Effects Of Co-based Catalysts On Hydrogen Storage Properties Of Li-B-N-H System And Its Mechanism

Development of safe, economic and efficient hydrogen storage technologies is the key issue for large-scale applications of hydrogen energy. In recent years, considerable attention has been paid to the Li-B-N-H hydrogen storage system due to their relatively high hydrogen capacity. However, the high dehydrogenation temperature and poor reversibility prevent it from practical applications. In this paper, to reduce the dehydrogenation and improve the hydrogen storagte reversibility, the effects of CoO, CO3O4, Co(OH)2and MOF-74-Co on hydrogen storage properties and mechanism of the Li-B-N-H system were systematically investigated.The LiBH4-2LiNH2-xCoO composites with x=0,0.0006,0.005,0.01,0.03,0.05,0.10and0.20were prepared by ball milling, and the hydrogen storage properties of the as-prepared samples were investigated. It was found that the sample with0.05mol CoO behaved the best hydrogen storage performances. With the addition of0.05mol CoO, the composite released about9.1wt%of hydrogen within10min at200℃, whereas there was no detectable hydrogen desorption for the additive-free sample under the same conditions. Thermodynamic and kinetic measurements revealed that adding CoO changed the heat flow behaviors of hydrogen desorption and decreased the activation energy. Further XAFS analyses indicated that CoO was reduced to metallic Co during the initial heating stage, and the newly formed metallic Co played a role as the actual active catalytic species in favor of the creation of B-N bonding on the surface of metallic Co. Moreover, the dehydrogenated CoO-added sample exhibited improved hydrogen storage reversibility, absorbing about1.1wt%of hydrogen at350℃and a hydrogen pressure of110bar.The LiBH4-2LiNH2-x/3Co3O4composites with x=0,0.01,0.03,0.05,0.08and0.10were prepared by ball milling, and the hydrogen storage properties of the as-prepared samples were investigated systematically. It was found that the presence of Co3O4in the LiBH4-2LiNH2system significantly reduced the dehydrogenation operating temperatures and enhanced the dehydrogenation kinetics. The LiBH4-2LiNH2-0.05/3Co3O4composite exhibited optimal hydrogen storage properties. It released～8.2wt%of hydrogen within60min at200℃. Hydrogen desorption from the Co3O4-added LiBH4-2LiNH2system was a four-step reaction, and the first and third steps of dehydrogenation are endothermic in nature, exhibiting favourable thermodynamics for reversible hydrogen storage. The apparent activation energies of the four dehydrogenation steps were all lower than that of the pristine LiBH4-2LiNH2sample. XRD and FTIR analyses revealed that the added Co3O4was first converted to Li1.47Co3O4and then formed Li2.57Co0.43N and Co3B7O13NO3. Finally, the metallic Co was identified in the resultant dehydrogenation product. Such a transformation not only changed the dehydrogenation thermodynamics but also decreased the energy barriers, consequently improving the dehydrogenation properties of the Co3O4-added sample. Further hydrogenation examinations revealed that the dehydrogenated Co3O4-added sample exhibited a partial reversibility because it absorbed～1.7wt%of hydrogen at220℃and110bar of hydrogen pressure.In-depth investigations were conducted on the hydrogen storage properties and mechanism of the LiBH4-2LiNH2-xCo(OH)2composites with x=0,0.0004,0.01,0.03,0.05,0.08,0.10and0.20. During ball milling, a chemical reaction among Co(OH)2, LiBH4and LiNH2readily occurred to generate H2. The presence of Co(OH)2in the LiBH4-2LiNH2system significantly enhanced the dehydrogenation kinetics. The LiBH4-2LiNH2-0.05Co(OH)2composite exhibited optimal hydrogen storage properties. It released～9.1wt%of hydrogen within20min at200℃. Kinetic measurements revealed that adding Co(OH)2decreased the activation energy by25%. XRD analyses showed that Co(OH)2were converted to Co after dehydrogenation, and the newly formed metallic Co played a role as the actual active catalytic species. EDS measurements showed that the in situ generated Co uniformly distributed in the system which is in favor of the creation of Li3BN2, nucleation and growth on the surface of metallic Co. Further hydrogenation examinations reveal that the dehydrogenated Co(OH)2-added sample exhibited a partial reversibility because it absorbs～1.3wt%of hydrogen at350℃and110bar of hydrogen pressure.The effects of MOF(MOF-74-Co) on the hydrogen storage properties of LiBH4-2LiNH2system were further investigated. It was found that the sample with5wt%MOF-74-Co exhibited the best hydrogen storage performances. With the addition of5wt%MOF-74-Co, the composite released about9.0wt%of hydrogen’ within10min at200℃, which is87%of the total amount of hydrogen (10.4wt%). Kinetic measurements revealed that adding MOF-74-Co decreased the activation energy by26%. XRD and EDS measurements showed that MOF-74-Co was reduced to Co, which is uniformly distributed in the system throughout the whole dehydrogenation process. SEM observation found a loosened porous morphology for the dehydrogenated (MOF-74-Co)-added sample, which facilitates the transport and diffusion of hydrogen. Further hydrogenation examinations reveal that the dehydrogenated (MOF-74-Co)-added sample exhibited a partial reversibility because it absorbed～1.7wt%of hydrogen at220℃and110bar of hydrogen pressure.