Microstructure of Catalyst Layers in PEM Fuel Cells Redefined: A Computational Approach

This work comprises an extensive coarse-grained molecular dynamics study of self-organization processes that define the mesoscopic structure of catalyst layers used in polymer electrolyte fuel cells. The detailed structural analysis focuses on agglomeration of Pt-decorated primary particles of graphitized carbon black, formation of ionomer domains, emergence of the porous network, and formation of interfaces between the distinct phases. Insights obtained enable us to decisively redraw the existing structural picture of the catalyst layer. As a key result, we found that ionomer forms a thin adhesive film, which partially covers agglomerates of Pt/carbon. Densely arranged charged side chains of ionomer form a highly ordered array on the ionomer film surface. The preferential orientation of these charged side chains depends on the surface wetting properties of the agglomerates. As a major consequence, results on ionomer structure and distribution, presented in this work, seem to invalidate the classical electrolyte-flooded agglomerate model that has been widely applied to catalyst layers in polymer electrolyte fuel cells. Instead, the structural analysis provided defines a need for novel models of proton transport, water distribution, and Pt effectiveness that account for the thin-film morphology of ionomer and the specific arrangement of surface groups.