Nano to Micro Scale Characterization of Water Uptake in The Catalyst Coated Membrane Measured by Soft X-ray Scanning Transmission X-ray Microscopy

We describe a method of quantitative mapping the water content in the catalyst coated membranes (CCM) of proton exchange membrane (PEM) fuel cells with high spatial resolution (~30 nm) using soft X-ray Scanning Transmission X-ray Microscopy. Detailed component chemical maps of CCMs at different environmental conditions (relative humidity (RH)) were generated from O 1s optical density measurements by fitting to the soft X-ray absorption reference spectra for liquid and gaseous water and those of the electrode and membrane materials. Under high RH conditions, different distributions of water (vapor and liquid) were found for the cathode layer and the membrane. These results are related to differences in hydrophilicity / hydrophobicity of different regions.