Tunable hydrogen storage in magnesium–transition metal compounds: First-principles calculations

Abstract

TUnable hydrogen storage in magnesium-transition metal compounds: First-principles calculationsSuleym an E r,1 D hirendra T iw ari,1 G illes A. Magnesium dihydride (MgH2) stores 7.7 wt % hydrogen but it suffers from a high thermodynamic stability and slow (de)hydrogenation kinetics. Alloying Mg with lightweight transition metals (TM) (=Sc,Ti,V ,Cr) aims at improving the thermodynamic and kinetic properties. We study the structure and stability of MgxTM1-xH2 compounds, x = [0 -1 ], by first-principles calculations at the level of density functional theory. We find that the experimentally observed sharp decrease in hydrogenation rates for x a 0.8 correlates with a phase transition of MgxTM1-xH2 from a fluorite to a rutile phase. The stability of these compounds decreases along the series Sc, Ti, V, and Cr. Varying the TM and the composition x , the formation enthalpy of MgxTM1-xH2 can be tuned over the substantial range of 0-2 eV/f.u. Assuming however that the alloy MgxTM1-x does not decompose upon dehydrogenation, the enthalpy associated with reversible hydrogenation of compounds with a high magnesium content (x=0.75) is close to that of pure Mg.

New FCC Mg–Zr and Mg–Zr–ti deuterides obtained by reactive milling

“…Several approaches, e.g. elemental substitutions [22][23][24][25][26] , mechanical alloying 22,25,27 and high-pressure synthesis 28 , have been applied separately or jointly resulting in a formation of a number of improved and promising compounds but none of them reveal outstanding kinetic or/and thermodynamic properties. Novel Mg-TM hydrides (TM = transition metal) have been previously obtained from MgH 2 and different TM or TM hydrides in an anvil cell under mechanical pressure of several GPa 29 30,36 .…”

Divacancies and the hydrogenation of Mg-Ti films with short range chemical order

“…6 The combined effect of divacancies present and relatively large local lattice relaxations may effectively lower the migration barriers for hydrogen diffusion in the fluorite MgH 2 phase, leading to the fast hydrogen transport. Indeed, recent ab initio modeling studies [7][8][9] point to the relevance of vacancy-mediated hydrogen diffusion in MgH 2 and related metal hydrides, since this provides a major contribution to the hydrogen mobility.…”

“…6 Ab initio studies indicate that vacancies form a key factor in the hydrogen diffusion of both the rutile and fluorite MgH 2 phases. [7][8][9] Further insight into the occurrence of vacancies and their possible role in the hydrogenation kinetics in MgTi-H films is thus strongly warranted.In recent studies on Mg, Mg 2 Ni, and Mg/Si films, positron annihilation emerged as a sensitive tool to monitor the hydrogenation and phase transitions in Mg-based metal films in a depth-resolved manner. 10 Here, we provide direct evidence for chemical segregation in Mg-Ti films using the positron Doppler broadening depth profiling method.…”

“…6 Ab initio studies indicate that vacancies form a key factor in the hydrogen diffusion of both the rutile and fluorite MgH 2 phases. [7][8][9] Further insight into the occurrence of vacancies and their possible role in the hydrogenation kinetics in MgTi-H films is thus strongly warranted.…”

First-principles study of the optical properties ofMgxTi1−xH2

“…Density-functional theory ͑DFT͒ calculations confirm the dependence of the relative stability of the rutile and fluorite structures on the Mg-Ti composition. 17,23,24 Interestingly, the kinetics of the hydrogen ab/desorption reactions are much faster in the alloys with a high Ti content. 25 The origin of the optical black state in hydrogenated Mg-Ti thin films is not understood.…”