Current understanding of catalyst/ionomer interfacial structure and phenomena affecting the oxygen reduction reaction in cathode catalyst layers of proton exchange membrane fuel cells

To improve the performance of membrane electrode assemblies used in proton exchange membrane fuel cells, a better understanding is necessitated regarding the nano/microstructure of the catalyst layer and the physicochemical phenomena responsible for the oxygen reduction reaction (ORR) occurring on this layer. In particular, it is very important to understand catalyst/ionomer interfaces in the cathode catalyst layer to apply the advanced ORR catalysts to the cathode catalyst layer in membrane electrode assemblies, which have solid-phase electrolytes; these catalysts are primarily developed under liquid electrolyte conditions. A closer observation of the catalyst/ionomer interfacial structure shows that all the transport processes required for ORR are controlled by the ionomer thin film covering the catalyst. Therefore, this review addresses this issue and introduces recent studies on catalyst/ionomer interfaces. We discuss the current understanding of the structure of the catalyst/ionomer interface, which depends on the surface characteristics of the catalyst and the ionomer, as well as transport of water, ions, and gas; these factors are in turn dependent on the structure of the interface. In addition, we introduce research efforts for improving the properties of catalyst inks, which form the basis for controlling the catalyst/ionomer interfacial structure. Based on the findings of these studies, we propose further opportunities and challenges in the study of catalyst/ionomer interfaces.