Hydrogen Storage in Nano-Materials

Why Nano materials for Hydrogen Storage

• Nano materials have many unique features among them the important ones for hydrogen storage are

1) Favourable surface to volume ratio

2) Geometrical features with appropriate active sites

3) Large container volume within the material like nano tubes, nano cones

Evidence of nano state Improving Hydrogen Storage

• The thermodynamics and Kinetics both are altered in the nano state.

• Nano-scale catalysis with proper dispersion can generate systems which will reduce the barrier for hydrogen absorptiion

• In the nano state one can generate new absorbing systems in the form of composites, functional nano-composites, or through nano-scale reactions

Experimental data to show that in the nano state Mg based alloys absorbs more hydrogen as compared to normal state

Effect of ball-milling of Mg2NiH4 on the desorption temperatureof hydrogen, measured by differential scanning calorimetry (heating rate 10 K/min). (a) As-hydrogenated; (b) after ball-milling of the hydride

Rate of hydrogen absorption by LaNi5. (a) Polycrystalline; (b) nanocrystalline; (c) nano-crystalline with catalyst

Rate of hydrogen absorption and desorption at 220◦C for a mixture of hydrides (65 wt.% of MgH2 and 35 wt.% of Mg2 NiH4) after ballmilling in the hydrogenated state

ACHIEVEMENTS

1. How nano-scale processing may be used in a generic way to enhance the performance of known hydrides and to develop entirely new systems. 2. Exploitation of these techniques is still in its early days, and one may expect in the next few years to see a continued refinement of metal hydrides, with more and more materials intermetallics, composites, MOFs and carbon materials being developed to meet specific performance criteria. 3. Furthermore, the underlying process – ball milling – is already used in industrial production, implying that upscaling the process to large volume should not present any insuperable obstacles.4. Newer methods using vapour deposition and sputtering technologies will be extensively used for generation of new nano materials for hydrogen storage applications.

Some Perspectives• Even though the enhancement of hydrogen absorption in the nano state is established

unequivocally, the extent of enhancement and if it will reach the level specified by DOE can be realized are still questions to be answered.

• The factors like surface to volume ratio and also residual surface tension forces are favourable for higher hydrogen absorption, the precise manner of operation and how to optimize these residual forces are not yet clearly delineated.

• . Hydrogen absorption itself is intriguing since it is the smallest atom, the storage is due to simple trapping by van der Waals forces or due to formation of specific bonds.

• The optimum size of the nano materials and also the extent of dispersion of nano catalysts have to be established with respect to individual species.

• In the case of carbon nano materials the hydrogen storage may have many other implications as stated previously since dimensionality and also possibility of carbon nano tubes, nano rods, nano cones all of them can present various \textbf{active sites} and thus show enhanced hydrogen absorption. These aspects have to be establiished unequivocally.

• Various levels of theory have been employed at the level of DFT to understand and rationalize the hydrogen absorption in variety of materials and these will also be intensified to formulate and design new materials.

• And many more