On some challenging avenues in hydrogen storage

Abstract

Hydrogen storage is a key enabling technology for the introduction of hydrogen economy and the use of hydrogen as energy carrier. The benchmark requires 6.5 weight percent hydrogen. Intermetallic compounds (AB, AB2, and AB5) can store hydrogen at ambient conditions with fast absorption/desorption kinetics (Ex. LaNi5, FeTi). But hydrogen storage capacity is limited to about 3 wt%. Light metal hydrides like magnesium and its alloys, alanates and borohydride show a high potential for reversible hydrogen storage because of their high hydrogen storage capacity (> 5 wt. %). However, kinetics of hydriding and dehydriding of Mg and its alloys is slow. The potential benefits of using alanates/borohydrides as hydrogen storage systems were overshadowed by the fact that the synthesis of alanates is a difficult process and the kinetics of the reaction used for storing hydrogen is slow. More over, high temperatures are needed to desorb stored hydrogen. Various attempts have been made in the past to facilitate (or even enhance the kinetic parameters or decrease the activation parameters) desorption process by addition of suitable materials. However, the role of these additives has to be carefully understood, since they do not only function as catalysts but have also to facilitate the transport of the species to the surface before desorption. In the present study, nanosized Mg2Ni alloy particles have been synthesized by polyol reduction method followed by annealing. The hydriding/dehydriding kinetics as well as electrochemical characteristics of this material were investigated. The effect of carbon materials (commercial carbons, carbon nanofibers, nitrogen containing carbon nanotubes) as additives to the LiAlH4 has been examined. Carbon nanofibers (CNFs) were prepared by catalyzed decomposition of gaseous ethylene on NiCu/H-ZSM-5. Nitrogen containing carbon nanotubes (NCNT) were synthesized by using PVP as carbon source and alumina membrane as template. These materials were characterized by XRD, SEM, TEM and FT-IR techniques. Dehydrogenation characteristics of LiAlH4 admixed with Vulcan XC72R, CDX975, Mesoporous carbon and CNFs were studied and compared with that of pure lithium aluminum hydride. Carbon nanofibers have shown prominent effect on the dehydrogenation properties of LiAlH4. On the basis of the dehydrogenation characteristics a possible mechanism for the improved dehydriding of LiAlH4 upon adding carbon materials has been considered. Effect of NCNT on the hydrogenation/ dehydrogenation properties of NaAlH4 and LiAlH4 was studied. Thermal decomposition properties of alanate-NCNT composites were examined by DSC analysis and in-situ DRIFT-IR studies. NaAlH4-NCNT composite was capable of taking reversibly 4 wt. %.of hydrogen. Effect of hetero atom (B and N) containing carbon material on hydrogen sorption properties was studied. Nitrogen containing carbon increases hydrogen absorption and desorption kinetics compared to boron containing and also results in the decrease of dehydriding temperature of MgH2 by 20 K. Cobalt based catalysts (Co, Co-Ni, Co-Cr, Co-Mo) were employed to study the hydrogen evolution reaction form alkali stabilized sodium borohydride. Among examined catalysts Co-Mo system is found to be highly active for the hydrolysis of sodium borohydride. The rate of hydrogen evolution was measured as a function of the concentration of NaOH, as well as the reaction temperature. The activation energies for hydrogen evolution reaction were calculated.