Experimental Studies Of Hydrogen Storage By Carbon Nanotubes Cnts

Of the problems to be solved for the utilization of hydrogen energy, how to store hydrogen easily and inexpensively, has been given high priority. A vehicle powered by a fuel cell would currently require ~3.1 kg of hydrogen for a 500 km range. This amount of hydrogen stored in the weight and volume of a typical petrol tank (50 L) requires system densities approaching 6.5 wt% or 63 kg H2/m3 [19]. Therefore, the U.S. Department of Energy Hydrogen Plan has set this as the standard for providing a commercially significant reversible hydrogen storage technology [7]. No storage technology is currently capable of meeting these goals [17]. Meeting this target requires major advances in storage density, and the energy required for storage without compromising safety and cost. Most of these issues would be resolved by a lightweight material capable of reversibly storing and releasing hydrogen in a modest range of near-ambient temperatures and pressures. Novel lightweight carbon nanotubes were seen to be an ideal candidate as it was assumed that hydrogen molecules could be physisorbed both inside the tubes and in the interstitial pore spaces between the tubes.

A large number of hydrogen adsorption experiments have been conducted to measure the storage capacity of carbon nanotubes. At the same time, many researchers have calculated the adsorption performance of the carbon nan-otubes by means of various models and theories. Here, we first summarize the recent experimental and theoretical results, then briefly discuss the interaction between hydrogen and carbon nanotubes for elucidating the hydrogen adsorption nature within carbon nanotubes. Currently, there are two major methods of hydrogen storage in carbon nan-otubes being studied: gaseous storage and electrochemical storage.

The three main methods for the preparation of carbon nanotubes are electric arc discharge, chemical vapor deposition, and laser ablation. All of these methods rely on creating highly energetic carbon clusters that condense to form carbon materials such as fullerenes and nanotubes. Recent advances have been to use metals as condensation catalysts that template and promote certain types of materials. The prepared nanotubes can then be purified and/or treated in some manner to open the nanotube ends. It is interesting to note that high-capacity room-temperature adsorption was first demonstrated for arc-generated SWNTs, and not laser-produced nanotubes. This may be attributed to a much smaller number of ends or defects in the laser produced tubes and/or an enhancement in their stability toward opening procedures or cutting [1]. Significant research has been undertaken to improve synthesis methods, and to more accurately characterize the unique mechanical and chemical properties of carbon nanotubes. A discussion of these topics is outside the scope of this chapter, but the reader can be directed to a wealth of related literature [20-25].

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