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

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

Free Powerpoint template from 3 H2H2 Hydrogen storage: a brief history Hydrogen Facts: Hydrogen is an odorless and colorless gas. BP of o C. Density of grams/liter. The most abundant element on earth but less than 1% is in the form of H 2 Ways to produce H 2 : electrolysis, thermal dissociation of H 2 O, or photochemical splitting of H 2 O A clean synthetic fuel H 2 O 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.

Free Powerpoint template from 4 Saftey issues of hydrogen vs. other fuels Lower risk of explosion Nontoxic! PropertyGasolineMethaneHydrogen Density (Kg/M 3 ) Diffusion Coefficient In Air (Cm 2 /Sec) Specific Heat at Constant Pressure (J/Gk) Ignition Limits In Air (vol %) Ignition Energy In Air (Mj) Ignition Temperature ( o C) Explosion Energy (G TNT/kj) Flame Emissivity (%) Toxicity High No No

How large of a gas tank do you want? Schlapbach & Züttel, Nature, 15 Nov Volume Comparisons for 4 kg Vehicular H 2 Storage Storage remains a problem! Electric car with fuel cell (4kg H) Combustion engine (8kg H) Combustion engine (24 kg petrol) 400 km

Free Powerpoint template from 6 Pressurized gas - Must be intensely pressurized to several hundred atmospheres (200 bar or more) - Stored in pressure vessel Condensed liquid state - Liquifying H 2 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)

Free Powerpoint template from 7 MaterialH-atoms per cm 3 ( ) H 2 gas (200 bar)0.99 H 2 liquid (20K)4.2 H 2 solid (4.2K)5.3 MgH Mg 2 NiH TiFeH LaNi 5 H 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 8 Reax FF: general principles Time Distance ÅngstromKilometres years QC ab initio, DFT, HF Electrons Bond formation MD Empirical force fields Atoms Molecular conformations MESO FEA Design Grains Grids ReaxFF

Free Powerpoint template from body multibody 3-body4-body System energy description

Free Powerpoint template from 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

Free Powerpoint template from 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

Free Powerpoint template from 12 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 Parameterization of ReaxFF:

Free Powerpoint template from 13 Training set Bonds Mg-H -Normal, under-, and overcoordinated systems Angles H-Mg-H H-H-Mg Mg-H-Mg H-Mg-Mg HCP BCC FCC SC diamond  -MgH 2  -MgH 2  -MgH 2 CaF 2 -MgH2 Cluster:Condensed phase:

Free Powerpoint template from 14 BIOGRF 200 DESCRP mgh2_b1.2 RUTYPE NORMAL RUN BOND RESTRAINT 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 Mg HETATM 2 H H HETATM 3 H H END BIOGRF 200 DESCRP mgh2_a140 RUTYPE NORMAL RUN ANGLE RESTRAINT 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 Mg HETATM 2 H H HETATM 3 H H END XTLGRF 200 DESCRP diamond-mgh2_opt RUTYPE CELL OPT 0 CRYSTX 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 H HETATM 2 Mg Mg HETATM 3 H H HETATM 4 H H HETATM 5 H H HETATM 6 Mg Mg FORMAT CONECT (a6,12i6) END File Format: geo trainset.in geo CHARGES mgh mgh ENDCHARGES GEOMETRY mgh mgh ENDGEOMETRY ENERGY #Mg1-H3 (Mg-H 1.71) dissociation MgH mgh2 /1 - mgh2_b1.2 / mgh2 /1 - mgh2_b1.4 / mgh2 /1 - mgh2_b1.5 / mgh2 /1 - mgh2_b1.6 / mgh2 /1 - mgh2_b2.0 / mgh2 /1 - mgh2_b4.1 / #H-Mg-H angle in mgh mgh2 /1 - mgh2_a160 / mgh2 /1 - mgh2_a140 / mgh2 /1 - mgh2_a120 / mgh2 /1 - mgh2_a100 / mgh2 /1 - mgh2_a80 / mgh2 /1 - mgh2_a60 / mgh2 /1 - mgh2_a40 / # Relative Energy for Clusters mg2h4 /2 - mgh2 / # Mg hcp (EOS) hcp0 /2 - hcp14 / hcp0 /2 - hcp17 / hcp0 /2 - hcp20 / hcp0 /2 - hcp_eq/ hcp0 /2 - hcp27 / hcp0 /2 - hcp31 / hcp0 /2 - hcp35 / ENDENERGY trainset.in

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

Free Powerpoint template from ReaxFF gives a fair description for the Mg-H bond dissocation Bond distance (Å) Energy (kcal/mol) Mg (3s) 2 Results: 2. MgH/MgH2 bond dissociation

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

Free Powerpoint template from 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 (Å 3 ) Energy (kcal/mole-Mg) Results: 4. Mg bulk metal

Free Powerpoint template from ReaxFF reproduces the EOS for the stable phases (  -MgH 2,  -MgH 2,  -MgH 2 ) Volume/MgH 2 (Å 3 ) Energy (kcal/mol-MgH 2 ) Results: 4. Magnesium hydride crystal

Free Powerpoint template from 20 Mg metal PhaseE ref (kcal/Mg atom) E Reax  ref  Reax HCP BCC FCC SC diamond Relative stabilities of Mg bulk phase and Mg Hydride crystals Mg Hydride crystals PhaseE ref (kcal/Mg atom) E Reax  ref  Reax a-MgH g-MgH b-MgH e-MgH fluorite diamond 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 21 H-Atomic Adsorption Adsorption SiteHeight ( Å ) Literature* (kcal/mol) ReaxFF (kcal/mol) Top Bridge Centre-FCC Centre-HCP Centre-FCCBridgeTopCentre-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).

Free Powerpoint template from 22 Mg-particle aggregation MgH 2 -particle anneal (300-0K) Cook-off simulations on MgH 2 -particles Strategy for improving hydrogen adsorption and desorption process Reduction of H 2 dissociation barrier via Pt catalyst Applications

Free Powerpoint template from 23 Mg 87 -particles (300K NVT-MD) Mg-particle aggregation

Free Powerpoint template from 24 Mg 87 -particles (300K NVT-MD) MgH 2 -particle aggregation

Free Powerpoint template from 25 MD-heatup of Mg 123 H 246 -cluster. Start temperature: 300K heatup rate K/fs Cook-off simulations on MgH 2 -particles

Free Powerpoint template from 26 HMg * Mg -Modify Mg * -H, Mg * -Mg * and Mg * -Mg force field parameters to optimize H 2 -release from nanoparticle -Find element that fits with optimal Mg * -characteristics Designer catalysts for H 2 -release

Free Powerpoint template from 27 E(Mg * -H)=0.75*E(Mg-H) Mg * Mg -Weakened Mg * -H bond reduces H 2 -release temperature by about 150K Temperature regime: 300 to 1300K in 2.5 ps Comparison Mg 0.7 Mg 0.3 * H 2 and MgH 2 -cookoff runs