In a proton exchange membrane fuel cell (PEMFC), the catalyst layer has the most complicated transport phenomena taking place within the smallest component dimension. The transport of charged species (H+, electrons and radicals) and non-charged species (gaseous H2O, O2, H2 N2 and liquid water) occur in their own pathways and transport among the phases occurs across the phase interfaces. Most existing models for the PEMFC catalyst layer are developed based on the volume-average method, e.g. pseudo-homogeneous model, agglomerate model and its variants, which rely on lumped parameters to account for the transport with the representative element volume (REV). The major drawback of such models is the lacking of information of the micro-structure of the catalyst layer, thus these models have limited predictive capabilities. The objective of the present study is two-fold, i.e., to develop a rigorous theoretical framework based on which the transport in the micro-structural level can be modeled, and to construct a pore-scale model that resolves the geometry of the phases (carbon, ionomer and gas pores) for which direct numerical simulation can be performed. The theoretical framework is developed by employing the volume-averaging techniques for multi-phase porous media. The complete set of the conservation equations for all species in all phases are derived and every interfacial transport is accounted. The problem of model-closure on the terms in the transport equations is addressed by the pore-scale model reported in the present study. A 3-D pore-scale model is constructed by a solid model that consists of packing spherical carbon particles and simulated ionomer coating on these carbon aggregates. The index system of the pore-scale model allows easy identification of volumetric pathway, interfaces and triple phase boundaries. The transport of charged and non-charged species is simulated by solving the equations based on first principle in the entire REV domain. The computational domain contains typically several million cells and a parallelized, iterative sparse matrix solver, GMRES, is employed to solve the coupled transport with complex geometries. Preliminary results based on the pore-scale model have shown that the effective transport properties of the species are strongly affected by the micro-structure, e.g. morphology and phase-connectivity. Further simulations and investigation on the coupling effects of the transport are underway. Combination of the proposed theoretical framework and pore-scale model will lay a foundation for the construction of multi-scale modeling of the PEMFC catalyst layer. On the one hand, the pore-scale model helps close the macroscopic volume-averaged equations in the framework. On the other hand, the pore-scale model provides a platform to include microscopic or atomistic simulations.

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