Bridging structure and dynamics: mechanistic insights into CMK-3 incorporated amide–hydride composite systems for solid-state hydrogen storage

Abstract

The influence of ordered mesoporous carbon (CMK-3) on the hydrogen storage properties of the 6Mg(NH₂)₂–9LiH–2LiBH₄ reactive hydride composite (6 : 9 : 2-RHC) is systematically investigated. Incorporation of 10 wt% CMK-3 into the pristine composite (6 : 9 : 2-RHC + 0.1CMK3) leads to substantial improvements in hydrogen sorption kinetics. Under identical thermodynamic conditions, the CMK-3 incorporated composite exhibits significantly enhanced reaction kinetics, with the absorption time reduced from ∼30 minutes to only 3 minutes (≈90% improvement) and the full desorption completed within 25 minutes instead of 60 minutes (≈58% improvement) compared to the pristine system. Moreover, the modified composite maintains excellent cycling stability, with 98% capacity retention over the first ten dehydrogenation/hydrogenation cycles compared to 90% for the pure composite. To elucidate the mechanism underlying this kinetic enhancement, we use a combination of advanced characterization techniques, including differential thermal analysis (DTA), synchrotron radiation X-ray powder diffraction (SR-XRPD), Fourier transform infrared (FT-IR) spectroscopy, small-angle and ultra-small-angle neutron scattering (SANS/USANS) and quasi-elastic neutron scattering (QENS). DTA analysis shows an 11 kJ mol⁻¹ decrease in apparent activation energy barrier upon CMK-3 addition. SANS/USANS results reveal that CMK-3 effectively suppresses particle agglomeration during the dehydrogenation/hydrogenation cycle and maintains structural integrity, preserving the high accessible surface area for long-term cycling stability. QENS measurements confirm that the incorporation of mesoporous carbon lowers the reorientational energy barriers of [BH₄]⁻ anions, leading to faster local dynamics. This enhanced mobility facilitates the elementary reaction steps involved in hydrogen release and uptake, which contributes to the improved sorption kinetics of the CMK-3 modified composite.