Pore network modeling of fibrous gas diffusion layers for polymer electrolyte membrane fuel cells

Abstract

A pore network model of the gas diffusion layer (GDL) in a polymer electrolyte membrane fuel cell is developed and validated. The model idealizes the GDL as a regular cubic network of pore bodies and pore throats following respective size distributions. Geometric parameters of the pore network model are calibrated with respect to porosimetry and gas permeability measurements for two common GDL materials and the model is subsequently used to compute the pore-scale distribution of water and gas under drainage conditions using an invasion percolation algorithm. From this information, the relative permeability of water and gas and the effective gas diffusivity are computed as functions of water saturation using resistor-network theory. Comparison of the model predictions with those obtained from constitutive relationships commonly used in current PEMFC models indicates that the latter may significantly overestimate the gas phase transport properties. Alternative relationships are suggested that better match the pore network model results. The pore network model is also used to calculate the limiting current in a PEMFC under operating conditions for which transport through the GDL dominates mass transfer resistance. The results suggest that a dry GDL does not limit the performance of a PEMFC, but it may become a significant source of concentration polarization as the GDL becomes increasingly saturated with water.3

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

“…For this reason, it is recommended that a 5 th -order dependence is set to be the maximum that can be obtained; the order dependence of around 2 to 5 agrees with recent pore-network modeling studies [19][20][21]. It should be noted that for this paper, the differential impacts of changing properties is being explored and not the exact magnitude.…”

“…Often, a Bruggeman-type relation is used where n = 1.5; however, recent microscopic simulations and experimental studies have suggested that the value is closer to between 2 and 5 for typical GDLs [5,14,19,20,24,39,40,64]. Although, it should be noted that some of these studies change the volume fraction by compression, which alters the pore network and is different than examining a change in tortuosity and volume fraction due to pore filling.…”

“…This is typically done by calculating a saturation or liquid pore-volume fraction using relationships such as the Leverett J-function [5][6][7][8][9], and then using the saturation to modify the intrinsic or dry-medium transport properties using relations like those of Corey [10], (Brooks-Corey) [11], Van Genuchten [12], or Bruggemann [13][14][15]. The more intricate porelevel modeling [16][17][18][19][20][21][22][23][24] has allowed the calculation of various transport properties from a handful of statistical structural properties such as pore-and throat-size distributions, porosity, and bulktransport measurements including saturated permeabilities. While these models can provide some fundamental understanding of water percolation and movement, they are limited by a lack of measurements of the nonuniform chemical (i.e., wettability) distribution at the pore level.…”

Modeling and Diagnostics of Fuel Cell Porous Media for Improving Water Transport

“…The pressure-volume function may be constant (7,11,12), may vary as a function of the meniscus position inside the pore mimicking pore throat effect (16), or by defining an irregular pore shape (15).…”

Water Management and Experimental Diagnostics in Polymer Electrolyte Fuel Cell

“…Subsequently, Pasaogullari and Wang (Pasaogullari & Wang, 2004a) developed a theory describing liquid water transport in hydrophobic GDL, and explored the effect of GDL wettability on liquid water transport. Recently, Sinha and Wang (Sinha & Wang, 2007, Gostick et al (Gostick et al, 2007) and Rebai and Prat (Rebai & Prat, 2009) have developed a pore-network model to understand the liquid water transport in a hydrophobic GDL with the GDL morphology taken into account.…”