Unstructured Pore Network Modeling with Heterogeneous PEMFC GDL Porosity Distributions

Abstract

This is the first investigation of the liquid water saturation profile dependence on empirically determined heterogeneous polymer electrolyte membrane fuel cell ͑PEMFC͒ gas diffusion layer ͑GDL͒ porosity distributions. An unstructured, two-dimensional pore network model using an invasion percolation algorithm is presented. Random fiber placements are based on the heterogeneous porosity distributions of six commercially available GDL materials recently obtained through X-ray-computed tomography visualizations. The pore space is characterized with a Voronoi diagram, and simulations are performed with a single inlet liquid water cluster. Saturation profiles are also computed for GDLs with uniform, sinusoidal, and square-wave porosity distributions. Liquid water tends to accumulate in regions of high porosity due to the associated lower capillary pressures. The results of this work suggest that GDLs tailored to have smooth porosity distributions have fewer pockets of high saturation levels within the bulk of the material. Finally, a study on theoretical surface modifications demonstrates that low porosity surface treatments at the catalyst layer͉GDL interface result in greatly reduced overall saturation levels of the material.To operate polymer electrolyte membrane fuel cells ͑PEMFCs͒ over a wide range of operating conditions, liquid water within the fuel cell must be actively or passively managed, maximizing reactant diffusion pathways and preventing premature mass transport limitations. 1 Water is introduced into the cathode of the PEMFC at the catalyst layer ͑CL͒ both as a product of the electrochemical reaction and via electro-osmotic drag through the polymer electrolyte membrane. Water is also introduced at the flow field ͑FF͒ inlet from humidified oxidant streams. In the studies of Basu et al. 2 and Yamada et al., 3 saturated relative humidity levels have been predicted at the gas diffusion layer ͑GDL͉͒CL boundary, indicating that liquid water streams within the cathode may originate near the GDL͉CL boundary under specific operating conditions. Pore network models have been employed to describe the liquid water saturation patterns generated from the invasion of hydrophobic, porous materials, 4-14 where heterogeneity is often provided by randomizing pore and throat radii. Two-phase flow for GDL materials has recently been modeled within three-dimensional pore spaces found from either microcomputed tomography 15 or stochastic geometry generation. [15][16][17] The modeling techniques employed were the Lattice-Boltzmann method 16,17 and pore morphology modeling. 15,17 While pore space heterogeneity is obtained intrinsically with these methods, the detailed, three-dimensional images are required for all simulations and transport calculations, which can result in high computational costs. More recently, a topologically equivalent pore network model by Luo et al. 18 has been developed for three-dimensional stochastic models of GDL materials that applies the maximal ball method 19 to reduce the pore space into a pore n...

Synchrotron Investigation of Microporous Layer Thickness on Liquid Water Distribution in a PEM Fuel Cell

“…All parameters including number injection points and pore size distribution were consistent between two cases. The details on generating a pore network and performing invasion-percolation algorithm were described by Hinebaugh et al 28 According to the simulation, the 50-μm MPL pertained to four individual clusters, each with a breakthrough location, while the 150-μm MPL had only two. Moreover, it is shown that the total content of water increases with a thicker MPL, because greater number of pores must be occupied for breakthrough compared to a thinner MPL.…”

Validation of pore network simulations of ex-situ water distributions in a gas diffusion layer of proton exchange membrane fuel cells with X-ray tomographic images

“…The experimental data are 3D microscopic liquid water distributions from water injection experiments obtained by X-ray tomographic microscopy [11]. X-ray tomography has been used in several works to visualize liquid water in GDLs [23,24,43e49].…”

Liquid Invasion from Multiple Inlet Sources and Optimal Gas Access in a Two-Layer Thin Porous Medium

“…As for the present work, many of these studies are based on pore network models (PNM) (e.g., Sinha and Wang 2007;Markicevic et al 2007;Bazylak et al 2008;Hinebaugh et al 2010;Lee et al 2009Lee et al , 2010Lee et al , 2014Gostick 2013;Wu et al 2013;Fazeli et al 2015;Quin 2015).…”