Effective participation of Li4(NH2)3BH4 in the dehydrogenation pathway of the Mg(NH2)2–2LiH composite

Abstract

Lithium fast-ion conductors have shown positive effects on the hydrogen storage properties of the Li–Mg–N–H system. In the present work, Li₄(NH₂)₃BH₄ doped Mg(NH₂)₂–2LiH was formed by milling the 2LiNH₂–MgH₂–0.2LiBH₄ composite and posterior annealing under hydrogen pressure to reduce the kinetic barrier of the Li–Mg–N–H system. The effect of repetitive dehydrogenation/rehydrogenation cycles on the kinetic and thermodynamic performance was evaluated. The dehydrogenation rate in the doped composite was twice that in the un-doped sample at 200 °C, while hydrogenation was 20 times faster. The activation energy decreases by 9% due to the presence of Li₄(NH₂)₃BH₄ compared to the un-doped composite, evidencing its catalytic role. The presence of Li₄(NH₂)₃BH₄ in the composite stabilized the hydrogen storage capacity after successive sorption cycles. Thermodynamic studies revealed a variation in the pressure composition isotherm curves between the first dehydrogenation cycle and the subsequent. The Li₄(NH₂)₃BH₄ doped composite showed a sloped plateau region at higher equilibrium pressure in regard to the flat plateau of the un-doped composite. Detailed structural investigations revealed the effective influence of Li₄(NH₂)₃BH₄ in different reactions: the irreversible dehydrogenation in the presence of MgH₂ and the reversible hydrogen release when it reacts with Li₂Mg₂(NH)₃. The role of Li₄(NH₂)₃BH₄ in improving the dehydrogenation kinetics is associated with the weakening of the N–H bond and the mobile small ion mass transfer enhancement.