(49a) Equilibrium and Kinetics of Hydrogen Storage in Nano-Palladium
A major challenge in the use of hydrogen as an energy source is efficient and effective storage of hydrogen. Among several potential approaches, high pressure gas cylinders and cryogenic liquid systems has been eliminated due to economic and technical reasons. It is doubtful that physical adsorption with high surface area solids will ever meet the performance requirements although it is still being considered. Chemical storage systems and metal hydrides are the most promising technologies. Chemical storage systems release hydrogen upon addition of water and are not easily reversible. While metal hydride systems based on absorption/dissolution of hydrogen in a metal crystal structure is reversible by reducing pressure and providing energy for desorption. Metal hydrides are the subject of this work.
Metal hydrides have been recognized a long time ago. Majority of hydrogen is absorbed/evolved during metal-to-hydride phase change. The storage performance of metal hydrides is determined by equilibrium; i.e. 1) P-T level or isotherms, and 2) the energy released during the phase change, and by kinetics. We have been pursuing nano-structuring of metals to manipulate its hydrogenation properties. In particular, we have concentrated on the common palladium-hydrogen system primarily since its bulk metal hydrogenation properties are well-documented.
In this paper, we report our results on equilibrium and kinetics of nano-structured palladium hydride system and compare it to bulk metal. There are very substantial changes in the shape of the isotherms making the nano-structured metal much more useful for storage purposes. More importantly the heat of sorption is reduced by four (4) fold substantially reducing heat transfer requirements in a storage application. These two important effects on the equilibrium may be somewhat unexpected. On kinetics, the nano-structured system is at least an order of magnitude faster as would be expected. These results clearly show a great potential from manipulating the particle size at nano-scale. Further evaluation of hydrogen storage feasibility needs to be performed for more realistic metals, larger quantities and total designs.