The Hydrogen Storage Performance Of New Metal Nitrides Composites

An efficient hydrogen storage material is one of the key issues for hydrogen energy application. Li-Mg-N-H hydrogen storage system exhibits more and more potential superiority due to its moderate operation temperatures, good reversibility, and relatively high capacity. However, the slow kinetics makes it unpractical when used alone.Catalysis and compounding has demonstrated their valuable efficiency on improving kinetics and wakening thermodynamics respectively. In this paper, a kinetics-enhanced Li-Mg-N-H system with kinds of Co based catalyst for hydrogen storage is prepared by high-energy ball milling. Followingly, the Li-Mg-N-H/NaAlH4/LiBH4 composite is synthesized to improve the thermodynamics of Li-Mg-N-H by ball milling Li-Mg-N-H and NaAlH4 or LiBH4 togetherly. Founding on above results, the ZrCoH3 catalyzed Li-Mg-N-H/LiBH4 composite which has a superior overall performance for hrydrogen storage is successfully synthesized. Finally, the Sievert's type apparatus, Temperature-programmed-desorption (TPD) tests and Differential scanning calorimeter (DSC) were applied to measure the kinetics, thermodynamics and cycling performance for hydrogen storage. The XRD and SEM were taken to analyze the phase structure and micromorphology. The FTIR and XPS were taken conbinedly to study the catalytic mechanism.A series of Co-based catalysts (Co, ZrCoH3, CoF2 and Co(OH)2) catalyzed Li-Mg-N-H composites were prepared by high-energy ball milling. All of new balled composites exhibit an improved kinetics, and the Li-Mg-N-H system with ZrCoH3 or Co(OH)2 do the best, which can desorb 3.24wt% and 3.62wt% of hydrogen under 180℃and 0.1 MPa H2 within 100 minutes, compared with 2.8wt% of hydrogen for the sample without any catalysts addition, respectively. Reducing the temperature to 160℃, the dynamic catalysis of Co(OH)2 is still significant. The XRD and SEM results indicate that ZrCoH3 keep intact but Co(OH)2 has descomposed to metal Co which are very dispersive in the matrix during hydrogenation and dehydrogenation. The IR analysis reveals a wakened N-H bond in ZrCoH3 and Co(OH)2 catalyzed Li-Mg-N-H system, which may be the main factor for kinetics enhancement.Co based catalysts are effective to improve the kinetics of Li-Mg-N-Hsystem, but the catalysis is limited in improving the thermodynamics. Therefor, catalytic complex demonstrates its necessityThe Li-Mg-N-H/NaAlH4 composite synthesized by ball milling demonstrates an enhanced kinetics and lowered dehydrogenation temperature than the pure NaAlH4 and Li-Mg-N-H, and can desorb a total of 5 wt% hydrogen through a three-step reaction. It is found that the 10-minutes ball milled composite has a good overall hydrogen storage properties, which begins to desorb hydrogen below 100℃with a hydrogen capacity of 4.35wt%. The H/D testing shows that the composite has a low desorption capacity at lower temperature and poor cycle performance at eveluated temperature. It is speculated that the stable dehydrogenation products AlN and Mg3N2 restict the reversibility of Li-Mg-N-H/NaAlH4 system.Co based catalysts or complex hydrides are effective to improve the kinetics or thermodynamics of Li-Mg-N-Hsystem, but a single catalyst or compound has limitations. Therefor, catalytic complex demonstrates its necessity. The 5wt% ZrCoH3 catalyzed Li-Mg-N-H/LiBH4 composite is prepared by high energy ball milling on basis of above results. The new balled composite shows a good overall hydrogen storage performance, which can desorb 3.6wt% of hydrogen under 150℃and 0.1 MPa H2 within 100 minutes, compared with 3.05wt% of hydrogen for the sample without any catalysts addition. A better performance that 4.2wt% of hydrogen desorption within 100 minutes and 5.2wt% of hydrogen absorption in 10 mins at 150℃will be got by optimazing the catalytic process of ZrCoH3. The best composite has a more reduced Ea and increased rate constant k. It is deduced from the XRD and SEM results that the optimized catalytic process will gain a more dispersive particle system and some size-reduced ZrCoH3 particles which not only promotes the mass transfer but also provide numerous nucleation sites and promotes the nucleation, therefor, an enhanced kinetics is got.