ReaxFF for Magnesium Hydrides Sam Cheung, Weiqiao Deng, Adri van Duin FF-subgroup meeting 9 Dec....

ReaxFF for Magnesium Hydrides

Sam Cheung, Weiqiao Deng, Adri van DuinFF-subgroup meeting 9 Dec. 2003

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• Hydrogen storage: a brief history

• Objectives• ReaxFF: general principles• Building the ReaxFF for Mg-hydride • File Format• Applications• Conclusion

Topic Overview


3

H2

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.


4

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 (%)

Toxicity

4.40

0.05

1.20

1.0-7.6

0.24

228-471

2197

0.25

34-43

High

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

How large of a gas tank do you want?

Schlapbach & Züttel, Nature, 15 Nov. 2001

Volume Comparisons for 4 kg Vehicular H2 Storage

Storage remains a problem!Electric car with fuel cell (4kg H)

Combustion engine (8kg H)

Combustion engine (24 kg petrol)

400 km


6

• 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

Storing Hydrogen

From Patrovic & Milliken (2003)


7

Material H-atoms per cm3 (10-22)

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

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


8

Reax FF: general principlesT

ime

DistanceÅngstrom Kilometres

10-15

years

QC

ab initio,DFT,HF

ElectronsBond formation

MD

Empiricalforce fields

AtomsMolecular

conformations

MESO

FEA

Design

Grains

Grids

ReaxFF


9

underover

torsvalCoulombvdWaalsbondsystem

EE

EEEEEE

2-body

multibody

3-body 4-body

System energy description


10

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.

Key Features


11

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.

General Rules


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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 EOSPeriodic system (CASTEP GGA-PBE 4x4x2 k-space KE cutoff 380eV)

Step 3 -FFopt and ReaxFF fittings

Step 4 -Applications

Parameterization of ReaxFF:


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Training set

Bonds•Mg-H -Normal, under-, and overcoordinated systems

Angles•H-Mg-H•H-H-Mg

•Mg-H-Mg•H-Mg-Mg

HCPBCCFCCSCdiamond

-MgH2

-MgH2

-MgH2

CaF2-MgH2

Cluster: Condensed phase:

H

-MgH2 (rutile)

Mg


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BIOGRF 200DESCRP mgh2_b1.2 RUTYPE NORMAL RUNBOND RESTRAINT 1 3 1.2000 7500.00 0.50000 0.0000000FORMAT 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 0.00000HETATM 2 H 0.00000 0.00000 1.62594 H 1 1 0.00000HETATM 3 H 0.00000 0.00000 -1.19525 H 1 1 0.00000END

BIOGRF 200DESCRP mgh2_a140RUTYPE NORMAL RUNANGLE RESTRAINT 2 1 3 140.00 2500.00 1.0000 0.000000FORMAT 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 0.00000HETATM 2 H -0.00006 0.00000 1.71361 H 1 1 0.00000HETATM 3 H 1.10148 0.00000 -1.31278 H 1 1 0.00000END

XTLGRF 200DESCRP diamond-mgh2_opt RUTYPE CELL OPT 0CRYSTX 3.93314 3.93314 3.93314 90.00000 90.00000 90.00000FORMAT 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 0.00000HETATM 2 Mg 1.96646 1.96644 1.96644 Mg 1 1 0.00000HETATM 3 H 0.98315 0.94017 1.02607 H 1 1 0.00000HETATM 4 H 0.98321 2.99259 2.90679 H 1 1 0.00000HETATM 5 H 2.94977 1.02602 2.99268 H 1 1 0.00000HETATM 6 Mg -0.00011 -0.00013 -0.00012 Mg 1 1 0.00000FORMAT CONECT (a6,12i6)END

File Format: geo trainset.in geo

CHARGESmgh2 0.05 1 0.2519mgh2 0.05 2 -0.1260ENDCHARGESGEOMETRYmgh2 0.020 1 2 1.707mgh2 0.500 2 1 3 179.000ENDGEOMETRYENERGY#Mg1-H3 (Mg-H 1.71) dissociation MgH210.0 + mgh2 /1 - mgh2_b1.2 /1 -51.57.0 + mgh2 /1 - mgh2_b1.4 /1 -14.05.0 + mgh2 /1 - mgh2_b1.5 /1 -5.42.0 + mgh2 /1 - mgh2_b1.6 /1 -1.22.0 + 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 -13.4710.0 + mgh2 /1 - mgh2_a100 / -25.7210.0 + mgh2 /1 - mgh2_a80 /1 -44.5725.0 + mgh2 /1 - mgh2_a60 /1 -73.4725.0 + 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 -17.610.0 + 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 -10.8ENDENERGY

trainset.in

0

20

40

60

80

0.5 1.5 2.5 3.5 4.5

0

40

80

50 150 250

H Mg H


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Results: 1. Charge AnalysisM

ulik

en C

harg

es

(Debye)

Atom number

QCReaxFF

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1 2

Mg1 H2

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1 2 3

Mg1 H3H2

-0.3

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1 2 3 4 5 6

Mg1 Mg2H4

H3

H5

H6

-0.3

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1 2 3 4 5 6

Mg2 Mg3

Mg1

HHH

H5 H

Mg4

H

H

H6

-ReaxFF reproduces charge for clusters.


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

Bond distance (Å)

En

erg

y (

kca

l/m

ol)

MgH2

0

10

20

30

40

50

60

70

80

90

100

0.5 1.5 2.5 3.5 4.5

QC-singlet

QC-triplet

ReaxFF

MgH

0

10

20

30

40

50

60

70

80

90

100

0.5 1.5 2.5 3.5 4.5

Mg (3s)2

Results: 2. MgH/MgH2 bond dissociation


17

Results: 3. H-Mg-H Angle Bend Curve

MgH2

-5

0

5

10

15

20

25

30

35

40

45

50

50 100 150 200 250 300

H-Mg-H angle, degrees

Ene

rgy

(kca

l/mol

)

QC

ReaxFF


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QC

0

10

20

30

40

50

60

70

80

90

100

10 15 20 25 30 35 40

Reax FF

0

10

20

30

40

50

60

70

80

90

100

10 15 20 25 30 35 40

HCP

BCC

FCC

SC

diamond

-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.

Volume/atom (Å3)

En

erg

y (

kca

l/m

ole

-Mg)

Results: 4. Mg bulk metal

-1

-0.5

0

0.5

1

1.5

2

19 20 21 22 23 24 25


19

-ReaxFF reproduces the EOS for the stable phases (-MgH2, -MgH2, -MgH2)

Volume/MgH2 (Å3)

En

erg

y (

kca

l/m

ol-

MgH

2)

Results: 4. Magnesium hydride crystal

ReaxFF

0

10

20

30

40

50

60

15 20 25 30 35 40 45 50 55

a-mgh2

b-mgh2

g-mgh2

CaF2

QC

0

10

20

30

40

50

60

15 20 25 30 35 40 45 50 55


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Mg metal

Phase Eref

(kcal/Mg atom)

EReax ref Reax

HCP 0.00 0.00 1.73 1.73

BCC 1.64 0.40 1.62 1.75

FCC 1.81 -0.24 1.72 1.74

SC 10.94 8.70 1.46 1.66

diamond 19.00 17.30 1.14 1.19

Relative stabilities of Mg bulk phase and Mg Hydride crystals

Mg Hydride crystals

Phase Eref

(kcal/Mg atom)

EReax ref Reax

a-MgH2 0.00 0.00 1.42 1.505

g-MgH2 0.05 0.40 1.44 1.445

b-MgH2 2.38 2.36 1.56 1.535

e-MgH2 7.13 3.19 1.74 1.485

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

Adsorption Site Height

(Å)

Literature*

(kcal/mol)

ReaxFF

(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

Centre-FCCBridgeTop Centre-HCP

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

Calculated atomic energies, equilibrium bonding heights (above the top layer Mg atoms) for H absorption on the high-symmetry sites of Mg (0001).


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• 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

Applications


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Mg87-particles (300K NVT-MD)

Mg-particle aggregation


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Mg87-particles (300K NVT-MD)

MgH2-particle aggregation


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MD-heatup of Mg123H246-cluster. Start temperature: 300Kheatup rate 0.002 K/fs

Cook-off simulations on MgH2-particles


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H Mg* Mg

-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

Designer catalysts for H2-release


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

Mg*

Mg

-Weakened Mg*-H bond reduces H2-release temperature by about 150KTemperature regime:

300 to 1300K in 2.5 ps

Comparison Mg0.7Mg0.3*H2 and MgH2-cookoff

runs

ReaxFF for Magnesium Hydrides Sam Cheung, Weiqiao Deng, Adri van Duin FF-subgroup meeting 9 Dec....

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