Droplet Evolution from a Single Hydrophobic Pore in the Diffusion Media of Polymer Electrolyte Fuel Cells: A Conceptual Analysis

Maintaining a well-balanced water distribution is crucial for the operation of polymer electrolyte fuel cells. The water distribution depends to a large extent on the ease of liquid water transport in the diffusion media (DM) and in the flow field (FF) on the cathode side and, especially, on processes in boundary regions between these media. In this article, the droplet dynamics at the DM-FF interface are studied by a mechano-hydrodynamic pore model. The presented treatment allows droplet detachment to be rationalized, in dependence of pore radius and air flow velocity. Large pore radius and high air flow velocity favor droplet detachment during an earlier stage of growth, at which the contact line of the droplet remains pinned at the pore opening while the droplet volume expands during growth. We evaluate the trend of detachment time and detachment height of a droplet. Furthermore, we analyze the influence of the air flow velocity on the time-averaged liquid pressure at the pore exit. The physics governing the flooding behavior of the pore is discussed. The presented model-based analyses provide theoretical insights into material properties and operating conditions that improve water removal.