Dynamic Percolation and Droplet Growth Behavior in Porous Electrodes of Polymer Electrolyte Fuel Cells
The percolating flow of liquid water in the gas-diffusion layer (GDL) of polymer electrolyte fuel cells (PEFCs) was studied ex situ using a simple water-injection experiment. Water was injected into the top of the sample so the droplet was free to detach from the bottom, allowing for uninterrupted study of the dynamic cycle of droplet appearance, growth, and detachment. Although droplets emerged from a single point on the GDL, the measured pressure response in the water phase was clearly not equivalent to a single needle. The behavior of the system was explained by the simultaneous filling and inflating of many menisci, resulting in extended periods with no droplet activity at the GDL surface, followed by the sudden eruption of a droplet at the breakthrough site as all interfaces deflated and their stored water was directed toward the droplet. A simple numerical model was presented that could qualitatively explain the observed behavior. Tests were performed on GDLs, with and without microporous layers (MPLs), and all observed behavior could be interpreted in terms of the proposed model. MPLs shifted the behavior to a more needlelike behavior which was consistent with the MPL reducing the number of invading liquid clusters in the system.