Microstructural observation of the swollen catalyst layers of fuel cells by cryo-TEM.

It is important to understand and control the fine structure of the fuel cell catalyst layer in order to improve the battery characteristics of the fuel cell. A major challenge in observing the microstructure of the catalyst layer by electron microscopy is the visualization of ionomers, which have low contrast and are susceptible to damage by electron beam irradiation. Previous papers have reported transmission electron microscopy (TEM) observations of ionomers neutralized with cesium (Cs) ions. However, this approach involves chemical reactions and indirect visualization of ionomers. In contrast, we have previously revealed the microstructure of ionomers in frozen catalyst inks by cryogenic (cryo) scanning electron microscopy and cryo-TEM. In general, ionomers are basically used under high-temperature and humid conditions while the fuel cell is operating. Therefore, in this study, ultrathin sections prepared from the fuel cell catalyst layer (membrane electrode assemblies) were incubated in a chamber under high-temperature and humid conditions and then rapidly frozen for observation by cryo-TEM. As a result, we succeeded in observing the pore structure of the catalyst layer in the swollen state of the ionomer. The swollen ionomer surrounded and enclosed the Pt/C aggregates and bridged over the pores in the catalyst layer.