Remarkable hydrogen desorption properties and mechanisms of the Mg2FeH6@MgH2 core-shell nanostructure

Mg2FeH6@MgH2 dual-metal hydrides with a core-shell nanostructure were synthesized via ball-milling and heat treatment methods using Mg and Fe as raw materials assisted by diethyl ether addition. Systematic investigations of the association between the microstructure and hydrogen desorption properties of the Mg2FeH6@MgH2 core-shell hydride were performed. It is found that the as-synthesized Mg2FeH6@MgH2 is comprised of the Mg2FeH6-core with a particle size of 40-60 nm and the MgH2-shell with a thickness of 5 nm. The hydrogen desorption of the Mg2FeH6@MgH2 core-shell nanoparticle starts at 220 degrees C, which is similar to 45 degrees C lower than that of the Mg2FeH6@MgH2 micrometer particle. Compared to the as-synthesized Mg2FeH6@MgH2 micrometer particle, the Mg2FeH6@MgH2 core-shell sample exhibited faster hydrogen desorption kinetics, which released more than 5.0 wt% H-2 within 50 min at 280 degrees C. The desorption activation energy of the core-shell Mg2FeH6@MgH2 was reduced to 115.7 kJ mol(-1) H-2, while the desorption reaction enthalpy and entropy were calculated to be -80.6 +/- 7.4 kJ mol(-1) H-2 and -140.0 +/- 11.9 J K-1 mol(-1) H-2, respectively. It is proposed that the improvements of both hydrogen desorption kinetics and thermodynamics are due to the special core-shell nanostructure of Mg2FeH6@MgH2. More remarkably, it is demonstrated that the core-shell nanostructure could be recovered after rehydrogenation, leading to excellent cycling hydrogen desorption properties of Mg2FeH6@MgH2. In addition, the suggested dehydrogenation mechanism involves the dehydrogenation of the MgH2-shell followed by the decomposition of the Mg2FeH6-core into Mg and Fe according to the three-dimensional phase-boundary process.