Hydrogen storage in magnesium clusters: quantum chemical study.

Area of Science:

• Materials Science
• Computational Chemistry
• Energy Storage

Background:

• Magnesium hydride (MgH2) is a promising, cost-effective hydrogen storage material due to its high hydrogen content (7.7 wt%).
• High hydrogen desorption temperatures (≥300°C) for bulk MgH2 hinder its practical application.
• Literature suggests lower desorption temperatures may be achievable in disordered materials and thin films.

Purpose of the Study:

• To systematically investigate the influence of crystal grain size on the thermodynamic stability of magnesium and magnesium hydride.
• To determine the relationship between crystallite size and hydrogen desorption temperatures.
• To explore the potential of nanoscale MgH2 for improved hydrogen storage applications.

Main Methods:

• Utilized ab initio Hartree-Fock and density functional theory calculations.
• Investigated the effect of decreasing cluster sizes on magnesium and magnesium hydride stability.
• Analyzed the stepwise hydrogen desorption process in detail.

Main Results:

• Both magnesium and magnesium hydride exhibit reduced thermodynamic stability with decreasing cluster size, particularly below 20 atoms.
• Magnesium hydride destabilizes more significantly than magnesium as size decreases.
• A substantial decrease in hydrogen desorption energy was observed for crystallite sizes below approximately 1.3 nm.
• A MgH2 crystallite size of 0.9 nm predicts a desorption temperature as low as 200°C.

Conclusions:

• Decreasing the crystal grain size of magnesium hydride effectively lowers its hydrogen desorption temperature.
• Nanoscale magnesium hydride shows potential for overcoming thermodynamic limitations in hydrogen storage.
• This work represents a significant advancement towards the practical application of magnesium as a hydrogen storage material.