Materials for hydrogen storage: ab-initio Molecular Dynamics study in the ENEA-GRID environment M. Celino and S. Giusepponi ENEA Italian Agency for New Technologies, Energy and Environment Physical Technologies and New Materials Department Casaccia Research Centre Rome, Italy
Introduction: MgH 2 It can store significant quantities of hydrogen (7.7 wt% of hydrogen) Low cost of production High abundance BUT Too high temperature of decomposition Slow decomposition kinetics
Introduction: MgH 2 Improvements comes from: High energy ball milling Adding small amounts of catalytic metals High density of crystal defects Fine dispersion of second phase particles
Experimental results Now the problem is to control at the atomic level the phase transformation of the milled samples
Experimental results It is possible to perform SEM observation at high spatial resolution of the phases distribution in partially decomposed Mg-MgH 2 containing heavier catalyzing particles The addition of Fe particles induces a nucleation process diffused in the material giving raise to a strongly interconnected microstructure Mg/MgH 2 10h milled MgH 2 Mg/MgH 2 Fe (10%)10h milled Fe Mg Thanks to A. Montone, ENEA
Molecular Dynamics Simulations input N atoms 3N degrees of freedom t=1,t max Distances among the atoms Forces among all the atoms New atomic positions Compute physical quantities m dx dt 2 i 2 = i V ({ x }) i Data storage output
CPMD: code performance Time Speedup 120 IBM SP5 400 100 80 CRESCO 300 60 40 Mesh: 72x576x180 200 100 20 0 0 50 100 150 200 250 300 Processing Elements 0 0 10 20 30 40 50 60 70 Processing Elements
CPMD: code performance 500 400 Speedup Rotate system: mesh 576 on x axis 300 200 100 IBM SP5: ~ 8 sec - 64 PE CRESCO: ~ 8 sec 150 PE ~ 3 sec 576 PE 0 0 200 400 600 800 1000 Processing elements
Hydrogen desorption: the MgH 2 -Mg interface Starting configuration Idrogeno Magnesio Mg: 72 atoms Mg surface MgH 2 : 60 Mg atoms + 120 H atoms MgH 2 surface L x = 6.21 Å L y = 50.30 Å Interface L z = 15.10 Å
Molecular dynamics at T= 700 K Starting configuration T= 700 K
Molecular dynamics (NVT ensemble) T= 700 K Starting configuration T= 800 K T= 900 K
Mg-MgH 2 interface : Fe Starting configuration with a Fe atom near the interface: Increase of Hydrogen mobility Lower desorption temperature Fe in POS 1 Fe T= 500 K
Mg-MgH 2 interface : Fe Fe in POS 2 Fe in POS 3
Mg-MgH 2 interface : Fe Fe in POS 1 Fe in POS 2 T= 500 K Fe in POS 3
Mg-MgH 2 interface : Fe Fe in POS 1 T= 400 K Fe in POS 2 Fe in POS 3
T= 400 K POS 2 Mg-MgH 2 interface : Fe Fe atom: first and second shell of Mg coordination
Mg-MgH 2 interface : Fe T= 400 K POS 3
Conclusions It is possible to characterize experimentally and numerically the hydrogen desorption from MgH 2 The addition of catalyzing particles induces a nucleation process diffused in the material giving raise to a strongly interconnected microstructure The interplay of interfaces and catalyzing particles lowers Hydrogen desorption temperature: Fe atoms induce a depression region able to lower the bonding between H and Mg