| NaAlH4 is a promising material of hydrogen storage with high reversible capacity(5.6wt%)of hydrogen storage and mild temperature conditions(100-200?).Although the kinetic properties of hydriding/dehydriding are not good,it can be improved obviously by adding a catalyst.Most of the traditional catalysts are ionic,such as TiCl3,which can enhance the kinetics of hydriding/dehydriding obviously,but there are inert by-products of sodium halides.It will reduce the capacity of hydrogen storage.Nonionic catalysts,such as TiB2 are difficult to refine to nanometer scale under traditional process conditions,so the catalytic effect is not sufficient.In this paper,we hope that by optimizing the process of preparation and doping methods,the catalyst can be cut in size,the dispersion uniformity can be improved and the degree of coupling with substrate can be increased,thereby increasing the efficiency of catalytic.Considering that Al is prone to adhesion during ball milling,we adopt a step ball milling process based on ceramic tank and ceramic ball and then in-situ reaction ball milling in high pressure of hydrogen atmosphere.It has been found that this process can effectively decrease the adhesion of Al and the loss of initial reactants during ball milling.The powder after pre-milling has a high degree of dispersion and a good coupling relationship.And then in-situ synthesis is significantly better.Therefore,we will exploit this process to develop the following research work.Firstly,a single doping of TiAl3,TiH2 and Ti were studied.Experiments show that these dopants additions are conducive to in-situ synthesis of NaAlH4.Sample which synthesized with Ti powder has the highest amounts of initial hydrogen release.The effect of TiAl3 on the kinetics of hydrogen is slightly better than that of TiH2.Adding Ti is the best,which may be due to the formation of TiAl3 and TiH2 in the presence of Ti added and then they have a good coupling relationship in in-situ ball milling,thus a synergistic catalytic effect on the process of hydriding/dehydriding was obtained.It was also noted that from the second hydrogen release,the capacity of hydrogen storage on the three samples was significantly lower than the first time.It may be caused by excessive growth of Al crystal grains during the first process of hydriding/dehydriding.Then,the same process was used to investigate samples of Al+NaH+xTi(x=0.01,0.02,0.03,0.04).It was found that initial hydrogen release of sample which synthesized by added 2mol%Ti powder was the highest(5.13wt%)and 4mol%Ti added situation was the least(4.84wt%),but its temperature of the first and second dehydrogenation reaction was the lowest and dynamic performances was the best too.It is also found that the higher the doping amount of Ti is,the faster the kinetics of hydrogen for the synthesized samples is and the better the reversible hydrogen storage is,but the lower the capacity of hydrogen storage is.Addition of Ti with better comprehensive performance may be near 2mol%from our current experiments.Secondly,a single doping of Y was investigated.It was found that there were obvious YH2 diffraction peaks in the products after pre-milled when Y and YH2 were added alone,and synthesis process of NaAlH4 was much worse than that of Ti additions.In view of this,we conduct the melting of Al+x Y(x= 0.02,0.04)to obtain a dual phase structure of Al/YAl3.It was pointed out that in the case of adding YAl3 the synthesis of NaAlH4 was improved compared to Y and YH2,but it was still less than Ti additives.From the kinetic test of the sample,we can see that when the amount of Y is high,the sample has a lower temperature of initial hydrogen release and the kinetics of hydriding/dehydriding are better,but the capacity of hydrogen storage is also significantly affected.For the amounts of hydrogen release,the sample with 2mol%Y was 4.56wt%,which was significantly higher than that of adding 4mol%Y(3.48wt%).That is to say,the amount of the first hydrogen release is larger when Y addition is small.The kinetics of cyclic hydriding/dehydriding showed that YAl3 was helpful to improve of cycling stability.Comparison shows that Ti additions are superior to Y-Al in the rate of hydrogen release,kinetics and temperature of the initial dehydrogenation,but the temperature difference of the two dehydrogenation reactions is smaller and capacity of the cyclic is better when adding Y-Al.Finally,the same technology and related operation were used to investigate the co-doping of Al-Ti-Y by mixing powder directly and by smelting first then pre-ball milling.After 35 hours of high energy ball milling,the diffraction patterns of the sample which without melting is basically only NaAlH4,it is indicated that the synthesis of the samples was sufficient;The samples after melting,NaAlH4 has been synthesized after 35 hours high energy ball milling in Ti-rich condition.When Y is rich,it is difficult to synthesize until the milling time reaches 75 hours.Besides,the kinetics of hydrogen releases of the samples treated by melted is better than that of the unmelted.The results of cycle hydrogen discharge experiments show that the cyclic stability of NaAlH4 can be improved obviously by co-doping of Ti-Y than that in a single doping of Ti.However,the samples after smelted is better.Compared with single doped Y,kinetics of hydrogen absorption and desorption is significantly improved.Therefore,in the case of Ti-Y co-doping,samples not only has the advantages of Ti in kinetics but also has the advantages of Y in cycling performance.Although the best ratio of Ti-Y remains to be explored,it can be expected to be achieved in Ti-rich conditions. |
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