characteristics) in the literature.