ReaxFF for Magnesium Hydrides
Sam Cheung, Weiqiao Deng, Adri van Duin
FF-subgroup meeting 9 Dec. 2003
Topic Overview
• Hydrogen storage: a brief history
•
•
•
•
•
•
Objectives
ReaxFF: general principles
Building the ReaxFF for Mg-hydride
File Format
Applications
Conclusion
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Hydrogen storage: a brief history
Hydrogen Facts:
•
•
•
•
Hydrogen is an odorless and colorless gas.
BP of -252.77o C.
Density of 0.0899 grams/liter.
The most abundant element on earth but less than 1%
is in the form of H2
H2
• Ways to produce H2: electrolysis, thermal dissociation of H2O, or
photochemical splitting of H2O
• A clean synthetic fuel
• H2O vapour as the only exhaust gas
• Energy density by weight
• Chemical energy per mass of Hydrogen (142 MJ/kg)
vs. that of other chemical fuels (liquid hydrocarbons ~ 47 MJ/kg)
• 1 Kg of hydrogen contains the same amount of energy as 2.1 Kg
of natural gas or 2.8 Kg of gasoline.
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Saftey issues of hydrogen vs. other fuels
• Lower risk of explosion
• Nontoxic!
Property
Gasoline Methane Hydrogen
Density (Kg/M3)
Diffusion Coefficient In Air (Cm2/Sec)
Specific Heat at Constant Pressure (J/Gk)
Ignition Limits In Air (vol %)
Ignition Energy In Air (Mj)
Ignition Temperature (oC)
Explosion Energy (G TNT/kj)
Flame Emissivity (%)
4.40
0.05
1.20
1.0-7.6
0.24
228-471
2197
0.25
34-43
High
Toxicity
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0.65
0.16
2.22
5.3-15.0
0.29
540
1875
0.19
25-33
No
0.084
0.610
14.89
4.0-75.0
0.02
585
2045
0.17
17-25
No
4
How large of a gas tank do you want?
Storage remains a problem!
Electric car with fuel cell (4kg H)
Combustion engine (8kg H)
Combustion engine (24 kg petrol)
400 km
Volume Comparisons for 4 kg Vehicular H2 Storage
Schlapbach & Züttel, Nature, 15 Nov. 2001
Storing Hydrogen
• Pressurized gas
- Must be intensely pressurized to several hundred atmospheres
(200 bar or more)
-Stored in pressure vessel
• Condensed liquid state
- Liquifying H2 requires substantial energy
- Boil-off is an issue for non-pressurized insulated tanks
- Insulation is bulky
From Patrovic & Milliken (2003)
• Solid or liquid state as chemical hydrogen-rich compunds
- methanol, methane, carbon
- metal hydrides
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Materials with High Weight Hydrogen
Material
H-atoms per
cm3
H2 gas (200 bar)
0.99
H2 liquid (20K)
4.2
H2 solid (4.2K)
5.3
MgH2
6.5
Mg2NiH4
5.9
TiFeH2
6.0
LaNi5H6
5.5
(10-22)
Mg hydrides
• light weight
• low manufacture cost
• high hydrogen-storage capacity
• reversible reaction
Limitations
• High dehydriding temperature
• Slow adsorption kinetics
• Surface oxidation of magnesium
• Stability of the MgH2.
Possible solutions
• Milling
• Catalyst
• Alloying with other metals
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Reax FF: general principles
Design
FEA
Time
years
Atoms
Molecular
conformations
Electrons
Bond formation
MESO
Grids
Grains
MD
ReaxFF
Empirical
QC
10-15
force fields
ab initio,
DFT,
HF
Ångstrom
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Kilometres
8
System energy description
Esystem  Ebond  EvdW aals  ECoulomb  Eval  Etors
2-body
3-body
4-body
 Eover  Eunder
multibody
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Key Features
1. To get a smooth transition from nonbonded to single, double and
triple bonded systems ReaxFF employs a bond length/bond
order relationship. Bond orders are updated every iteration.
2. Nonbonded interactions (van der Waals, Coulomb) are calculated
between every atom pair, irrespective of connectivity. Excessive
close-range nonbonded interactions are avoided by shielding.
3. All connectivity-dependent interactions (i.e. valence and torsion
angles) are made bond-order dependent, ensuring that their
energy contributions disappear upon bond dissociation.
4. ReaxFF uses a geometry-dependent charge calculation scheme
that accounts for polarization effects.
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General Rules
1. MD-force field; no discontinuities in energy or forces even during
reactions.
2. User should not have to pre-define reactive sites or reaction
pathways; potential functions should be able to automatically
handle coordination changes associated with reactions.
3. Each element is represented by only 1 atom type in the force field;
force field should be able to determine equilibrium bond lengths,
valence angles etc. from chemical environment.
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Parameterization of ReaxFF:
Strategy for parameterizing ReaxFF
Step 1
-Identify interactions to be optimized
-Identify relevant systems
Step 2
-Build QC-trainset for bond breaking and angle bending
cases for all relevant small cluster
Cluster (DFT B3LYP 6-31G**++)
-Perform QC simulations on condensed phases to obtain EOS
Periodic system (CASTEP GGA-PBE 4x4x2 k-space KE cutoff 380eV)
Step 3
-FFopt and ReaxFF fittings
Step 4
-Applications
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Training set
Cluster:
Condensed phase:
Bonds
•Mg-H
-Normal, under-, and overcoordinated systems
HCP
BCC
FCC
SC
diamond
a-MgH2
g-MgH2
b-MgH2
CaF2-MgH2
Mg
H
a-MgH2 (rutile)
Angles
•H-Mg-H
•H-H-Mg
•Mg-H-Mg
•H-Mg-Mg
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File Format: geo trainset.in
geo
trainset.in
BIOGRF 200
DESCRP mgh2_b1.2
RUTYPE NORMAL RUN
BOND RESTRAINT 1 3 1.2000 7500.00 0.50000 0.0000000
FORMAT ATOM (a6,1x,i5,1x,a5,1x,a3,1x,a1,1x,a5,3f10.5,1x,a5,i3,i2,1x,f8.5)
HETATM 1 Mg
0.00000 0.00000 0.02469 Mg 1 1 0.00000
HETATM 2 H
0.00000 0.00000 1.62594 H 1 1 0.00000
HETATM 3 H
0.00000 0.00000 -1.19525 H 1 1 0.00000
END
CHARGES
mgh2 0.05 1 0.2519
mgh2 0.05 2 -0.1260
ENDCHARGES
GEOMETRY
mgh2
0.020 1 2
1.707
mgh2
0.500 2 1 3 179.000
ENDGEOMETRY
ENERGY
#Mg1-H3 (Mg-H 1.71) dissociation MgH2
BIOGRF 200
10.0 + mgh2 /1 - mgh2_b1.2 /1 -51.5
DESCRP mgh2_a140
7.0 + mgh2 /1 - mgh2_b1.4 /1 -14.0
RUTYPE NORMAL RUN
5.0 + mgh2 /1 - mgh2_b1.5 /1
-5.4
ANGLE RESTRAINT 2 1 3 140.00 2500.00 1.0000 0.000000
2.0 + mgh2 /1 - mgh2_b1.6 /1
-1.2
FORMAT ATOM (a6,1x,i5,1x,a5,1x,a3,1x,a1,1x,a5,3f10.5,1x,a5,i3,i2,1x,f8.5)
2.0 + mgh2 /1 - mgh2_b2.0 /1
-6.8
HETATM 1 Mg
-0.00006 0.00000 -0.00002 Mg 1 1 0.00000
1.0 + mgh2 /1 - mgh2_b4.1 /1 -73.1
HETATM 2 H
-0.00006 0.00000 1.71361 H 1 1 0.00000
#H-Mg-H angle in mgh2
HETATM 3 H
1.10148 0.00000 -1.31278 H 1 1 0.00000
1.0 + mgh2 /1 - mgh2_a160 /1 -1.41
END
2.0 + mgh2 /1 - mgh2_a140 /1 -5.74
4.0 + mgh2 /1 - mgh2_a120 /1 -13.47
XTLGRF 200
10.0 + mgh2 /1 - mgh2_a100 / -25.72
DESCRP diamond-mgh2_opt
10.0 + mgh2 /1 - mgh2_a80 /1 -44.57
RUTYPE CELL OPT 0
25.0 + mgh2 /1 - mgh2_a60 /1 -73.47
CRYSTX 3.93314 3.93314 3.93314 90.00000 90.00000 90.00000
25.0 + mgh2 /1 - mgh2_a40 /1 -73.29
FORMAT ATOM (a6,1x,i5,1x,a5,1x,a3,1x,a1,1x,a5,3f10.5,1x,a5,i3,i2,1x,f8.5)
# Relative Energy for Clusters
HETATM 1 H
2.94972 2.90674 0.94026 H 1 1 0.00000
2.0 + mg2h4 /2 - mgh2 /1
-14.21
HETATM 2 Mg
1.96646 1.96644 1.96644 Mg 1 1 0.00000
# Mg hcp (EOS)
HETATM 3 H
0.98315 0.94017 1.02607 H 1 1 0.00000
20.0 + hcp0 /2 - hcp14 /2 -17.6
HETATM 4 H
0.98321 2.99259 2.90679 H 1 1 0.00000
10.0 + hcp0 /2 - hcp17 /2 -6.2
HETATM 5 H
2.94977 1.02602 2.99268 H 1 1 0.00000
2.0 + hcp0 /2 - hcp20 /2 -1.2
HETATM 6 Mg
-0.00011 -0.00013 -0.00012 Mg 1 1 0.00000
2.0 + hcp0 /2 - hcp_eq/2 -0.001
FORMAT CONECT (a6,12i6)
2.0 + hcp0 /2 - hcp27 /2 -1.3
END
5.0 + hcp0 /2 - hcp31 /2 -7.6
+ hcp0 /2 - hcp35 /2 -10.8
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ENDENERGY
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80
60
40
20
0
0.5
1.5
2.5
3.5
4.5

80

H
Mg
H
40
0
50
150
14
250
Results: 1. Charge Analysis
0.7
0.7
0.6
0.6
0.5
0.5
Muliken Charges (Debye)
0.4
0.4
Mg1 H 2
0.3
0.3
0.2
0.2
0.1
0.1
0
H2
M g1
H3
0
1
-0.1
2
1
-0.1
-0.2
-0.2
1.5
1.5
1.3
1.3
2
3
QC
ReaxFF
H
H3
1.1
0.9
H 4 Mg 1
0.7
Mg 2
0.5
0.3
0.3
0.1
0.1
-0.3
2
3
4
5
6
-0.1
Mg4
H
Mg 1
0.7
H5
1
H6
0.9
H6
0.5
-0.1
H5
1.1
Mg2
H
1
2
H
Mg3
H
3
4
5
H
6
-0.3
Atom number
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Results: 2. MgH/MgH2 bond dissociation
Mg (3s)2
Energy (kcal/mol)
MgH
MgH2
100
100
90
90
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
QC-singlet
QC-triplet
ReaxFF
0
0
0.5
1.5
2.5
3.5
4.5
0.5
1.5
2.5
3.5
4.5
Bond distance (Å)
-ReaxFF gives a fair descriptionFree
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Results: 3. H-Mg-H Angle Bend Curve
MgH2
50
45
Energy (kcal/mol)
40
35
30
25
QC
20
ReaxFF
15
10
5
0
-5 50
100
150
200
250
300
H-Mg-H angle, degrees
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Results: 4. Mg bulk metal
100
100
2
Energy (kcal/mole-Mg)
1.5
90
90
80
80
HCP
1
BCC
0.5
FCC
0
19
20
21
22
23
24
25
-0.5
70
SC
70
QC
60
-1
Reax FF
60
50
50
40
40
30
30
20
20
10
10
0
diamond
0
10
15
20
25
30
35
40
10
Volume/atom
15
20
25
30
35
(Å3)
-ReaxFF reproduces the EOS for the stable phases (BCC)
-ReaxFF properly predicts the instability of the low-coordination phases (SC, Diamond)
-Discrepancy in relative stabilityFree
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FCC can be
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Energy (kcal/mol-MgH2)
Results: 4. Magnesium hydride crystal
60
60
50
50
ReaxFF
QC
40
40
a-mgh2
30
b-mgh2
30
g-mgh2
CaF2
20
20
10
10
0
0
15
20
25
30
35
40
45
50
55
15
20
25
30
35
40
45
50
55
Volume/MgH2 (Å3)
-ReaxFF reproduces the EOS for the stable phases (a-MgH2, g-MgH2, a-MgH2)
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Relative stabilities of Mg bulk phase and Mg Hydride
crystals
Mg Hydride crystals
Mg metal
Phase
Eref
EReax
rref
rReax
Phase
Eref
EReax
rref
rReax
(kcal/Mg atom)
(kcal/Mg atom)
HCP
0.00
0.00
1.73
1.73
a-MgH2
0.00
0.00
1.42
1.505
BCC
1.64
0.40
1.62
1.75
g-MgH2
0.05
0.40
1.44
1.445
FCC
1.81
-0.24
1.72
1.74
b-MgH2
2.38
2.36
1.56
1.535
SC
10.94
8.70
1.46
1.66
e-MgH2
7.13
3.19
1.74
1.485
diamond
19.00
17.30
1.14
1.19
fluorite
8.78
7.62
1.60
1.325
diamond
9.88
0.52
1.43
1.420
-ReaxFF gives a fair description of the relative stability of Mg bulk phase and Mg-hydride
crystal phases (longer ffopt run needed for better description)
-ReaxFF properly predicts the instability of the low-coordination phases (SC, Diamond)
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H-Atomic Adsorption
Calculated atomic energies, equilibrium bonding heights (above the top layer Mg atoms)
for H absorption on the high-symmetry sites of Mg (0001).
Adsorption Site Height Literature* ReaxFF
(Å)
(kcal/mol) (kcal/mol)
Top
2.66
61.29
40.69
Bridge
3.28
75.57
63.70
Centre-FCC
3.46
79.72
78.14
Centre-HCP
3.44
79.26
80.37
* M.C. Payne et. al., Chemical Physics Letters, Vol 212, p. 518
Top
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Centre-HCP
21
Applications
•
•
•
•
Mg-particle aggregation
MgH2-particle anneal (300-0K)
Cook-off simulations on MgH2-particles
Strategy for improving hydrogen adsorption and
desorption process
• Reduction of H2 dissociation barrier via Pt catalyst
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Mg-particle aggregation
Mg87-particles (300K NVT-MD)
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MgH2-particle aggregation
Mg87-particles (300K NVT-MD)
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Cook-off simulations on MgH2-particles
MD-heatup of Mg123H246-cluster.
Start temperature: 300K
heatup rate 0.002 K/fs
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Designer catalysts for H2-release
-Modify Mg*-H, Mg*-Mg* and
Mg*-Mg force field parameters to
optimize H2-release from
nanoparticle
-Find element that fits with
optimal Mg*-characteristics
H
Mg*
Mg
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Comparison Mg0.7Mg0.3*H2 and MgH2-cookoff
runs
E(Mg*-H)=0.75*E(Mg-H)
Mg*
Mg
Temperature regime:
300 to 1300K in 2.5 ps
-Weakened Mg*-H bond reduces
H2-release temperature by about
150K
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