Serge Nyallang Nyamsi

Microstructure and improved hydrogen storage properties of Mg based alloy powders prepared by modified milling method

Abstract In this study, the modified preparation method of combining planetary and vibratory ball milling was proposed to prepare Mg based hydrogen storage alloy powders. The comparison of micromorphology and hydrogen storage behaviour between Mg2Ni prepared using the modified and conventional preparation methods were investigated experimentally. The comparison results showed that the combination of first planetary and then vibratory ball milling has more favourable effect on improving both the kinetics and the thermodynamics of ball milled Mg2Ni alloys. The sample synthesised by first planetary milling for 40 h and then vibratory milling for 30 h has faster hydrogen absorption kinetics and lower dehydriding onset temperature than those prepared by the single method of planetary or vibratory milling and hydriding combustion synthesis owing to its popcorn-like microstructure. Moreover, this kind of modified method reduces the reaction enthalpy and activation energy by up to ∼18 and 22% respectively.

An outstanding effect of graphite in nano-MgH2–TiH2 on hydrogen storage performance

TiH2-modified MgH2 was prepared by high energy reactive ball milling (HRBM) of Mg and Ti in hydrogen and showed high weight H storage capacity and fast hydrogenation/dehydrogenation kinetics. However, a decrease in the reversible H storage capacity on cycling at high temperatures takes place and is a major obstacle for its use in hydrogen and heat storage applications. Reversible hydrogen absorption/desorption cycling of the materials requires use of the working temperature ≥330 °C and results in a partial step-by-step loss of the recoverable hydrogen storage capacity, with less significant changes in the rates of hydrogenation/dehydrogenation. After hydrogen desorption at 330–350 °C, hydrogen absorption can proceed at much lower temperatures, down to 24 °C. However, a significant decay in the reversible hydrogen capacity takes place with increasing number of cycles. The observed deterioration is caused by cycling-induced drastic morphological changes in the studied composite material leading to a segregation of TiH2 particles in the cycled samples instead of their initial homogeneous distribution. However, the introduction of 5 wt% of graphite into the MgH2–TiH2 composite system prepared by HRBM leads to an outstanding improvement of the hydrogen storage performance. Indeed, hydrogen absorption and desorption characteristics remain stable through 100 hydrogen absorption/desorption cycles and are related to an effect of the added graphite. The TEM study showed that carbon is uniformly distributed between the MgH2 grains covering segregated TiH2, preventing the grain growth and thus keeping the reversible storage capacity and the rates of hydrogen charge and discharge unchanged. Modelling of the kinetics of hydrogen absorption and desorption in the Mg–Ti and Mg–Ti–C composites showed that the reaction mechanisms significantly change depending on the presence or absence of graphite, the number of absorption–desorption cycles and the operating temperature.