Title

Development of nano-engineered powders of LiNH2 + LiH for solid state hydrogen storage

Date of Completion

January 2009

Abstract

Solid-state hydrogen storage offers the potential of storing hydrogen onboard vehicles in a volumetrically dense fashion at modest temperatures and pressures. The LiNH2 + LiH system has a 6.5 wt% H2 capacity; however, ammonia emission and kinetic issues currently limit the applicability of this material for onboard storage. These limitations are addressed by using high-energy ball milling to generate powders with crystallite sizes near 20nm and specific surface areas more than 10× larger than commercially available powders. This milling process also causes tremendous mixing of the two phases, thereby preventing the escape of ammonia (an intermediate species of the dehydrogenation reaction) from the system. The kinetics of the dehydrogenation reaction have been analyzed and exhibit diffusion controlled behavior. By developing a low temperature milling technique, the defect concentration in the milled particles is shown to increase, which in turn increases the diffusion constant of NH3 through the Li2NH product layer by 450%. Although this provides a 41% increase in the effective reaction rate, the microstructure generated by low temperature milling would typically not be stable at operating temperatures of 285°C (0.9 Tm of LiNH2). However, due to the fine mixture of independent phases and the low packing density caused by ball milling, substantial microstructural evolution is inhibited. As a result, the kinetic improvements gained from the nano-engineering approach are stable through 60 charge and discharge cycles (approximately 200 hours at 285°C). The significance of this stability extends beyond the LiNH2 + LiH system as stability of nanostructures is a key element in many solid-state hydrogen storage systems. ^