The performance of proton exchange membrane (PEM) fuel cells depends on the composition and structure of the catalyst layers. Experimental observations reveal that state-of-the-art catalyst layers consist of microporous agglomerates of carbon black- supported catalyst sites bound together by polymer electrolyte. In between the agglomerates are macropores which provide pathways for the transport of gaseous reactants. The active surface is limited to the catalyst sites located on the surfaces of the agglomerates in contact with polymer electrolyte. Improving the performance of PEM fuel cells depends on the optimisation of catalyst layer composition and structure for large active surfaces and low transport resistances. This optimisation requires a detailed modelling of the reactions and mass transport in catalyst layers in order to find ways to increase the effectiveness of the catalyst layers for a given precious metal loading.

In this work, three-dimensional, multicomponent and multiphase transport simulations are performed using a new PEM fuel cell implementation (Li & Becker, 2004) in the general purpose commercial computational fluid dynamics (CFD) software package FLUENT™ (Fluent Inc., 2001), which has been further improved by taking into account the detailed composition and structure of the catalyst layers using a multiple thin-film agglomerate model. In this model, it is assumed that thin films of polymer electrolyte and liquid water surround the catalyst sites and, therefore, that the reactants in the gas phase must dissolve into the water and diffuse across both the water and polymer electrolyte films, before reacting at the catalyst sites on the surfaces of the agglomerates in contact with polymer electrolyte.

From previous modelling studies, it is well known that PEM fuel cell performance is affected by the transport limitations associated with the low concentration of oxygen in air and the restriction of the porous media to gas transport. In the multiple thin-film agglomerate model, there are further transport limitations associated with the thin films of polymer electrolyte and liquid water. The effects of the thin films of polymer electrolyte and liquid water on PEM fuel cell performance are explored by varying the thickness of the thin films in the CFD simulations. It is found that the presence of the thin film of polymer electrolyte has a substantial negative effect on PEM fuel cell performance. For polymer electrolyte films greater than 1000 nm in thickness, current densities become negligible. Also, although the transport limitation associated with the thickness of the thin film of liquid water is found to be small compared to that associated with the thickness of the thin film of polymer electrolyte, the presence of liquid water in the cathode gas diffusion and catalyst layers decreases the volumetric fraction available for the transport of gaseous reactants and has a substantial negative effect on PEM fuel cell performance. As liquid water saturation in the cathode approaches one (i.e. the gas diffusion and catalyst layers are fully flooded) CFD simulations predict that current densities become negligible.

From previous modelling studies, it is also well known that the distribution of electrochemical reactions in the catalyst layers is highly dependent on the complex interaction of activation and ohmic effects as well as the contributions from transport limitations and the variations in local and overall current densities. Available data on catalyst layer composition and structure are used in the CFD simulations to predict reaction rate distributions in the catalyst layers. Based on these results, variations in local catalyst loading are implemented in the CFD simulations for a given precious metal loading in an attempt to improve PEM fuel cell performance. Improved performance is obtained for increased catalyst loading adjacent to the membrane at low and medium current densities. However, in general, PEM fuel cell performance is higher for uniform catalyst loading. Thus, optimising platinum loading and reducing costs through better catalyst utilisation is accomplished primarily by causing the reaction regions to expand and fill the entire catalyst layers.