Metallurgical processing of Mg alloys and MgH2 for hydrogen storage

Abstract

The reversible reaction of hydrogen with metallic Mg to form MgH2 has been evaluated by more than 70 years, with particular attention in the last 20–30 years for the exceptional properties of this hydride to store energy. The main challenges to the practical use of these properties were soon identified; the high thermodynamical stability of the hydride, resulting in high temperatures for absorption / desorption of hydrogen; the low diffusion rate of hydrogen in Mg and in MgH2, which requires short diffusion distances and therefore, large surface, interface or interphase areas and the presence of catalyst to improve the absorption and desorption kinetics. Thermodynamic stability cannot be modified by the microstructure, which can, however, affect the other above-mentioned challenges. The first Mg samples to react with hydrogen were prepared by conventional metallurgical routes, but with the important addition of being crushed to be smaller than a certain particle size. Turnings of bulk Mg or Mg alloys samples were also used, resulting in the same increase in surface and interfaces area as the crushed particles. Alloys of different compositions, including eutectic alloys were designed to result in a combination of phases, with one of them in conditions to form a hydride and the other to act as catalyst, or most likely just supplying extra interphases for hydrogen diffusion. But since melting of Mg and Mg alloys is not a friendly process, mechanical alloying was considered simpler and more adequate to prepare Mg powders, resulting in the identification of the exceptional properties of nano powders to improve kinetics but also stressing the difficulties of handling nano powders and the deleterious effect of surface contamination. Severe plastic deformation was then used to prepare “bulk” nanograined material with the expectation to design microstructures with special textures and special types of grain boundaries and to avoid the problem associated with surface reactivity. Because of kinetics limitation, the SPD techniques had to be combined with other processing step, such as filing, cold rolling or milling, basically to increase the surface / interface area. Extensive plastic deformation via co-lamination had already been used but the necessity to complement SPD processing led to the use of conventional metallurgical processing such as cold rolling and cold forging to process Mg and MgH2. Such metallurgical routes prove to be simpler and cost-effective to deliver microstructures satisfying the requirement for Mg and MgH2 to act as hydrogen storage material. Other processing routes or combination of routes that could result in refined microstructure have also been studied, including melt spinning, melt spinning combined with HEBM or with cold-rolling, friction stir welding, mechanical grinding and filing and chips from lathe turnings. In this review we discuss the evolution of metallurgical processing of Mg and MgH2 based hydrogen storage material and how each processing route can lead to microstructures with the required characteristics.