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

Abstract

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 °C, which is ∼45 °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% H₂ within 50 min at 280 °C. The desorption activation energy of the core–shell Mg₂FeH₆@MgH₂ was reduced to 115.7 kJ mol⁻¹ H₂, while the desorption reaction enthalpy and entropy were calculated to be −80.6 ± 7.4 kJ mol⁻¹ H₂ and −140.0 ± 11.9 J K⁻¹ mol⁻¹ H₂, 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.