The Preparation And Hydrogen Storage Properties Of Advanced Mg-based Composites

Hydrogen production, security storage as well as efficient applications are the focus in the field of hydrogen energy research. Among them, the safe and efficient storage of hydrogen is the most difficult obstacle to realize the scale application of hydrogen energy. Therefore, the research and development of new high-capacity hydrogen storage materials has important theoretical significance and application value. Recently, considerable attentions have been paid on MgH2,0which is one of the most promising hydrogen storage materials due to its high gravimetric (7.6wt%H2) density and volumetric (110kg m-3) density. However, the practical application of MgH2is limited by high thermal stability and sluggish sorption kinetics. Based on the overview of the process in MgH2as the hydrogen storage medium, several Mg-based hydrogen storage composites were systematically investigated and discussed.(1) Highly crumpled graphene nanosheets (GNS) were fabricated by a thermal exfoliation method, and then a systematic investigation was performed on the hydrogen sorption properties of MgH2-GNS nanocomposites acquired by ball-milling. It was found that the as-synthesized GNS exhibited a superior catalytic effect on dehydrogenation/hydrogenation of MgH2. It was found that both hydrogen sorption capacity and dehydrogenation/hydrogenation kinetics of the composites improved with increasing milling time. The composites MgH2-GNS milled for20h can absorb6.6wt%H2within1min and release6.1wt%H2at300℃, and there was no difference of hydrogen storage capacity after6th hydrogenation/dehydrogenation cycling. It was also demonstrated that MgH2-GNS-20h could absorb6.0wt%H2within180min even at150℃. The fitting results of JMA model revealed that the absorption process in the experimental temperature range and desorption process at300℃were controlled by one-dimensional growth with constant interface velocity, while at350and320℃, the desorption process was controlled by two-dimensional nucleation and growth. In addition, microstructure measurements revealed that the grain size of thus-prepared MgH2-GNS nanocomposites decreased with increasing milling time and the graphene layers were broken into smaller graphene nanosheets. Furthermore, the graphene nanosheets dispersed disorderly and irregularly among the MgH2particles during the hydrogenation/dehydrogenation cycling. It was confirmed that these smaller graphene nanosheets on the composite surface, providing more edge sites and hydrogen diffusion channels, prevented the nanograins sintering and agglomeration, thus, leading to promote the de/hydrogenation kinetics and cycling stability of MgH2.(2) Porous Ni@GNS nanocomposite was successfully prepared by ethylene glycol method followed by an annealing process. MgH2-5wt%Ni@GNS composite acquired by ball milling exhibited improved faster sorption kinetics and relatively lower sorption temperature than pure MgH2. The MgH2-5wt%Ni@GNS composite could release6.0wt.%H2within10min at300℃even after nine cycles, it can also desorb5.07wt%H2within120min at230℃. In addition, the composite had good hydrogen absorption kinetics and it can absorb5.3wt%H2within25min at150℃. The kinetic analysis revealed that the best fit for both hydrogenation and dehydrogenation based on the Mampel model formulated through the random nucleation approach. The activation energy (Ea) decreased significantly compared to pure MgH2and the presence of few layer graphene nanosheets on the MgH2surface prevented the nanograins sintering and agglomeration during cycling, which enhanced the MgH2decomposition and cycling stability. It was confirmed that the porous Ni@GNS composite has a synergetic effect on the MgH2hydrogen sorption properties.(3) The effect of NiB on hydrogen desorption properties of MgH2was investigated. Measurements using temperature-programmed desorption system (TPD) and volumetric pressure-composition isotherm (PCI) revealed that both the desorption temperature and desorption kinetics have been improved by adding amorphous NiB and the10wt%NiB addition had the best hydrogen desorption performance. For example, the MgH2-10wt%NiB mixture started to release hydrogen at180℃and a hydrogen desorption capacity of6.0wt%was reached within10min at300℃, while the desorption temperature lowerd to230℃, the mixture can also release4.79wt%H2within120min. Further cyclic kinetics investigation using high-pressure differential scanning calorimetry technique (HP-DSC) indicated that the composite had good cycle stability. An activation energy of59.7kJ/mol for the MgH2/NiB composite had been obtained from the desorption data, the enhanced kinetics possibly due to the formed Mg2Ni and MgB2during desorption process, which can reduce the barrier and lowered the driving forces for nucleation, thus, improving the desorption kinetics and cycling stability.(4) The catalytic effects of TiB2and TiB2/GNS on the hydrogen desorption of MgH2were investigated. It was found that TiB2/GNS exhibited more excellent catalytic effect on the dehydrogenation of MgH2. The MgH2-5wt%TiB2/GNS composite started to release hydrogen at215℃and a hydrogen desorption capacity of6.5wt%was reached within10min at300℃, while the desorption temperature lowerd to240℃, the mixture can also release5.8wt%H2within120min. The kinetic analysis based on JMA model assumed a two-dimensional nucleation and growth of MgH2decomposition. Above hydrogen desorption kinetics as well as microstructure analysis confirmed that TiB2/GNS had a synergetic effect on the MgH2hydrogen sorption properties. It was suggested that TiB2enhanced the hydrogen desorption kinetics and GNS had positive effect on the hydrogen storage capacity of MgH2-5wt%TiB2/GNS composite.