Mg-containing multicomponent alloys produced by high-energy ball milling for hydrogen storage

Abstract

Hydrogen is a promising energy carrier that allows the use of energy in a sustainable form. However, safe and efficient hydrogen storage is a scientific and technological challenge that still has to be overcome. Recently it was reported that some high entropy alloys (HEAs), multicomponent alloys that crystallize as extended solid solutions with simple crystalline structures (BCC or FCC, for instance), present promising hydrogen storage properties. For example, TiVZrNbHf alloy, which forms a BCC single-phase structure, presented higher storage capacity than conventional hydrides. However, most of the papers published so far report compositions based only on transition metal elements, which limit the gravimetric capacities due to their densities. Since Mg is a low-density element promising for hydrogen storage, the study of Mg-containing multicomponent compositions is opportune. Recently, our research group studied an Mg-containing A2B type HEA system, namely, MgTiZrFe0.5Co0.5Ni0.5. This alloy formed a BCC structure when milled under argon, and this phase absorbs up to 1.2 wt.% of H2 before it undergoes a phase transition to FCC hydride during absorption kinetics. The gravimetric capacity of the alloy would have been 3.5 wt.% H2 (hydrogen over metal atoms - H/M=2) if the transformation to the dihydride phase had happened. This master project aimed to study the hydrogen storage behavior of new alloys for the Mg-Ti-Nb-Cr-Mn-Ni and Mg-Ti-Nb-Ni systems. For alloys selection, a thermodynamic model that allows predicting which compositions have the highest tendency to form single-phase microstructures based on extended solid solutions was tested. The selected alloys were produced by high-energy ball milling (HEBM) and evaluated in terms of phase formation and stability and hydrogen storage behavior. All the synthesized alloys formed solid solutions, but no single-phase was obtained and the formation of elemental segregation was observed. The Mg22Ti22Nb22Cr11Mn11Ni11 alloy synthesized under argon atmosphere and under hydrogen pressure absorbed 1.18 wt.% of H2 and desorbed 1.6 wt.% of H2, respectively. The Mg21Ti31Nb31Ni17 alloy synthesized under argon atmosphere and under hydrogen pressure absorbed 1.3 wt.% of H2 and desorbed 2.26 wt.% of H2, respectively. Based on these results, the method of alloy selection and the approach of using Mg-containing compositions could be analyzed.