Synthesis Of V-and Ti-based Mxenes And Their Effects On The Hydrogen Storage Performances Of Magnesium Hydride

Magnesium hydride(MgH₂)is an extremely promising material for hydrogen storage.It has received extensive attention because of its high hydrogen storage capacity,good reversibility,abundant reserves and low cost.However,the slow kinetic property and high thermodynamic stability severely restrict the practical application of MgH₂ in the field of hydrogen storage.In this thesis,V₂C,Ti₃C₂ and Ti₃CN MXenes(layered metal carbides/nitrides)were first synthesized by selectively HF-etching the Al layers from V₂Al C,Ti₃Al C₂ and Ti₃Al CN,respectively and then introduced into MgH₂by ball milling to improve the hydrogen storage performances of MgH₂.The hydrogen storage properties,mechanisms and microstructures of the samples were comprehensively studied by a variety of characterization methods.The research results are as follows.1.The initial hydrogen desorption temperature of MgH₂-10 wt%V₂C is reduced to 190°C.It can release 6.4 wt%of H₂ within 10 min at 300°C and hardly loss any capacity for up to 10 cycles.The activation energy for the hydrogen desorption reaction of MgH₂ with V₂C addition was calculated to be 92.1 k J mol^-1 by Arrhenius's equation and 87.6 k J mol^-1 by Kissinger's equation.The hydrogen desorption reaction enthalpy of MgH₂-10 wt%V₂C was estimated by van't Hoff equation to be 75.8 k J mol^-1 H₂^-1,which is slightly lower than pure MgH₂(77.9 k J mol^-1 H₂^-1).The first-principles density functional theory(DFT)calculations demonstrated that the bond length of Mg-H can be extended from 1.71Afor pure MgH₂ to 2.14Afor MgH₂ with V₂C addition,and the formation energy can be reduced from-4.36 e V for pure MgH₂ to-3.07 e V for MgH₂ with V₂C addition,which will benefit the breaking of Mg-H bond.Microstructure studies by XPS,TEM,SAED,and SEM showed that V₂C acts as an efficient catalyst for the hydrogen desorption reaction of MgH₂.2.On the basis of the research above,Ti₃C₂ was first prepared and then introduced jointly with V₂C into MgH₂ to tailor the hydrogen absorption and desorption performance of MgH₂.MgH₂ with 10 wt%of 2V₂C/Ti₃C₂ addition starts to release hydrogen at about 180°C.It can release 5.1 wt%of hydrogen within 60 min at 225°C and 5.8 wt%of hydrogen within 2 min at 300°C.With an initial hydrogen pressure of6 MPa,the dehydrided MgH₂-2V₂C/Ti₃C₂ can start to absorb hydrogen even at room temperature and a hydrogen absorption capacity of 5.1 wt%is obtained within 20 s at a constant temperature of 40°C.The reversible hydrogen storage capacity(6.3 wt%)does not decline for up to 10 cycles,which shows excellent cycling stability.2V₂C/Ti₃C₂ addition can remarkably reduce the hydrogen desorption activation energy of MgH₂ by 37%and slightly lowers the hydrogen desorption reaction enthalpy of MgH₂ by 2 k J mol^-1H₂^-1.It was demonstrated that combination of V₂C and Ti₃C₂ is beneficial to the hydrogen desorption process of MgH₂ compared with solo V₂C or Ti₃C₂ addition,while Ti₃C₂ impacts MgH₂ more significantly than V₂C concerning the hydrogen absorption process of MgH₂ at ambient temperatures.A possible hydrogen sorption mechanism of the MgH₂-V₂C-Ti₃C₂ system was proposed that the hydrogen atoms or molecules may preferentially transfer through the MgH₂/V₂C/Ti₃C₂ triple grain boundaries during the desorption process and through the Mg/Ti₃C₂ interfaces during the absorption process.3.Since the above research results have proven that MXene can significantly improve the hydrogen storage performances of MgH₂,we synthesized another MXene,which is Ti₃CN,and introduced it into MgH₂.The result demonstrated Ti₃CN also can evidently lower the hydrogen desorpton temperature of MgH₂.MgH₂-7.5 wt%Ti₃CN has the best performance whose onset temperature was reduced to 210°C.It can start to suck up hydrogen at room temperature and fill up with hydrogen before 200°C.The activation energy for the hydrogen desorption reaction of MgH₂with Ti₃CN addition was calculated to be 91.5 k J mol^-1 by Kissinger's equation and 82.0 k J mol^-1 by Arrhenius's equation.The hydrogen desorption reaction enthalpy was estimated by van't Hoff equation to be 77.4 k J mol^-1 H₂^-1.Therefore,Ti₃CN can significantly improve the hydrogen desorption kinetics of MgH₂.In general,V-and Ti-based MXenes both show positive effects on the hydrogen sorption thermodynamics and kinetics of MgH₂.Especially,the hydrogen desorption and absorption kinetic properties of MgH₂ can be significantly improved by MXenes addition.The MgH₂-MXene system can generally start releasing hydrogen at below200°C and absorb hydrogen even at room temperature,with very good cycling stability.This work will help understand the hydrogen storage performances and mechanisms of the MgH₂-MXenes system.