Preparation Of Two-Dimensional Layered MXene Material And Its Catalytic Modification Of Hydrogen Storage Performance Of Coordinated Aluminum Hydride

Coordination hydride is one of the high-capacity hydrogen storage materials which attracts widely concern.Among all the coordination hydride,NaAlH₄ has always been a research hotspot.In order to improve the application of coordination hydrides in the field of hydrogen storage in the future,it is necessary to improve its reversible hydrogen absorption and release temperature,kinetic performance,and cycle performance to adapt to different application scenarios.In this work,based on previous research,a variety of new two-dimensional materials mxene were prepared in a safe and controllable way,then MXene doped NaAlH₄ systems were prepared by reaction ball milling.The hydrogen absorption and desorption performance of the system was verified by a Sievert instrument.Afterwarfs,microstructure of the samples are comprehensively analyzed by differential scanning calorimetry,X-ray diffraction,scanning electron microscopy,energy dispersive X-ray spectroscopy,X-ray photoelectron spectroscopy,Fourier transform infrared spectroscopy,etc.?1?The preparation of layered MXene material Ti₃C₂ was achieved through the MILD method?using hydrochloric acid and fluoride salt as raw materials?in order to prevent the danger in synthesis.The NaH/Al-Ti₃C₂ composite was obtained by reactive ball milling.The NaH/Al-Ti₃C₂ composite shows better performance than directly doped NaAlH₄-Ti₃C₂ composite in hydrogen storage and cycling.Hydrogenated NaH/Al-8 wt%Ti₃C₂ sample can start dehydrogenation at 76°C,and release 4.9 wt%of hydrogen in 73 minutes at 130°C at last.From the DSC results,it can be seen that the peak temperatures of dehydrogenation in the first and second steps of NaH/Al-8 wt%Ti₃C₂ is 145°C and 154.3°C,respectively,the apparent activation energy of the first and the second step of hydrogen evolution is 61.2±4.6 KJ/mol and 58.1±2.9 KJ/mol,respectively.Dehydrogenated NaH/Al-8 wt%Ti₃C₂ sample can absorb 4.2 wt%hydrogen at 4.5°C at 110°C.NaH/Al-8 wt%Ti₃C₂ maintains 99%of its initial capacity after ten cycles.The mechanism analysis shows that during the ball milling process,Ti₃C₂ decomposes and reacts with fluoride ions on the surface to form Ti/TiF_x nanoclusters attached to the NaH/Al matrix,providing active sites for rapid hydrogen absorption and desorption reactions,thereby improving the system kinetic performance of hydrogen storage.?2?On the basis of the research above,M₂X-MXene Ti₂C is synthesized by the same method.By ball-milling,NaH/Al-Ti₂C composite is successfully synthesized within 48 h under 30 bar hydrogen pressure.NaH/Al-Ti₂C exhibits excellent hydrogen absorption and desorption performance.The sample can start dehydrogenation at 56°C.The peak dehydrogenation temperatures of the first and second steps of NaH/Al-7wt%Ti₂C are 127.1°C and 150.6°C,respectively.Fully Dehydrogenated NaH/Al-7wt%Ti₂C can absorb 5.2 wt%hydrogen in 90 seconds at 120°C.In the cycle experiment,after ten cycles,the capacity of the NaH/Al-7 wt%Ti₂C composite was only 0.2 wt%attenuated compared with its initial capacity.Structural analysis showed that Ti₂C also decomposed during the ball milling process and reacts with NaH/Al,generating nanoclusters containing Ti⁰ and Ti³⁺.These nanoclusters acted as fast channels for hydrogenation and dehydrogenation and improved the performance of NaH/Al-7 wt%Ti₂C composite.After kinetic fitting calculation,the kinetic models of the first and second steps of dehydrogenation of NaAlH₄-Ti₂C are the two-dimensional phase interface migration model and the Avrami-Erofe'ev model,respectively.The reaction rate in the two-dimensional phase interface migration reaction depends on the rate of phase interface migration.The two-dimensional characteristics of Ti₂C provide a phase interface for the phase interface migration,thereby increasing the velocity of hydrogen absorption and desorption reactions.?3?The niobium-based MXene material Nb₂C was prepared safely and controbally.the NaAlH₄-Nb₂C composite material was prepared by ball milling method.NaAlH₄-7wt%Nb₂C release 4.25 wt%hydrogen at 150°C in 20 min.After complete dehydrogenation,NaAlH₄-7 wt%Nb₂C absorbs 3.96 wt%H₂ at 140°C within 21 min.After ten cycles,NaAlH₄-7 wt%Nb₂C maintained 97.6%of its initial capacity.During the ball milling process,Nb₂C combines with the hydrogen atoms in the NaAlH₄ unit cell to form NbH,which may be the actual catalytic component.According to the kinetic model fitting calculation,the two-step dehydrogenation reaction of NaAlH₄-₇wt%Nb₂C is a three-dimensional diffusion reaction,but it follows different equations.