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Effect of Nano-Technology in Hydrogen Storage

A new era of Hydrogen fuel storageR.Siva kumar1, T.Vijayprathap2.

B.E.Mechanical(III-Year), Sona College of Technology,Salem-5.

Contact no: 8526899442, Email id: kumarrsiva35@gmail.com1Email id: nationvijayprathap@gmail.com2.

Abstract

Hydrogen is the most abundant element is the universe on the Earth its mostly found as water. Hydrogen can easily separated from Oxygen in water via Electrolysis. This process is about 67% efficient. The alternative is to store hydrogen in a solid metal by forcing it into the gaps between the atoms. By a strange property of physics this actually holds more hydrogen in a smaller volume than possible in liquid form. For this purpose the lightest element Titanium is used in our study.

Properties:

As a liquid its energy density per unit volume is 1000 times higher.

Hydrogen is the lightest of the elements with an atomic weight of 1.0. Liquid hydrogen has a density of 0.07 grams per cubic centimeter, whereas water has a density of 1.0 g/cc. and gasoline about 0.75 g/cc.

When hydrogen is burned in air the main product is water. Some nitrogen compounds may also be produced and may have to be controlled.

Recent Development:

Japanese scientists are working on a method of splitting water using laser light beamed from a satellite in orbit.

The alternative is to store hydrogen in a solid metal by forcing it into the gaps between the atoms. By a strange property of physics this actually holds more hydrogen in a smaller volume than possible in liquid form. For this purpose the lightest element Titanium is used in our study.

Literature Review Use of Hydrogen as Fuel:

It is believed that hydrogen will within a few years become the fuel that powers most vehicles and portable devices. The reason is the depletion of oil and the relatively facile production of hydrogen from various renewable source of energy- hydroelectric, wind, solar, geothermal- with water being the only raw material needed. To release the energy, hydrogen can be burned in an efficient and clean way in a fuel cell to form water again, or made to drive an electrochemical cell as in the commonly used nickel hydride battery. As concerns over air pollution and global warming increase, the incentive to switch to clean and efficient hydrogen economy becomes grater and the transition may occur well before oil reserves are depleted. While hydrogen has many inherent advantages, there remains a problem with storage and transportation. Pressurized hydrogen gas takes a great deal of volume compared with, for ex, Gasoline with equal energy content- about 30 times bigger volume at 100 atm gas pressure. Condensed hydrogen is about ten times denser but is much too expensive to produce and maintain. There are also obvious safety concerns with the use of pressurized or liquefied hydrogen in vehicles. Metal hydrides as Hydrogen storage devices:

Metals can absorb hydrogen in atomic form and thereby act as hydrogen sponges. Around 50 metallic elements of the periodic table can absorb hydrogen in a great quantity and the possible choices of hydrogen storage materials are, therefore, enormous. Many scientific and engineering studies have been carried out of the absorption/desorption of hydrogen in metals and development of such hydrogen storage devices. Daimler-Benz, for example, produced in the early 1980s a car fuelled by hydrogen where the storage tank was a chunk of Fe-Ti metal alloy. The volume of this storage device is less than a factor of two greater than the equivalent gasoline tank, but the problem is that hydride is 20 times heavier. The only successful commercial large scale application of metal hydrides as hydrogen storage so far is the metal hydride battery, which has supplied battery power to many small electrical appliances such as mobile phones and portable computers. Metal hydrides have so far not become useful as storage devices for hydrogen gas even though they have some distinct advantages over pressurized hydrogen gas, both improved safety and reduced volume.

Mechanical Alloying;

Mechanical Alloying (MA) is a high-energy ball milling technique; in which elemental blends are tend to achieve alloying at the atomic level. In addition to elemental blends, pre-alloyed powders and ceramics, such as oxides, nitrides, etc., can also be used to produce alloys and composites by these techniques. This technique was developed around 1996 by Benjamin and his co-workers as a part of the program to produce oxide dispersion strengthened (ODS) NI-base super alloys for gas turbine applications. The initial experiments by Benjamins group were aimed at coating the oxide particles with Ni by ball milling. Such a process was known 40 years earlier from the work of Hoyt, who reported coating of WC with Co by ball milling. In 1966, Benjamins group to the production of alloy turned attention by high-energy ball milling. The first experiments on this direction were on the production of thoria dispersed nickel (commonly known as TD nickel) and NI-Cr-Al-Ti alloy with Thoria dispersions. The success of both these experiments led to the first patent on this process. The process was initially referred to as milling/mixing. The term mechanical alloying was actually coined by Ewan C. McQueen.

Experimental work

TiH2 in the powder form is taken as the starting material and is milled in a ball mill under dry conditions for various time intervals (2h, 5h, 7h and 10h) with Ball to Powder ratio of 20:1. Heat treatment is carried out in a muffle type furnace for 1h in each case. Characterisation of the metal hydride is done using XRD and DSC.

X-ray diffraction results of as received and milled samples are shown in figure 1. XRD results confirmed the as received sample as TiH2 as in Fig 1. Even up to 10h of milling at BPR ratio of 20:1 there is no formation of new phase except the decrease in the intensity of TiH2 peaks with the addition of tungsten carbide peaks as impurity. Hence DSC is carried out with the milled and the as received samples. XRD plot of unmilled and milled heat-treated samples are shown in figure 2, which clearly indicates that the dehydrogenation process is a two-step process. TiHx forms at lower temperatures and the formation of Ti shows the end of the dehydrogenation process. The peak transformation temperatures are well separated in milled samples when compare to unmilled samples and the separation increases with milling time. The peak temperatures decrease with increase in the milling time.

The observed effects of milling of the hydride are due to the decrease in dehydrogenation temperatures and separation between the peak temperatures. The separation in the peak transformation temperatures is clearly observed from the DSC plots shown in Figure 3. The peak transformation temperature for the (-Ti transformation and the TiHx transformation is given in Table 1. Table 1 indicates that the peak transformation temperature for (-Ti transformation in the unmilled and 10h milled powder differs by 65(C whereas the peak transformation temperature for TiHx transformation differs by a margin of 170(C. The TiHx thus formed as a result of the first stage dehydrogenation is more stable when compared to TiH2.

A possible explanation for the decrease in the first transformation temperature in the ease of decomposition of the high hydrogen containing TiH2 phase due to decrease in the particle size, increase in the specific surface area and increase in the defect concentration due to cold work.Figures

Fig 1. XRD plot of milled TiH2 without Fig 2.a.XRD plot of unmilled TiH2Heat treatment heat treated at diff. temp.

Fig 2.b. XRD plot of milled TiH2 (10h)

Fig 3. DSC plot of TiH2 heat treated at diff. temp

Table 1 Peak transformation temperatures for TiHx and (-Ti formation

Hours of MillingPeak Temperatures of TiHx Transformation ((C)Peak Temperatures of (-Ti Transformation ((C)

0528625

2440602

5366591

7360582

10355560

Discussion

Nano TiH2 used as fuel should be first ignited to 355C and the exothermic reactions thus produced will ignite the remaining TiH2. It functions as an auto catalytic mechanism. This ignition can be brought about very easily by the passage of electricity because Hydrogen can be easily dissociated from Titanium. This dissociated hydrogen is immediately used for propulsion of the engine and hence the safety hazard is eliminated. Why TiH2 and why not cheaper MgH2?

The automobile industry has set 5% by weight as a target for efficient hydrogen storage in metals. The hydride, MgH2 can store upto 7% by weight (approx.) and TiH2 can store upto 5% by weight of hydrogen (approx.). Hence it is apparent that both TiH2 and MgH2 can store hydrogen for automobile industries. But the problems encountered with MgH2 are that

The rate of absorption and desorption of hydrogen is too low

Hydrogen molecules donot readily dissociate at the surface of Mg

Hydrogen atoms bind too strongly with magnesium atoms

The above problems can be overcome by using TiH2.

The enthalpy of formation of TiH2 from Ti and hydrogen is lower when compared to Magnesium.

Hydrogen molecules readily dissociate at the surface of Titanium, being an active metal

Bonding of hydrogen with Titanium is strong only after dissociation of 50% of hydrogen (i.e., after the formation of TiH0.7-1.1 depending upon the dissociation process)

Why TiH2 and why not cheaper AlH2?

In case of Aluminium the formation of hydride is too difficult comparing with the hydride formation in Titanium. Because the rate of formation aluminium hydride from aluminium and hydrogen is lower than that of TiH2. Again the dissociation of hydrogen is difficult in the case of aluminium.

Oxide formation is more facilitated in Aluminium, than the formation of hydride. As oxygen will be deposited at the surface of aluminium, the reactivity of aluminium is minimum at the surface than at the core portion and hence hydride formation is more difficult.

Hence by comparing TiH2with light materials, it is shown that TiH2 is well suited for hydrogen storage. This serves as a big break through in the usage of hydrogen as a potential source of fuel.Conclusion

Present situation requires immediate care to be taken for the problem of environmental pollution caused by the combustion of fuels. Using of Hydrogen gives a major advantage of creating a greener environment since the end product of combustion is water that can be used to produce hydrogen again.Major results of this research work reveal the following datas.

1) TiH2 dissociation occurs at some 200C earlier for nano particles when compared with conventional particles which facilitates the use of TiH2 in the storage of hydrogen.

2) The dissociation oh hydrogen is easier in the nano level when it is produced by mechanical milling because of the two following reasons.

a) A part of the energy is supplied by the process of mechanical activation.

b) Enthalpy formation is also facilitated by the process of mechanical

activation.Hence TiH2 can be used as a good source of hydrogen storage device which facilitates the usage of hydrogen as a fuel. Reference

www.nano.org.uk www.nanowerk.com www.hydrogen.energy.gov www.interscience.willy.com_1329835906.bin

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