1 Department of Chemistry, Technical University of Denmark2 Energy and Materials, Department of Chemistry, Technical University of Denmark3 Department of Chemical and Biochemical Engineering, Technical University of Denmark4 Department of Energy Conversion and Storage, Technical University of Denmark
Hydrogen storage for practical applications is under intense scrutiny worldwide since hopes are prevalent of being able to use hydrogen as energy vector in a continually difficult time in terms of having access to clean and affordable energy in the world. Hydrogen can be stored in compressed or liquid form, technologies that are well developed and usable, but not energy efficient. Certain metals and alloys are able to contain hydrogen within practical pressure and temperature ranges very efficient volume-wise, but they are too heavy for use in cars. Recently, attention has turned to the so-called complex hydrides, which contain hydrogen bound covalently often in very light materials involving elements such as lithium, sodium, nitrogen and aluminum. While these materials typically have high decomposition temperatures, the combination with other compounds helps to destabilize the material resulting in lowered effective dehydrogenation temperatures. From the discovery in 1996 by Borislav Bogdanović and his group that catalyzed sodium alanate, NaAlH4, can release hydrogen reversibly below 200 °C relatively fast, hydrogen storage in nitrogen-containing compounds beginning with lithium nitride, Li3N, was considered a next major step in the succession of research in complex hydrides. Many complex hydrides involving nitrogen are presently under examination. This thesis reviews some of the results so far and embarks on a study of hydrogen storage in some of the compounds. Following a brief introduction in Chapter 1, Chapter 2 of the text deals with general principles and an overview for hydrogen storage in solid materials. Chapter 3-5 deals with the development of an in-house high pressure microbalance in a glovebox built from scratch for the use of characterizing new hydrogen storage materials including giving an example of characterization on a well-known hydrogen storage material, CaNi5. Chapter 6 contains results on a new system based on Li, Al and N for hydrogen storage. It was shown that Li3AlN2 can be synthesized from Li3N and Al under nitrogen pressure. Furthermore, the compound was proven to be able to store hydrogen reversibly. Chapter 7 describes first time results for a new hydrogen system based on Li, Si, and N. It discusses the synthesis of Li5SiN3 and Li2SiN2. Li5SiN3 was treated in-depth and was seen to be able to store hydrogen reversibly at fairly moderate conditions. Furthermore, the effect of doping a system of lithium amide and silicon, LiNH2+Si, with TiCl3 was examined, showing vastly improved desorption conditions with increased doping loads. Chapter 8 is about the newly publicized “hydrogen” pill, which in this work was attempted to be turned into a real hydrogen pill as opposed to an ammonia pill. The findings point to the possibility of combining the material in the ammonia pill with other compounds, which make it possible to store hydrogen reversibly.
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Jensen, Jens Oluf, Bjerrum, Niels, Barner, Jens H. Von