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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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Reaxff for magnesium hydrides l.jpg

ReaxFF for Magnesium Hydrides

Sam Cheung, Weiqiao Deng, Adri van Duin

FF-subgroup meeting 9 Dec. 2003


Topic overview l.jpg
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 l.jpg
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

      • 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.

H2

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Slide4 l.jpg

Saftey issues of hydrogen vs. other fuels

  • Lower risk of explosion

  • Nontoxic!

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How large of a gas tank do you want l.jpg
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 l.jpg
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

  • Solid or liquid state as chemical hydrogen-rich compunds

  • - methanol, methane, carbon

  • - metal hydrides

From Patrovic & Milliken (2003)

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Materials with high weight hydrogen l.jpg
Materials with High Weight Hydrogen

  • 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 l.jpg
Reax FF: general principles

Atoms

Molecular

conformations

Design

years

Electrons

Bond formation

FEA

Time

MESO

Grids

MD

Grains

ReaxFF

QC

Empirical

force fields

10-15

ab initio,

DFT,

HF

Ångstrom

Kilometres

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Distance


System energy description l.jpg
System energy description

2-body

3-body

4-body

multibody

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Key Features

  • 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 l.jpg
General Rules

  • 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.

  • 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 l.jpg
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 l.jpg
Training set

Cluster:

Condensed phase:

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File format geo trainset in l.jpg
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

BIOGRF 200

DESCRP mgh2_a140

RUTYPE NORMAL RUN

ANGLE RESTRAINT 2 1 3 140.00 2500.00 1.0000 0.000000

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 -0.00006 0.00000 -0.00002 Mg 1 1 0.00000

HETATM 2 H -0.00006 0.00000 1.71361 H 1 1 0.00000

HETATM 3 H 1.10148 0.00000 -1.31278 H 1 1 0.00000

END

XTLGRF 200

DESCRP diamond-mgh2_opt

RUTYPE CELL OPT 0

CRYSTX 3.93314 3.93314 3.93314 90.00000 90.00000 90.00000

FORMAT ATOM (a6,1x,i5,1x,a5,1x,a3,1x,a1,1x,a5,3f10.5,1x,a5,i3,i2,1x,f8.5)

HETATM 1 H 2.94972 2.90674 0.94026 H 1 1 0.00000

HETATM 2 Mg 1.96646 1.96644 1.96644 Mg 1 1 0.00000

HETATM 3 H 0.98315 0.94017 1.02607 H 1 1 0.00000

HETATM 4 H 0.98321 2.99259 2.90679 H 1 1 0.00000

HETATM 5 H 2.94977 1.02602 2.99268 H 1 1 0.00000

HETATM 6 Mg -0.00011 -0.00013 -0.00012 Mg 1 1 0.00000

FORMAT CONECT (a6,12i6)

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

10.0 + mgh2 /1 - mgh2_b1.2 /1 -51.5

7.0 + mgh2 /1 - mgh2_b1.4 /1 -14.0

5.0 + mgh2 /1 - mgh2_b1.5 /1 -5.4

2.0 + mgh2 /1 - mgh2_b1.6 /1 -1.2

2.0 + mgh2 /1 - mgh2_b2.0 /1 -6.8

1.0 + mgh2 /1 - mgh2_b4.1 /1 -73.1

#H-Mg-H angle in mgh2

1.0 + mgh2 /1 - mgh2_a160 /1 -1.41

2.0 + mgh2 /1 - mgh2_a140 /1 -5.74

4.0 + mgh2 /1 - mgh2_a120 /1 -13.47

10.0 + mgh2 /1 - mgh2_a100 / -25.72

10.0 + mgh2 /1 - mgh2_a80 /1 -44.57

25.0 + mgh2 /1 - mgh2_a60 /1 -73.47

25.0 + mgh2 /1 - mgh2_a40 /1 -73.29

# Relative Energy for Clusters

2.0 + mg2h4 /2 - mgh2 /1 -14.21

# Mg hcp (EOS)

20.0 + hcp0 /2 - hcp14 /2 -17.6

10.0 + hcp0 /2 - hcp17 /2 -6.2

2.0 + hcp0 /2 - hcp20 /2 -1.2

2.0 + hcp0 /2 - hcp_eq/2 -0.001

2.0 + hcp0 /2 - hcp27 /2 -1.3

5.0 + hcp0 /2 - hcp31 /2 -7.6

5.0 + hcp0 /2 - hcp35 /2 -10.8

ENDENERGY

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Results 1 charge analysis l.jpg
Results: 1. Charge Analysis

QC

ReaxFF

Muliken Charges (Debye)

Atom number

  • ReaxFF reproduces charge for clusters.

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Results 2 mgh mgh2 bond dissociation l.jpg
Results: 2. MgH/MgH2 bond dissociation

Mg (3s)2

Energy (kcal/mol)

Bond distance (Å)

  • ReaxFF gives a fair description for the Mg-H bond dissocation

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Results 3 h mg h angle bend curve l.jpg
Results: 3. H-Mg-H Angle Bend Curve

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Results 4 mg bulk metal l.jpg
Results: 4. Mg bulk metal

Energy (kcal/mole-Mg)

Volume/atom (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 stability of FCC can be solved by further optimization.

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Results 4 magnesium hydride crystal l.jpg
Results: 4. Magnesium hydride crystal

Energy (kcal/mol-MgH2)

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 l.jpg
Relative stabilities of Mg bulk phase and Mg Hydride crystals

  • 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 l.jpg
H-Atomic Adsorption crystals

Calculated atomic energies, equilibrium bonding heights (above the top layer Mg atoms)

for H absorption on the high-symmetry sites of Mg (0001).

* M.C. Payne et. al., Chemical Physics Letters, Vol 212, p. 518

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Top

Bridge

Centre-FCC

Centre-HCP


Applications l.jpg
Applications crystals

  • 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 l.jpg
Mg-particle aggregation crystals

Mg87-particles (300K NVT-MD)

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Mgh 2 particle aggregation l.jpg
MgH crystals2-particle aggregation

Mg87-particles (300K NVT-MD)

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Cook off simulations on mgh 2 particles l.jpg
Cook-off simulations on MgH crystals2-particles

MD-heatup of Mg123H246-cluster. Start temperature: 300K

heatup rate 0.002 K/fs

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Designer catalysts for h 2 release l.jpg
Designer catalysts for H crystals2-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 mg 0 7 mg 0 3 h 2 and mgh 2 cookoff runs l.jpg
Comparison Mg crystals0.7Mg0.3*H2 and MgH2-cookoff runs

E(Mg*-H)=0.75*E(Mg-H)

Mg*

Mg

  • Weakened Mg*-H bond reduces H2-release temperature by about 150K

Temperature regime:

300 to 1300K in 2.5 ps

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