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ReaxFF for Magnesium Hydrides

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

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  1. ReaxFF for Magnesium Hydrides Sam Cheung, Weiqiao Deng, Adri van Duin FF-subgroup meeting 9 Dec. 2003

  2. Topic Overview • Hydrogen storage: a brief history • Objectives • ReaxFF: general principles • Building the ReaxFF for Mg-hydride • File Format • Applications • Conclusion Free Powerpoint template from www.brainybetty.com

  3. 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 Free Powerpoint template from www.brainybetty.com

  4. Saftey issues of hydrogen vs. other fuels • Lower risk of explosion • Nontoxic! Free Powerpoint template from www.brainybetty.com

  5. 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

  6. 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) Free Powerpoint template from www.brainybetty.com

  7. 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 Free Powerpoint template from www.brainybetty.com

  8. 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 Free Powerpoint template from www.brainybetty.com Distance

  9. System energy description 2-body 3-body 4-body multibody Free Powerpoint template from www.brainybetty.com

  10. 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. Free Powerpoint template from www.brainybetty.com

  11. 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. Free Powerpoint template from www.brainybetty.com

  12. 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 Free Powerpoint template from www.brainybetty.com

  13. Training set Cluster: Condensed phase: Free Powerpoint template from www.brainybetty.com

  14. 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 Free Powerpoint template from www.brainybetty.com

  15. Results: 1. Charge Analysis QC ReaxFF Muliken Charges (Debye) Atom number • ReaxFF reproduces charge for clusters. Free Powerpoint template from www.brainybetty.com

  16. 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 Free Powerpoint template from www.brainybetty.com

  17. Results: 3. H-Mg-H Angle Bend Curve Free Powerpoint template from www.brainybetty.com

  18. 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. Free Powerpoint template from www.brainybetty.com

  19. 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) Free Powerpoint template from www.brainybetty.com

  20. 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) Free Powerpoint template from www.brainybetty.com

  21. 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). * M.C. Payne et. al., Chemical Physics Letters, Vol 212, p. 518 Free Powerpoint template from www.brainybetty.com Top Bridge Centre-FCC Centre-HCP

  22. 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 Free Powerpoint template from www.brainybetty.com

  23. Mg-particle aggregation Mg87-particles (300K NVT-MD) Free Powerpoint template from www.brainybetty.com

  24. MgH2-particle aggregation Mg87-particles (300K NVT-MD) Free Powerpoint template from www.brainybetty.com

  25. Cook-off simulations on MgH2-particles MD-heatup of Mg123H246-cluster. Start temperature: 300K heatup rate 0.002 K/fs Free Powerpoint template from www.brainybetty.com

  26. 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 Free Powerpoint template from www.brainybetty.com

  27. Comparison Mg0.7Mg0.3*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 Free Powerpoint template from www.brainybetty.com

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