With the recent development of high-performing Pt-based electrocatalysts, fuel cells for automotive applications are becoming increasingly viable, and understanding the multiple mechanisms contributing to catalyst degradation is a priority to enable further catalyst optimization. In this respect, the 3D structure of the electrocatalyst, catalyst support, and ionomer within actual catalyst layers of membrane electrode assemblies (MEAs) must be fully resolved, complete with compositional profiles, and correlated with the structural and chemical evolution of the material components during fuel cell use.

Scanning transmission electron microscopy (STEM)-based tomography methods represent an effective way to interrogate the microstructure of the starting materials (particularly the Pt-based catalysts and carbon supports) and the resulting catalyst layer architectures, despite multiple inherent microscopy challenges, most of which are coupled to long data acquisition times involved and can include beam damage, contamination, insufficient angle collection ranges/increments for the data reconstruction, and the quality of the algorithms employed for producing an accurate tomogram from the tilt series.  In addition, varying image contrast levels produced by the different material constituents within the catalyst layer can complicate their specific segmentation for a precise analysis of catalyst dispersion and morphology, pore sizes, ionomer distributions, carbon corrosion, catalyst dealloying/leaching, and catalyst nanoparticle coalescence within the catalyst layer, all of which are important factors contributing to fuel cell degradation.

In this work, 3D electron tomography will be used to couple the distinct morphological characteristics of various electrocatalyst and their support structures (e.g., Pt and Pt-allloy catalysts supported on various types of carbon supports) with the resultant nm-scale structures within actual PEM fuel cell catalyst layers.  This work will highlight results of 3D STEM-tomography that address the problems of missing wedge artifacts and distinguishing the low contrast of the carbon support from the background.  Tilt-series reconstructions of electron energy loss spectroscopy (EELS) and/or energy dispersive X-ray spectroscopy (EDS) elemental maps acquired from the catalyst layers will be presented that provide information regarding porosity and ionomer distributions within catalyst layers as a function of the type of electrocatalyst and catalyst support material employed.   Quantifying changes in the structure and composition of the individual materials comprising cathode catalyst layers during fuel cell testing will be similarly evaluated.

Research supported by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE) and as part of a user project through Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences, which is a U.S. DOE Office of Science user facility.