460
Molecular Dynamics Study of Hydrogen Storage in FeTi
Seifollah Jalili
Department of Chemistry, K. N. Toosi University of Technology, P.O. Box 16315-1618, Tehran, Iran and Computational Physical Sciences Research Laboratory, Department of Nano-Science, Institute for Studies
in Theoretical Physics and Mathematics (IPM), P.O. Box 19395-5531, Tehran, Iran
Abstract
We have used molecular dynamics simulation to study the adsorption isotherms of molecular hydrogen on FeTi at several temperatures ranging from 60 to 100 K.
Adsorption coverage, isosteric heat, and binding energy were calculated at different temperatures and pressures. Our results indicate that FeTi can be an ideal hydrogen storage material.
Keywords: FeTi, molecular dynamics simulation, adsorption, isotherm
Introduction
Hydrogen as an ideal energy carrier has attracted a great deal of attention in recent years. Its application in vehicles and portable electronics have limited by the difficulty of achieving a capable storage method until now. There are several hydrogen storage methods, including compressed gas, liquefaction, metal hydrides, and physisorption.
Despite numerous research efforts and many obvious developments, problems with storage and transportation of hydrogen remain unsolved. In the beginning metal alloys have been suggested as proper material for hydrogen storage. FeTi was selected for practical employment, because it is lighter and cheaper than other rare earth metals.
In this paper, the adsorption isotherms of molecular hydrogen were obtained using molecular dynamics simulation. Adsorption coverage, isosteric heat, and binding energy were measured.
Method
Molecular dynamics simulations were performed using a modified Tinker 4.2 package [13] in the NPT ensemble. We used Buckingham force field for describing the interaction between different atoms. The potential is defined as
461
6 ζ
E = D
eζ (1−ρ ) −
ρ −6 .
0 ζ − 6 ζ − 6
The periodic boundary conditions were imposed in all three dimensions and Van der Waals cutoff was chosen 12 Å. Our supercell of FeTi crystal contains 250 atoms, corresponding to 5 unit cells along x, y and z directions. The volume of simulation box is chosen equal to (50×50×50) Å3. The initial configuration is randomly generated with the number of hydrogen molecules, and the simulation boxes include 25 to 1000 hydrogen molecules. First, the initial configurations were minimized then the system was equilibrated for 100 ps followed by a 200 ps production run. The equations of motion were integrated by the Beeman method. The integration time step is 1fs. A Nose-Hoover extended system thermostat was used for the temperature control.
Results and Discussion
A monolayer of H2 on the external surfaces of FeTi is observed at low pressure. The first layer is filled completely by successive increase of H2 after that second, third, and higher layers are formed. Adsorption isotherms of H2 on FeTi for different number of molecular hydrogen at various temperatures and pressures were considered in this work.
From our results, the adsorption coverage for the first adsorption layer decrease by increase of temperature (Fig 1) and increase by pressure increasing (Fig 2).
Figure 1. Adsorption coverage as a Figure 2. Adsorption coverage as a
function of temperature. function of pressure.
The isosteric heat increases with coverage due to attractive interacting with neighbor atoms. The isosteric heat reaches a maximum at about 1660 J/mol for θ = 0.23 H2- FeTi.
462
After the maximum value, the heat of adsorption decreased to 1500 J/mol, due to the repulsive forces among adsorbed hydrogen molecules. When the binding energy is determined from the isosteric heat for low coverage, it found that the binding energy of H
2 on FeTi has a value of about 430 J/mol.
In summery amount of hydrogen adsorption indicates that FeTi has a good hydrogen storage capacity. This alloy has certainly desirable characteristics as an adsorbent due to its isosteric heat of adsorption and binding energy.
References
1. S. Jalili and R. Alizadeh, JICS. (in press)
2. J.W. Ponder, Tinker: software tools for molecular design, version 4.2, Saint Louis. Mo. 2001.
3. M. Jurczyk, L. Smardz, M. Makowiecka, E. Jankowska, K. Smardz, J. Phys. and Chem. Solids, 65 (2004) 545.
463
