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...

specific situation, only the wetting phase may need to be considered, (e.g., the Richards equation), or the NWP Effective hydraulic properties of porous media such as the capillary is considered additionally. pressure-saturation relation and the hydraulic conductivity functionClearly, the constitutive relations are a direct manifesare a direct manifestation of the underlying pore geometry. The porous tation of the complicated geometry of the underlying structure of a macroscopically homogeneous porous medium (sintered glass) was measured in detail using X-ray microtomography. We invessimple and uniform surface properties. Following such J. Tö lke and M. Krafczyk, Institute for Computer Applications in Civil a path to determine hydraulic material properties is Engineering, TU Braunschweig, Pockelsstr. 3, 38106 Braunschweig, attractive, since structure is directly observable. Con-Germany.

In this paper, we give a complete description of the process of determining two-phase material parameters for a gas diffusion layer: Starting from a 3D tomography image of the gas diffusion layer the distribution of gas and water phases is determined using the pore morphology method. Using these 3D phase distributions, we are able to determine permeability, diffusivity, and heat conductivity as a function of the saturation of the porous medium with comparatively low numerical costs. Using a reduced model for the compression of the gas diffusion layer, the influence of the compression on the parameter values is studied.

Gas diffusion electrodes are commonly used in high energy density metalair batteries for the supply of oxygen. Hydrophobic binder materials ensure the coexistence of gas and liquid phase in the pore network. The phase distribution has a strong influence on transport processes and electrochemical reactions. In this article we present 2D and 3D Rothman-Keller type multiphase Lattice-Boltzmann models which take into account the hetero- for an efficient development of improved gas diffusion electrodes.

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We present electrospinning as a versatile technique to design and fabricate tailored polymer electrolyte membrane (PEM) fuel cell gas diffusion layers (GDLs) with a pore-size gradient (increasing from catalyst layer to flow field) to enhance the high current density performance and water management behavior of a PEM fuel cell. The novel graded electrospun GDL exhibits highly robust performance over a range of inlet gas relative humidities (RH). At relatively dry (50% RH) inlet conditions that exacerbate ohmic losses, the graded GDL lowers ohmic resistance and improves high current density performance compared to a uniform GDL with larger pores and fiber diameters. Specifically, the graded GDL facilitates a beneficial degree of liquid water retention at the catalyst layer/GDL interface due to the high capillary pressure inherent in its microstructure, thereby improving membrane hydration. Additionally, enhanced graphitization and connectivity of the graded electrospun fibers improves heat dissipation from the catalyst layer interface compared to the GDL with larger fiber diameters, thereby reducing membrane dehydration. When the inlet RH is raised to fully humid (100% RH) conditions, the graded GDL mitigates liquid water accumulation and lowers mass transport resistance. Specifically, the pore size gradient directs the removal of liquid water from the GDL, resulting in superior performance at high current densities.

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Since the first publications by Hazlett (Transp Porous Med, 20:21-35, 1995) and Hilpert and Miller (Adv Water Res, 24:243-255, 2001), the pore-morphology-based method has been widely applied to determine the capillary pressure-saturation curves of porous media. The main advantage of the method is the simulation of a primary drainage process for large binary images using moderate computational time and memory compared to other two-phase flow simulations. Until now, the pore morphology model was restricted to totally wetting materials or those with a constant contact angle. Here, we introduce a similarly computationally efficient extension of the model that now enables the calculation of capillary pressure-saturation curves for porous media, where the contact angle varies locally within, due to a composite structure.