Modeling of Two-Phase Behavior in the Gas Diffusion Medium of PEFCs via Full Morphology Approach Volker Paul Schulz, a,z Jürgen Becker, a Andreas Wiegmann, a Partha P. Mukherjee, b, * and Chao-Yang Wang b, ** a Fraunhofer-Institut für Techno- und Wirtschaftsmathematik (ITWM), Kaiserslautern, Germany b Electrochemical Engine Center (ECEC), and Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA 16802, USA A full morphology FM model has been developed for studying the two-phase characteristics of the gas diffusion medium in a polymer electrolyte fuel cell PEFC. The three-dimensional 3D fibrous microstructure for the nonwoven gas diffusion layer GDL 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. © 2007 The Electrochemical Society. DOI: 10.1149/1.2472547 All rights reserved. Manuscript submitted July 11, 2006; revised manuscript received December 13, 2006. Available electronically February 21, 2007. The polymer electrolyte fuel cells PEFCs, which convert the chemical energy of hydrogen directly into electrical energy, are con- sidered as the most promising alternative energy-conversion devices in the 21st century for several applications including automotive, stationary and portable power. The electrochemical reaction occur- ring in the cathode catalyst layer CL, referred to as the oxygen reduction reaction combines protons, resulting from hydrogen oxi- dation 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 GDLs 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 manage- ment issue with particular attention to GDL in significant details. However, all of the above-mentioned macroscopic two-phase mod- els 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 me- dium characterized by woven or nonwoven fibrous structures. Due to the lack of reliable two phase correlations, these models often deploy a generic curve-fitted capillary pressure-saturation data origi- nally obtained by Udell 18 in the form of Leverett-J function from imbibition in water-wet unconsolidated sand. 19 In order to enhance the fidelity of the two-phase computational fuel cell dynamics mod- els, it is imperative to investigate the interplay between underlying microstructure and two-phase characteristics of the porous gas dif- fusion medium in polymer electrolyte fuel cells. Recently, Gostick et al. 20 reported capillary pressure-saturation data for various GDL materials from drainage experiments using standard porosimetry technique. However, hardly any modeling ef- forts in estimating two-phase characteristics of the GDL in terms of capillary pressure as a function of liquid water saturation have been reported in the literature. In the present study, digital computer models of nonwoven car- bon paper GDLs are reconstructed using a stochastic generation method 21,22 with structural inputs, namely fiber diameter, fiber ori- entation, and porosity, obtained from two-dimensional scanning electron microscopy SEM micrographs of the GDL and related available data from literature. A full morphology FM model 23-25 is subsequently developed, which is again the attempt to investigate two-phase behavior in the reconstructed GDL microstructures by simulating a drainage process. Capillary pressure as function of liq- uid water saturation is estimated for two different nonwoven GDL materials. The capillary pressure correlations predicted by the FM model are subsequently compared with experimental data available in the literature. 20 Furthermore, the effect of cell clamping pressure on the two-phase characteristics of a carbon paper GDL is investi- gated. The importance of clamping pressure on fuel cell perfor- mance has been studied by several researchers. Notable works in- clude Mathias et al., 1 Wilde et al., 26 and Ihonen et al.. 27 Mathias et al. 1 reported compression and flexural behavior of carbon paper and carbon cloth GDLs and indicated the effect of compressive charac- teristics on the channel flow-field pressure drop. Wilde et al. 26 stud- ied the impact of compression force on the GDL properties, namely electrical resistivity, pore size, and permeability for different mate- rials and briefly described the resulting influence on PEFC perfor- mance. Ihonen and co-workers, 27 on the other hand, tried to assess experimentally the influence of clamping pressure on the flooding behavior of the GDL. However, none of these studies focused on the dependency of capillary pressure-saturation relation on clamping pressure through its tight coupling on the underlying GDL structure. In the present study, we report the variation of two-phase correla- tions in terms of capillary pressure-water saturation relation for the GDL with various levels of cell compression. Finally, the predictive capability of the present model in estimating realistic two-phase correlations for input into macroscopic two-phase fuel cell dynamics models is emphasized. Experimental The construction of a realistic GDL pore morphology is the es- sential prerequisite for unveiling the influence of the underlying structure on the two-phase behavior. This can be achieved either by three-dimensional 3D volume imaging or by constructing a digital microstructure based on stochastic models. Noninvasive experimen- tal techniques, such as X-ray and magnetic resonance computed * Electrochemical Society Student Member. ** Electrochemical Society Active Member. z E-mail: volker.schulz@itwm.fraunhofer.de Journal of The Electrochemical Society, 154 4 B419-B426 2007 0013-4651/2007/1544/B419/8/$20.00 © The Electrochemical Society B419