Morphology‐Dependent Stability of Complex Metal Hydrides and Their Intermediates Using First‐Principles Calculations

Abstract

Complex light metal hydrides are promising candidates for efficient, compact solid-state hydrogen storage. (De)hydrogenation of these materials often proceeds via multiple reaction intermediates, the energetics of which determine reversibility and kinetics. At the solid-state reaction front, molecular-level chemistry eventually drives the formation of bulk product phases. Therefore, a better understanding of realistic (de)hydrogenation behavior requires considering possible reaction products along all stages of morphological evolution, from molecular to bulk crystalline. Here, we use first-principles calculations to explore the interplay between intermediate morphology and reaction pathways. Employing representative complex metal hydride systems, we investi-gate the relative energetics of three distinct morphological stages that can be expressed by intermediates during solidstate reactions: i) dispersed molecules; ii) clustered molecular chains; and iii) condensed-phase crystals. Our results verify that the effective reaction energy landscape strongly depends on the morphological features and associated chemical environment, offering a possible explanation for observed discrepancies between X-ray diffraction and nuclear magnetic resonance measurements. Our theoretical understanding also provides physical and chemical insight into phase nucleation kinetics upon (de)hydrogenation of complex metal hydrides.

Boron Hydrogen Compounds for Hydrogen Storage and as Solid Ionic Conductors

“…In parallel, new theoretical approaches to predict the morphology of potential reaction intermediates have been developed and allow to explore the energetics of reaction pathways in this system. [82] 4. Closo Hydridoborates and Related Species as Solid Ionic Conductors…”

A Mechanistic Analysis of Phase Evolution and Hydrogen Storage Behavior in Nanocrystalline Mg(BH4)2 within Reduced Graphene Oxide

“…Explicit modeling of the phase boundaries between Mg(BH 4 ) 2 polymorphs is extremely challenging due to the inherent structural complexity of the polymorphs and the phase boundaries arising from lattice mismatch, lattice misorientation, and the local orientations and arrangements of BH 4 − anions. However, the energy variation of α-, β-, and γ-Mg(BH 4 ) 2 polymorphs (∼0.16 eV) is much smaller than the cohesive energy (∼1.37 eV)a condensation driving force from an isolated molecule to bulk crystalline 41 implying that the local clustering and covalent bonds between Mg-BH 4 are more significant for stabilizing the Mg(BH 4 ) 2 phase than the long-range order. Moreover, at the phase boundary where the symmetry is broken, the Mg 2+ and BH 4 − units may rearrange and reorient to stabilize the phase boundary.…”

Beyond Idealized Models of Nanoscale Metal Hydrides for Hydrogen Storage

“…In general, simple metal hydrides, such as MgB 2 and Li 3 N, have larger surface energies accompanying dangling bonds than those of complex metal hydrides, such as Mg­(BH 4 ) 2 and MgB 12 H 12 , which feature looser packing without cleavage of surface covalent bonds. The trend in surface energies, β-Mg­(BH 4 ) 2 ≫ Mg­(B 3 H 8 ) 2 ∼ MgB 10 H 10 > MgB 12 H 12 , matches the trend in energy differences between bulk crystalline metal hydride compounds and their lower-dimensional variants, as recently reported by our team . This implies that the surface energies of these materials can be mainly attributed to the cohesion between cations and molecular anions.…”