Tuning the reaction mechanism and hydrogenation/dehydrogenation properties of 6Mg(NH_2)_2-9LiH system by adding LiBH_4

摘要

The hydrogen storage properties of 6Mg(NH2)(2)-9LiH-x(LiBH4) (x = 0, 0.5, 1, 2) system and the role of LiBH4 on the kinetic behaviour and the dehydrogenation/hydrogenation reaction mechanism were herein systematically investigated. Among the studied compositions, 6Mg(NH2)(2)-9LiH-2LiBH(4) showed the best hydrogen storage properties. The presence of 2 mol of LiBH4 improved the thermal behaviour of the 6Mg(NH2)(2)-9LiH by lowering the dehydrogenation peak temperature nearly 25 degrees C and by reducing the apparent dehydrogenation activation energy of about 40 kJ/mol. Furthermore, this material exhibited fast dehydrogenation (10 min) and hydrogenation kinetics (3 min) and excellent cycling stability with a reversible hydrogen capacity of 3.5 wt % at isothermal 180 degrees C. Investigations on the reaction pathway indicated that the observed superior kinetic behaviour likely related to the formation of Li-4(BH4)(NH2)(3). Studies on the rate-limiting steps hinted that the sluggish kinetic behaviour of the 6Mg(NH2)(2)-9LiH pristine material are attributed to an interface-controlled mechanism. On the contrary, LiBH4-containing samples show a diffusion-controlled mechanism. During the first dehydrogenation reaction, the possible formation of Li-4(BH4)(NH2)(3) accelerates the reaction rates at the interface. Upon hydrogenation, this 'liquid like' of Li-4(BH4)(NH2)(3) phase assists the diffusion of small ions into the interfaces of the amide-hydride matrix. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.