Development of High Storage Capacity Complex Hydrides for Reversible Hydrogen Storage

Abstract

In this study, novel light weight complex hydrides for reversible hydrogen storage are developed and synthesized. Metal hydrides are generally known to have a good hydrogen storage capacity but possess slow kinetics, irreversibility and high activation energy barriers. Therefore, metal hydrides often require very high temperature (>350° C) to release hydrogen. This behavior makes their wide usage in hydrogen storage applications difficult. We have extensively employed a unique synthesis approach of solid state mechano-chemical process, via high energy ball milling. From this synthesis process, eight new complex hydrides have been prepared and each of them are characterized using high pressure Sievert's type volumetric apparatus, residual gas or mass spectrometric analysis (RGA/MS), and the Fourier transform infrared spectroscopy (FT-IR) to determine their chemical and hydrogen sorption properties. The as-synthesized new complex hydrides with light weight elements such as lithium, magnesium, boron, aluminum etc. have demonstrated excellent physical and chemical behavior in terms of lower activation energies, higher reversible hydrogen storage capacity (i.e., ≥6wt%) at operating temperatures below 350° C, and faster reaction kinetics than their conventional metal hydride counterparts. Unfortunately, the residual gas analysis of these materials have revealed the evolution of some undesirable and toxic gases such as ammonia (NH3) and di-borane (B2H6) in addition to the release of hydrogen. The release of these toxic gases was eventually suppressed by catalysts added and destabilization proce- dures are mentioned explicitly. Among the various complex hydrides developed, two novel systems namely, Li-nMg-B-N-H and BNH6-nMgH2 have shown distinguishing properties as efficient reversible hydrogen storage materials.