Superior Reversible Hydrogen Storage of the LiBH4 + MgH2 system

Hydrogen storage is the tailback for the implementation of a so-called “hydrogen economy”, which could replace the fossil fuels used today with hydrogen generated by renewable energy. Lithium borohydride (LiBH₄) is a promising hydrogen storage material for some applications. In 2003, a study was published that hypothesized the theoretical hydrogen storage capacity of a mixture of LiBH₄ and MgH₂ (magnesium hydride) at 11.45 wt % H₂ at certain temperature. It is rather unfortunate that since that discovery, there have been few studies on the dehydrogenation and rehydrogenation of LiBH₄ + MgH₂ system below the melting point of LiBH₄ (275 °C). Of the few available publications, a recent study has shown that nano-LiBH₄ + nano-MgH₂ mixture produced from ball milling with aerosol spraying (BMAS) can release 4.11 wt % H₂ in the first dehydrogenation at a temperature ≤265 °C.

All factors considered, the prospects of LiBH₄ + MgH₂ mixture has been limited by its sluggish kinetics despite its excellent hydrogen storage capacity theoretically. To address this, researchers from the Department of Mechanical, Materials and Aerospace Engineering at Illinois Institute of Technology: Dr. Zhao Ding and Professor Leon Shaw evaluated the superior hydrogen release behavior of the solid-state BMAS powder with 50% LiBH₄ from the viewpoints of both thermodynamics and chemical kinetics. Their goal was to demonstrate that the BMAS approach recently presented could not only tune the thermodynamics but also improve the kinetics for hydrogen release from a LiBH₄ + MgH₂ mixture. Their work is currently published in the research journal, ACS Sustainable Chemistry & Engineering.

Given the significantly improved hydrogen storage capacity for the LiBH₄ + MgH₂ system obtained via BMAS, in-depth assessment of the background processes becomes a necessity to further improve on the entire system. As such, the two scholars evaluated the improved thermodynamics from the viewpoint of the significantly heightened dissociation pressure. In fact, the research pair used nine different kinetics models to analyze the solid-state dehydrogenation behavior of the BMAS powder with 50% LiBH₄ at 265 °C.

The kinetics analysis revealed that the rate limiting step of this BMAS powder was initially controlled by the nucleation/growth process but then changed to moving-phase boundary control and finally to diffusion control as the number of dehydrogenation/rehydrogenation cycles increased. Moreover, thermal analysis results indicated that the apparent activation energy of the BMAS powder was reduced by 23.3 and 30.6 kJ/mol when compared to that of bulk LiBH₄ and ball-milled MgH₂ + C mixtures, respectively, thus revealing BMAS as an effective method to promote hydrogen release from LiBH₄ + MgH₂ mixtures.

In summary, the study by Dr. Zhao Ding and Professor Leon Shaw looked carefully on the unusually high hydrogen release through the solid-state BMAS powder with 50% LiBH₄ at 265 °C from the viewpoints of the thermodynamic driving force, as well as the chemical kinetics of the dehydrogenation process. Remarkably, it was established that the dissociation pressures for the BMAS sample were ∼700 and ∼1910% higher than those of the commercial LiBH₄ and commercial MgH₂, respectively. In a statement to Advances in Engineering, Dr. Zhao Ding commented that the new insights in the solid-state dehydration kinetics and the associated reaction mechanisms established in their study could lay a scientific foundation for further understanding and improvements in enhancing the solid-state dehydrogenation/rehydrogenation properties of LiBH₄ + MgH₂ mixtures in the future.