Assignment-2
Hydrogen storage
Introduction
Hydrogen, the most abundant element in the universe, has great potential as an energy source. It is non-polluting, and forms water as a by-product during use. Hydrogen storage includes physical storage via compression or liquefaction, and storage in materials via reversible sorption processes or chemical reaction. Metal hydrides, Chemical hydrides and Carbon based materials are recent research topics for hydrogen storage.
Metal hydrides
The hydrogen storage alloys in common use occur in four different forms: AB5 (e.g., LaNi5), AB (e.g., FeTi), A2B (e.g., Mg2Ni) and AB2 (e.g., ZrV2).Research is being conducted to modify the compositions of these base alloys by alloying with various other elements. Such modifications can enhance their stability during cycles of charging and discharging, allow them to undergo cycles at ambient pressure and temperature, increase their hydrogen storage capacity, and increase their hydrogen absorption/desorption rate.
Sodium aluminohydride (NaAlH4) is presently the most promising reversible complex hydride for hydrogen storage. It has a reversible storage capacity of 5.5 wt. %, whereas its total hydrogen content amounts to 7.4 wt. %. Reversible thermal dissociation occurs in two steps

3NaAlH4 ⇄ Na3AlH6 + 2Al + 3H2 (1) Na3AlH6 ⇄ 3NaH + Al + 1.5H2 (2)
The decomposition of NaH, is possible at much higher temperatures.

Doping agents and methods
Titanium trichloride (TiCl3) is generally the standard doping agent for NaAlH4. The doping of NaAlH4 with TiCl3 is expressed as- 3NaAlH4 + TiCl3 → 3Al + Ti + 3NaCl + 6H2 (3)
Titanium and aluminium are both reduced to the zero-valent state by the hydride species, which results in irreversible hydrogen evolution. For this reason, apart from the additional dead weight of the dopant, the theoretical capacity of 5.5 wt. % cannot be attained with doped samples. Therefore, the optimum doping amount is always a compromise. The more dopant that is added, the higher are the kinetic rates, but the lower is the capacity. Usually, doping levels of 2–4 mol% are considered as reasonable. Apart from the doping procedure, also the nature of the titanium precursor is important for the performance of the material. Compounds of elements other than titanium have shown good prospects as doping agents; ScCl3 and CeCl3 are prime examples.
Although the location of the titanium and its oxidation state in the system has been investigated very thoroughly, the mechanism of the facilitated hydrogen uptake and release has yet to be fully clarified. The first problem in understanding the processes occurring is the spatial distance between the active species, the Ti–Al solid solution and the hydrogen-storage material (NaAlH4).
Conclusion
The advances in the research on complex metal hydrides for hydrogen storage over the last couple of years are remarkable. The properties of some systems are approaching the requirements for large-scale applications. Doped NaAlH4 is still the most promising material to meet most of these targets. It decomposes sufficiently quickly to provide adequate hydrogen during all operation states of the fuel cell, which include peak power demand.