Hydrogen Storage Performance And Micromechanism Of Ti₃C₂ MXene And Its Composites Enhanced NaAlH₄

As a representative of light metal complex hydride,NaAlH₄ has high theoretical hydrogen storage capacity and can absorb and release hydrogen reversibly under mild conditions.Therefore,it is considered to be one of the most promising hydrogen storage materials in the 21st Century.However,it also has some problems,such as high temperature of hydrogen absorption and desorption,slow kinetics,and so on.These problems seriously impede its practical application.It is found that adding catalyst can effectively improve the kinetic and cyclic stability of hydrogen absorption and desorption of NaAlH₄.Ti-based transition metal carbide two-dimensional material?MXene?endow some prerequisites to be optimal additives in terms of unique two-dimensional structures and controllable morphologies.In this paper,NaAlH₄ is selected as the subject,and two-dimensional Ti₃C₂ and its composites were used as additives to study the influence of them on the hydrogen storage properties of NaAlH₄and their micro-mechanism.The main research content are as follows:Two-dimensional Ti₃C₂ was successfully synthesized by hydrofluoric acid etching of Ti₃AlC₂,and the hydrogen storage properties of NaAlH₄ doped with different amount of Ti₃C₂ were tested.The results showed that the initial hydrogen desorption temperature of NaAlH₄ decreased with increaseing the amount of Ti₃C₂,and the capacity of hydrogen desorption also decreased.NaAlH₄+10 wt%Ti₃C₂ composite has superior hydrogen storage properties.After adding 10 wt%Ti₃C₂,the initial dehydrogenation temperature of NaAlH₄ is reduced to 90°C,and it can release 3.14wt%of hydrogen within 180 min at 100°C.At 140°C,it can rapidly release more than3.7 wt%of hydrogen within 17 min,and 4.5 wt%hydrogen can be released within 200min.In addition,the sample showed good cycling stability and reversibility.After 5cycles,dehydrogenation temperature and hydrogen capacity remained stable,and the total hydrogen capacity can reach 4.45 wt%,which is the 92.7%of that in the first cycle.The hydrogen storage properties of NaAlH₄ with 10 wt%Ti₃C₂ is obviously improved,which shows that two-dimensional Ti₃C₂ has better catalytic activity to NaAlH₄.Two-dimensional Ti₃C₂ annealed under different atmosphere were successfully prepared by heat-treatment method,and the hydrogen storage properties of NaAlH₄with Ti₃C₂ annealed under different gas ambients were tested.The results showed that NaAlH₄+10 wt%Ti₃C₂ annealed under C₂H₂ atmosphere had better hydrogen storage properties.Adding 10 wt%Ti₃C₂ annealed under C₂H₂ atmosphere can remarkably decrease the operating temperature and improve the cycling stability and reversibility for NaAlH₄.The onset temperature for dehydrogenation was lowered by 70°C relative to that of the pristine sample,and the hydrogen capacity of the first cycle is up to 4.97wt%.At 140°C,3.7 wt%of hydrogen can be released rapidly within 9 min,and 4.91wt%of hydrogen can be released within 200 min.Even at 100°C,the amount of hydrogen desorption can reach 3.5 wt%within 180 min.In addition,4.65 wt%hydrogen can be reversibly released after 5 cycles,which is 93.6%of that in the first cycle.The study shows that the synergistic effect of Ti-containing species and high carbon content catalyzes the decomposition reaction of NaAlH₄.Two dimensional Ti₃C₂ loaded CeF₃ nanopowders?named Ti₃C₂/CeF₃?were successfully prepared by solution self-assembly,and the effects of Ti₃C₂/CeF₃ on the hydrogen storage properties of NaAlH₄ were tested.The results showed that adding 10wt%Ti₃C₂/CeF₃ can significantly improve the hydrogen storage properties of NaAlH₄,and the operation temperature was significantly reduced,and the kinetics of hydrogen release was greatly improved.Compared with pure NaAlH₄,the initial dehydrogenation temperature of NaAlH₄ doped with 10 wt%Ti₃C₂/CeF₃ decreased by 68°C,the amount of hydrogen release could reach 5.25 wt%in the first cycle,which is 0.45 wt%higher than that of NaAlH₄ doped with 10 wt%Ti₃C₂,and 3.7 wt%of hydrogen was released from the first step.At 140°C,3.7 wt%of hydrogen can be released rapidly within 13min,and 4.78 wt%hydrogen can be released within 200 min.Even at 100°C,it can release 3.27 wt%hydrogen within 180 min.The cycling measurement shows that NaAlH₄ doped with 10 wt%Ti₃C₂/CeF₃ sample can maintain 4.8 wt%of hydrogen capacity after 5 cycles,which is the 91.4%of that in the first cycle.The study shows that two dimensional Ti₃C₂/CeF₃ can effectively improve the hydrogen storage properties of NaAlH₄.