Agglomerates in Polymer Electrolyte Fuel Cell Electrodes: Part I. Structural Characterization

This two-part study characterizes the structural and transport properties of polymer electrolyte fuel cell (PEFC) cathode catalyst electrode. The agglomerates comprising the electrode are characterized with nano scale resolution X-ray computed tomography (nano-CT). A hybrid reconstruction technique is used to further refine the 3-D agglomerate microstructure and obtain 1-nm voxel resolution of the constituent catalyst, ionomer, carbon support and primary and secondary pore phases. Our analysis of nano-CT data shows that the electrode composition is heterogeneous and the agglomerates have structural heterogeneities in terms of size and composition. There is a strong correlation between the ionomer to carbon weight ratio (I/C) and agglomerate microstructure, suggesting that ionomer plays role in formation of larger agglomerates. With the hybrid reconstructions, we show that ionomer film thickness is spatially non-uniform inside the agglomerate and is strongly dependent on the I/C. Smaller agglomerate size and high I/C are shown to favor the ionomer impregnation. We demonstrate that the majority of the primary pores are smaller than 15 nm for I/C = 0.8 and higher I/C reduces the primary pore size. We also characterize the uniformity of the catalyst distribution and show that the majority of the catalyst is located away from the agglomerate surface.