Numerical simulation of hydrogen dynamics at a Mg-MgH 2 interface

1. Numerical simulation of hydrogen dynamics at a Mg-MgH2 interface Computational MAterials Science and Technology Lab CMAST Laboratory : www.afs.enea.it/project/cmast Simone Giusepponi and Massimo Celino ENEA – C. R. Casaccia Via Anguillarese 301 00123 Rome, Italy Email: simone.giusepponi@enea.it massimo.celino@enea.it COST WG4 Meeting Rome, 14.2.2012

2. Introduction: MgH2 • It can store significant quantities of hydrogen • (7.7 wt% of hydrogen) • Low cost of production • High abundance BUT • Too high temperature of decomposition • Slow decomposition kinetics

3. High density of crystal defects • Increased surface area • Formation of micro/nanostructures High energy ball milling Adding small amounts of metal additives which act as catalysts and are used to destabilize the hydrid Introduction: MgH2 Improvements comes from: Thanks to Amelia Montone, ENEA TEPSI Project

4. Experimental results It is possible to perform SEM observations at high spatial resolution to characterize phase distributions in partially decomposed Mg-MgH2 containing Fe catalyst The addition of Fe particles induces a nucleation process diffused in the material giving raise to a strongly interconnected microstructure Mg/MgH2 Fe (10%) 10h milled Mg/MgH2 10h milled MgH2 Fe Mg 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 (Mg2, MgH, H2) and on crystalline structures of Mg and MgH2 Constant temperature and constant volume MD simulations Experimentally MgH2 transforms in the β-MgH2 before the onset of hydrogen desorption

6. H Mg Hydrogen desorption: the MgH2-Mg interface Starting configuration Mg: 72 atoms Mg surface MgH2: 60 Mg atoms + 120 H atoms MgH2 surface Mg-MgH2: 132 Mg atoms + 120 H atoms Interface Lx= 6.21 Å Ly= 15.10 Å Lz= 50.30 Å

7. Molecular dynamics simulations Starting configuration Optimization moving rigidly in all directions the Mg part keeping fixed the MgH2 one. MgH2 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. T= 700 K T= 800 K T= 900 K MD at constant temperature Starting configuration At T< 700 K no diffusion is detected

9. Average distance covered by hydrogen atoms at the interface in three different temperature conditions. Rx, with x = 1, 2, 3 and 4 are groups of five H atoms (near the interface) belonging to same line in the MgH2 side as shown in the inset. RB are the remaining H atoms in the MgH2 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. MgH2-Mginterface : Fe Fe in POS 1 Fe in POS 2 Fe in POS 3

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

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

14. Snapshot of the MgH2-Mg interface with Fe in POS2 at T= 500 K H atoms are in white, Mg atoms (MgH2 side) are light grey Mg atoms (Mg side) are dark grey Fe atom is black. 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

15. Mg nanoclusters R3 = 12 Å 305 Mg atoms R1 = 10 Å 183 Mg atoms R2 = 11 Å 251Mg atoms Eb = -1.1237 eV/at Eb = -1.1611 eV/at Eb = -1.2024 eV/at Ionic relaxation Eb = -1.1317 eV/at Eb = -1.1669 eV/at Eb = -1.2071 eV/at

16. Mg nanoclusters R1 = 10 Å r1 =3.6 Å 170 Mg atoms r2 =4.6 Å 164 Mg atoms r3 =5.6 Å 144 Mg atoms 183 Mg atoms Ionic relaxation Eb = -1.1237 eV/at Eb = -1.0553 eV/at Eb = -1.0268 eV/at Eb = -0.9059 eV/at Eb = -1.1317 eV/at Eb = -1.0676 eV/at Eb = -1.0437 eV/at Eb = -0.9285 eV/at

17. Mg nanoclusters R1 = 11 Å r1 =3.6 Å 238 Mg atoms r2 =4.6 Å 232 Mg atoms r3 =5.6 Å 212 Mg atoms 251 Mg atoms Ionic relaxation Eb = -1.1611 eV/at Eb = -1.1116 eV/at Eb = -1.0947 eV/at Eb = -1.0201 eV/at Eb = -1.1669 eV/at Eb = -1.1224 eV/at Eb = -1.1068 eV/at Eb = -1.0870 eV/at

18. Mg nanoclusters R1 = 12 Å r1 =3.6 Å 292 Mg atoms r2 =4.6 Å 286 Mg atoms r3 =5.6 Å 266 Mg atoms 305 Mg atoms Ionic relaxation Eb = -1.2024 eV/at Eb = -1.1641 eV/at Eb = -1.1491 eV/at Eb = -1.0933 eV/at Eb = -1.2071 eV/at Eb = -1.1723 eV/at Eb = -1.1593 eV/at Eb = -1.1080 eV/at

19. r1 =5.6 Å r1 =4.6 Å r1 =3.6 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