Improvements in the hydrogen storage properties of the Mg(NH2)2–LiH composite by KOH addition

Abstract

Potassium-containing compounds, such as KH, KOH, KNH₂ and different potassium halides, have shown positive effects on the dehydrogenation properties of the Li–Mg–N–H system. However, it is still discussed whether the K-compounds modify the thermodynamics of the system or if they have only a catalytic effect. In this work the impact of the addition of two K-containing compounds (0.08 mol% of KCl and KOH) on the hydrogen storage performance of the Mg(NH₂)₂–LiH composite was studied. The KOH incorporation reduced the dehydrogenation temperature from 197 °C to 154 °C, beginning the process at low temperature (∼70 °C). The doped sample was able to reversibly absorb and desorb 4.6 wt% of hydrogen with improved kinetics; dehydrogenation rates were increased four times, whereas absorptions required 20% less time to be completed in comparison to the pristine material. The thermodynamic destabilization of the Mg(NH₂)₂–2LiH composite by the addition of a small amount of KOH was demonstrated by an increment of 30% in the dehydrogenation equilibrium pressure. According to detailed structural investigations, the KH formed by the KOH decomposition through milling and thermal treatment, can replace LiH and react with Mg(NH₂)₂ to produce a mixed potassium–lithium amide (Li₃K(NH₂)₄). The KH role is not limited to catalysis, but rather it is responsible for the thermodynamic destabilization of the Mg(NH₂)₂–LiH composite and it is actively involved in the dehydrogenation process.