Low temperature thermolysis and hydrolysis of MgH2 generated from a titanium-mediated hydrogenation of Mg2Si

Abstract

The magnesium-silicon hydrogen storage system (Mg₂Si+H₂↔MgH₂+Si) has attracted considerable attention with the potential of achieving near-room temperature hydrogen sorption since the reaction enthalpy is only ∼36.8 kJ/mol H₂, offering much better thermodynamic properties over MgH₂. However, the rehydrogenation of Mg₂Si faces significant kinetic barriers, restricting its practical utilization. In this work, hydrogen-assisted high-energy ball milling (HHBM) was employed to hydrogenate Mg₂Si. XRD and HRTEM results confirmed that Mg₂Si was successfully converted into MgH₂ under the influence of titanium (Ti), achieving a conversion rate up to 76%. Aberration-corrected TEM (AC-TEM) revealed that in situ formed MgH₂ and titanium silicide (TiSi₂) were homogenously mixed with robust interfaces. Such a growth mode reduces the migration path of Si atoms and the stability of Mg₂Si, thus promoting the formation of MgH₂. Furthermore, the as-synthesized MgH₂ particles with ultrafine particle size and adjacent catalysts show superior thermolysis and hydrolysis performances. Consequently, the composite exhibits excellent hydrogen storage properties, absorbing 50% of its total hydrogen capacity at room temperature and initiating dehydrogenation at 114.3 °C. In situ synchrotron X-ray diffraction (ISXRD) confirms the dehydrogenation of in situ generated ultrafine MgH₂ and part of TiSi₂ is converted to Mg₂Si at 283 °C. In addition, the ultrafine, defect-rich MgH₂ particles show favorable hydrolysis kinetics and high conversion efficiency at relatively low temperatures (91.9% of the hydrogen yield can be achieved at 5 °C). This method developed in this work exhibits a new way to synthesize high-performance hydrogen storage materials.