The effects of halide modifiers on the sorption kinetics of the Li-Mg-N-H System

Abstract

The effects of different transition metal halides (TiCl 3 , VCl 3 , ScCl 3 and NiCl 2 ) on the sorption properties of the 1:1 molar ratio of LiNH 2 to MgH 2 are investigated. The modified mixtures were found to contain LiNH 2 , MgH 2 and LiCl. TGA results showed that the hydrogen desorption temperature was reduced with the modifier addition in this order: TiCl 3 > ScCl 3 > VCl 3 > NiCl 2 . Ammonia release was not significantly reduced resulting in a weight loss greater than the theoretical hydrogen storage capacity of the material. The isothermal sorption kinetics of the modified systems showed little improvement after the first dehydrogenation cycle over the unmodified system but showed drastic improvement in rehydrogenation cycles. X-ray diffraction and Raman spectroscopy identified the cycled material to be composed of LiH, MgH 2 , Mg(NH 2 ) 2 and Mg 3 N 2 .

Affects of Mechanical Milling and Metal Oxide Additives on Sorption Kinetics of 1:1 LiNH2/MgH2 Mixture

“…Both samples exhibited the same reduction in dehydrogenation capacity seen in our previous work [30]. This reduced capacity was attributed the reduction in diffusion kinetics for the transition metal halide modified mixtures to the formation of lithium salts, irreversible Mg 3 N 2 formation, the loss of essential ammonia and the agglomeration of particles during the high temperature dehydrogenation [30].…”

“…This reduced capacity was attributed the reduction in diffusion kinetics for the transition metal halide modified mixtures to the formation of lithium salts, irreversible Mg 3 N 2 formation, the loss of essential ammonia and the agglomeration of particles during the high temperature dehydrogenation [30]. However, in this experiment, the reduction kinetics can be attributed to the favorable reaction between LiH and N to form Li 2 NH [37], essentially hindering the formation of necessary intermediate steps.…”

“…As discussed in our previous publication [30] the unmodified Fritsch milled sample started to desorb hydrogen at approximately 135 °C during ramping from RT to 400 °C at with heating at a rate of 5 °C/min [30]. From the TGA results in Figure 3 and RGA data given in Figure 4A,B, hydrogen desorption for the Fritsch-milled unmodified material starts at approximately 135 °C with three hydrogen peaks at 205 °C, 260 °C, and 340 °C, indicating that the sample decomposed in three steps.…”

“…The following cycling processes were used: dehydrogenation at 200 °C for 6 h into 1 bar of H 2 back pressure, and rehydrogenation under 100 bar of H 2 at 180 °C for 6 h. After observing the significant formation of Mg 3 N 2 from dehydrogenation at 260 °C [30], the temperature was reduced to 200 °C to prevent ammonia loss. Previous investigations have shown that decreasing the temperature for hydrogenation/dehydrogenation effectively restrains the particle sizes of the samples, enhancing kinetics during cycling [29].…”

Recent Progress and New Perspectives on Metal Amide and Imide Systems for Solid-State Hydrogen Storage

“…Therefore, to improve the kinetics at temperatures lower than 200 • C, the reaction needed to be catalyzed with suitable dopants or additives. During the last years, many attempts have been made to improve the sorption kinetics of the 2LiNH 2 + MgH 2 system and, in this context, a large number of papers have been published [62][63][64][65][66][67][68][69][70][71][72][73][74]. Luo et al reported faster absorption kinetics by doping with less than 4 mol.…”