Abstract:Hydrogen has attracted people because of its cleanest, sustainable and renewable energy carrier with a remarkably reduced impact on the environment. In the future, hydrogen will replace the existing fossil fuels. Although it has number of advantages, it creates some difficulties in utilization. The most famous of them are production and storage. Carbon nanotubes (CNTs) shows a good adsorbent material as a high and reversible hydrogen capacity.
This paper will broadly discuss the carbon nanotube production and storage of hydrogen in carbon nanotubes.IntroductionNowadays researches have been focusing on main two problems: Global warming(environment) and energy sources. The environment is polluted due to emission of vehicles. Due to this problem it attracted to think about other energy resources1. It is widely expected that carbon free use of fuels will decrease the environment problems. That is why sustainable energy use exponentially increase in coming times. Hydrogen is the cleanest, sustainable and renewable energy carrier with remarkably reduced impact on the environment.
Hydrogen gas is cleanest, highly abundant and non-hazardous renewable fuel.2 It releases water vapour into environment when it burns. It has larger chemical energy per mass (142 MJ) compare to other hydro carbon fuel. In term of the efficiency hydrogen has 60%, gasoline has 22% and diesel has 45%.
1–22 Though we are having this much benefits of hydrogen we are facing two main problems like production at low cost and storage. It’s in explosive in nature and larger in volume are two main issue for storage and transportation purposes. The new forms of carbon ke Carbon nanotubes have attracted people due to its properties. Carbon nanotubes found by Iijima 7 in 1991 as a hollow tubular structure of nanometer diameter rolled with graphite plates with having high length to diameter ratio.
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It is kind of similar to honeycomb arrangements of carbon atoms in graphite sheets. Carbon nanotubes (CNTs) can be divided into two parts Single walled carbon nanotubes (SWNTs) and multi walled carbon nanotubes (MWNTs). SWNTs are made up of consisting seamless graphene sheet and have several microns length. A MWNTs are made up of different thickness layer of graphite sheets. On the basis of rolling CNTs further divided into three types 1) armchair, 2) Zigzag and 3) chiral types (fig 1.
). Among all nanotubes armchair nanotubes are truly metallic while other tubes the zigzag is narrow gap and chiral are wide gap semiconductors.13 Here its shown the schematic diagram of single walled carbon nanotubes and multiwalled carbon nanotubes.
Fig 1. Schematic of Different SWNTs: (a) armchair, (b) zigzag (3) chiral Fig 2. Schematic of MWNT.
3. Production of Hydrogen Everyday the demand of hydrogen is increasing so new routes of hydrogen production must be found. Nowadays there are four traditional methods to produce the hydrogen: 1) Water electrolysis 2) Oxidation reaction of heavy oil 3) CH steam reforming reactions and 4) Gasification reactions. Among all these process the best renewable and eco friendly procedure is water electrolysis. In this method the water splitting method is very famous nowadays but it is in initial stage2.
4. Storage of Hydrogen Safety, effective and cheap in cost are major problems in storage of hydrogen. Ongoing result is focused on these problems.
There are few major technologies for hydrogen storage for example Cryogenic liquid(liquefaction), Compressed gas, form of metal hydrides, Underground storage , Adsorbed on high surface area materials e.g. metal-organic frame works (MOFs) and electrochemical hydrogen storage.13 The compressed gas and cryogenic liquid, these two alternatives are either not safe or expensive and it has some practical difficulties in storing technique related to tank volume, transportation and waste. Atomic hydrogen can be stored best by metal hydride. Compare to hydrogen gas or liquid hydrogen the storage density is higher in Metal hydrides. 6 Among all the methods, electrochemical hydrogen storage is very convenient.
In this method hydrogen is adsorbed in that hydrogen storage materials during electrochemical decomposition of an aqueous medium.6 A.C.
Dillon 3 measured the Hydrogen adsorption capacity at temperature 133 K with SMNTs. They plotted the value and absorptivity of hydrogen was 5 to 10 weight % and predicted. Fig 3. Hydrogen adsorption (top view) in (5,5) carbon nanotube with ball and stick forms (a) clean (b) arch type, (c) zigzag type (d) molecular hydrogen inside CNT 21 Adsorption Geometry To observe the geometry Seiung Mi Lee21 and his colleague analysis the various hydrogen adsorption in CNTs.
As we can see in fig 3a, it is a carbon nanotube with diameter of 6.88 Å and the average length between C-C bond is 1.44 Å.
Hydrogen ions are actively available in electrolyte in electro chemical process. On top sites of carbon atoms on tube well covered with that hydrogen ions and will form an arch-type fig 3b. Here the hydrogen absorptivity observed was 7.74 wright % and the C-C bond length has been enlarged and noticed that 1.54 Å whereas the C-H bond length is 1.
12 Å. In the zigzag type CNTs, the stable geometry was shown in fig 3c. Figure 3d also indicate another stable geometry in which the molecular hydrogen are kept inside the CNTs.
