High-Yield Direct Synthesis of Mg₂FeH₆ from the Elements by Reactive Milling

Abstract

Magnesium complex hydrides as Mg2FeH6 are interesting phases for hydrogen storage in the solid state, mainly due to its high gravimetric and volumetric densities of H2. However, the synthesis of this hydride is not trivial because the intermetallic phase Mg2Fe does not exist and Mg and Fe are virtually immiscible under equilibrium conditions. In this study, we have systematically studied the influence of the most important processing parameters in reactive milling under hydrogen (RM) for Mg2FeH6 synthesis: milling time, ball-to-powder weight ratio (BPR), hydrogen pressure and type of mill. Low cost 2Mg-Fe mixtures were used as raw materials. An important control of the Mg2FeH6 direct synthesis by RM was attained. In optimized combinations of the processing parameters, very high proportions of the complex hydride could be obtained.

Investigation of Effect of Milling Atmosphere and Starting Composition on Mg2FeH6 Formation

“…The synthesis of Mg 2 FeH 6 by ball milling has been studied by several authors using different milling parameters, such as: type of mill; ball to powder ratio; number and size of balls; milling time; rotational speed; milling atmosphere and hydrogen pressure [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Irrespective of the milling conditions, some iron was always detected in the milled powder representing an incomplete Mg 2 FeH 6 formation.…”

“…Some authors reported that Mg 2 FeH 6 is formed from the metallic elements [2,4,19,24], while others claimed that MgH 2 is the Mg 2 FeH 6 precursor [9,20,21,25,26]. Recently, we studied the transformation of phases during hydrogen absorption of a ball milled 2Mg + Fe mixture [27].…”

Controlled mechanochemical synthesis and hydrogen desorption mechanisms of nanostructured Mg2CoH5

“…The rapid and practically full synthesis of Mg 2 CoH 5 by RM from the metallic elements is attributed to an appropriate choice of milling parameters. As reported by Leiva et al [12,13], a high yield synthesis of the complex hydride Mg 2 FeH 6 , which belongs to the same complex hydride family as Mg 2 CoH 5 , requires high energy transfer rates during ball milling. The results obtained for Mg 2 FeH 6 [12,13] and Mg 2 CoH 5 (this study) are supported by the model proposed by Bab and Mendoza-Z elis [14,15] for hydride formation kinetics during RM.…”

Formation reaction of Mg2FeH6: effect of hydrogen absorption/desorption kinetics

“…Some authors suggested that Mg 2 FeH 6 is formed directly from the reaction of 2Mg+Fe+3H 2 [1,2,6,8] , while others reported that the reaction involves two steps: the first one is the formation of MgH 2 , and secondly, the reaction of magnesium hydride with Fe [5,[9][10][11] . Polanski et al 11 using in-situ synchrotron X-ray diffraction showed that MgH 2 is the Mg 2 FeH 6 precursor.…”