Phase transition and hydrogen storage properties of Mg–Ga alloy

Highlights: • A fully reversible transformation in Mg–Ga–H system with reduced dehydrogenation enthalpy is realized. • The mechanism of phase transformation in the de/hydrogenation of Mg–Ga alloy is revealed. • The de/hydrogenation process of Mg₅Ga₂ compound is expressed as: Mg₅Ga₂ + H₂ ↔ 2Mg₂Ga + MgH₂. - Abstract: Mg-based alloys are viewed as one of the most promising candidates for hydrogen storage; however, high desorption temperature and the sluggish kinetics of MgH₂ hinder their practical application. Alloying and changing the reaction pathway are effective methods to solve these issues. As the solid solubility of Ga in Mg is 5 wt% at 573 K, the preparation of a Mg(Ga) solid solution at relatively high temperatures was designed in this paper. The phase transition and hydrogen storage properties of the MgH₂ and Mg₅Ga₂ composite (hereafter referred to as Mg–Ga alloy) were investigated by X-ray diffraction (XRD), pressure–composition-isotherm (PCI) measurements, and differential scanning calorimetry (DSC). The reversible hydrogen storage capacity of Mg–Ga alloy is 5.7 wt% H₂. During the dehydrogenation process of Mg–Ga alloy, Mg₂Ga reacts with MgH₂, initially releasing H₂ and forming Mg₅Ga₂; subsequently, MgH₂ decomposes into Mg with further release of H₂. The phase transition mechanism of the Mg₅Ga₂ compound during the dehydrogenation process was also investigated by using in situ XRD analysis. In addition, the dehydrogenation enthalpy and entropy changes, and the apparent activation energy were also calculated