This paper proposed an ex-situ experimental test to investigate the transport of liquid water through a gas diffusion layer (GDL) in a proton exchange membrane fuel cell. A transparent test cell was made which allowed the visualization of water droplet formation and detachment on the surface of a GDL through a CCD camera. The breakthrough pressure, at which the liquid water penetrates the GDL and starts to form a droplet, was measured. The breakthrough pressure was found to be different for the GDLs with different porosities. The equilibrium pressure, at which the speed of the liquid droplet formation equals to the speed of liquid droplet detachment, was also recorded. For the same GDL, the equilibrium pressure was found to be much lower than the breakthrough pressure. The difference between these two pressures might be due to the surface tension between the liquid water and the GDL. The results of this paper will be useful in re-examining the mathematical models describing the liquid water transport in the porous GDL.

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