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
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
Introduction: MgH2
Improvements comes from:
High energy ball milling
Adding small amounts of
metal additives which act
as catalysts and are used
to destabilize the
hydrid
Thanks to Amelia Montone,
ENEA TEPSI Project
• High density of crystal
defects
• Increased surface area
• Formation of
micro/nanostructures
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 10h milled
Mg/MgH2 Fe (10%) 10h milled
MgH2
Mg
Fe
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 (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
Hydrogen desorption: the MgH2-Mg interface
H
Starting configuration
Mg
Mg: 72 atoms
Mg
surface
MgH2:
60 Mg atoms +
120 H atoms
Mg-MgH2:
132 Mg atoms +
120 H atoms
MgH2
surface
Interface
Lx= 6.21 Å
Ly= 15.10 Å
Lz= 50.30 Å
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.
MD at constant temperature
Starting configuration
T= 700 K
T= 800 K
At T< 700 K no
diffusion is detected
T= 900 K
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 1
Fe in POS 2
Fe in POS 3
Hydrogen diffusion
Fe in POS 1
T= 400 K
Fe in POS 2
Hydrogen rows from the interface
first row
second row
third row
fourth row
bulk rows
Fe in POS 3
Average distance
covered by rows of
hydrogen atoms
near the interface
Hydrogen diffusion
Fe in POS 1
T= 500 K
Fe in POS 2
Hydrogen rows from the interface
first row
second row
third row
fourth row
bulk rows
• Increase of Hydrogen mobility
• Lower desorption temperature
Fe in POS 3
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
Ionic relaxation
Mg nanoclusters
R1 = 10 Å
183 Mg atoms
R2 = 11 Å
251Mg atoms
R3 = 12 Å
305 Mg atoms
Eb = -1.1237 eV/at
Eb = -1.1611 eV/at
Eb = -1.2024 eV/at
Eb = -1.1317 eV/at
Eb = -1.1669 eV/at
Eb = -1.2071 eV/at
Mg nanoclusters
Ionic relaxation
R1 = 10 Å
183 Mg atoms
r1 =3.6 Å
170 Mg atoms
r2 =4.6 Å
164 Mg atoms
r3 =5.6 Å
144 Mg atoms
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
Mg nanoclusters
Ionic relaxation
R1 = 11 Å
251 Mg atoms
r1 =3.6 Å
238 Mg atoms
r2 =4.6 Å
232 Mg atoms
r3 =5.6 Å
212 Mg atoms
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
Mg nanoclusters
Ionic relaxation
R1 = 12 Å
305 Mg atoms
r1 =3.6 Å
292 Mg atoms
r2 =4.6 Å
286 Mg atoms
r3 =5.6 Å
266 Mg atoms
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
r1 =5.6 Å
r1 =3.6 Å
r1 =4.6 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.
Thank you for your attention