Effect Of Ti-V-Based Hydrogen Storage Alloys On Synthesis And Hydrogen Storage Properties Of NaAlH₄

Among the solid-state hydrogen storage materials,NaAlH₄ had gained much attention due to its moderate dehydrogenation temperature with a theoretical capacity of up to 7.4 wt%.However,NaAlH₄ also suffered from poor reversibility and poor dehydrogenation/hydrogenation kinetic performance.It had been found that doped nanoscale catalysts could significantly enhance the hydrogen storage performance of NaAlH₄.However,nanocatalysts suffered from many problems such as complicated preparation process and high cost.Meanwhile,some BCC phase hydrogen storage alloys had been shown to have good catalytic effect on NaAlH₄.Therefore,in this paper,three Ti-V-based hydrogen storage alloys with simple preparation process and moderate cost were selected as catalysts to investigate the effect of Ti-V-based hydrogen storage alloys on the synthesis and hydrogen storage performance of NaAlH₄,and the following work was carried out to investigate:The work Ⅰ,investigated how to obtain Ti-V-based alloy powders with different hydrogen contents and the effect of the hydrogen content of the alloy on the composite system.It was found that hydrogen-assisted comminution could effectively refine the highly ductile Ti_0.35V_0.65 alloy and the resulting alloy powder was close to hydrogen absorption saturation;the alloy powder was vacuumed at 270℃ and 600℃ to obtain the semi-saturated and hydrogen-absorbing state and the hydrogen-free state,respectively.Then the Ti_0.35V_0.65H_x（x=0,1,and 2）alloy powder was added at the 10 mol%for pre-ball milling and reaction ball milling to synthesise the composite system.It was found that all three alloys with different hydrogen contents could reduce the hydrogen pressure required for the in-situ synthesis of NaAlH₄;all were able to significantly improve the kinetic performance of the NaAlH₄dehydrogenation/hydrogenation process.In the present work,the alloy with the highest hydrogen content,x=2,showed the best catalytic effect;the corresponding sample achieved a first warming dehydrogenation capacity of4.67 wt%;moreover,the corresponding sample essentially completed the hydrogenation process within 70 min at 130℃ and 9 MPa hydrogen pressure.The work Ⅱ,investigated how to avoid passivation to obtain Ti-V-based alloy powders with high reversible hydrogen storage capacity at room temperature and the effect of alloy doping on the hydrogen storage performance of the composite system.The low toughness(Ti_0.35V_0.65)_0.86Fe_0.14 ternary alloy was selected and the(Ti_0.35V_0.65)_0.86Fe_0.14H_y alloy powder was prepared by melting,annealing and H₂-assisted-crushed methods,and it was found that the H₂-assisted-crushed method could significantly promote its particle refinement and avoid passivation.The same two-step ball milling method was used to synthesize the composite system in situ.It was found that after 190 h of ball milling alone,some of the alloys showed lattice distortion and the reversible hydrogen storage capacity at room temperature dropped significantly to 1.10wt%.However,after prolonged composite ball milling,the alloy still significantly reduced the hydrogen pressure required for the synthesis of NaAlH₄;in addition,the(Ti_0.35V_0.65)_0.86Fe_0.14H_y alloy was effective in reducing the dehydrogenation/hydrogenation temperature of NaAlH₄ and improving the kinetic properties.In this study,the 5 mol%was the optimum alloy addition ratio,corresponding to a nonisothermal dehydrogenation capacity of 5.04 wt%for the first sample and higher than 4.40 wt%during the second and subsequent dehydrogenation cycles.The work Ⅲ,explored how to obtain finer particle size of Ti-V based alloys and whether the composite system has better hydrogen storage properties at finer particle size.The Ti_0.35V_0.35Nb_0.30 ternary alloy with higher brittleness was selected,and the alloy was wet milled at different times to find that wet ball milling was beneficial to the refinement of the alloy particles.5 h and 10 h wet ball milling could refine the particle size from 51.5μm to 7.6μm and 5.2μm,but the alloy showed lattice distortion at 10 h wet ball milling.In addition,the alloy could not absorb hydrogen after wet milling because it was passivated by water oxygen in the solvent,but it could absorb hydrogen again after evacuation at 600℃.The rehydrogenated alloy at the addition ratio of the 5 mol%with Na H-Al was ball milled in an Ar environment and hydrogenated to a composite system at 120℃ and 8 MPa hydrogen pressure.It was found that although wet grinding was effective in reducing the alloy granularity,it also introduced lattice distortions and these distortions were difficult to fully recover at 600℃.The catalytic effect of the alloy on the matrix deteriorated significantly after passivation due to the influence of the solvent;however,the catalytic effect of the alloy was significantly improved after evacuation at 600℃.In this work,wet ball milling for 5 h was the optimum time for wet ball milling to refine the alloy,probably because significant particle refinement with less lattice distortion was achieved;the first warming dehydrogenation capacity of the composite system in this case reached 4.42 wt%;this sample was able to completely dehydrogenate at 170℃.