Review—Wetting Phenomena in Catalyst Layers of PEM Fuel Cells: Novel Approaches for Modeling and Materials Research

The development of high performance polymer electrolyte fuel cells increasingly relies on modeling to optimally tune cathode catalyst layers (CCL) to desired properties. This includes models to rationalize the role of water as promoter and asphyxiant to the oxygen reduction reaction. Existing models are able to reproduce or predict, using assumed parameters, the performance of the cell. However, consideration of the wetting properties of the composite has remained elusive. Experiments to characterize these properties are difficult to perform. There is thus a gap in theory for relating material choices with wetting properties. This article elaborates on this gap and presents a novel conceptual approach to close it. Fundamental modeling approaches, molecular dynamics studies and experimental works have shown that the interaction of ionomer with the Pt/C surface exerts a major impact on wetting behavior and water sorption properties of the porous CCL composite. In our approach, the state of molecular alignment of ionomer sidechains and backbones is linked to the structural characteristics of the Pt/C catalyst. From this rationalization, wetting properties of the CCL can be deduced. An analysis of these correlations supports a crucial hypothesis: lowering the platinum loading leaves the CCL more prone to flooding.