Hydrogen Storage Capacity Improvement of Nanostructured Materials
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Hydrogen storage capacity improvement of nanostructured materials Jeremy Lawrence and Gu Xu Department of Materials Science and Engineering, McMaster University, Hamilton, ON L8S 4L7, Canada
ABSTRACT Safe, lightweight, and cost-effective materials are required to practically store hydrogen for use in portable fuel cell applications. Compressed hydrogen and on-board hydrocarbon reforming present certain advantages, but their limitations must ultimately render them insufficient. Storage in hydrides and adsorption systems show promise in models and experimentation, but a practical medium remains unavailable. To study hydrogen storage properties a new volumetric testing apparatus was designed and constructed. Adsorption conditions are evaluated up to pressures exceeding 250 bar and a broad range of temperatures. RF sputtering was used to introduce metals to carbon nanotubes with the aim to enhance hydrogen storage. Here we show a significant improvement in the gravimetric storage density over that of as-prepared single-wall nanotube samples that may be due to the unique interface introduced.
INTRODUCTION The discovery of hydrogen adsorption on nanotubes initiated extensive research activity into this intriguing property due to the enormous potential market for safe, efficient hydrogen storage [1-5]. An effective hydrogen storage method will allow the widespread use of hydrogen powered fuel cells in portable electronics and transportation applications. Fuel cells run on hydrogen generate only water and heat by-products and therefore have an environmental advantage over other portable fuel types such as hydrocarbons, and batteries. Although hydrogen has a high energy density, its use is limited by its low mass density, even in the liquid state (71 kg/m3). High pressures are therefore required to achieve useful volumetric storage. Metal hydrides offer an alternative storage method, but are largely limited by their high relative mass. Another alternative is solid adsorbents, whereby the presence of a solid interface generates an attraction potential for hydrogen adsorption. This feature could be used to reduce the pressure required to achieve significant hydrogen storage. Modeling suggests that high surface area materials that are structured such that more than one surface potential is able to interact with a given adsorbed hydrogen molecule generate a larger attraction than planar surfaces [6,7]. Furthermore, the level of excess adsorption is expected to be highly dependent on the spacing of adjacent surfaces. A large number of adsorbents with varying pore size have been studied, and appear to support these findings [8]. Models also suggest that significant adsorption may only occur at low temperatures where the thermal energy of the gaseous hydrogen molecules is reduced and are therefore better trapped by the potential wells of surfaces [7]. There is some experimental evidence, however, to suggest that the behavior of some nanostructures, in particular W8.5.1
carbon nanotubes and nanofibers [9,10], deviate from unders
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