Bridging Structure and Dynamics: Mechanistic Insights into CMK-3 Incorporated Amide-Hydride Composite System for Solid-State Hydrogen Storage

Abstract

The influence of ordered mesoporous carbon (CMK-3) on the hydrogen storage properties of 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 transformed infrared spectroscopy (FT-IR), small-angle and ultra-small-angle neutron scattering (SANS/USANS) and quasi-elastic neutron scattering (QENS). DTA analysis shows an 11 kJ/mol decrease in apparent activation energy barrier upon CMK-3 addition. SANS/USANS results reveal that CMK-3 effectively suppresses particle agglomeration during dehydrogenation/hydrogenation cycle and maintaining the structural integrity, preserving 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 contribute to the improved sorption kinetics of the CMK-3 modified composite.