Due to the negative environmental influences arising upon using fossil fuel, as well as its diminishing reserves, developing new energy is becoming increasingly important for achieving sustainable development. Among the vairous potential solutions, the clean and recyclable energy carrier --- hydrogen is highly apppreciated. Commerialization of hydrogen energy involves mass production, storage/transportation and utilization of hydrogen, among which the intermediate “hydrogen storage” step is generally recognized as the most challenging one. This is evidenced by the lack of progress even after decades of entensive worldwide reaserch ettorts on hydrogen storage system/method. Mg has been attracting considerable interest as a viable hydrogen storage medium due to its high H-capacity (with a theoretical value of 7.6 wt.%), high abundance and low cost. However, its commercial application has been largely hindered by its high operation temperature and sluggish sorption kinetics. A recent main strategy used to address these problems is preparation of nano-structured Mg-based composite. NbF5 provides a source of both high valence Nb cation and active F anion; Single–walled carbon nanotube (SWNT) is characterized by its novel one-dimensional nano-structure and unique electronic structure. The present work focuses on utilization of these two novel materials in Mg-based nano-composites. The high effectiveness of NbF5 and SWNT in catalytically enhancing absorption/desorption processes of Mg is substantiated by the observed property improvement.
In this thesis work, the hydrogen storage properties of mechanically prepared Mg/NbF5 and Mg/NbF5+SWNTs composites were systematically investigated by using a self-made Sievelt’s apparatus, which allows a simultaneous and precise collection of pressure and temperature signals. Furthermore, the catalytic mechanism involved in the reversible absorption/desorption processes of Mg was investigated on the basis of combined property/phase/microstructure investigations.
1. Preparation, structure and property of Mg/NbF5 composites
MgH2 was mechanically milled with different amounts of NbF5 for different periods under argon atmosphere using a SPEX8000 mill，and hydrogen storage performance of thus-prepared MgH2/NbF5 composites was examined. It was found that the hydrogen sorption capacity and sorption kinetics of the composites were dependent on the addition amount of NbF5 as well as the milling time. An optimal property was obtained when using 2 mol% NbF5 and 5 h milling time. At 573 K, the MgH2/2mol%NbF5 composite can absorb 5 wt.% hydrogen in 12 s and 6 wt.% in 60 min, and desorb 4.4 wt.% in 10 min and 5 wt.% in 60 min. Combined X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses indicate the presence of Nb, MgF2 and some unclear Nbx+(0
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