High-current-density performance of polymer electrolyte fuel cells ͑PEFCs͒ is known to be limited by transport of reactants and products. In addition, at high current densities, excessive amount of water is generated and condenses, filling the pores of electrodes with liquid water, and hence limiting the reactant transport to active catalyst. This phenomenon known as ''flooding'' is an important limiting factor of PEFC performance. In this work, the governing physics of water transport in both hydrophilic and hydrophobic diffusion media is described along with one-dimensional analytical solutions of related transport processes. It is found that liquid water transport across the gas diffusion layer ͑GDL͒ is controlled by capillary forces resulting from the gradient in phase saturation. A one-dimensional analytical solution of liquid water transport across the GDL is derived, and liquid saturation in excess of 10% is predicted for a local current density of 1.4 A/cm 2 . Effect of GDL wettability on liquid water transport is explored in detail for the first time. Furthermore, the effect of flooding on oxygen transport and cell performance is investigated and it is seen that flooding diminishes the cell performance as a result of decreased oxygen transport and surface coverage of active catalyst by liquid water.

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A newly developed theory of liquid water transport in hydrophobic gas diffusion layers is applied to simulate flooding in polymer electrolyte fuel cells ͑PEFCs͒ and its effects on performance. The numerical model accounts for simultaneous two-phase flow and transport of species and electrochemical kinetics, utilizing the well-established multiphase mixture formulation to efficiently model the two-phase transport processes. The two-phase model is developed in a single domain, yielding a single set of governing equations valid in all components of a PEFC. The model is used to explore the two-phase flow physics in the cathode gas diffusion layer. Multidimensional simulations reveal that flooding of the porous cathode reduces the rate of oxygen transport to the cathode catalyst layer and causes a substantial increase in cathode polarization. Furthermore, the humidification level and flow rate of reactant streams are key parameters controlling PEFC performance and two-phase flow and transport characteristics. It is also found that minimization of performance limitations such as membrane dry-out and electrode flooding depends not only on material characteristics but also on the optimization of these operating parameters.

A two-phase, full cell model based on the multiphase mixture ͑M 2 ͒ framework is developed to analyze the two-phase transport in polymer electrolyte fuel cells with bilayer cathode gas diffusion media ͑GDM͒, consisting of a coarse gas diffusion layer ͑GDL͒ with an average pore size around 10-30 m and a microporous layer ͑MPL͒ with an average pore size ranging from 0.1 to 1 m. Effects of the relevant properties of the MPL on liquid water transport are examined, including average pore size, wettability, thickness, and porosity. It is quantitatively shown that the MPL increases the rate of water back-flow across the membrane toward the anode by increasing the hydraulic pressure differential across the membrane, consequently reducing the net amount of water to be removed from the cathode. Furthermore, it is seen that different microporous and wetting characteristics of the MPL cause a discontinuity in the liquid saturation profile at the MPL-GDL interface, which in turn reduces the amount of liquid water in the catalyst layer-MPL interface. Our analyses show that the back-flow of liquid water increases with increasing hydrophobicity and thickness, and decreasing pore size and porosity of the MPL.A main limitation in polymer electrolyte fuel cell ͑PEFC͒ performance results from the transport of reactants from the channel to the catalyst layer, referred to as the mass-transport limitation. This limitation is further amplified by the presence of liquid water, which blocks some of the open pores in the gas diffusion media ͑GDM͒ and thus reduces the available paths for the transport of reactant species. This phenomenon, commonly referred to as flooding, is more severe in the cathode because the slower oxygen reduction reaction ͑ORR͒ is more susceptible to the negative impact caused by flooding. Recently, a bilayer GDM, consisting of a coarse gas diffusion layer ͑GDL͒ and a finer microporous layer ͑MPL͒, has been employed by practitioners to reduce flooding in the porous cathode and to enhance water management in PEFCs by increasing the backflow tendency of liquid water across the membrane toward the anode. It has been shown that highly hydrophobic MPLs usually exhibit better performance. 1-13 Although the exact mechanisms are yet to be fully elucidated, the performance enhancement is usually associated with better water management capabilities of MPLs. In phosphoric acid fuel cells, Hara et al. 2 wetproofed the GDL using a fluorinated polyethylene film, which has much smaller pore size than commonly used polytetrafluoroethylene ͑PTFE͒ particles. It was found that this additional layer, which is similar to the MPL in PEFCs, improved oxygen reduction by reducing the flooding. Passalacqua et al. 3 also showed that introducing a hydrophobic layer between the carbon paper and the catalyst layer improves cell performance. The thin hydrophobic layer of about 2 mg/cm 2 of carbon ͑Vulcan XC-72͒, containing 40% of PTFE, substantially improved cell performance both in air and pure oxygen operation, by reducing the ohmic losses and i...

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