Influence of Crystal Structure of Bulk Phase on the Stability of Nanoscale Phases: Investigation on MgH₂ Derived Nanostructures

Abstract

Phase stability of α-, β-, γ-, and δ′-MgH 2 -derived nanostructures have been investigated using ab initio projected augmented plane wave method. Structural optimizations based on total energy calculations predicted that β-derived nanoparticles and α-derived nanowhiskers are more stable than those derived from other polymorphs. Present study indicates that the crystal structure of the bulk phase plays a significant role on the stabilization of different forms (nanoparticles or nanowhiskers) of nanophases. The predicted critical sizes of the stable β-nanoparticle and α-nanowhisker for MgH 2 are 2.4 and 1.5 nm, respectively. The calculated hydrogen site energies suggest that it is relatively easier to remove hydrogen from the surface of the nanoparticles and nanowhiskers compared to that from bulk crystals. Among the considered nano-objects, removing hydrogen from the β-derived nanocluster is much easier, and hence, the present study suggests that one can use these nanoparticles for practical applications. NMR-related parameters are calculated for different sizes of the nanoclusters and nanowhiskers.

Metallization of magnesium polyhydrides under pressure

“…The potential for reversible hydrogen storage has resulted in much interest not only in crystalline MgH 2 , but also in nanoparticles based on the structure of the various phases of this solid [40][41][42][43][44]. At 1 atm α-MgH 2 assumes a TiO 2 -rutile-type geometry and it undergoes a series of structural transitions, α (P 4 2 /mnm) → γ (P bcn) → β (P a3) → δ (P bc2 1 ) → (P nma), at 0.39, 3.84, 6.73 and 10.26 GPa [45,46].…”

The Size Dependence of Hydrogen Mobility and Sorption Kinetics for Carbon‐Supported MgH2 Particles

“…Examples of these systems include Mg–Al or Mg 2 NiH 4 , but the enthalpy required to decompose the hydrides is still high in the range of 60 kJ mol −1 H 2 . Computational studies have predicted that MgH 2 ‐clusters consisting of few atoms would have lower thermodynamic stability . However, experimental results only show modest changes in the thermodynamic equilibrium, since the less positive value for the enthalpy for desorption is being counteracted by a less positive value for the entropy change …”

The kinetics of lightweight solid-state hydrogen storage materials: A review

“…Then, hydrogen diffuses along the metal interface rather than through the hydride layer [67]. Jain et al [60] and Vajeeston et al [68] recommended using powder or small magnesium particles rather than the bulk magnesium phase to achieve easier uptake and release of H from the stable nanocluster. Therefore, in the bulk of pure magnesium, the formation of MgH 2 becomes difficult.…”

“…Preparing these nanosized particles using the existing data-based/ experimental methods might be difficult and challenging in practice. Encapsulating such nanoparticles in scaffold materials can fix this problem, as suggested by Vajeeston et al [68], but still requires further research and exploration.…”