Energy 2.0

Storage Of Hydrogen In Carbon Nanotube

Posted on: June 23, 2008

Many new methods use carbon as a storage medium and bring us a step closer to the widespread use of hydrogen as a fuel source. Scientists are using various approaches to shape carbon into microscopic cylindrical structures known as nanotubes.


The first method of producing nanotubes uses an electric arc to vaporize a metal-impregnated carbon electrode.


The second method uses a laser to vaporize a heated carbon target that has been treated with a metal such as nickel, cobalt or iron.


The third method is known as catalytic chemical vapor deposition (CCVD), and researchers at Washington University in St. Louis believe this is the most promising approach. In the CCVD technique, a heated metal element breaks down a hydrocarbon gas (such as methane, ethylene, acetylene, etc.) into carbon and hydrogen. The hydrogen gas is released while the carbon is extruded as a nanofiber. The advantage of CCVD is that it is a low-temperature technique and is suitable for large-scale production.


One of the critical factors in nanotubes’ usefulness as a hydrogen storage medium is the ratio of stored hydrogen to carbon. According to the US Department of Energy, a carbon material needs to store 6.5% of its own weight in hydrogen to make fuel cells practical in cars. Such fuel cell cars could then travel 300 miles between refueling stops.


Researchers at MIT claim to have produced nanotube clusters with the ability to store 4.2% of their own weight in hydrogen. In recent months, scientists from the National University of Singapore have released figures for nanotubes and nanofibers that can store 10-20% of their weight in hydrogen. These results, when combined with new car manufacturing technologies have the potential of transforming our transportation industries.Single-walled carbon nanotubes are remarkable forms of elemental carbon. Their unique properties have stimulated the imaginations of many scientists and engineers to propose a wide range of applications.


Nanotubes do have a dramatic visual Impact. If beauty rests on symmetry, nanotubes have inherent beauty. Further, their cylindrical structures led to suggestions that they would be ideal gas storage materials. The appearance of these potential storage materials conveniently coincided with the revivification of interest in the hydrogen economy. The potential for coupling carbon-based storage materials to supply pure hydrogen to automotive fuel cell power plants was quickly seen.


Initial reports of experiments showing high levels of hydrogen storage were encouraging. Theoreticians were then quick to calculate the possible amounts of hydrogen that could be stored using arrays of tubes of various sizes and packing parameters. Since the appearance of the initial reports, the results have been varied and controversial. Some are higher, some lower; some imply physisorption, and some chemisorption. It is clear that storage is a complex issue, partly because the, materials are more far complex than the visual comprehension of the single ideal nanotube would allow.


Studies have been conducted and it has been found that purified Multi walled carbon nanotubes (MWNT) can be used for bulk storage of hydrogen. Multi walled carbon nanotubes have been synthesised by catalytic decomposition of hydrocarbon using a floating catalyst method. The mean diameter of the MWNTs was found to be 5.1 nm.


The MWNTs are then purified and hydrogen storage techniques are used. It is found that the gravimetric hydrogen storage capacity of purified MWNTs is much higher than that of as-prepared one which means that purification process is very important for hydrogen storage. This could be attributed to the fact that there is more exposure to more surfaces of the multiwalled nanotubes. The ends were seen to be opened up. This allowed hydrogen to more easily move into the hollow core of MWNTs. XPS spectra of C1s of the purified sample is narrower and has no notable peak in the range of high electron binding energy. This indicates that the sample is in simple chemical state. This simple chemical state of C and lower oxygen contained groups correspond higher hydrogen storage capacity of carbon nanotubes.


There are many questions that must still be answered regarding nanotube hydrogen storage: How do we make process more efficient at lower temperatures in order to increase supply and decrease cost? What is the capacity loss with each storage cycle? Can other forms of carbon produce the same results just as effectively? What additional applications can increase demand and research into nanotubes?


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