1. Numerical simulation of hydrogen dynamics at a Mg-MgH 2 interface Simone Giusepponi and Massimo Celino ENEA – C. R. Casaccia Via Anguillarese 301 00123 Rome, Italy Email: simone.giusepponi@enea.it massimo.celino@enea.it Computational MAterials Science and Technology Lab CMAST Laboratory : www.afs.enea.it/project/cmast COST WG4 Meeting Rome, 14.2.2012

2. 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

3. 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

4. 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

5. 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

6. 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 + 120 H atoms L x = 6.21 ÅL z = 50.30 ÅL y = 15.10 Å Mg-MgH 2 : 132 Mg atoms + 120 H atoms

7. 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.

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

9. 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.

10. 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.

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

12. 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

13. 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

14. 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.

15. R 1 = 10 Å 183 Mg atoms E b = -1.1237 eV/at E b = -1.1317 eV/at R 2 = 11 Å 251Mg atoms E b = -1.1611 eV/at E b = -1.1669 eV/at R 3 = 12 Å 305 Mg atoms E b = -1.2024 eV/at E b = -1.2071 eV/at Ionic relaxation Mg nanoclusters

16. R 1 = 10 Å 183 Mg atoms E b = -1.1237 eV/at E b = -1.1317 eV/at r 1 =3.6 Å 170 Mg atoms E b = -1.0553 eV/at E b = -1.0676 eV/at r 2 =4.6 Å 164 Mg atoms E b = -1.0268 eV/at E b = -1.0437 eV/at r 3 =5.6 Å 144 Mg atoms E b = -0.9059 eV/at E b = -0.9285 eV/at Ionic relaxation Mg nanoclusters

17. R 1 = 11 Å 251 Mg atoms E b = -1.1611 eV/at E b = -1.1669 eV/at r 1 =3.6 Å 238 Mg atoms E b = -1.1116 eV/at E b = -1.1224 eV/at r 2 =4.6 Å 232 Mg atoms E b = -1.0947 eV/at E b = -1.1068 eV/at r 3 =5.6 Å 212 Mg atoms E b = -1.0201 eV/at E b = -1.0870 eV/at Ionic relaxation Mg nanoclusters

18. Ionic relaxation R 1 = 12 Å 305 Mg atoms E b = -1.2024 eV/at E b = -1.2071 eV/at r 1 =3.6 Å 292 Mg atoms E b = -1.1641 eV/at E b = -1.1723 eV/at r 2 =4.6 Å 286 Mg atoms E b = -1.1491 eV/at E b = -1.1593 eV/at r 3 =5.6 Å 266 Mg atoms E b = -1.0933 eV/at E b = -1.1080 eV/at Mg nanoclusters

19. r 1 =3.6 Å r 1 =4.6 Å r 1 =5.6 Å

20. 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 www.cresco.enea.it 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.

21. Thank you for your attention