Multiscale Modeling of the Membrane Electrode Assembly in Polymer Electrolyte Fuel Cells: Interaction of Pore Size and Wettability of Porous Layers

Abstract

Design of the membrane electrode assembly (MEA) plays a crucial role in polymer electrolyte membrane fuel cells (PEMFCs). However, predictive modeling of mass, charge and heat transport considering primary microstructural information rather than previously measured effective transport properties is challenged by the multiscale pore structure found in an MEA, which spans around 4-5 orders of magnitude (from tens of nanometers in the catalyst layer to tens of micrometers in the gas diffusion layer). It is not clear how to incorporate microstructural information to guide design, while preserving moderate computational cost (see log-scale pore size distribution in an MEA in Figure a). In this work, a 3D two-phase, non-isothermal model is presented, which combines continuum and pore network (PN) formulations in a single CFD framework. Transport at fine pore scales of the catalyst layer and the microporous layer (below 1 um) is modeled by means of a continuum bundle-of-capillary-tubes (BCT) submodel, and transport at coarse pore scales of the gas diffusion layer and manufacturing defects above 1 um (cracks, gaps, etc.) by means of a continuum-based PN submodel. The BCT submodel accounts for microscopic (<20 nm), mesoscopic (20-300 nm) and macroscopic (300 nm-1 um) pore scales considering a subdivision of pore size distributions in the integral volume-averaged formulation. The interplay between pore size distribution and wettability of the various porous layers is analyzed, with a focus on the role of microporous layer wettability and defects, depending on operating relative humidity and temperature (see water distribution in Figure b from an invasion-percolation simulation). Best design practices of porous assemblies are extracted for maximum performance from a large computational campaign, including stochastic effects from different sample realizations. Analysis of degradation phenomena will be the focus of subsequent work to determine optimal microstructures arising from a trade-off between performance and durability. Figure 1