Computacional study of hydrogen storage in carbon-based materials

A. SIGAL; M.I. ROJAS; E.P.M. LEIVA

La Plata

Congreso; Humboldt Kolleg, Internacional Conference on Physics; 2011

Internacional Conference on Physics

<!-- @page { margin: 2cm } P { margin-bottom: 0.21cm } --> The future of hydrogen economy strongly depends on the development of hydrogen storage systems for vehicle applications. In this respect, carbonaceous materials appear very attractive, since they provide a light, high surface area and low- cost option. In recent times, the addition of metal atoms (transition, earth-alkali) has been suggested as a way to increase their capacity for hydrogen storage, since these materials alone do not meet the targets suggested by organizations like the Department of Energy of USA (DOE). In the present work we analyze by means of density functional (DFT) calculations the possibility of hydrogen storage in carbonaceous materials decorated by metals, as well as the role placed by interferents present in the air. Particularly important is the case of oxygen, since recent studies show that this molecule appears as the main competitor and inhibitor for hydrogen adsorption [1, 2]. As a model system, we consider a graphene layer decorated by Ni, Ti, Cu, Pt and K atoms, among others. Local minima are determined by the technique of conjugate gradients, as implemented in the SIESTA [3] computer code. The reaction coordinate for different reactions (oxidation, hydride formation) is obtained using the nudged elastic band method [4]. Some of the difficulties found for the experimental implementation of these storage media can be understood from the present results. In the case of the Ti decoration, it is found that oxygen adsorbs spontaneous and irreversibly on the Ti adatom, strongly inhibiting hydrogen adsorption. In the case of the Ni decoration, the results are more encouraging but oxygen still represents an important hindrance for hydrogen storage. The entropic contribution for hydrogen adsorption is also analyzed, showing that it also deliver an important counterbalance to the favorable hydrogen adsorption energy. [1] M.I. Rojas, E.P.M. Leiva, Phys. Rev. B 76 155415 (2007). [2] A. Sigal, M.I. Rojas, E.P.M. Leiva, Inter. J. of Hydrogen Energy (2011), doi:10.1016/j.ijhydene.2010.12.024 [3] J. M. Soler, E. Artacho, J. D. Gale, A. García, J. Junquera, P. Ordejón, D. Sánchez-Portal, J. Phys.: Condens. Matter 14 2745 (2002). [4] G. Henkelman, B.P. Uberuaga, H. Jonsson, J. Chem. Phys. 113 (2000) 9901-04. G. Henkelman, H. Jonsson, J. Chem. Phys. 113 (2000) 9978-85.