Amorphous alloys for hydrogen storage

Metal hydrides are promising materials for solid-state hydrogen storage, however, their gravimetric hydrogen storage density is generally low. In addition, they may also exhibit poor activity, sluggish de/hydrogenation kinetics and high thermodynamic stability, in particular for metal hydrides with high storage capacity. Because of the long-range disordered atomic structure, the amorphous structure, showing a wider interstitial configuration diversity, can provide hydrogen with more types of occupation sites. Such unique property leads to much larger hydrogen storage capacity, faster de/hydrogenation kinetics, local thermodynamic destabilization and even destruction resistance. Therefore, making use of amorphous structure is one of the attractive strategies for promoting the performance of metal hydrides. To develop amorphous hydrogen storage alloys, composition design is the first issue, and two main factors should be considered. One is the glass forming ability of alloys and the other is hydrogen storage ability of the alloys. Comparing with the crystalline counterparts, the amorphous hydrogen storage alloys exhibit some unique features: (1) the non-Arrhenius hydrogen diffusion; (2) the deviation from the Sieverts’ law; (3) protean hydrogen-induced phase separation/crystallization; (4) plateau-free pressure-composition-isothermal curve; (5) excessed storage capacity; etc. Till now, the developed hydrogen storage amorphous alloys are mainly Ti-based, Zr-based and Mg-based alloys. Some amorphous alloys do show attractive performance. In particular, Mg-based amorphous alloys have been investigated extensively in recent years. The hydrogen storage capacity in Mg-based amorphous alloys can be readily higher than 3.0 wt%-H, and with addition of other elements, one can also obtain reasonable de/hydrogenation rate and temperature during electrochemical or gaseous process. Though the de/hydrogenation capacity and kinetics of the amorphous alloys can be much better than the crystalline counterparts, there are still great challenges, such as amorphous structure stability, long-cycle reversible de/hydrogenation and irreversible hydrogen-induced amorphous phase transformation. With further exploration of alloy composition and material processing, there are great chances in using hydrogen storage amorphous alloys as energy storage material.