Advances in Structural Characterization Using Soft X-ray Scanning Transmission Microscopy (STXM): Mapping and Measuring Porosity in PEM-FC Catalyst Layers

A new method for characterizing porosity in polymer electrolyte membrane fuel cell (PEM-FC) catalyst layers (CL) is presented. The CL porosity is derived from the distribution of TTE, a non-swelling embedding epoxy, which is discriminated by STXM. Analysis of carbon 1s image sequences of TTE-embedded PEM FC microtome sections measured by STXM generates thickness maps of the CL major components: carbon support, ionomer, TTE, and Pt. The sum of all maps yields a 2D model of the CL total thickness. The TTE-porosity map is the ratio of the TTE thickness map to the total thickness map. We show that TTE-porosity maps are sensitive to CL processing. Examples illustrating how TTE-porosity can help optimize CL processing are presented.