| The theoretical hydrogen storage capacity of NaAlH4 is as high as 5.6wt%and it can release hydrogen under 100-200 centigrade conditions,therefore it is regard as a potential hydrogen storage material with application prospects.However,the phase separation of NaH and Al will occur during the dehydrogenation products of NaAlH4,resulting in segregation of components and coarseness of particles,resulting in very low activity and little contact between the products,as a result,re-hydrogenation is hard to achieve.It was found that re-hydrogenation can be achieved by doped with catalysts,and the dehydrogenation temperature also reduce at the same time,which bring the practicality to a big step.Common catalysts are rare earth chlorides,such as CeCl4,PrCl3,etc.These catalysts can significantly improve the hydrogen absorption and desorption performance of NaAlH4,but the hydrogen storage capacity is reduced by inactivity product NaCl.In addition,metallic additive,such as CeAl4 and SmAl3 alloys,are difficult to refine under general ball milling conditions,which make it can not fully contact with the matrix.Therefore,the catalytic effect is limited,and special preparation methods are needed to refine their particle size.In this paper,doping and preparation methods would be changing in order to avoid the generation of ineffective hydrogen storage materials,and promote more contact area between catalysts and substrates.Two steps in-situ milling preparation method based on ceramic medium was adopted,NaH,Al and additive were pre-ball milling firstly,then they were milling under high pressure hydrogen gas.Element additive was used,in this way,no inactive by-product will be introduced firstly;secondly,reversible capacity is not decrease obviously due to more small molecular weight;thirdly,reaction between element additive and the matrix is easier to take place,and the catalyst formed during in-situ milling can be coupled with the matrix more highly.In the pre-ball-milling stage,Al,NaH and element additive were milling with ceramic pots which can avoid the adhesion of Al obviously during ball-milling,improve the efficiency and effect of milling.On the other hand,the reaction between elemental additive and matrix is improved,and the coupling between catalyst and Al(or NaH)is improved too,which will make the matrix more activated.In the subsequent hydrogenation ball milling stage,the as-actived Al and NaH in-situ reactive with high-pressure hydrogen and catalyst-NaAlH4 system formed,which is expected to improve the hydrogen absorption and desorption performance of NaAlH4 more effectively.In addition,the existence of the interface can become a channel for the rapid diffusion of substances,inhibit the long-range diffusion of substances,and prevent the agglomeration of dehydrogenation products.Three works were carried out:Firstly,the hydrogen storage properties of NaAlH4 in-situ synthesized by X(X = Sc,Ce,Pr,Sm)were studied.It was found that these rare earth elements can promote the synthesis of NaAlH4,and XHy(C = Ce,Pr,Sm)was produced in the pre-ball milling stage,and always exists in the process of hydrogen absorption and desorption;which mean that Al and NaH were active by X simultaneously.As for the semi rare earth element Sc situation,there was not ScHx produced,which mean that only Al was activated.The dehydrogenation capacity in the first cycle in Sc added situation was 5.2wt%which was the highest among all the situations;besides the hydronation kinetics was also the best,and the dehydronation kinetics ranked the second.The kinetics of dehydrogenation with Ce added was the best among all the situations,the initial dehydrogenation temperature was 90 degree C,and the hydrogen absorption kinetics ranked the second,the dehydrogenation kinetics in Sm added situation was close to that of Pr,and the hydronation kinetics was better than that of Pr.In addition,the cycle performance in rare earth elements added situations were better than that in Sc added situation;among which Sm added situation was the best and the capacity retention rate in five cycles is 95.8%.Secondly,the Co-doping of Ce and graphene was studied to improve its cycle performance.CeHx was also found in the pre-ball milling stage,but H is only exist in NaH in the system,which indicated that Ce reacted with NaH.Through theoretical and experimental studies,it was found that Ce captured part of H in NaH and many non-equilibrium H vacancies produced in NaH.In pre-ball milling,the diffraction peaks of A1 shifts to a low angle,when combined with First-principles calculations,it was founded that Ce dissolves into Al during ball milling which limits the slip of dislocations,and large number of non-equilibrium Al vacancies produced,i.e.Al was well activated too.Moreover,the activation effect in Ce added situation was from the body to the surface,which was obviously better than that of CeHx added situation(the latter is mainly activated on the surface).Through experimental and theoretical studies,it was found that during pre-ball milling Al and NaH were active better when Ce and Graphene added together,besides during hydro-ball milling stage higher activity of NaAlH4 was produced,what was more,reversible hydrogen storage capacity,hydrogen absorption and desorption kinetics and cycle life were significant improved.Finally,in view of the good hydrogen absorption kinetics of Sc,different dosages of Sc were studied,and binary doping of Sc+X(X=Si,Ti,Ce)were studied too.The second component is Si,Ti and Ce,which are mainly aimed at improving cycle performance and lowering dehydrogenation temperature.It was found that hydriding/dehydriding kinetics was improved obviously when Sc-Ti was added;growth of dehydrogenation products was inhibited and cycle proformance was improved in Sc-Si added situation;the effect of Sc-Ce was similar with Sc-Ti,but the cycle performance in the former one was better.The experimental results show that,in the hydriding/dehydriding kinetics aspect,the Si-Ti binary doping system was the best;and in the cyclic performance aspect,Sc-Ce binary doping system was the best. |
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