A pore scale model for the transport phenomena in the catalyst layer of a PEM fuel cell

In a proton exchange membrane fuel cell (PEMFC), the catalyst layer is a porous medium made of carbon-supported catalysts and solid electrolyte, and has a thickness in the order of 10 mu m. Within this layer, complex transport phenomena take place: transport of charged species (H(+), electrons and ionic radicals), non-charged species (gaseous H(2)O, O(2), H(2), N(2) and liquid water) and heat transfer occur in their own pathways. Furthermore, phase change of water and physiochemical/electrochemical reactions also take place on phase boundaries. These transport process take place in an intertwined network of materials having characteristic length scale ranging from nano-meters to micro-meters. 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 modelled, 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 representative element volume (REV) domain. The computational domain contains typically several million cells and a parallelized, iterative solver, GMRES, is employed to solve the coupled transport with complex geometries. Computational results based on the pore-scale model show 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 modelling 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.