Tailoring Thermodynamics of Hydrogen Storage Materials

Hydrogen has the potential to serve as clean and versatile energy carrier, but efficient, reliable and safe storage systems have to be developed for a future hydrogen economy. Besides pressurized gas or liquid hydrogen tanks, solid state storage systems are a viable alternative. However, the materials investigated so far for solid state storage still show too low capacity at sufficiently low working temperatures to meet the targets set by the automotive industry. This is not necessarily due to the amount of stored hydrogen but thermodynamic and kinetic reasons restraining the application. Two alternative pathways to adjust thermodynamic properties are currently discussed: (i) restriction of particle sizes of the active material or (ii) the use of a reaction system introducing a second partner to alter the final products and thus the reaction enthalpy [1,2]. Among the materials with the largest amount of hydrogen stored are complex hydrides such as aluminohydrides (e.g NaAlH4) or borohydrides (e.g. Mg(BH4)2, LiBH4) or amides (Mg(NH2)2 + x LiH composites). They have in common that hydrogen is released in a solid state reaction and the desorbed state is comprised of two or (more) segregated solid phases. Upon rehydrogenation the opposite reaction has to take place and the reaction kinetics is mainly determined by mass transfer. That is the reason why initial investigations on free standing small particles of NaAlH4 suffered from insufficient reversibility. One possibility to circumvent this problem is the encapsulation of a complex hydride in a mesoporous host [3,4,5,6].JRC.DDG.F.2-Cleaner energ