Enhanced hydrogen diffusion in magnesium based hydride induced by strain and doping from first principle study

Magnesium based hydride (MgH2) is a promising hydrogen storage material. However, the sluggish sorption rate impedes its practical applications. To improve the dehydrogenation kinetics of MgH2, the influences of external strains with different dimension (uniaxial/biaxial/triaxial), direction (tensile/compressive) and magnitude (-3% similar to +3%) on H atom diffusion properties within MgH2 lattice are investigated for the first time using first-principles calculations method. In addition, the synergistic effects of strain and doping of transition metal Nb are further studied. The results show that the diffusion energy barrier of H atom is sensitive to the strain applied to MgH2 lattice. The energy barrier value decreases significantly upon no matter uniaxial, biaxial or triaxial strain, leading to the remarkable increase of H atom diffusion rate. By contrast, the uniaxial tensile strains of epsilon >= 0.5% along [100] or [010] crystal direction, biaxial and triaxial compressive strains of epsilon <= -2% are more beneficial to improving the dehydrogenation kinetics of MgH2. What's more, the strain and Nb doping play the synergistic enhancement effect in increasing H atom diffusion rate, which is superior to their separate role. Analysis of electronic structures reveals that the dehydrogenation kinetics of different MgH2 systems is associated with the energy gap value near Fermi level and the covalent bond strength between hydrogen and metal ions within MgH2 lattice. These results prompt a new insight to enhance the dehydrogenation kinetics of MgH2 by synergistically introducing external strain and transition metal catalysts. (C) 2016 Elsevier B.V. All rights reserved.