Wettability and capillary behavior of fibrous gas diffusion media for polymer electrolyte membrane fuel cells

Abstract

The relationship of capillary pressure to liquid saturation for the water-air fluid pair in two different types of gas diffusion media (GDM) used in polymer electrolyte membrane fuel cell (PEMFC) electrodes is elucidated. It is experimentally demonstrated that GDM samples with and without treatment with poly(tetrafluoroethylene) (PTFE) ubiquitously display permanent capillary pressure hysteresis. Water does not imbibe spontaneously into a dry GDM, neither is it ejected spontaneously from a water-saturated GDM.Rather, positive displacement pressure is required to force both water and air into GDMs, whereas the main effect of adding PTFE is to increase the amount of work required for forcing water into the GDM, and to decrease the work required for water removal.Irrespective of PTFE content, the GDM samples tested are generally shown to behave as materials of intermediate (neutral) wettability. The US Bureau of Mines (USBM) wettability index nevertheless shows that water is the preferentially non-wetting phase in PTFE-treated GDMs and the preferentially wetting phase in untreated GDMs. Water-air capillary pressure curves are found to depend on sample thickness, clearly demonstrating that finite-size effects are important. Finally, compression of the GDM is found to increase the capillary pressures for water injection and decrease the capillary pressures required for water withdrawal. These results should aid the design of GDMs with improved water management properties and the modeling of PEMFC electrodes in general.3

Improved modeling and understanding of diffusion-media wettability on polymer-electrolyte-fuel-cell performance

“…This is in agreement with several groups that note that a certain pressure is necessary to overcome the surface resistance, but when it has, the water flows and easily through the layer [35,80,83]. In addition, the hysteresis is in agreement with surface and fiber contact-angle hysteresis seen with these materials [30,37].…”

“…Also given in Figure 2 are the drainage and filling curves for a SGL GDL with 5 wt-% Teflonization. The increase in the hydrophobic treatment causes a shift of the curves to the right as expected [34,80]. This shift causes the angle in the CAD to also shift to higher values with around an increase of 5° for the 5 wt-% Teflon-content increase.…”

“…Even if one guesses the distributions, they only represent part of the chemical nature of the DM since, due to the DM intermediate wettability caused by the constituent Teflon and graphite moieties, pore necks, fiber orientation, and other related effects [38,80]. Since there is no independent measurement of the CAD, it is best determined by fitting the available capillary pressure -saturation relationships.…”

“…Using the fits from Figure 1 and Table I for the SGL10BA (similar GDL, but no MPL as in Figure 1) and Toray GDLs, the CADs were fit to the cycled capillary pressure versus saturation data from the literature [28,29,34,80] as shown in Figure 2. From the data, one can see that both an irreducible liquid saturation and a large hysteresis exist.…”

“…Also, this boundary condition is expected to increase with increased DM hydrophobic treatment and agrees with experimental breakthrough pressure analysis and GDLs having intermediate wettability. [35,80] The water flux from the catalyst layer is taken to be known and is a varied parameter (in full-cell simulations, this flux is related to the water production and net water transport through the membrane). Finally, either the oxygen flux or concentration is taken to be known at the catalyst layer; in either case, the oxygen flux can be related to a current density through Faraday's law…”