Nano-scale 4D mapping by Scanning Transmission X-ray Microscopy

Abstract

Proton exchange membrane fuel cells (PEMFC) are a promising green energy resource for automotive applications. The cathode is a key rate limiting component of PEMFC. Typical cathodes are composed of Pt catalyst decorated carbon support particles, mixed with a perfluorosulfonic acid (PFSA) polymer. As the proton conduction medium in the cathode, the distribution of the PFSA ionomer will affect PEMFC efficiency, Pt utilization, and degradation kinetics. Optimization of ionomer loadings and distributions is a major goal of PEMFC research. In this thesis, Scanning transmission X-ray microscopy (STXM) has been used to measure the porosity and component distributions in 3D. Initial studies of PEMFC materials by STXM tomography have been presented. The high energy resolution of STXM collects an extra dimension of component information to be added to the 3 dimension of volume imaging. Multi chemical 3D imaging is defined as 4D imaging. To reduce the radiation damage to PFSA, the compressed sensing (CS) algorithm has been used. The results show that the total radiation damage can be reduced below the critical dose, and better reconstruction results with less measured angles are achieved using CS algorithm. PFSA materials were also measured using ptychography STXM tomography to improve the spatial resolution. The spatial resolution was improved from 30 nm to < 15 nm at ~700 eV. 4D imaging using ptychography STXM tomography is presented. Highly porous materials with functional coatings were also characterized by soft X-ray STXM-spectro-tomography and spectro-ptycho-tomography. The sample were Al2O3 aerogels coated with ZnO or TiO2 by atomic layer deposition (ALD). Quantitative analysis shows the ZnO ALD coatings are non uniform. Comparisons are made to electron microscopy imaging and X-ray fluorescence analysis of the same ZnO/ Al2O3 aerogel material. Together the results provide useful feedback for optimization of ALD coated alumina aerogels.