Evidence of the hydrogen release mechanism in bulk MgH2

Abstract

Hydrogen has the potential to power much of the modern world with only water as a by-product, but storing hydrogen safely and efficiently in solid form such as magnesium hydride remains a major obstacle. A significant challenge has been the difficulty of proving the hydriding/dehydriding mechanisms and, therefore, the mechanisms have long been the subject of debate. Here we use in situ ultra-high voltage transmission electron microscopy (TEM) to directly verify the mechanisms of the hydride decomposition of bulk MgH2 in Mg-Ni alloys. We find that the hydrogen release mechanism from bulk (2 μm) MgH2 particles is based on the growth of multiple pre-existing Mg crystallites within the MgH2 matrix, present due to the difficulty of fully transforming all Mg during a hydrogenation cycle whereas, in thin samples analogous to nano-powders, dehydriding occurs by a ‘shrinking core' mechanism.

Reply to ‘Comments on “Evidence of the hydrogen release mechanism in bulk MgH2”’

“…Furthermore the detailed crystallography of our sample can be found from the supporting Synchrotron XRD data which is discussed in the supplementary section of our paper (as Figure S3)2. This confirms that the as–prepared bulk sample contains mainly MgH 2 and a small amount of other phases including Mg, Mg 2 Ni, Mg 2 NiH 4 but not MgO.…”

“…al. 1 on our recently published study2 contains two main assertions:The MgH 2 sample we studied was not MgH 2 at any time but rather MgO.The transformation we observed was the formation of Kirkendall voids due to the outward diffusion of Mg.…”

Tribochemical Decomposition of Light Ionic Hydrides at Room Temperature

“…25,48 In those cases, desorption of H 2 from MgH 2 was also interpreted on the base of a nucleation and growth mechanism, particularly a "shrinking core" model. 49 In summary, we have observed that dehydriding of MgH 2 could be achieved at room temperature by mechanical deformation of pellets in vacuum. The evidence of this comes from the analysis of H 2 emission using MSGE mass spectrometry and characterization of mechanically affected zones by confocal-microRaman.…”

The kinetics of lightweight solid-state hydrogen storage materials: A review

“…All of these models are applicable depending on the properties or behavior of a material. In the most recent study, Nogita et al [74] found that the mechanism for hydrogen desorption from bulk (2 mm) MgH 2 particles is affected by the growth of crystalline Mg within the MgH 2 matrix. This effect occurred due to the difficulty of fully changing all Mg during an absorption cycle.…”

“…Through the rapid and incredible improvements and understanding, knowledge regarding their structures, thermodynamics and chemical reactions has been attained, but they still suffer from sluggish kinetics and highly stable materials. In addition to these recognized light metal-based materials, there are other candidates that may be practicable for hydrogen storage in the future, e.g., hydride complexes such as Mg 2 FeH 6 [212], NH 3 BH 3 [213], and Mg 3 Pr [74]; Ni(macrocycle) complexes (Ni(cyclam) (BH 4 ) 2 ); Ln 4 MH n ; and others. There have also been some candidate adsorbents (AX-21, Pt/AC-IRMOF 8, and MOF 5), chemical hydrides (NH 3 BH 3 , H 3 BH 3 , AlH 3 , and LiAlH 4 ) and metal hydrides (NaAlH 4 and TiCr(Mn)H 2 ) that are currently being evaluated for the DOE by the Hydrogen Storage Engineering Center of Excellence (HSECoE) [214].…”