Destabilization of Boron-Based Compounds for Hydrogen Storage in the Solid-State: Recent Advances

Boron-based materials have been widely studied for hydrogen storage applications. Examples of these compounds are borohydrides and boranes. However, all of these present some disadvantages that have hindered their potential application as hydrogen storage materials in the solid-state. Thus, different strategies have been developed to improve the dehydrogenation properties of these materials. The purpose of this review is to provide an overview of recent advances (for the period 2015–2021) in the destabilization strategies that have been considered for selected boron-based compounds. With this aim, we selected seven of the most investigated boron-based compounds for hydrogen storage applications: lithium borohydride, sodium borohydride, magnesium borohydride, calcium borohydride, ammonia borane, hydrazine borane and hydrazine bisborane. The destabilization strategies include the use of additives, the chemical modification and the nanosizing of these compounds. These approaches were analyzed for each one of the selected boron-based compounds and these are discussed in the present review.

Disorder induced polymorphic transitions in the high hydrogen density compound Sr(BH4)2(NH3BH3)2

The presence of two stable configurations of the ammonia borane molecule at elevated temperature induces a polymorphic phase transition to lower symmetry.

In-Situ and Real-time Monitoring of Mechanochemical Preparation of Li2 Mg(NH2 BH3 )4 and Na2 Mg(NH2 BH3 )4 and Their Thermal Dehydrogenation

For the first time, in situ monitoring of uninterrupted mechanochemical synthesis of two bimetallic amidoboranes, M₂ Mg(NH₂ BH₃ )₄ (M=Li, Na), by means of Raman spectroscopy, has been applied. This approach allowed real-time observation of key intermediate phases, and a straightforward follow-up of the reaction course. Detailed analysis of time-dependent spectra revealed a two-step mechanism through MNH₂ BH₃ ⋅NH₃ BH₃ adducts as key intermediate phases which further reacted with MgH₂ , giving M₂ Mg(NH₂ BH₃ )₄ as final products. The intermediates partially take a competitive pathway toward the oligomeric M(BH₃ NH₂ BH₂ NH₂ BH₃ ) phases. The crystal structure of the novel bimetallic amidoborane Li₂ Mg(NH₂ BH₃ )₄ was solved from high-resolution powder diffraction data and showed an analogous metal coordination to Na₂ Mg(NH₂ BH₃ )₄ , but a significantly different crystal packing. Li₂ Mg(NH₂ BH₃ )₄ thermally dehydrogenates releasing highly pure H₂ in the amount of 7 wt.%, and at a lower temperature then its sodium analogue, making it significantly more viable for practical applications.

Amidoboranes of rubidium and caesium: the last missing members of the alkali metal amidoborane family

The nature of temperature-induced phase transitions of rubidium and caesium amidoboranes was elucidated using structural and spectroscopic methods.