Improvement Of The Hydrogen Storage Properties Of Ca（BH₄）₂-based Hydrogen Storage Materials By Reactive Combination And Catalysis

As a kind of abundant, environmental friendly and renewable green energy, hydrogen has been considered as one of the most promising alternatives to fossil fuels, but the requirement for safe and efficient hydrogen storage materials is one of the key challenges for the extensive application of hydrogen as a practical substitution for fossil fuels. In recent years, Calcium Borohydride, Ca(BH4)2, with a relatively high hydrogen capacity of 11.6 wt%, has been considered as one of the most promising candidates to meet the requirements for hydrogen storage materials. However, the rather high desorption temperature, sluggish kinetics and poor reversibility prevent it from practical applications. In this paper, starting from the modification of thermodynamics and kinetics, we systematically investigate the hydrogen storage properties and mechanism of the Ca(BH4)2-xNH4Cl system. Meanwhile, the positive role of CoO and NiO on hydrogen storage properties and mechanisms of the Ca(BH4)2-4LiNH2 system were also studied.The effect of the ball milling regime and the ratio of Ca(BH4)2 and NH4Cl, on the dehydrogenation properties of the Ca(BH4)2-xNH4Cl system were investigated. It is found that ball milling Ca(BH4)2 and NH4Cl mixtures with a ratio of 1:1.5, at 300 rpm for 4h exhibits the optimal dehydrogenation properties. The onset dehydrogenation temperature of Ca(BH4)2/1.5NH4Cl system was as low as 60℃, and about 4.5 wt% H2 (>99 mol%) was achieved below 150℃. Furthermore, The isothermal dehydrogenation results of sample shows that favorable dehydrogenation kinetics can be obtained at temperature above 110 ℃, from which hydrogen capacities of 4.4 wt% can be released within 15 min at 140 ℃. It shows that the interaction between the two hydrogen storage systems, based on a promoted recombination reaction of Hδ- in [BHx] and Hδ+ in [NHx] sources, destabilizes the complex system.Investigations were conducted on the effect of the addition of different contents of CoO or NiO on the hydrogen storage performances of the Ca(BH4)2-4LiNH2 system. It is found that the addition of CoO or NiO evidently lowered the dehydrogenation temperatures of the system. The Ca(BH4)2-4LiNH2-5 wt%CoO composite starts to release hydrogen at 160 ℃, and shows the main desorption temperature of 210 ℃, which is lowered by 110 ℃ compared to the Ca(BH4)2-4LiNH2 system. The Ca(BH4)2-4LiNH2-5 wt% NiO composite released 7.5 wt% of hydrogen within 5 min at 200℃. Further evaluations on the kinetics revealed that the apparent activation energy for the hydrogen desorption from the Ca(BH4)2-4LiNH2-5 wt%CoO/NiO samples were decreased. XRD analyses showed that NiO convert to Ni during the initial heating stage, and the newly formed metallic Ni played a role as the actual active catalytic species, and the Ni3B phase was observed upon heating the sample up to 400℃.