The relationship between activated H2 bond length and adsorption distance on MXenes identified with graph neural network and resonating valence bond theory.

Motivated by the recent experimental study on hydrogen storage in MXene multilayers [Liu et al., Nat. Nanotechnol. 16, 331 (2021)], for the first time we propose a workflow to computationally screen 23 857 compounds of MXene to explore the general relation between the activated H2 bond length and adsorption distance. By using density functional theory we generate a dataset to investigate the adsorption geometries of hydrogen on MXenes, based on which we train physics-informed atomistic line graph neural networks (ALIGNNs) to predict adsorption parameters. To fit the results, we further derived a formula that quantitatively reproduces the dependence of H2 bond length on the adsorption distance from MXenes within the framework of Pauling's resonating valence bond theory, revealing the impact of transition metal's ligancy and valence on activating dihydrogen in H2 storage.

Hydrogen Adsorption on Type-5 Penta-MgN8 Sheet, Nanotube, and 3D Porous Structure.

Motivated by the recent report on penta-MgN8 sheet [Mater. Today Phys. 2023, 38, 101259] that is the first realization of type-5 pentagonal 2D tessellation with exposed regularly distributed Mg ions, we carried out density functional theory studies on the interactions of H2 molecules with 1D penta-nanotube, 2D penta-sheet, and 3D porous structures based on penta-MgN8. We found that when the penta-MgN8 sheet is assembled to a 3D porous structure or curved to a nanotube, the bandgap increases from 1.18 to 1.35 and 1.88 eV, and the resulting derivatives are stable dynamically. When H2 molecules are introduced, the nanotube behaves best in adsorption, where each Mg ion can adsorb three H2 molecules: two on the outer surface and one on the inner surface. The curved geometry of the nanotube makes the Mg ion on the outer surface more exposed as compared with the situations of the 2D sheet and 3D porous structure, resulting in stronger adsorptions to H2. The gravimetric capacity (volumetric capacity) is 4.25 wt % (63 g/L) and 4.25 wt % (65 g/L) for the studied penta-sheet and penta-nanotube, and the corresponding desorption temperature is 115 and 162 K at 1 atm pressure, respectively, while for the 3D porous structure, the adsorption performance is poor due to the limited space and the less curvature, leading to strong steric hindrance and less exposed configuration for Mg ions. Moreover, the effects of temperature and pressure on adsorption are also discussed.