Modeling of Two-Phase Behavior in the Gas Diffusion Medium of PEFCs via Full Morphology Approach

Abstract

A full morphology ͑FM͒ model has been developed for studying the two-phase characteristics of the gas diffusion medium in a polymer electrolyte fuel cell ͑PEFC͒. The three-dimensional ͑3D͒ fibrous microstructure for the nonwoven gas diffusion layer ͑GDL͒ microstructure has been reconstructed using a stochastic technique for Toray090 and SGL10BA carbon papers. The FM model directly solves for the capillary pressure-saturation relations on the detailed morphology of the reconstructed GDL from drainage simulations. The estimated capillary pressure-saturation curves can be used as valuable inputs to macroscopic two-phase models. Additionally, 3D visualization of the water distribution in the gas diffusion medium suggests that only a small number of pores are occupied by liquid water at breakthrough. Based on a reduced compression model, the two-phase behavior of the GDL under mechanical load is also investigated and the capillary pressure-saturation relations are evaluated for different compression levels.The polymer electrolyte fuel cells ͑PEFCs͒, which convert the chemical energy of hydrogen directly into electrical energy, are considered as the most promising alternative energy-conversion devices in the 21st century for several applications including automotive, stationary and portable power. The electrochemical reaction occurring in the cathode catalyst layer ͑CL͒, referred to as the oxygen reduction reaction combines protons, resulting from hydrogen oxidation in the anode catalyst layer, with oxygen to produce water and waste heat. Although tremendous progress has been made in recent years in enhancing overall performance of the PEFC, one major performance-limiting step is the coverage of the reaction sites in the CLs as well as the blockage of the reactant-transporting networks in the porous gas diffusion layers ͑GDLs͒ due to liquid water, which hinders the oxidant from reaching the active reaction sites in the CLs at high current density operation. The GDL plays a crucial role in the overall water management which requires a delicate balance between reactant transport from the gas channels and water removal from the electrochemically active sites. Mathias et al. 1 provided a comprehensive overview of GDL structure and functions.Several studies have been attempted in recent years to model two-phase behavior and flooding phenomena in polymer electrolyte fuel cells in various degrees of complexities. 2-15 Recent reviews by Wang 16 and Weber and Newman 17 provide comprehensive overview of various two-phase PEFC models and address the water management issue with particular attention to GDL in significant details. However, all of the above-mentioned macroscopic two-phase models are plagued with the scarcity of realistic two-phase correlations, in terms of capillary pressure and relative permeability as functions of water saturation, tailored specifically for actual gas diffusion medium characterized by woven or nonwoven fibrous structures. Due to the lack of reliable two phase correlations, these models often deploy...

Versatile and efficient pore network extraction method using marker-based watershed segmentation

“…The size distributions shown in Fig. 13 are in good agreement with accepted values for similar materials [59][60][61][62].…”

Modeling Water Management in Polymer-Electrolyte Fuel Cells

“…[165][166][167][168][169] Such microscopic modeling in general is still in the infancy stage and mechanistic understanding of how water moves through different porous media, i.e., as thin films, slugs, droplets, or some combination thereof, is still being studied for both steady-state and transient conditions. 170,171 Nonetheless, some preliminary attempts have been made to derive constitutive relations-namely permeability and capillary-pressure-saturation curves-via direct treatment of microphysiochemical structure.…”

3D analysis, modeling and simulation of transport processes in compressed fibrous microstructures, using the Lattice Boltzmann method

“…In this model straight fibers of non-woven GDLs are generated by a 3D dilated Poisson line process. Its development was continued by Schulz et al [50] and applied to characterize carbon papers. The basic assumptions are overlapping fibers which are long compared to the sample size.…”