Sunday, January 10, 2010

Carbon nanotubes for hydrogen storage

Hydrogen is considered to be the most promising alternative energy carrier in the global energy balance of the future. The use of carbon-based fossil fuels have caused measurable and catastrophic alterations to the earth's climate. It is widely hoped that the use of carbon-free energy carriers would reverse or decelerate the “greenhouse” phenomenon. Hydrogen is one such carrier. Although hydrogen possesses significant advantages, it also exhibits major drawbacks in its utilization. The most important one being its storage characteristics, which limits its application.

There are four major technologies for hydrogen storage: (1) compressed gas, (2) cryogenic liquid, (3) in the form of metal hydrides and (4) adsorbed on high surface area materials. The first two alternatives are hampered with problems related to tank volume, compression requirements, safety and loss due to evaporation. On the other hand, metal hydrides posses the disadvantages of large weight, excessive cost of manufacture and high temperatures of decomposition.

Adsorption of hydrogen on the surface of porous materials of high surface area could be a viable alternative for hydrogen storage with the potential to meet the capacity goals set by US DOE (6.5 wt.%) as well as the advantages of low weight and ease of desorption. Among the materials examined in this respect, carbon nanotubes (CNT) possess a prominent position.

It has been proposed that hydrogen can be absorbed on carbon nanotubes by physisorption and/or chemisorption. Physisorption occurs when hydrogen maintains its molecular structure and “it is trapped” in the CNT with Van der Waals forces. In chemisorption, atoms of hydrogen create chemical bonds with the carbon of the nanotubes. Many theoretical studies of the mechanisms of hydrogen adsorption on CNT have appeared in the literature. However, the precise mechanism of hydrogen adsorption on carbon nanotubes is not ascertained. Consequently, it is difficult to determine if hydrogen is adsorbed exclusively by physisorption or chemisorption also takes place.

Hydrogen can be stored on the inside area of CNT, shaping a cylindrical monolayer form or at the outer surface or between the nanotubes in the case of bundles of carbon nanotubes. Three different places of bonding have been proposed: above the carbon atom (top), in the middle of the C–C bonds (bridge) or in the centre of a hexagonal of carbons (hollow). Moreover, various configurations of hydrogen with respect to its interaction with the walls of the nanotubes are considered, namely perpendicular, longitudinal and transversal.

The discrepancies concerning hydrogen storage capacity of CNT are mainly attributed to: (a) The interaction potential models used to describe the gas-solid interaction, in the case of theoretical analysis. (b) Different experimental conditions employed, primarily temperature. (c) Different production methods of CNT and consequently different specific surface areas. For example, the CVD method seems to result in CNT of higher surface areas. (d) Different pre-treatment of CNT prior to adsorption (heating, vacuum). (e) Different configurations of CNT, such as tube diameters, tube lengths and inter tube spacing, which permit hydrogen either to move into the tube or to be confined in the interstitial pores. Also, if CNT are opened in the edge, they are able to adsorb hydrogen on both the inside and outside walls, whereas closed CNTs do not. (f) Different means of purification of CNT.

Read article by Ioannatos et. al.

Gerasimos E. Ioannatos and Xenophon E. Verykios, “H2 storage on single- and multi-walled carbon nanotubes”, International Journal of Hydrogen Energy, Volume 35, Issue 2, January 2010, Pages 622-628



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