*Review* **Intermetallic Compounds Synthesized by Mechanical Alloying for Solid-State Hydrogen Storage: A Review**

**Yuchen Liu \*, Djafar Chabane and Omar Elkedim**

FEMTO-ST Institute, University Bourgogne Franche-Comté, UTBM, CNRS, 90000 Belfort, France; djafar.chabane@utbm.fr (D.C.); omar.elkedim@utbm.fr (O.E.)

**\*** Correspondence: yuchen.liu@utbm.fr

**Abstract:** Hydrogen energy is a very attractive option in dealing with the existing energy crisis. For the development of a hydrogen energy economy, hydrogen storage technology must be improved to over the storage limitations. Compared with traditional hydrogen storage technology, the prospect of hydrogen storage materials is broader. Among all types of hydrogen storage materials, solid hydrogen storage materials are most promising and have the most safety security. Solid hydrogen storage materials include high surface area physical adsorption materials and interstitial and non-interstitial hydrides. Among them, interstitial hydrides, also called intermetallic hydrides, are hydrides formed by transition metals or their alloys. The main alloy types are A2B, AB, AB2, AB3, A2B7, AB5, and BCC. A is a hydride that easily forms metal (such as Ti, V, Zr, and Y), while B is a non-hydride forming metal (such as Cr, Mn, and Fe). The development of intermetallic compounds as hydrogen storage materials is very attractive because their volumetric capacity is much higher (80–160 kgH2m<sup>−</sup>3) than the gaseous storage method and the liquid storage method in a cryogenic tank (40 and 71 kgH2m<sup>−</sup>3). Additionally, for hydrogen absorption and desorption reactions, the environmental requirements are lower than that of physical adsorption materials (ultra-low temperature) and the simplicity of the procedure is higher than that of non-interstitial hydrogen storage materials (multiple steps and a complex catalyst). In addition, there are abundant raw materials and diverse ingredients. For the synthesis and optimization of intermetallic compounds, in addition to traditional melting methods, mechanical alloying is a very important synthesis method, which has a unique synthesis mechanism and advantages. This review focuses on the application of mechanical alloying methods in the field of solid hydrogen storage materials.

**Keywords:** mechanical alloying; intermetallic compounds; solid hydrogen storage
