The Energetics of Hydrogen Adsorbed in Nanoporous Silicon. An ab initio Simulational Study
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0971-Z07-08
The Energetics of Hydrogen Adsorbed in Nanoporous Silicon. An ab initio Simulational Study Ariel A. Valladares1, Alexander Valladares2, R. M. Valladares2, and A. G. Calles2 1 Condensed Matter, Instituto de Investigaciones en Materiales, UNAM, Apartado Postal 70-360, Mexico, D.F., 04510, Mexico 2 Physics Department, Facultad de Ciencias, UNAM, Apartado Postal 70-542, Mexico, D.F., 04510, Mexico
ABSTRACT Porous silicon may be an interesting alternative to store hydrogen. Unlike carbon, its bonding multiplicity is limited, and because of this, the probability of having more dangling bonds on the pore surface is larger than in carbon. Using nanoporous silicon periodic supercells with 216 atoms and 50 % porosity, constructed with a novel ab initio approach devised by us, the dangling bonds of the silicon atoms were first saturated with hydrogen, then relaxed and its total energy calculated. Next the same number of hydrogen atoms was placed within the pore in the pure silicon supercell, then the sample relaxed, and finally its total energy calculated, with and without hydrogens. From these results the average energy per hydrogen atom is obtained. We compare our results to SiH bond energies and to previous results for hydrogenated carbon; conclusions are drawn concerning the possibility of using porous silicon as a fuel tank for hydrogen. Keywords: Porous silicon; hydrogen adsorption; hydrogen storage, INTRODUCTION The possibility of using porous materials as fuel tanks for hydrogen storage is a subject of much activity in recent years since hydrogen has been considered the fuel of the future, but its use in large scale applications has been hindered because of the difficulty and potential danger in the storage process. Two main lines are worth mentioning; one line moves along the synthesis of crystalline materials with large regular pores that can serve this purpose; the other consists in investigating the potential applicability of amorphous or crystalline porous materials like carbon or silicon. Recently the structure and catalytic activity of a crystalline highly porous silicogermanate was reported [1]. Corma and coworkers have synthesized this silicogermanate zeolite identified as ITQ-33 that could be a molecular sieve with 18 and 10 member rings. The zeolite obtained is very small, of the order of micrometers, and this could pose an obstacle for some applications. On the other hand adsorption of hydrogen in carbon has been a topic frequently referred to and worked on; not long ago it was the subject of an ample experimental investigation for different carbon nanostructures [2]. Our group has also looked at this problem and computationally analysed the energetics of hydrogen stored/adsorbed in the irregular pores of porous carbon [3][4]. Carbon has been in the forefront of this type of investigation and different techniques have been tried (and can be reviewed in Refs. 5-7) to develop an efficient and economically attractive way to store this element for fuel cell applications. The porous state of carbon
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