Hydrolysis of Mg–salt and MgH2–salt mixtures prepared by ball milling for hydrogen production

It is well known that the reaction between Mg or MgH2 and water produces hydrogen. However, this reaction stops rapidly, due to the formation of a passive Mg(OH)2 layer onto the reactive material. The milling of Mg or MgH2 with solid salts appears very effective to improve their hydrolysis reactivity. The milling time (0.5, 3 and 10 h), the amount of salt (1, 3 and 10 mol%) and the salt composition (KCl, NaCl, LiCl and MgCl2) are all parameters that control the hydrolysis reaction in terms of conversion yield and kinetics. The salt additive acts as a process control agent upon milling that increases the specific surface area of the Mg powder (not valid for more brittle MgH2 powder). In addition, the salt dissolution during the hydrolysis is assumed to break down the passive Mg(OH)2 layer, favouring the reaction between Mg or MgH2 and water. The driving force related to the exothermic dissolution of the salt additive such as MgCl2 is also a key factor for accentuating the MgH2 and Mg hydrolysis. The 0.5 h milled MgH2–3 mol% MgCl2 composite displays the best compromise in terms of milling duration and hydrogen production performance with 964 ml of hydrogen produced per gram of composite.