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.

Trends in the Series of Ammine Rare-Earth-Metal Borohydrides: Relating Structural and Thermal Properties

Ammine metal borohydrides display extreme structural and compositional diversity and show potential applications for solid-state hydrogen and ammonia storage and as solid-state electrolytes. Thirty-two new compounds are reported in this work, and trends in the full series of ammine rare-earth-metal borohydrides are discussed. The majority of the rare-earth metals (RE) form trivalent RE(BH₄)₃·xNH₃ (x = 7-1) compounds, which possess an intriguing crystal chemistry changing with the number of ammonia ligands, varying from structures built from complex ions (x = 5-7), to molecular structures (x = 3, 4), one-dimensional chains (x = 2), and structures built from two-dimensional layers (x = 1). Divalent RE(BH₄)₂·xNH₃ (x = 4, 2, 1) compounds are observed for RE²⁺ = Sm, Eu, Yb, with structures varying from molecular structures (x = 4) to two-dimensional layered (x = 2, 1) and three-dimensional structures (Yb(BH₄)₂·NH₃). The crystal structure and composition of the compounds depend on the volume of the rare-earth ion. In all structures, NH₃ coordinates to the metal, while BH₄^- has a more flexible coordination and is observed as a bridging and terminal ligand and as a counterion. RE(BH₄)₃·xNH₃ (x = 7-5, 4) releases NH₃ stepwise during thermal treatment, while mainly H₂ is released for x ≤ 3. In contrast, only NH₃ is released from RE(BH₄)₂·xNH₃ due to the lower charge density on the RE²⁺ ion and higher stability of RE(BH₄)₂. The thermal stability of RE(BH₄)₃·xNH₃ increase with increasing cation charge density for x = 5, 7, while it decreases for x = 4, 6. For x = 3, the thermal stability decreases with increasing charge density, due to the destabilization of the BH₄^- group, making it more reactive toward NH₃. This research provides a large number of novel compounds and new insight into trends in the crystal chemistry of ammine metal borohydrides and reveals a correlation between the local metal coordination and the thermal stability.

Structural Diversity and Trends in Properties of an Array of Hydrogen-Rich Ammonium Metal Borohydrides

Metal borohydrides are a fascinating and continuously expanding class of materials, showing promising applications within many different fields of research. This study presents 17 derivatives of the hydrogen-rich ammonium borohydride, NH₄BH₄, which all exhibit high gravimetric hydrogen densities (>9.2 wt % of H₂). A detailed insight into the crystal structures combining X-ray diffraction and density functional theory calculations exposes an intriguing structural variety ranging from three-dimensional (3D) frameworks, 2D-layered, and 1D-chainlike structures to structures built from isolated complex anions, in all cases containing NH₄⁺ countercations. Dihydrogen interactions between complex NH₄⁺ and BH₄^- ions contribute to the structural diversity and flexibility, while inducing an inherent instability facilitating hydrogen release. The thermal stability of the ammonium metal borohydrides, as a function of a range of structural properties, is analyzed in detail. The Pauling electronegativity of the metal, the structural dimensionality, the dihydrogen bond length, the relative amount of NH₄⁺ to BH₄^-, and the nearest coordination sphere of NH₄⁺ are among the most important factors. Hydrogen release usually occurs in three steps, involving new intermediate compounds, observed as crystalline, polymeric, and amorphous materials. This research provides new opportunities for the design and tailoring of novel functional materials with interesting properties.