Hydrogen storage in Mg2Fe(Ni)H6 nanowires synthesized from coarse-grained Mg and nano sized γ-Fe(Ni) precursors

Abstract

In this work, Mg2Fe(Ni)H6 was synthesized under relatively mild conditions using precursors of coarse-grained Mg powder and Fe(Ni) composite containing γ-Fe(Ni) nano particles prepared through arc plasma method. The microstructure, composition, phase components and the hydrogen storage properties of the Mg–Fe(Ni) composite were carefully investigated. It is observed that the Mg2Fe(Ni)H6 formed from the Mg–Fe(Ni) composite has a “tangled nanowire” morphology with the wire diameter of 100–240 nm. In contrast, a typical columnar morphology was observed for the Mg2FeH6 produced from hydrogenation of coarse-grained Mg and pure α-Fe nano particles. A promotion effect on the synthesis of Mg2FeH6 is found for the utilization of γ-Fe(Ni) precursor instead of pure α-Fe. Indeed, γ-Fe(Ni) has the same fcc lattice as Mg2FeH6, which may remarkably shorten the diffusion distance of Fe during the formation of Mg2FeH6 from MgH2 and Fe. Meantime, the existence of Ni in Fe would catalyze the hydrogenation and influence the growing of Mg2Fe(Ni)H6/γ-Fe(Ni) interface, leading to the formation of Mg2Fe(Ni)H6 tangled nanowires. For hydrogen release, the peak desorption temperature of Mg2Fe(Ni)H6 is 593 K, which is 22 K lower than that of the Mg2FeH6. The absorption and desorption enthalpies of Mg2Fe(Ni)H6 were measured to be −68.8 ± 3.0 and 69.2 ± 3.2 kJ/mol H2, respectively. The improvements in both hydrogen sorption kinetics and thermodynamics can be attributed to destabilization effect of Ni substitution for Fe in the Mg2FeH6.