Numerical simulation of hydrogen dynamics at a Mg-MgH2 interface

Simone Giusepponi and Massimo Celino

ENEA – C. R. CasacciaVia Anguillarese 30100123 Rome, Italy

Email: simone.giusepponi@enea.itmassimo.celino@enea.it

Computational MAterials Science and Technology LabCMAST Laboratory : www.afs.enea.it/project/cmast

COST WG4 MeetingRome, 14.2.2012

Introduction: MgH2

• 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: MgH2

Improvements comes from:

Adding small amounts of metal additives which act as catalysts and are usedto 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-MgH2 containing Fe catalyst

Mg/MgH2 Fe (10%) 10h milledMg/MgH2 10h milled

Mg

MgH2

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 pseudopotentials80 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

Mg: 72 atoms

Hydrogen desorption: the MgH2-Mg interface

Starting configuration

Mg surface

MgH2 surface

Interface

H

Mg

MgH2:

60 Mg atoms + 120 H atoms

Lx= 6.21 Å Lz= 50.30 ÅLy= 15.10 Å

Mg-MgH2:

132 Mg atoms + 120 H atoms

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.

Starting configurationT= 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. 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.

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.

MgH2-Mg interface : Fe

Fe in POS 3

Fe in POS 1

Fe in POS 2

Fe in POS 1 Fe in POS 2 Fe 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 rowsecond rowthird rowfourth rowbulk 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 MgH2-Mg interface with Fe in POS2 at T= 500 K

H atoms are in white, Mg atoms (MgH2 side) are light greyMg atoms (Mg side) are dark grey Fe atom is black.

R1 = 10 Å183 Mg atoms

Eb = -1.1237 eV/at

Eb = -1.1317 eV/at

R2 = 11 Å251Mg atoms

Eb = -1.1611 eV/at

Eb = -1.1669 eV/at

R3 = 12 Å305 Mg atoms

Eb = -1.2024 eV/at

Eb = -1.2071 eV/at

Ioni

c re

laxa

tion

Mg nanoclusters

R1 = 10 Å

183 Mg atoms

Eb = -1.1237 eV/at

Eb = -1.1317 eV/at

r1 =3.6 Å170 Mg atoms

Eb = -1.0553 eV/at

Eb = -1.0676 eV/at

r2 =4.6 Å164 Mg atoms

Eb = -1.0268 eV/at

Eb = -1.0437 eV/at

r3 =5.6 Å144 Mg atoms

Eb = -0.9059 eV/at

Eb = -0.9285 eV/at

Ioni

c re

laxa

tion

Mg nanoclusters

R1 = 11 Å

251 Mg atoms

Eb = -1.1611 eV/at

Eb = -1.1669 eV/at

r1 =3.6 Å238 Mg atoms

Eb = -1.1116 eV/at

Eb = -1.1224 eV/at

r2 =4.6 Å232 Mg atoms

Eb = -1.0947 eV/at

Eb = -1.1068 eV/at

r3 =5.6 Å212 Mg atoms

Eb = -1.0201 eV/at

Eb = -1.0870 eV/at

Ioni

c re

laxa

tion

Mg nanoclusters

Ioni

c re

laxa

tion

R1 = 12 Å

305 Mg atoms

Eb = -1.2024 eV/at

Eb = -1.2071 eV/at

r1 =3.6 Å292 Mg atoms

Eb = -1.1641 eV/at

Eb = -1.1723 eV/at

r2 =4.6 Å286 Mg atoms

Eb = -1.1491 eV/at

Eb = -1.1593 eV/at

r3 =5.6 Å266 Mg atoms

Eb = -1.0933 eV/at

Eb = -1.1080 eV/at

Mg nanoclusters

r1 =3.6 År1 =4.6 Å

r1 =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 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.

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