Storage Of Hydrogen In Carbon Nanotube

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.

Storage

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?

Is carbon trading an Easy Way out?

In the era of sustainable operations and growth momentums, emission legislations play an important role by being checks and balances of the system. The United States being the largest producer and consumer of energy has an effective role in this. But, with President Bush do away of federal monitoring systems and the Kyoto protocol; it has become the prerogative of individual states/organizations and the preachers of sustainability, to show some action in regulation of Greenhouse gas (GHG) emission through self regulation and advocation of best practice methods. State Governments for a sustainable economic growth have started imbibing strict emission regulations but the effects of regulation has been interpreted by authors on one end as a driving force for innovation and efficacy in industries; but on the other as an instigating tool of increased cost.

Action in terms of state based federal self-regulation and emission control is presently through the Chicago Climate Exchange (CCE). The mission and vision of CEC is to provide members both from private and public sectors with cost-effective methods for reducing their GHG emissions through building and operating a market-based emission reduction and trading program. The member group operates on a cap-and-trade system at trading prices between $1.15 and $1.35 per Carbon Financial Instrument (CFI) with the base year as 2003. Compliance is through internal reductions, purchase of allowances by members facing emission limitations from others, or purchase of credits from Emission Reduction (ER) projects that meet specific criteria.

But, emission reduction programs / tradables are not “region specific”; by that we mean ER programs are not run at the point of pollution but at “convenient locations” and so are tradables. It can be better explained by the following example (Figure 1). Consider, a Thermal power plant A with X amount of excess over emission regulation; it has the option of,
a) Buying the emission tradable certificates for the excess X (or)
b) Investing in a ER project / renewable portfolio standard (RPS) to provide a sink for excess X
The problem with the scenarios depicted is, they are not region specific; that is, the tradeables or the ER program can be bought/done from any member with no criteria on region of actual pollution control, thus providing no benefits for the stakeholders in the pollution region. This is like “outsourcing” the Pollution Control instead of regulating it at point of pollution.

The future scenario should involve
a) Providing subsidiary and Tax incentives to innovative preventive pollution systems, rather then providing subsidiary across the board for RPS systems.
b) Broader system scope involving pollution abatement technologies executed at point of pollution with stakeholders involvement
c) Basic GHG policy agreeable to all regions at the national level to create universal standards of operation.

As Thomas. L. Friedman puts it as the movement of the herd in his book, “Lexus and the Olive Tree”; one section is looking in to how to abate pollution through innovative technologies and sustainable policies (the Lexus) while, others are involved in stagnant polices involved in subsidized RPS and outsourced carbon sequestering.

– Pramodh Panchanadam (2005)