Numerical simulation of hydrogen dynamics at a Mg-MgH 2 interface Simone Giusepponi and Massimo Celino ENEA – C. R. Casaccia Via Anguillarese Rome, Italy Computational MAterials Science and Technology Lab CMAST Laboratory : COST WG4 Meeting Rome,

Introduction: MgH 2 It can store significant quantities of hydrogen (7.7 wt% of hydrogen) Low cost of production High abundance Too high temperature of decomposition Slow decomposition kinetics BUT

Introduction: MgH 2 Improvements comes from: Adding small amounts of metal additives which act as catalysts and are used to destabilize the hydrid High energy ball milling High density of crystal defects Increased surface area Formation of micro/nanostructures Thanks to Amelia Montone, ENEA TEPSI Project

It is possible to perform SEM observations at high spatial resolution to characterize phase distributions in partially decomposed Mg-MgH 2 containing Fe catalyst Mg/MgH 2 Fe (10%) 10h milled Mg/MgH 2 10h milled Mg MgH 2 Fe The addition of Fe particles induces a nucleation process diffused in the material giving raise to a strongly interconnected microstructure Experimental results Thanks to Amelia Montone, ENEA TEPSI Project

Molecular dynamics simulations: Car-Parrinello CPMD molecular dynamics code Goedecker-Teter-Hutter pseudopotentials 80 Ry cutoff tested on simple molecules (Mg 2, MgH, H 2 ) and on crystalline structures of Mg and MgH 2 Constant temperature and constant volume MD simulations Experimentally MgH 2 transforms in the β-MgH 2 before the onset of hydrogen desorption

Mg: 72 atoms Hydrogen desorption: the MgH 2 -Mg interface Starting configuration Mg surface MgH 2 surface Interface H Mg MgH 2 : 60 Mg atoms H atoms L x = 6.21 ÅL z = ÅL y = Å Mg-MgH 2 : 132 Mg atoms H atoms

Molecular dynamics simulations Starting configuration Optimization moving rigidly in all directions the Mg part keeping fixed the MgH 2 one. MgH 2 atoms at the interface prefer sites that continue the hexagonal sequence of the magnesium hcp bulk across the interface Low temperature CP molecular dynamics to optimize locally the atomic configuration.

Starting configuration T= 700 K T= 800 K T= 900 K MD at constant temperature At T< 700 K no diffusion is detected

Average distance covered by hydrogen atoms at the interface in three different temperature conditions. R x, with x = 1, 2, 3 and 4 are groups of five H atoms (near the interface) belonging to same line in the MgH 2 side as shown in the inset. R B are the remaining H atoms in the MgH 2 side that feel a bulk environment.

Molecular dynamics at T= 700 K When a stationary configuration is reached hydrogen atoms at the interface are eliminated. The restarted simulation show that Mg atoms at the interface in the hydride side adapt themselves to continue the hcp symmetry freeing behind them another row of hydrogen atoms in the new interface.

MgH 2 -Mg interface : Fe Fe in POS 3 Fe in POS 1 Fe in POS 2

Fe in POS 1 Fe in POS 2Fe in POS 3 T= 400 KHydrogen diffusion first row second row third row fourth row bulk rows Average distance covered by rows of hydrogen atoms near the interface Hydrogen rows from the interface

T= 500 K Fe in POS 1 Fe in POS 2 Fe in POS 3 Hydrogen diffusion first row second row third row fourth row bulk rows Increase of Hydrogen mobility Lower desorption temperature Hydrogen rows from the interface

Large transparent circles are used to indicate the first H-shell of an Mg atom (up circle) and of the Fe atom (bottom circle). These circles enlight the different first-shell coordination of the two atoms Snapshot of the MgH 2 -Mg interface with Fe in POS2 at T= 500 K H atoms are in white, Mg atoms (MgH 2 side) are light grey Mg atoms (Mg side) are dark grey Fe atom is black.

R 1 = 10 Å 183 Mg atoms E b = eV/at E b = eV/at R 2 = 11 Å 251Mg atoms E b = eV/at E b = eV/at R 3 = 12 Å 305 Mg atoms E b = eV/at E b = eV/at Ionic relaxation Mg nanoclusters

R 1 = 10 Å 183 Mg atoms E b = eV/at E b = eV/at r 1 =3.6 Å 170 Mg atoms E b = eV/at E b = eV/at r 2 =4.6 Å 164 Mg atoms E b = eV/at E b = eV/at r 3 =5.6 Å 144 Mg atoms E b = eV/at E b = eV/at Ionic relaxation Mg nanoclusters

R 1 = 11 Å 251 Mg atoms E b = eV/at E b = eV/at r 1 =3.6 Å 238 Mg atoms E b = eV/at E b = eV/at r 2 =4.6 Å 232 Mg atoms E b = eV/at E b = eV/at r 3 =5.6 Å 212 Mg atoms E b = eV/at E b = eV/at Ionic relaxation Mg nanoclusters

Ionic relaxation R 1 = 12 Å 305 Mg atoms E b = eV/at E b = eV/at r 1 =3.6 Å 292 Mg atoms E b = eV/at E b = eV/at r 2 =4.6 Å 286 Mg atoms E b = eV/at E b = eV/at r 3 =5.6 Å 266 Mg atoms E b = eV/at E b = eV/at Mg nanoclusters

r 1 =3.6 Å r 1 =4.6 Å r 1 =5.6 Å

Acknowledgment The computing resources and the related technical support used for this work have been provided by CRESCO-ENEAGRID High Performance Computing infrastructure and its staff; see for information. CRESCO-ENEAGRID High Performance Computing infrastructure is funded by ENEA, the “Italian National Agency for New Technologies, Energy and Sustainable Economic Development” and by national and European research programs.

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