Hydrogen economy is being promoted as a solution to the world’s energy, climate and environmental problems. 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 “bottle neck”. After decades of extensive research, the hydrogen-rich boron-containing compounds have received extensive attention as potential hydrogen storage media for vehicular applications. Recently, reactive hydride composites (known as Li-Mg-B-H) have been attracting considerable interest as a viable hydrogen storage medium due to its extremely high H-capacity (with a theoretical value of 11.8 wt.%) and relatively good reversibility. 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 composite. Single–walled carbon nanotube (SWNT) is characterized by its novel one-dimensional nano-structure and unique electronic structure; TiF3 provides a source of both high valence Ti cation and active F anion. The present work focuses on utilization of these two novel materials in catalytically enhancing absorption/desorption processes of Li-Mg-B-H system.
In this thesis work, the hydrogen storage properties of mechanically prepared Li-Mg-B-H/SWNTs and Li-Mg-B-H/TiF3 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 was investigated on the basis of combined property/phase/microstructure investigations.
1. Preparation, structure and property of Li-Mg-B-H/SWNTs composite
LiBH4 and MgH2 (2:1 mole ratio) were mechanically milled with different amounts of SWNTs for 1 h under argon atmosphere，and hydrogen storage performance of thus-prepared Li-Mg-B-H/SWNTs composite was examined. It was found that the hydrogen sorption capacity and sorption kinetics of the composites were dependent on the addition amount of SWNTs. An optimal property was obtained when using 10 wt.% SWNTs. At 450℃, the composite can desorb 10 wt.% in 20 min. The dehydriding rate was 2 times faster than that of neat composite, and capacity penalty was alleviated by about 20%. Combined X-ray diffraction (XRD) analysis and a series of designed experiments indicate that Ni powder that in situ incorporated into as-prepared SWNTs exerts negative effect on the host material. It formed a new phase of MgNi2.5B2, which is stable in the subsequent reactions, and is responsible for the observed kinetic degradation. Then this property improvement completely came from the special qusi-one dimensional structured SWNTs, which forms net-like structure and exerts micro-confinement effect on the host material.
2. Preparation, structure and property of Li-Mg-B-H/TiF3 composite
Hydrogen storage performances of Li-Mg-B-H/TiF3 were investigated. It was found that hydrogen desorption kinetics of Li-Mg-B-H can be markedly improved by mechanical milling with 1mol% TiF3 addtive for 5 h. At 400℃, it can desorb 8 wt.% hydrogen within 25 min and still maintain over 95% of the total H-capacity during the subsequent dehydrogenation/rehydrogenation cycles. Based on the X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy analysis as well as compared experiments, this thermodynamic and kinetic improvement was ascribed to both Ti cation and F anion. The former is stabilized as catalytically active TiH2 nanophase, whereas the state and function of F anion are still unclear. This result further experimentally demonstrates the previously point of “functional anion”, and provide a new way in tailoring the hydrogen storage performance of reactive hydride composites.
The above results help to further understand Li-Mg-B-H system, improve its hydrogen storage properties and provide reference and instruction on exploring other high H-capacity complex hydride.
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