Effect Of Two Kind Of Ti-V-based BCC Hydrogen Storage Alloys On Hydrogen Storage Properties Of NaAlH₄

The theoretical hydrogen storage capacity of NaAlH₄is as high as 7.6 wt%,which has attracted the attention of researchers.Although kinetics,thermodynamics and reversibility performance of pure NaAlH₄is not well,its hydrogen storage performance can be improved effectively through catalytic modification.Considering that the traditional catalyst will reduce the hydrogen storage capacity of the system,or pay high preparation cost to obtain efficient catalyst;in this thesis,we hoped to prepare a high-performance NaAlH₄composite hydrogen storage system without by-products through low-cost ball milling process.Two kinds of Ti-V-based BCC hydrogen storage alloy(V₃₀Ti₃₅Cr₂₅Fe₁₀and V₅₅Ti_22.5Cr_16.1Fe_6.4)which has high reversible hydrogen storage capacity at room temperature were adopted,and each of them was in-situ ball milling together with Na H and Al to prepare BCC/NaAlH₄composite system respectively.In this way,(de)hydriding kinetic of NaAlH₄is hoped to be improved,while the reversible hydrogen storage capacity of NaAlH₄is not obviously sacrificed.Four studies had been carried out:Firstly,the passivation film on V₃₀Ti₃₅Cr₂₅Fe₁₀alloy was removed by high temperature annealing in hydrogen atmosphere,at the same improve the composition uniformity of it.However,the experimental results show that after annealing at high temperature(1200?)under hydrogen atmosphere part of BCC phase was disproportionated into Laves phase with lower reversible hydrogen storage capacity,resulting in a significant decrease in hydrogen storage capacity;while the disproportion did not take place when annealing at1200?under Ar atmosphere,and the hydrogen storage capacity of the alloy was improved.In order to avoid the alloy contact with air and refine these hard-to-broken alloys into powders,an high hydrogen pressure(8 MPa)aided mechanical crushing technology was used based on a self-made device.The products can absorb hydrogen directly after being evacuated at room temperature without activation.Generally,small particles are beneficial to hydrogen absorption and desorption for the alloy.The particles of the alloy which containing residual hydrogen were further refined with ball milling method,and It was found that quite a few BCC phase was disproportionated into Laves phase.The Laves phase hydride may be more stable than the BCC phase hydride,and high temperature and ball milling conditions provide favorable conditions for the formation of the stable phase.In the second study,(V₃₀Ti₃₅Cr₂₅Fe₁₀H_y)_x/100+Na H+Al+0.1G(x=4,8,12;G:grephene)were in-situ two steps ball milling.The alloy had been hydrided and then dehydriding at vacuum situation,and there was some hydrogen residual in it.It was found that with the help of BCC alloy,up to 97%of Al and Na H synthesized to NaAlH₄at a hydrogen pressure as lower as 2 MPa.the reversible degree,and absorption and desorption kinetics of NaAlH₄was improved with help of the BCC alloy.Especially,all the samples were able to dehydriding completely after five cycles.when x=4,the dedriding capacity was 5.15 wt%and it was the highest among the three samples.When x=8,the dehydriding capacity retention rate in the fifth cycle was 70.1%,which was the highest among all the samples.Thirdly,(V₅₅Ti_22.5Cr_16.1Fe_6.4H_y)_x/100+NaH+Al+0.1G(x=4,8,12)were in-situ2 steps ball milling.It was found that the average particle size of the alloy was obviously bigger than that of the BCC alloy in the secondary work.However,absorption and desorption kinetics of NaAlH₄was still improved;what was more,the dehydriding capacity retention rate were even higher than that in the secondary work when x=8 and 12.Comprehensive hydriding performance was best when x=4,its dehydriding capacity was 5.17 wt%which was close to 98%of the theoretical capacity,and it was the highest among all the samples.The retention rate in the fith cycle was 68.3%,which was very closed to the case of x=8 in the secondary work.In this work,the comprehensive hydrogen storage performance was slightly better than that of the previous work,although the particle size of BCC alloy was larger,which was probably due to its better distortion resistance.In the fourth study,the x value of V₃₀Ti₃₅Cr₂₅Fe₁₀H_xwas turned to adjusted the hydrogen content of the alloy.It was found that the hydrogen content of the alloy had important influence on(de)hydriging properties of the samples:the hydrogen release capacity in the first cycle increased from 4.68 wt%to 4.85wt%,when the hydrogen content of the alloy increased from 0 wt%to 1.5 wt%.Besides,adding amount of graphene was adjusted too,and it was found when the amount of graphene reduced from 10 mol%to 2 mol%,the capacity retention rate in the fifth cycle increased from 54.5%to 60.3%.