(Invited) Advanced Multi-Technique Characterization of Nanoporous Materials

Nanoporous materials are widely used across renewable energy technologies as catalysts, supports, and storage materials. However, they are intrinsically challenging to characterize since their bulk and surface properties are difficult to identify using traditional characterization methods. While it is common to focus on structural and morphological properties like porosity and crystallinity, determined using physisorption and x-ray diffraction respectively, a combination of multiple techniques is necessary to assess chemical composition of these materials in order to determine structure-property-performance correlations. In order to better understand these materials at a fundamental level, multi-technique characterization must be utilized strategically to identify chemical and elemental differences in the surface, sub-surface, and bulk. The most common techniques include x-ray photoelectron spectroscopy (XPS) for surface chemical information, transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) for identification of morphological differences, and energy dispersive x-ray spectroscopy (EDS) for nanoscale elemental distributions. Atom probe tomography (APT) can be used as a stepping stone to understanding these materials at an atomic-scale and to bridge the gap between the compositions in the surface, sub-surface and bulk of materials. APT is a technique that has traditionally been used to reconstruct metallic and ceramic samples in 3D with sub-nanometer-resolution to enhance the understanding of specific grain-boundaries and precipitates. Expanding this technique to nanoporous materials requires significant optimization of existing procedures and considerations. The ability to elucidate dopant locations, nano-scale contaminations, and other important features of nanoporous materials at sub-nm scale in 3D can provide unprecedented opportunities towards optimization of these materials. Several classes of porous materials will be discussed in this talk. Nitrogen-doped carbon nanospheres are used as a model, carbon-based catalyst support material in order to show the advantages of the proposed characterization approach and demonstrate feasibility of the APT analysis of porous nanomaterials. Understanding surface chemistry of these materials is essential for elucidation of effects of nitrogen dopants on activity and stability of metal catalyst nanoparticles. Initial progress in characterization will be presented along with challenges and possible solutions. Further analysis of these porous carbons modified with both nitrogen and transition metal sites as examples of model catalysts with atomically dispersed active sites will also be presented. These efforts aim to further improve 3D APT reconstructions of carbon-based porous materials. And finally, efforts towards characterization of porous hydrogen storage/carrier materials based on functionalized magnesium borohydride (MBH) materials will be discussed. While these materials present additional challenges in characterization due to their complex chemistries and reactivity, multi-technique characterization with sub-nm resolution could significantly advance understanding of hydrogen desorption/adsorption mechanisms. The integration of 2D and 3D data at multiple scales provides a pathway for understanding these three classes of porous materials and a wide variety of other complex nanoporous materials used in electrochemical applications.