An ammonia-stabilized mixed-cation borohydride: synthesis, structure and thermal decomposition behavior

Area of Science:

• Materials Science
• Inorganic Chemistry
• Solid-State Chemistry

Background:

• Developing novel materials for efficient hydrogen storage is crucial for clean energy technologies.
• Mixed-cation borohydrides offer potential for high hydrogen content but often require high temperatures for release.
• Ammonia's role in stabilizing mixed-cation structures and influencing hydrogen release properties is an area of active research.

Purpose of the Study:

• To synthesize and characterize a novel ammonia-stabilized mixed-cation borohydride, Li2Mg(BH4)4·6NH3.
• To investigate the crystal structure and thermal decomposition behavior of the synthesized compound.
• To elucidate the role of ammonia in stabilizing the mixed-cation structure and facilitating hydrogen release.

Main Methods:

• Mechanochemical synthesis of Li2Mg(BH4)4·6NH3 from Mg(BH4)2·6NH3 and LiBH4.
• X-ray diffraction for crystal structure determination.
• Thermogravimetric analysis and differential scanning calorimetry for thermal decomposition studies.
• Infrared spectroscopy and other techniques for mechanistic investigations.

Main Results:

• Successful synthesis of Li2Mg(BH4)4·6NH3 with a tetragonal P4(3)2(1)2 structure.
• Observation of short dihydrogen bonds facilitating low-temperature hydrogen release (onset at 80 °C).
• Identification of Li2Mg(BH4)4·3NH3 as a decomposition intermediate.
• Release of 11.1 wt% H2 and 3.07 mol NH3 per mole of Li2Mg(BH4)4·6NH3 through a three-step decomposition process.
• Enhanced H(δ+)-H(δ-) combination due to strengthened Mg-N bonds.

Conclusions:

• Ammonia plays a critical role in stabilizing the mixed-cation borohydride structure, enabling the coexistence of Li and Mg.
• The unique crystal structure and presence of short dihydrogen bonds lead to efficient, low-temperature hydrogen release.
• The findings provide valuable insights for designing new boron-nitrogen-based materials with high hydrogen storage capacity.