| MgH2 and NaAlH4 have attracted considerable attentions as the lightweight hydrogen storage materials because of their high gravimetric hydrogen densities. However, the high thermodynamic stability and poor kinetics for hdyrogen storage reaction prevent their practical applications. In this work, to reduce the operating temperature and improve the kinetics of MgH2 and NaAlH4, the effects of Ti3AlC2 and Ti3C2 on hydrogen storage behaviors were systematically studied, and their action mechanisms were investigated.The MgH2-x wt%Ti3AlC2 (x= 1,3,5,7 and 9) composites were prepared by ball milling, and hydrogen storage properties and mechanisms of the as-prepared samples were investigated. It was found that hydrogen storage performances depended strongly on the content of Ti3AIC2, and the sample with 7 wt% Ti3AlC2 behaved the optimum hydrogen storage performances. The MgH2-7 wt%Ti3AlC2 composite started to dehydrogenation at 196℃, and desorbed 6.9 wt% of hydrogen while being heated to 350 ℃. At 300℃,5.8 wt% of hydrogen was released from the composite within 5 min, and the dehydrogenation rate is deteremined to be 3.47 wt% min=1, which is 16.5 times faster than that of the pristine sample. The dehydrogenated Ti3AlC2-added sample absorbed 6.8 wt% of hydrogen at 50 bar hydrogen pressure and 200℃. Thermodynamic and kinetic measurements suggested that adding Ti3AlC2 decreased the activation energy of dehydrogenation of MgH2, but the enthalpy change remained almost unchanged. Further XRD analyses revealed that the Ti3AlC2 still preserved during dehydrogenation.The preparation of the Ti3C2 and its effects on hydrogen storage of MgH2 were investigated. Two-dimensional Ti3C2 were synthesized by the exfoliation of Ti3AlC2 with a HF solution. The prepared Ti3C2 was added in MgH2 by ball milling. The results showed that the catalytic activity of Ti3C2 was superior to Ti3AlC2. The onset temperature for the dehydrogenation of the MgH2-5 wt%Ti3C2 composite was reduced from 278℃ (the pristine MgH2) to 185℃, representing a 93℃ reduction. At 300℃, approximately 6.2 wt% of hydrogen was released from the composite within 1 min. Further hydrogenation examinations indicated that the dehyrogenated Ti3C2-added sample showed a good hydrogen storage reversibility. It absorbed 6.1 wt% of hydrogen within 30 s at a temperature low as 150 ℃. After 10 cycles,95% of capacity was remained, representing improved cycling stability. The apparent activation energy was calculated to be 98.9 kJ mol-1 for hydrogen desorption from the MgH2-5 wt%Ti3C2 composite, which is reduced by 36% in comparison with the pristine MgH2. Further XRD and XPS analyses indicated that the in situ formed nano Ti, which derived from the redox reaction of Ti3C2 and MgH2 in the ball milling process, played an important role to improve hydrogen storage properties of MgH2.The effects of Ti3C2 on hydrogen storage behaviors of NaAlH4 were further investigated. It was found that during ball milling, a small amount of NaAlH4 reacted with Ti3C2 to release hydrogen and convert to Na3AlH6 and Al. The NaAlH4-7 wt% Ti3C2 composite desorbed 4.4 wt% of hydrogen within 60 min at 140℃, whereas no appreciable hydrogen desorption was observed for the pristine NaAlH4 under same conditions. The dehydrogenated product started absorbing hydrogen at 50℃ and 100 bar of hydrogen pressure. At 150℃, approximately 4.9 wt% was recharged into the dehyodrngeated sample. After 10 cycles, the reversible hydrogen storage capacity stayed at 4.8 wt%. Moreover, the presence of Ti3C2 decreased the activation energy and the enthalpy change of hydrogen storage in MgH2, especially for the second step dehydrogenation. XPS and EDS mesurements revealed that during ball milling, Ti3C2 reacted with NaAlH4 in part to form nano Ti and Ti3+ compounds, which are uniformly distributed in the system, and thus improving the hydrogen storage performance of NaAlH4. |
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