Fig. 6: Thermodynamic convex hull analysis of H2 adsorption on γ-Mg(BH4)2 and schematic representation of (B−Hδ−)2···H−H···(Hδ−−B)2 connectivity on γ-Mg(BH4)2 and the H2 distribution.
From: Small-pore hydridic frameworks store densely packed hydrogen
a, Convex hull analysis for a concentration of H2 from zero to 1.33 H2 per γ-Mg(BH4)2. The DFT energies refer to the empty framework γ-Mg(BH4)2 and γ-Mg(BH4)2·1.33H2. Blue and purple crosses represent configurations of γ-Mg(BH4)2·xH2 with adsorption on the D11 and D22 sites, respectively. The y axis represents the energy per γ-Mg(BH4)2 formula unit (f.u.). b, Convex hull analysis for a concentration of H2 from zero to 2.33 H2 per γ-Mg(BH4)2. The DFT energies are referred to the empty framework γ-Mg(BH4)2 and γ-Mg(BH4)2·2.33H2. The γ-Mg(BH4)2·1.33H2 structure (full D11 site adsorption) is stable and on the convex hull. Blue crosses show the relative energy per γ-Mg(BH4)2 of the full D11 loading followed by D22, purple crosses show isolated adsorption on D22, and red crosses the mixed configurations where D11 and D22 sites are partially occupied. c, Adsorption mode of H2 on the D22 site in fully loaded γ-Mg(BH4)2·2.33H2. The hydrogen atoms of the hydrogen molecules are connected to Hδ− with shorter distances than the D11 sites. d–f, Sketches of energetically equivalent γ-Mg(BH4)2·2.33H2. Red and purple H2 represent D11 and D22 sites, respectively. Note that H2 directionality on the D22 sites does not vary.
