Proprieties, Structure And Electrochemical Reduction Of Aqueous Sodium Metaborate Borate Solution

Physico-chemical properties such as density, viscosity, diffusion coefficient and conductivity etc of aqueous solutions are closely related with liquid structure factor. In order to get the distribution, structure and hydration characteristics of polyborate anion in aqueous borate solution. All the research in the thesis were focused on the aqueous Na B(OH)4 solution, a potential raw amterial for NaBH4 electrochemical preparation, the structure, proprieties and electrochemical reduction of aqueous NaB(OH)4 solution were taken into consideration.1. Physicochemical properties such as density, electrical conductivity, viscosity, acidity and especially the polyborate distribution at moderate temperatures, affect the H2 generation and storage as well as the electrochemical recyclability of NaB(OH)4. Raman spectra of some concentrated solutions and their microcrystal were also summed. In order to assign Raman shifts of aqueous borate solutions deliberatively, the geometries, energies and vibrational frequencies of various polyborates were calculated at the B3LYP/aug-cc-pVDZ level. The calculated symmetrical stretching vibration(vs.) Raman shifts can be used as the characteristic frequency of polyborates in aqueous phase. Polyborates distribution in those solutions as a function of concentration was calculated using measured pH value and literature equilibrium constants; Density of aqueous NaB(OH)4 solutions were further measured as a functions of concentration from dilution to saturation at 293.15, 298.15, 303.15, 313.15 and 323.15 K, and the volumetric properties of NaB(OH)4 solutions have been determined from those precision density measurements. Conductivity and Viscosity data were analyzed, and both the volumetric and transport properties indicate the B(OH)4- behaves as a structure disordering anions.2. Further progresses of NaB(OH)4 electroreduction suffered a hiatus due to lack of research on the understanding of the complex electrode mechanism and electrode kinetics of the B(OH)4- electro-reduction reaction. The micro-structure of aqueous NaB(OH)4 solutions, especially the hydration of B(OH)4- may plays an important role in the B(OH)4- electro-reduction process. In present thesis, the structural analysis of NaB(OH)4 aqueous solutions(Na BO2?RH2O, R=10, 15, 20) by the rapid liquid θ-θ X-ray diffractometry with a highly effective X’celerator is reported at 298±0.5K. Radial distribution functions(RDF) and theoretical partial radial distributions functions for B-O, O-O, Na-O, Na-B and Na-Na atom pairs were obtained from precisely diffraction data processing. The solution structure was described in sodium ions, hydrated monborate anions and ion association three discrete chemical species, and the effects of concentration and temperature on the structure of the NaB(OH)4 solutions were discussed, then the form mechanism for crystallization seed were also suggested.3. In order to understand the detail micro-structure of the main anions in aqueous NaB(OH)4 solutions. The microscopic structure, energy parameters of [Na(H2O)5]+,(n =1-12) and [B(OH)4(H2O)n]-,(n=1-14) clusters were probed by a rigorous and systematical density functional theory(DFT) caculation in both gaseous and aqueous phase at B3LYP/aug-cc-pVDZ level. Car-Parrinello molecular dynamics simulation and Raman spectra were also studied to confirm our study results. All the simulation results were also consistent with our previous study of sodium metaborate by X-ray scattering.4. Tetrahydridoborates(BH4-) is considered as a versatile reducing agents in various organic and inorganic processes. Especially the high hydrogen presented and released makes NaBH4 the most attractive chemical hydride for H2 generation and storage in automotive fuel cell applications. While the extensive use of NaBH4 is impeded by the high price of Na BH4, and the disposal of large quantities of the by-product NaB(OH)4 also become a hot potato, electroreduction regeneration NaBH4 from NaB(OH)4 become a hot topic. In order to study the electrochemical hydroboration of B(OH)4-, an accuracy determination method for trace of BH4- in alkaline solution was given, the effect of electrode area, concentration of OH- and B(OH)4- were taken into consideration. BH4- can be indicated as low as 10-5 mol/L, and a linear dependence between the oxidation peak formed by Na BH4 direct oxidation and the concentration of NaBH4 was presented in the concentration range from 1×10-4 to 1×10-2 mol/L, which can used as a quantity electroanalysis method for BH4-; Then the electrochemical reduction mechanism was deliberated, and the researched LSVs shows none feature for the electrochemical reduction B(OH)4- to BH4- can be found which indicated an indirect electrochemical reduction mechanism; Plenty of attempts consider the high hydrogen evolution overpotential material(avoid the of H2 O reduction), and abnormity electrode and pulse-powered electrolysis(increase the chances of B(OH)4- reduction), and other methods were considered. Unfortunately, none of the electrochemical reduction experiments tried here gave convincing evidence of borohydride formation, combining the properties and structure of aqueous NaB(OH)4 solutions, the possible reasons were given.