Pillared Graphene: A New 3-D Network Nanostructure for Enhanced Hydrogen Storage

Abstract

A multiscale theoretical approach was used to investigate hydrogen storage in a novel three-dimensional carbon nanostructure. This novel nanoporous material has by design tunable pore sizes and surface areas. Its interaction with hydrogen was studied thoroughly via ab initio and grand canonical Monte Carlo calculations. Our results show that, if this material is doped with lithium cations, it can store up to 41 g H 2 /L under ambient conditions, almost reaching the DOE volumetric requirement for mobile applications.Hydrogen is considered to be one of the most promising energy fuels for automobiles, and its use can be further extended to smaller portable devices, like mobile phones and laptops. It can be stored in either liquid or gas phase, provided that an efficient storage device exists. United States' Department of Energy (D.O.E.) has established requirements that have to be met by 2010, regarding the reversible storage of hydrogen according to which the required gravimetric density should be 6 wt % and the volumetric capacity should be 45 g of H 2 /L.1 By moving in that direction, the appropriate material for hosting hydrogen has to be developed.Initially, metal alloys, such as LaNi 5 , TiFe, and MgNi, were proposed as storage tanks since by chemical hydrogenation they form metal hydrides. Hydrogen can then be released by dehydrogenation of the hydride. Regarding vehicle applications, metal hydrides can be distinguished into high or low temperature materials. This depends on the temperature at which hydrogen absorption or desorption is taking place and if this is above or below 150°C, respectively. La-based and Ti-based alloys are examples of some low temperature materials with their main drawback as the very low gravimetric capacity (<2 wt %) they provide. 2,3 Controversially, high temperature materials like Mg-based alloys can reach a theoretical maximum capacity of 7.6 wt %, suffering though from poor hydrogenation/ dehydrogenation kinetics and thermodynamics. [3][4][5] In every case, metal alloys, besides the cost, are heavy for commercial production focused on mobile applications.On the other hand, light nanoporous materials can store hydrogen by physisorption that allows fast loading and unloading. However, as the interaction between H 2 and host material is dominated by weak van der Waals forces, only a small amount of H 2 can be stored at room temperature. In this case, high surface area and appropriate pore size are key parameters for achieving high hydrogen storage. Nanoporous carbon structures fulfill this criterion, placing them since the beginning among of the best candidates for hydrogen storage media. [6][7][8][9] After the synthesis of carbon nanotubes (CNTs), 10 the scientific research focused on their storage capability. Initial studies showed indeed that CNTs can be considered as a good material for reversible hydrogen storage, 11-13 but it was revealed later that under ambient conditions, pristine CNTs are not that promising.14-16 Further studies showed that doping CNTs with lithi...

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