# Presentation: 853204

## Trapping the hydrogen genie: A reactive force field for NaH  

**Ojwang’ JGO**, *Adri van Duin, W. A. Goddard III*,  R. van Santen, G.J. Kramer

- Berkeley, 26-27 Oct., 2007

**APS meeting**
**California section**

## Aim

- Study structural and dynamical details of hydrogen ab/desorption
- processes in NaAlH4.
-      Long range transport mechanism of Al atoms/particle segregation.
-      Is titanium a dopant or a catalyst?

- Methodology:
-  Simulate large clusters/molecules containing NaAlH4 plus  catalysts atoms.

## Outline

- Theoretical background/calculations
- Results
- Conclusion
- Future Work

## How large of a gas tank do you want?

**How large of a gas tank do you want?**

**Schlapbach & Züttel, Nature, 15 Nov. 2001**

**Volume Comparisons for 4 kg Vehicular H****2**** Storage**

**Storage remains a problem!**

**Electric car with fuel cell (4kg H****2****)**
**Combustion engine (8kg H****2****)**
**Combustion engine (24 kg petrol)**

- 400 km

##    3NaAlH4          Na3AlH6 + 2Al + 3H2
        (3.7 wt % of H2 released)

**        ****(****3.7 wt % of H****2**** released****)**

-  Na3AlH6          3NaH + Al + 3/2H2
-    (1.8 wt % of H2 released)

- Theoretical reversible capacity (wt%) of NaAlH4

- Reversible process: Bogdanovic (1997)

- 2NaH              2Na +H2

**Ti**

**Ti**

## Force fields: general principles and potential functions

- Time

- Distance

- Ångstrom

- Kilometres

- 10-15

- years

- QC

- ab initio,
- DFT,
- HF

- Electrons
- Bond formation

-      MD

- Empirical
- force fields

- Atoms
- Molecular
- conformations

- MESO

- FEA

- Design

- Grains

- Grids

**Empirical methods:**
- - Allow large systems
- - Rigid connectivity
**QC methods:**
- - Allow reactions
- - Expensive, only
-   small systems

**ReaxFF**

- Simulate bond formation
- in larger molecular systems

## Force Field Parameterizations

**Force field** -  functional form and parameter sets used to describe the potential
-                       energy of a system of particles.

- 1

- 2

- 3

- 4

##        3NaAlH4          Na3AlH6 + 2Al + 3H2

- Bonding terms

- Stretching              Angle Bending                Torsions

- U =

* **K*

*r*

- –

*r*

- 0

- 2

- +

*H*

- 

- –

- 

- 0

- 2

- +

*T*

- 1

- +

- cos

*n*

- 

-        **3NaAlH****4****          Na****3****AlH****6**** + 2Al + 3H****2**

**Ti**

- Parameterized using experimental work or ab initio
- Non reactive force fields!!

**For ****reactive force fields**** use the ****Bond Order**** formalism of Tersoff and Brenner**

## Equations of state
Heats of formation
Charges
Equilibrium bond lengths
Binding energies

- Equations of state
- Heats of formation
- Charges
- Equilibrium bond lengths
- Binding energies

- NaAlH4, Na3AlH6, AlH3, NaH, H2, Na, Al, Ti, TiAl3,TiH2

- ReaxFF training set fittings:

** ****Obtained from DFT**** – ****VASP, CRYSTAL06**

**NaCl-type**

**CsCl-type**

## Results

- ReaxFF gives a fair description of the relative stability of  NaH bulk phases.

**Bulk values: **
**DFT              ****-10.19 kcal/mol**
**ReaxFF        ****-11.60 kcal/mol**
**Experiment  ****-13.49 kcal/mol**

## NaH Bond dissociation curves

- ReaxFF correctly reproduces both the short and long range behavior of Na-H dissociation curve. DFT – 1.93 Å, ReaxFF 1.91Å

## Heatup simulation of NaH-cluster

- Simulation preparation

- 1. Generate Na24H24 structure from crystal.
- 2. Remove periodic boundaries to generate cluster.
- 3. Energy minimize cluster to find nearest metastable configuration.(timestep = 0.25fs)
- 4. Perform MD-equilibration simulation at T=300K.

- Structure after MD-equilibration at T=300K

- Minimization

- Equilibration

- MD run

- Energy

- Conformational space

## Heatup simulation of Na24H24- cluster

-  Fall in PES associated with H2 release.
-  Surface effect? Entropy driven reaction?

- 10.225ps

- 10.275ps

**Heatup from 300K to 1800K in 100,000 iterations (total time 25 ps)**
**timestep 0.25fs**

**Heatup simulation of Na****24****H****24****- cluster**

## Abstraction of H2

-  Simulate successive abstraction of hydrogen by simulated annealing.
-  Equilibrate at 300K  for 50000 steps and anneal before each successive   abstraction of H2

- Annealed structures

- Annealed from 300K to 0K in 50000 steps

- Na48H48

- Na48

- Na48H24

## ∆E = [E(Na48Hn) + E(H2)] - E(Na48Hn + 2) kcal/Na

## Charge transfer

- -ReaxFF nicely captures transfer of charges.

- Na48H48

- Na48H36

- Na48H24

- Na48H6

## We’ve parameterized a force field for NaH.

Thermal decomposition of NaH has been studied using the parameterized force field.

H2 release during the decomposition is associated with a drop in PES.

Charge transfer is correctly reproduced.

The results puts us on a firm footing towards modeling a force field for NaAlH4.

- Thermal decomposition of NaH has been studied using the parameterized force field.
- H2 release during the decomposition is associated with a drop in PES.
- Charge transfer is correctly reproduced.
- The results puts us on a firm footing towards modeling a force field for NaAlH4.

- Conclusion

## Future work

-  Add more QM-data to training set:
-  Al2H6-clusters; dissociation to two AlH3-clusters.  √
-  Heats of formation of various NaxAlyHz -phases (especially NaAlH4) √
-  Surface energies for NaxAlyHz-phases (especially NaAlH4)
-  Other NaxAly-alloys (check phase diagram) √
-  Barriers for H2 dissociation on Na, Al and NaAl-surfaces.
-  Add other elements (TiCl3, LiAlH4)

## ‘Lousy potentials will give lousy results’

**Art Voter**
**(IPAM Tutorial)**

**Test, Test and Test**

**Ok potentials give ok results.......Good potentials give good results.**

## Thank you all!