Effect of the particle size evolution on the hydrogen storage performance of KH doped Mg(NH2)2 + 2LiH

Abstract

In recent years, many solid-state hydride-based materials have been considered as hydrogen storage systems for mobile and stationary applications. Due to a gravimetric hydrogen capacity of 5.6 wt% and a dehydrogenation enthalpy of 38.9 kJ/mol H2, Mg(NH2)2 + 2LiH is considered a potential hydrogen storage material for solid-state storage systems to be coupled with PEM fuel cell devices. One of the main challenges is the reduction of dehydrogenation temperature since this system requires high dehydrogenation temperatures (~ 200 °C). The addition of KH to this system significantly decreases the dehydrogenation onset temperature to 130 °C. On the one hand, the addition of KH stabilizes the hydrogen storage capacity. On the other hand, the capacity is reduced by 50% (from 4.1 to 2%) after the first 25 cycles. In this work, the particle sizes of the overall hydride matrix and the potassium-containing species are investigated during hydrogen cycling. Relation between particle size evolution of the additive and hydrogen storage kinetics is described by using an advanced synchrotron-based technique: Anomalous small-angle X-ray scattering, which was applied for the first time at the potassium K-edge for amide-hydride hydrogen storage systems. The outcomes from this investigation show that, the nanometric potassium-containing phases might be located at the reaction interfaces, limiting the particle coarsening. Average diameters of potassium-containing nanoparticles double after 25 cycles (from 10 to 20 nm). Therefore, reaction kinetics at subsequent cycles degrade. The deterioration of the reaction kinetics can be minimized by selecting lower absorption temperatures, which mitigates the particle size growth, resulting in two times faster reaction kinetics.