Study On Preparation And Hydrogen Sorption Mechanisms Of NaBH₄-based Reversible Hydrogen Storage Composite Materials

Hydrogen energy is a highly efficient,renewable clean energy.The great challenge in hydrogen commercialization for onboard and stationary applications is to develop a high efficient hydrogen storage material with high volumetric and gravimetric hydrogen density.One of the promising options regarding with safety and high volumetric capacity is storing hydrogen in metal hydrides.However,no single metal hydride can fulfill the requirements relating to the high H₂ capacity,reaction enthalpy and kinetics for H₂ storage in the required operating temperature range for fuel cell vehicles.And the reaction temperatures of the reported materials are still high with poor reversibility.The objective of this thesis is to tailor the thermodynamics and improve the de-/rehydrogenation kinetics of NaBH₄ by employing material design strategies such as the addition of destabilizers,catalytic doping and graphene encapsulation,so as to develop efficient reversible hydrogen storage materials.Firstly,a series of NaBH₄-based reversible hydrogen storage composite materials with high efficiency,fast hydrogen uptake and release kinetics,which can be reversible under relative mild condition were obtained through ball milling method,using LnF₃（Ln=La,Ce,Pr,Nd,Sm,Gd,Ho,Er and Yb）as destabilization agents.Secondly,nano-sized NaBH₄@graphene composite with record high reversible H₂ capacity was developed by wet chemistry method.The hydrogen storage performance and sorption mechanism of each composite were systematically investigated.The major results are summarized as follows:1.The hydrogen storage performances of the 3NaBH₄-LaF₃ and NaBH₄-LaH₂composites were compatatively studied,as well as the different effect of F-and H-during H₂ charge and discharge processes.Experimental results corroborated that both LaF₃ and LaH₂ improved the reversibility of NaBH₄,especially,a superior hydrogen storage performance were found in 3NaBH₄-LaF₃.The 3NaBH₄-LaF₃ cmposite can store H₂ at 238^oC with reversible capacity upto 3.0 wt%after the 6^th cycle.F-subsititution of H-results in a further improvement of reaction thermodynamics and kinetics.The detailed role of functional anions and cations,reaction mechanisms and nucleation modes of the two composites have been proposed based on experimental and theoretical analyses.2.For 3NaBH₄-PrF₃ composite,a fast hydrogen uptake at 400 ^oC with reversible H₂capacity to 3 wt%was achieved.After addition of 5 mol%VF₃,the de-/rehydrogenation kinetics of the 3NaBH₄-PrF₃ composite were significantly improved:the onset dehydriding temperature of the ternary 3NaBH₄-PrF₃-5 mol%VF₃ composite was lowered down to 46^oC;the dehydrogenation can be finished in 2 min at 400 ^oC.A capacity of 1.2 wt%capacity can be reached at 84 ^oC.The results demonstrated the efficient catalytic effect of VB₂formed during dehydrogenation.3.The hydrogen storage mechanism of the 3NaBH₄-NdF₃ composite was carefully investigated.According to the pressure-composition-temperature（PCT）measurements,the de-/rehydrogenation enthalpies of the 3NaBH₄-NdF₃ composite were calculated to be 86.4kJ mol^-1 H₂ and-13.2 kJ·mol^-1 H₂.It was revealed that NdB₆,Nd₂H₅ and NaF were formed in the decomposition products,which can be rehydrogenated to NaBH₄,NaNdF₄ and NdF₃.The formation of intermediate-B,Nd and NdB₄ during dehydrogenation were detected,which accounts for the asymmetric hydriding/dehydriding behaviors in the 3NaBH₄-NdF₃composite.4.The 3NaBH₄-LnF₃（Ln=Ce,Sm,Gd and Yb）composites were studied as extensions,and the mechanisms associated with different effects of LnF₃（Ln=La,Ce,Pr,Nd,Sm,Gd,Ho,Er and Yb）on reversible hydrogen uptake in NaBH₄ were summarized based on careful comparisons.The results reveal that the key factors contribute to the excellent properties of 3NaBH₄-LnF₃ are the electronegativityχ_p of Ln³⁺within the range of 1.23-1.54;the stable electronic configuration of Ln³⁺,and the formation of Ln-B phase during dehydrogenation,as well as its the unique crystal structure.Meanwhile,it revealed the formation of biometallic borohydrid-NaHo（BH₄）₄ during ball-milling in the 3NaBH₄-HoF₃ composite.It also found that the 3NaBH₄-GdF₃ composite exhibits the best overall hydrogen storage properties among all the studied 3NaBH₄-LnF₃ composites,with high cyclability up to 51 cycles and fast kinetics.5.The sate-of-the-art architecture structure-nanosized NaBH₄ wrapped in graphene-NaBH₄@graphene reversible hydrogen storage composite was successfully prepared using a simple and robust wet chemistry method.It is revealed that NaBH₄@graphene composite possesses an excellent H₂ storage performance:the onset dehydrogenation temperature was lowered down to 40 ^oC,relatively mild reversible condition,high stability,high reversible H₂ storage capacity,fast H₂ uptake/release kinetics and without any by-products.Such structure restricts the hydride phase segregation/agglomeration,prevents the dehydrogenation products,such as Na,B,from escape,so as to maximize the hydrogen storage capacity of the composite.In summary,the effects of lanthanum florides on hydrogen storage performance of NaBH₄ were systematically investigated,and a series of 3NaBH₄-LnF₃ reversible hydrogen storage composites were obtained.The hydrogen storage mechanisms of those composites were studied carefully,which afford us criterions to design new borohydride-based H₂-storage systems and to optimize their performances.That the high reversible H₂ capacity,high stability,relative mild rechargeable condition,fast reaction kinetics of NaBH₄@graphene composite enable it the promising for the application for the hydrogen fuel cell car.The encapsulated architecture structure of NaBH₄@graphene opens up a new way to design and synthesis of the new materials applied in various research areas.