Loading...
Advanced transmission electron microscopy characterization of complex nanomaterials for hydrogen storage and conversion
Fitzgerald, Margaret Anne
Fitzgerald, Margaret Anne
Citations
Altmetric:
Advisor
Editor
Date
Date Issued
2022
Date Submitted
Keywords
Collections
Research Projects
Organizational Units
Journal Issue
Embargo Expires
2024-04-22
Abstract
Increasing energy demands and high carbon emissions require society to move toward renewable energy sources like hydrogen; however, the optimization and cost reduction of nano-scale materials in hydrogen energy systems is critical to implementing a hydrogen-based energy economy. In hydrogen power generation, fuel cells require high amounts of costly catalyst materials, which have limited durability and reduce the cost efficiency and lifespan of the system. In parallel, hydrogen storage technologies would benefit from the development of materials that transport hydrogen at higher densities than current compression technologies. This work aims to fill these gaps through investigations of the effects of nitrogen doping in carbon supports for fuel cells as well as introducing and characterizing a novel hydrogen storage material.
Nitrogen-doped carbon supports have shown improvements in the catalyst-support interactions for fuel cell catalysts, but their inherent heterogeneity limits our ability to tune their performance. In the first study, the effect of pyridinic and graphitic nitrogen on metal nucleation is investigated. This work begins with computational analysis to assess the absorption energy and electronic occupation of twelve metals across the periodic table in the presence of pyridinic and graphitic nitrogen defects. The computations inform the down selection of metals to test experimentally, where non-platinum-group metals (Fe, Co, and Ni) are synthesized on pyridinic-rich and graphitic-rich nitrogen-doped carbons. Low-voltage scanning transmission electron microscopy (STEM) is then used to determine the extent of metal nucleation for Fe, Co, and Ni. Experimental and theoretical trends were then compared and demonstrated trends that can be traced back to the electronic structure of the metal and its interaction with the nitrogen defect.
Next, this dissertation investigates the effect of the same nitrogen-doped materials on the dispersion and stability of pre-formed Pt nanoparticles. Platinum nanoparticles are deposited using a polyol method onto four chemically-varied, nitrogen-doped carbon supports. Initial dispersion of the platinum particles is evaluated using high-resolution STEM. The resulting electron micrographs are analyzed using a machine-learning-based technique to identify and quantify the size of thousands of platinum particles, increasing the quantity of data from hand-selection methods. The stability of the platinum catalyst is then probed through electrochemical stress testing and imaged at various points in the test using identical location STEM. A combination of machine-learning-based image analysis and manual identical location analysis suggests better dispersion and stability of pre-formed Pt nanoparticles on nitrogen-doped carbons with a slightly higher percentage of graphitic N defects.
Lastly, a new, highly energetic material that allows for the rapid release of hydrogen is synthesized and characterized. A new set of protocols for the vapor phase addition of TiCl4, BBr3, and N2H2 to Mg(BH4)2 results in a new hybrid composite material that releases an impressive 7.6 wt % of hydrogen at temperatures as low as 100 °C. Differences in composition and morphology between the series of modified samples are demonstrated using identical location microscopy paired with other physical and chemical characterization techniques. This investigation determines how the modification of Mg(BH4)2 resulted in the presence of N2H5Br and N2H5Cl, which attributes to the mechanism of low-temperature hydrogen dehydrogenation. This work also spotlights the benefits and caveats of utilizing identical location microscopy for metal hydrides in the hydrogen storage field due to their air- and beam-sensitivity. The final chapter summarizes the findings seen in the discussed studies and proposes the suggested directions for future work.
Associated Publications
Rights
Copyright of the original work is retained by the author.