| As a chemical hydrogen storage material,sodium borohydride has the following advantages:high density of hydrogen storage,high purity of hydrogen production,low reaction temperature and high energy recycling efficiency.It can be used to manufacture direct borohydride fuel cell?DBFC?.The solvation and ion association of BH4-in the alkaline solution of NaBH4 play an important role in the stability of NaBH4 solution in the direct hydride fuel cell.The study of the structure of sodium borohydride alkaline solution is helpful to understand the hydrolysis mechanism of sodium borohydride and provide theoretical basis for the development of new fuel cell.In the present work,sodium borohydride alkaline solutions were used as the research object,and Raman spectroscopy was used to monitor the structural changes in the hydrolysis process and crystallization process of sodium borohydride alkaline solution.The density functional theory?DFT?method was used to optimize the micro-structure of sodium borohydride alkaline solution and explore its detailed structure information.The influence of sodium hydroxide concentration on the microstructure of sodium borohydride solution was studied by combining the X-ray scattering experiment with the model calculation:?1?The crystallization of sodium borohydride in alkaline solution and the hydration process of sodium borohydride crystal were tracked in situ by changing the environmental humidity.No new species formation was observed,but the possibility of failing to capture key information due to the limitation of Raman spectrum was not ruled out.With the increase of humidity,the interaction between water and B-H is weakened,leading to the decrease of the characteristic peak intensity of B-H.?2?The influence of sodium hydroxide content on the structure of sodium borohydride aqueous solution was analyzed in detail by means of X-ray scattering experiment and model calculation method.The peak at 0.96?represents O-H interaction,2.38?is Na-O?I,w?,3.33?belongs to O?W?-O?W?in the first hydration layer of sodium atom,and the peak at 3.70?is B-O?W?interaction,4.28?is Na-O?II,w?interaction in the second hydrated layer of sodium atom.The contribution of the two hydrogen bonds to the total scattering intensity is very small,and the model fitting result is 1.68-1.99?.With the increase of NaOH concentration,the hydration distance near Na+increases,and the hydrate number around Na+decreases,and the first hydrate layer of Na+is destroyed.The B-O?W?distance increased from 0.349 nm to 0.377 nm,and the number of hydroboration decreased from 5.89 to 3.21.The hydrogen bond is destroyed,which makes the solution more stable.The Na-B distance is between 0.280-0.254 nm.With the increase of NaOH concentration,the coordination number has no clear change rule.Because the contribution of Na+-BH4-contact ion to the total scattering intensity is very small,its radial distribution function only gives the average information of solution structure.Na-O?OH-?interaction was enhanced,the coordination number increased from 0.48to 0.63,and the distance decreased from 0.298 nm to 0.274 nm.The hydrogen bond in the body water is destroyed by hydrogen peroxide.?3?DFT calculation method is used to design and optimize the possible configuration of[Na3·BH4·?OH?2·?H2O?10]clusters,and the theoretical model of Raman spectrum is analyzed:optimize[Na3·BH4·?OH?2·?H2O?10]at the level of?B97XD/def2TZVP.Among the 20 possible configurations,NBOW10A,NBOW10B,NBOW10C,NBOW10D,NBOW10E and NBOW10F are the most likely.The average distance of Na-O?I,w?and Na Na is 0.236 nm and 0.336 nm,which are similar to the XRS results of BH10N5.The average distance of Na-B is 0.261 nm,the average distance of dihydrogen bond is 0.191 nm,the average distance of B-O?W?is 0.348 nm.DFT calculation and XRS experimental results complement each other,and verify each other that the results are reliable.The proportion of NBOW10A,NBOW10B,NBOW10C,NBOW10D,NBOW10E and NBOW10F in the system was72.82%,21.87%,3.86%,1.21%,0.23%and 0.01%,respectively. |
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