Kinetic rate-limiting steps in dehydrogenation of Li–N–H and Li–Mg–N–H systems – Effects of elemental Si and Al

The study is focused on the (LiNH2 + LiH) and the (2LiNH2 + MgH2) hydrogen storage systems. Efforts were made to achieve an in-depth understanding of the dehydrogenation mechanisms following the introduction of elemental Si and Al to both systems. A variety of analytical instruments were employed, such as Temperature-Programmed Desorption, Mass Spectrometry, X-ray diffraction (XRD) and InfraRed spectroscopy. Unlike the effect of elemental Al in the (LiNH2 + LiH) system, which showed no promising improvement in the dehydrogenation, a significant kinetic improvement in the (LiNH2 + LiH) system was achieved upon addition of elemental Si. Kinetic improvement by elemental Si was described as a result of the LiH destabilisation through the formation of a Li2Si phase and an increase in H− anion concentration. On the other hand, once elemental Al is added to the (2LiNH2 + MgH2) system, the overall dehydrogenation kinetics of the system is delayed through the formation of a LiAl Phase. The results suggest that dehydrogenation mechanism in both the (LiNH2 + LiH) and the (2LiNH2 + MgH2) systems are identical. Dehydrogenation reaction starts with the electrostatic interaction of the oppositely charged hydrogen atoms in amide and hydride and proceeds by mass transfer of reactant species across the product layer at the later stage of the dehydrogenation. However, it was particularly identified that each system has a unique kinetic rate limiting step. Dehydrogenation kinetics seems to be controlled by the diffusion of the H− anion in the (LiNH2 + LiH) system but by the diffusion of the Li+ cation in the (2LiNH2 + MgH2) system.

► Elemental Si can effectively improve dehydrogenation kinetics in the Li–N–H system by destabilising LiH.
► Dehydrogenation rate-limiting step for the Li–N–H system is suggested to be the diffusion of H− through the dehydrogenation layer.
► Effect of Al suggests that the diffusion of Li+ cations controls the dehydrogenation kinetic in the Li–Mg–N–H system.