Materials Characterization
Polymer electrolyte membrane fuel cell (PEFC) (Figure 2) and flow battery technologies require the use of gas diffusion layers (GDLs) which serve several purposes, mainly:
1.) Provides void space for reactant delivery and byproduct removal from catalyst layer
2.) Provides path for electrons and thermal conductiviy for heat removal
3.) Serves as structural support for the ion conductive membrane
GDLs are usually constructed of carbon fibers and the layers are on the order of 100 to 400 microns in thickness. In PEFC the byproduct is liquid water, which must travel laterally through the GDL fibers. This creates a complex two-phase interaction between flowing gas and liquid water which can have performance and degradation implications. If liquid water is not removed and flooding occurs, the cell performance may fall significantly. One method of easing liquid water removal is the addition of hydrophobic surfaces to the GDL by means of PTFE inclusion. Hydrophobic surfaces reduce the adhesion force between the liquid water and the GDL fiber surface making it easier to remove via gas flow.
PEM Fuel Cell schematic.
Scanning electron microscope (SEM) of GDL surafaces highlighting the complex non-homogenous texture caused by randomly distributed carbon fibers and PTFE. PTFE is added to enhance liquid water removal.
Breakthrough Pressure
Liquid water must travel transversely through GDL to be removed via reactant gas flow. A limiting factor to transport due to the small GDL pore size is breakthrough pressure which is related to capillary pressure and is defined as:
This work aims to characterize breakthrough pressure over a large range of GDL materials and PTFE weights as well as for various compression factors. Such information is valuable to fuel cell modelers and flow-field designers.
