A group of researchers from China and Japan has identified a key inhibitor to hydrogen desorption in magnesium hydride (MgH₂) solid-state hydrogen storage materials. Details of their findings were published in the Journal of Materials Chemistry A.

Currently, hydrogen is stored by three methods: high-pressure gaseous hydrogen storage; low-temperature liquid hydrogen storage; and solid-state hydrogen storage. Among solid-state hydrogen storage, solid-state materials are generally the safest and provide the most hydrogen storage density.

Metal hydrides have long been explored for their large hydrogen storage potentiality and their low cost. As these metals come into contact with gaseous hydrogen, hydrogen is absorbed onto the surface. Further energy input leads to hydrogen atoms finding their way into the metal's crystal lattices until the metal becomes saturated with hydrogen. From there, the material can absorb and desorb hydrogen in larger amounts.

Magnesium hydride (MgH₂) has shown immense promise for superior hydrogen storage capacity. However, a high temperature is necessary for MgH₂ to decompose and produce hydrogen. Furthermore, the material's complex hydrogen migration and desorption, which result in sluggish dehydrogenation kinetics, have stymied its commercial application.

For decades, scientists have debated why dehydrogenation within MgH₂ is so difficult. Now, the research group has uncovered an answer.

Herein, we study the sequential MgH₂ dehydrogenation mechanism by analyzing the kinetic and structural changes during the layer-by-layer hydrogen desorption process. Our results obtained by spin-polarized density functional theory calculations with van der Waals corrections (DFT-D3) unveiled an interesting “burst effect” during MgH₂ dehydrogenation.

We found that the initial dehydrogenation barriers (2.52 and 2.53 eV) are much higher than the subsequent reaction barriers (0.12–1.51 eV). The Mg–H bond analyses by the crystal orbital Hamilton population method indicate that the Mg–H bond strength decreases along the dehydrogenation process. Therefore, the subsequent H migration and hydrogen desorption become significantly easier, showing a “burst effect”.

Electronic structure analyses using the electron localization function show that the H vacancy still has a high degree of electronic localization when the first layer of atomic H exists. Furthermore, ab initio molecular dynamics simulations were performed to analyze the kinetic characteristics of MgH₂ after surface dehydrogenation to provide more evidence. This identified burst effect provides a theoretical basis for the dehydrogenation kinetics of MgH₂ and proposes important guidelines for modifying MgH₂-based hydrogen storage materials: promoting the initial dehydrogenation by structural engineering could be the key to facilitating the hydrogen desorption of MgH₂.

—Dong et al.

An interesting “burst effect” was found on the dehydrogenation of a typical solid-state hydrogen storage material, MgH₂. After the sluggish dehydrogenation at the first layer, hydrogen desorption from the subsequent layers will be much easier. © Hao Li et al.

The group carried out further bond analysis with the crystal orbital Hamilton population method, where the researchers confirmed the magnesium-hydride bond strength decreased as the dehydrogenation process continued.

Hydrogen migration and hydrogen desorption is much easier following the initial burst effect. Structural engineering tweaks that promote this desorption process could be the key to facilitating the hydrogen desorption of MgH₂. Our findings provide a theoretical basis for the MgH₂‘s dehydrogenation kinetics, providing important guidelines for modifying MgH₂-based hydrogen storage materials.

—Co-corresponding author Hao Li, associate professor at Tohoku University’s Advanced Institute for Materials Research (WPI-AIMR)

The researchers demonstrated that hydrogen vacancies maintained a high degree of electronic localization when the first layer of atomic hydrogen exists. Analyses of the kinetic characteristics of MgH₂ after surface dehydrogenation, performed by ab initio molecular dynamics simulations, also provided additional evidence.

Resources

• Shuai Dong, Chaoqun Li, Jinhui Wang, Hao Liu, Zhao Ding, Zhengyang Gao, Weijie Yang, Wei Lv, Li Wei, Ying Wu, and Hao Li (2022) “The “burst effect” of hydrogen desorption in MgH₂ dehydrogenation” Journal of Materials Chemistry A doi: 10.1039/D2TA06458H