טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentPopilevsky Larisa
SubjectSynthesis, Hydrogen Storage and Thermal Transport Properties
of the Composites Based on Mg and Carbonaceous
Additives
DepartmentDepartment of Energy
Supervisor Professor Eugen Rabkin
Full Thesis textFull thesis text - English Version


Abstract

Intensive research has been carried in the recent years toward the development of novel energy carriers alternative to oil. The hydrogen technology is considered as one of the promising directions in this respect. The major advantages of hydrogen as a carrier of energy are its abundance and an environmentally friendly way of energy recovery by reaction with oxygen. The safest way of storing hydrogen is in the form of metal hydride, since it is reversibly bound to the metal by chemical bonds.

Magnesium is considered as one of the most promising materials for reversible hydrogen storage. However, slow hydriding and dehydriding kinetics, high temperatures of hydrogen release, along with low heat conductivity of the hydride powder beds are the main drawbacks that prevent its use in hydrogen storage.

It has been reported that ball-milling magnesium with carbonaceous additives can accelerate H2 absorption/desorption kinetics. Yet, the mechanisms that are responsible for the kinetics acceleration remain unclear. The objective of this study is to reveal the role that the carbonaceous additives play during hydrogen absorption/desorption processes. In addition, the correlation between the ball-milling conditions, the microstructure, hydrogenation performance and the thermal conductivity properties of Mg???? additives composites was established.

The pelletized composites were prepared by high energy ball milling of Mg powder with either multiwall carbon nanotubes (MWCNTs) or graphite, followed by uniaxial compression and sintering in hydrogen ambient under mechanical constraint.

It was found that the presence and condition of carbon additives determine the morphology of Mg particles in the pellet. This in turn determines the mechanical stability of the pellets during hydrogenation cycling.

The best results in terms of a combination of hydrogenation kinetics, thermal conductivity, and mechanical integrity were obtained after 4h of co-milling Mg with 2wt.% MWCNTs. As result of ball-milling, the MWCNTs agglomerates were destroyed in favor of carbon nano-particles, that tend to align themselves into chains. These chains served as fast diffusion paths for H atoms. As result, fast hydrogen diffusion along the Mg-carbon interface leads to the growth of elongated MgH2 colonies aligned along the chains of carbon nanoparticles. The residual Mg trapped between these colonies forms an interpenetrated percolating network allowing a long-range heat transport through the pellets. On the contrary, most of the growing MgH2 nuclei in the hydrogenated Mg reference sample were isotropic. Upon their impingement, large isolated pockets of Mg were observed in the reference Mg pellets, reducing the contribution of the Mg phase to the overall thermal conductivity.

The thermal conductivities of the pellets of the Mg-2wt.% MWCNTs composite in both axial and radial directions were consistently higher than those of the reference Mg pellets in the whole studied temperature range of 300-523K.           At  523K the relative difference in thermal conductivities reached 20%.

Co-milling Mg with 2 wt%. MWCNTs for 4h, neither destabilizes MgH2, nor removes the kinetic bottleneck of the hydrogenation reaction. Thus, it can be concluded that carbon nano-particles chains accelerate the hydrogenation kinetics by improving heat transport and serving as fast diffusion paths for H atoms.