Study On Hydrogen Absorption/Desorption Properties Of NaAlH₄ Enhanced By Ti₃C₂-Based Composites

Hydrogen has become an ideal energy carrier because of its high energy density,wide sources,cleanness and pollution-free.Reversible hydrogen absorption/desorption of metal hydride is one of the methods for the separation and purification of industrial by-product hydrogen（coke oven gas,etc.）.NaAlH₄ is expected to be used for the separation and purification of industrial by-product hydrogen due to its high hydrogen storage capacity and mild hydrogen absorption/desorption conditions,but its hydrogen absorption/desorption kinetics performance needs to be improved.In this paper,Ti₃C₂-based composites（CeF₃/Ti₃C₂,nitrogen doped Ti₃C₂ and nitrogen doped carbon coated Ti₃C₂）were used as additives to improve the hydrogen absorption/desorption performance of NaAlH₄,and the effects of CO and CO₂ on the hydrogen storage performance of NaAlH₄ containing additives were studied to obtain NaAlH₄ hydrogen storage system with excellent hydrogen absorption/desorption performance and improve its hydrogen separation and purification performance.The main conclusions are as follows:（1）The CeF₃/Ti₃C₂ composites were prepared by combining Ce³⁺with the F-functional group on the surface of Ti₃C₂ via hydrothermal method.The effects of hydrothermal reaction temperature and the mass ratio of CeCl₃·7H₂O to Ti₃C₂ on the microstructure and catalytic activity of CeF₃/Ti₃C₂ were studied.The results indicate that the CeF₃/Ti₃C₂ composite synthesized at 80℃ when the mass ratio of CeCl₃·7H₂O to Ti₃C₂ is 14%exhibits the best catalytic performance,which is obviously superior to that of CeF₃ and Ti₃C₂.After adding 10 wt%CeF₃/Ti₃C₂,the onset dehydrogenation temperature of NaAlH₄ is lowered to 87℃ and 3.0 wt%hydrogen can be released in 80min at 100℃.After 10 cycles,the amount of hydrogen release is about 4.68 wt%with a capacity retention rate of 94.5%.Microscopic analysis shows that the Ti-F-Ce structure in CeF₃/Ti₃C₂ remains stable during ball milling and hydrogen absorption/desorption,which improves the stability of Ti⁰ active center and avoids its transformation into Ti-Al.The synergistic effect of uniformly distributed Ti⁰ species and stable Ti-F-Ce structure improves the hydrogen storage performance of NaAlH₄.（2）Nitrogen doped Ti₃C₂（N-Ti₃C₂）was prepared by modifying Ti₃C₂ with nitrogen element instead of carbon element through heat treatment of NH₃ atmosphere,and the effect of N-Ti₃C₂ on the hydrogen absorption/desorption performance of NaAlH₄ was studied.The results show that N-Ti₃C₂ prepared by heat treatment at500℃ under NH₃ atmosphere exhibits the optimum catalytic activity,and it can effectively reduce the dehydrogenation temperature and improve the dehydrogenation kinetics and cycle stability of NaAlH₄.With the addition of 10 wt%N-Ti₃C₂,the initial dehydrogenation temperature is reduced to 87℃,the first step of dehydrogenation is completed in 90 min at 100℃,and 4.4 wt%of hydrogen can be maintained after 15cycles.The doping of nitrogen can stabilize Ti⁰ and Ti³⁺formed in situ during ball milling.Stable Ti N is conducive to improve the dehydrogenation properties of NaAlH₄.Ti-species(TiN,Ti⁰ and Ti³⁺)and the interaction between Ti⁰ and pyridine nitrogen jointly improve the dehydrogenation kinetics,cycle stability and reversibility of NaAlH₄.（3）Nitrogen doped carbon coated Ti₃C₂（Ti₃C₂/NC）was prepared by surface coating of Ti₃C₂ via the combination of dopamine self-polymerization and heat treatment,and the effect of the Ti₃C₂/NC composite on the hydrogen storage performance of NaAlH₄ was studied.The results show that the addition of Ti₃C₂/NC significantly reduces the dehydrogenation temperature and improves the dehydrogenation kinetics and cycle stability of NaAlH₄.The initial dehydrogenation temperature of NaAlH₄+10 wt%Ti₃C₂/NC is lowered to 85℃,3.0 wt%hydrogen is released within 4 min at 140℃,and the first step of dehydrogenation is completed in57 min at 100℃.After 15 cycles,the hydrogen capacity is 4.66 wt%and the capacity retention rate is as high as 96.3%.The coated carbon layer can effectively stabilize the Ti-based active species(Ti⁰ and Ti³⁺)formed in situ during ball milling,which improves the hydrogen absorption/desorption cycle stability of NaAlH₄.The uniform distribution of Ti-species,the existence of coated carbon layer and the interaction between Ti⁰ and pyridine nitrogen are the main reasons for the significant improvement of hydrogen storage performance for NaAlH₄.（4）Taking NaAlH₄+10 wt%Ti₃C₂/NC system as the research object,the as-milled sample was treated at room temperature and the dehydrogenated sample was treated under simulated hydrogenation conditions using Ar/CO（92%/8%）and Ar/CO₂（97%/3%）,respectively,and the effects of CO and CO₂ on the hydrogen absorption and desorption performance of the NaAlH₄+10 wt%Ti₃C₂/NC system were studied.The results indicate that the dehydrogenation capacity of the as-milled sample after the treatment of Ar/CO is maintained at 4.45 wt%and the capacity retention rate is as high as 92%after 10 cycles,while the hydrogen release capacity of the dehydrogenated sample after the treatment of Ar/CO is reduced to 4.13 wt%and the capacity retention rate is reduced to 85%,and the hydrogen release capacity of the as-milled and dehydrogenated samples after the treatment of Ar/CO₂ is only 4.0 wt%after 10 cycles.Microscopic analysis shows that the presence of CO or CO₂ in inert atmosphere gas will significantly change the composition of the dehydrogenated NaAlH₄+10 wt% Ti₃C₂/NC samples,resulting in the decline of hydrogen absorption performance,which is the reason for the decrease of cycle reversibility and stability.