| The application of hydrogen energy, which has long been considered as one of the most promising alternatives to fossil fuels, still suffers from the shortage of safe, efficient, and economic storage techniques. In recent years, Li-B-N-H complex hydrides have attracted a worldwide attention due to their relatively high hydrogen capacities. However, the sluggish kinetics and poor reversibility still prevent them from practical applications. In this work, the Co/Ni-based transition metal catalysts were introduced into the Li-B-N-H systems to improve their hydrogen storage properties. The de-/hydrogenation behaviors of the Co/Ni-doped Li-B-N-H systems were systemtically investigated, and the corresponding mechanisms were discussed.First, the hydrogen storage properties of the CoO-added LiBH4-NH3-3LiH system were systematically investigated. It was found that the majority of hydrogen release occurred in the temperature range of 130～250℃ for the 0.1 CoO-added sample. The initiating and terminating temperature were 50 and 120℃ lower than those of the pristine sample, respectively. Approximately 8.5 wt% of hydrogen was released from the 0.1 CoO-added system when it was heated to 250℃. At 200℃,8.0 wt% of hydrogen was liberated from the 0.1 CoO-added system within 100 min, whereas only 4.1 wt% was released from the pristine sample under identical conditions. XAFS and SEM analyses indicated that CoO was reduced to metallic Co during the first dehydrogenation stage. The in-situ formed Co particles with large specific surface and good dispersity distributed homogenously in the sample, which played as the actual active species in promoting the combination of [BH4] and [NH2] groups and decreasing the dehydrogenation temperature of the LiBH4-NH3-3LiH system.Subsequently, the LiBH4-2LiNH2 system was chosen as the studied object. The ultrafine metallic Co nanoparticles with sizes less than 10 nm in diameter supported by carbon (nanosized Co@C) were successfully synthesized by calcining FA filled MOF-74(Co). The effects of the addition of Co@C on hydrogen storage properties of the LiBH4-2LiNH2 system were systematically investigated. It was found that the Co and C contents in the as-calcined Co@C composite were 83.8 and 16.2 wt%, respectively. After adding 5 wt% Co@C, the onset dehydrogenation temperature of the LiBH4-2LiNH2 system was reduced by 130℃ in comparisng with the pristine sample. Approximately 10.0 wt% of hydrogen was released in the temperature range of 130～ 200℃. At 200℃, hydrogen release of the LiBH4-2LiNH2-5 wt% Co@C composite finished within 50 min. Hydrogenation experiments revealed that the hydrogen storage reversibility of the LiBH4-2LiNH2-5 wt% Co@C sample was slightly improved, becasuse approximately 0.5 wt% of hydrogen was recharged into the dehydrogenation product when it was heated to 350℃ under 100 bar of hydrogen pressure. The nanosized Co particles remained in metallic state during the heating process, and worked as a catalyst for improving hydrogen storage properties of the Li-B-N-H system.The as-calcined Co@C composite was further heated to 500℃ in air to prepare Co3O4. Structural and morphological analyses showed the poor crystallization of the prepared Co3O4 particles with particle sizes in the range of 1～3μm. For the LiBH4-2LiNH2-0.05/3Co3O4 sample, the onset decomposition temperature was reduced to 145 ℃. Approximately 9.7 wt% of hydrogen was released when it was heated to 225℃, and 99% of hydrogen capacity was released within 18 min at 200℃. The dehydrogenated LiBH4-2LiNH2-0.05/3Co3O4 sample absorbed 0.75 wt% of hydrogen when it was heated to 350℃ under 100 bar of hydrogen pressure, exhibiting partial reversibility. Further EELS measurements indicated that Co3O4 was first reduced to metallic Co during the heating process, which worked as the actual active catalyst in decreasing the dehydrogenation temperature and improving the hydrogen storage reversibility of the LiBH4-2LiNH2 system.For understanding the role played by Co, first-principles calculations for the co-adsorptions of [LiBH4] and [LiNH2] molecules on the Co(111) surface were performed. Calculation results suggested that B, N and H adatoms were covalently bonded with Co surface atoms. Analyses on the charge distribution and density of state indicated that most of the valence electrons accumulated on B atoms in the [LiBH4] and [Li2BH4NH2] structures. In the adsorption models, charge depletion from all of the Li atoms, [BH4] and [NH2] groups and charge accumulation on the Co surface atoms were observed. The symmetry of [BH4] group was lowered from Td to Cs for the [LiBH4] absorbed structure. The amount of charge transfer from B atoms to the Co surface atoms was greatly increased in the co-adsorption structure. The inequivalent B-H bonds and charge depletion of B atoms were the two main reasons for the dissociation of two H atoms in [BH4] groups after Li atoms, [BH4] and [NH2] groups were co-absorbed on the Co(111) surface.Finally, metallic Ni nanoparticles with less than 30 nm in diameter supported by carbon (nanosized Ni@C) were successfully synthesized by calcining FA filled MOF-74(Ni). The Ni and C contents in the Ni@C composite were determined to be 64.2 and 35.8 wt%, respectively. The LiBH4-2LiNH2-10 wt% Ni@C composite released 10.0 wt% of hydrogen in the temperature range of 135～250℃. At 220℃, hydrogen release reaction finished within 20 min. Unfortunately, the reversibility for hydrogen storage in the LiBH4-2LiNH2-10 wt% Ni@C sample was still poor because only 0.2 wt% of hydrogen was absorbed by heating the dehydrogenated sample to 350℃ under 100 bar of hydrogen pressure. Structural analyses revealed that the nanosized Ni particles remained almost unchanged during the heating process, suggesting that it may only act as a catalyst for reducing the dehydrognaiton temperature of the Li-B-N-H system. |
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