This review deals with the destabilization methods for improvement of storage properties of metal hydrides. Both theoretical and experimental approaches were used to point out the influence of various types of defects on structure and stability of hydrides. As a case study, Mg, and Ni based hydrides has been investigated. Theoretical studies, mainly carried out within various implementations of DFT, are a powerful tool to study mostly MgH 2 based materials. By providing an insight on metalhydrogen bonding that governs both thermodynamics and hydrogen kinetics, they allow us to describe phenomena to which experimental methods have a limited access or do not have it at all: to follow the hydrogen sorption reaction on a specific metal surface and hydrogen induced phase transformations, to describe structure of phase boundaries or to explain the impact of defects or various additives on MgH 2 stability and hydrogen sorption kinetics. In several cases theoretical calculations reveal themselves as being able to predict new properties of materials, including the ways to modify Mg or MgH 2 that would lead to better characteristics in terms of hydrogen storage. The influence of ion irradiation and mechanical milling with and without additives has been discussed. Ion irradiation is the way to introduce a well-defined concentration of defects (Frankel pairs) at the surface and subsurface layers of a material. Defects at the surface play the main role in sorption reaction since they enhance the dissociation of hydrogen. On the other hand, ball-milling introduce defects through the entire sample volume, refine the structure and thus decrease the path for hydrogen diffusion. Two Severe Plastic Deformation techniques were used to better understand the hydrogenation/dehydrogenation kinetics of Mg-and Mg 2 Nibased alloys: Equal-Angular-Channel-Pressing and Fast-Forging. Successive ECAP passes leads to refinement of the microstructure of AZ31 ingots and to instalment therein of high densities of defects. Depending on mode, number and temperature of ECAP passes, the H-sorption kinetics have been improved satisfactorily without any additive for mass H-storage applications considering the relative speed of the shaping procedure. A qualitative understanding of the kinetic advanced principles has been built. Fast-Forging was used for a "quasiinstantaneous" synthesis of Mg/Mg 2 Ni-based composites. Hydrogenation of the as-received almost bi-phased materials remains rather slow as generally observed elsewhere, whatever are multiple and different techniques used to deliver the composite alloys. However, our preliminary results suggest that a synergic hydrogenation / dehydrogenation process should assist hydrogen transfers from Mg/Mg 2 Ni on one side to MgH 2 / Mg 2 NiH 4 on the other side via the rather stable a-Mg 2 NiH 0.3 , acting as in-situ catalyser.
The influence of different milling conditions obtained using two high-energy mills on hydrogen desorption from MgH 2 -WO 3 composites was investigated. The morphology, particle and crystallite size were studied as a function of milling speed, vial's volume, and ball-to-powder ratio. The vial's fill level, the number, and type of milling balls and additive's content kept constant. Changes in morphology and microstructure were correlated to desorption properties of materials. Higher milling speed reduced particle size but, there is no significant crystallite size reduction. On the other hand, additive distribution is similar regardless of the energy input. It has been noticed that different energy input on milling blend, which is the result of combined effects of above-mentioned factors, reflects on desorption temperature but not on the kinetics of desorption. In fact, desorption mechanism changes from 2D to 3D growth with constant nucleation rate, despite obtained changes in microstructure or chemical composition of the material.
LiAlH4 was modified by mechanical milling and with the addition of 5 wt.% Fe2O3 in order to improve its hydrogen desorption properties. The composite was milled for 1, 3, 5, 7 or 15min, and depending on the milling time, various phenomena took place. Up to a milling time of 5min, the particle size of the composite decreases. Further milling leads to the particles agglomeration reaching the size of the starting material after 15min. Moreover, the mechanical milling process leads to the transformation of AlH - 4 to AlH 3 - 6 structure as a result of partial hydrogen desorption. Hydrogen desorption during the milling is the most pronounced in the sample milled for 15min, so this sample has only one hydrogen desorption peak in the temperature-programmed desorption measurements.Mechanical milling with the addition of Fe2O3 for up to 15min improves LiAlH4 hydrogen desorption properties as hydrogen desorption temperature and apparent activation energies decrease.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.