Preparation Of V And Nb-based Transition Metal Catalysts And Their Effects On Hydrogen Storage Properties Of MgH₂

MgH2 is regarded as one of the most promising candidates for solid state hydrogen storage because of its high gravimetric energy density and good reversibility.Unfortunately,the practical use of MgH2 was greatly restricted by the high operating temperature and slow de/hydrogenation rate.Herein,we demonstrated the successful synthesis of V2O3@C,Nb2C and NbTiC and their effects on hydrogen storage behaviors of MgH2.Moreover,the underlying catalytic mechanisms were systematically investigated.Vanadium oxide nanoparticles supported on cubic carbon nanoboxes(nano-V2O3@C)were synthesized successfully by using MIL-47(V)as a precursor,and superior catalytic effects derived from the nano-V2O3@C composite towards the hydrogen storage reaction of MgH2 were demonstrated.The MgH2-9 wt%nano-V2O3@C sample started releasing hydrogen at 215 ?,which was 60 ? lower than that of the additive-free MgH2.At 275 ?,approximately 6.4 wt%of hydrogen was released from the MgH2-9 wt%V2O3@C sample within 20 min.The dehydrogenated sample completed rehydrogenation within 700 s at 150 ?.XRD and XPS measurements identified the existence of metallic V after ball milling.Further DFT calculations reveal that the presence of V facilitates the breaking of the Mg-H bond of the MgH2 unit,which was reasonably responsible for the significantly reduced operating temperatures and improved kinetics of the V-containing MgH2.The preparation of the Nb2C and its effects on hydrogen storage of MgH2 were investigated.Two-dimensional Nb2C were synthesized by the exfoliation of Nb2AlC using a chemical etching method.The prepared Nb2C was added in MgH2 by ball milling.The onset temperature for the dehydrogenation of the MgH2-9 wt%Nb2C composite was reduced from 275 0C(the pristine MgH2)to 230 0C,representing a 45 ? reduction.At 300 ?,approximately 6.3 wt%of hydrogen was released from the composite within 8 min.Further hydrogenation examinations indicated that the dehyrogenated Nb2C-added sample absorbed 5.3 wt%of hydrogen within 18 min at a temperature of 150 ?.Further XPS analyses showed the in situ formation of Nb nanoparticles due to the redox reaction between Nb2C and MgH2 during the ball milling process,which plays a significant role for improving hydrogen storage properties of MgH2.By using an NbTiC MXene as precursor,a nanocrystal NbTi was successfully synthesized.The in situ formed NbTi nanocrystals were measured to be approximately 5 nm in size,which offers the highly stable catalytic activity for hydrogen storage reaction of MgH2.The sample prepared from MgH2-9 wt%NbTiC started releasing hydrogen from 195 ?,which was 80 ? lower than that of the additive-free sample(275 0C).At 250 ?,approximately 5.9 wt%H2 was rapidly released within 30 min,and the fully dehydrogenated sample took up 4.0 wt%H2 within 16 min even at 50 ? under 50 bar of H2 pressure.DFT calculations revealed a charge transfer from Ti atoms to Nb atoms in NbTi and a lower absolute value of adsorption energy of H2 on NbTi.This facilitates not only the break of Mg-H bonding but also the detachment of H2 from NbTi surface,consequently contributing a good catalytic activity.