Pore-scale modeling and analysis of the polymer electrolyte fuel cell catalyst layer

The catalyst layer (CL) plays a crucial role in the overall performance of a polymer electrolyte fuel cell (PEFC) due to the sluggish oxygen reduction reaction as well as transport limitation in the presence of liquid water and ensuing flooding. Nevertheless, it is often treated either as a thin interface or a macrohomogeneous porous layer and the influence of underlying morphology and wetting characteristics on the catalyst layer performance and water transport is ignored. The macroscopic fuel cell models, therefore, employ effective transport properties for reactant and charge transport as well as arbitrary two-phase closure relations for capillary pressure and relative permeability, the experimental measurements of which are exceedingly difficult and might be impossible in the near future. The role of the catalyst layer flooding in the overall cell performance and the mechanisms of liquid water transport/removal from the CL remain unexplored.;Furthermore, a macroscopic model of CL flooding is devised based on a simplified structure-wettability representation and a physical description of water and heat balance. The role of evaporation in the CL liquid water saturation distribution and resulting flooding is elucidated. While the primary focus of the pore-scale modeling is to quantitatively estimate the transport parameters along with a detailed structure-transport-performance description, the macroscopic analysis reveals profound inter-relations of adjacent components and operating cell temperature with CL flooding.;In order to reveal the underlying structure-performance relationship and to predict reliable closure relations, a pore-scale modeling framework comprising of a stochastic microstructure reconstruction model, an electrochemistry coupled direct numerical simulation (DNS) model and a two-phase lattice Boltzmann (LB) model is developed. The stochastic reconstruction model generates 3-D, statistically meaningful catalyst layer microstructure based on inputs from transmission electron microscope (TEM) images of an actual catalyst layer. Pore-level description of charge and species transport within the complex CL microstructure is achieved through the direct numerical simulation (DNS) model. The main purpose of the DNS model is to unravel the CL compositional influence on the performance and enable composition optimization for better performance. The mesoscopic lattice Boltzmann (LB) model simulates the liquid water transport through the CL microstructure in order to gain insight into the influence of structure on the porescale two-phase dynamics as well as to evaluate the two-phase constitutive relations in terms of capillary pressure and relative permeability as functions of liquid water saturation. A quantitative estimate of the detrimental effect of liquid water on the CL electrochemical performance in terms of the pore blockage and catalytic site coverage effects, which cannot be evaluated experimentally at present, is predicted from the combined LB and DNS models. These transport parameters can be used as reliable closure relations in macroscopic fuel cell models.