Thermodynamic and kinetic study on hydrolysis of concentrated sodium borohydride solution

The hydrolysis of sodium borohydride (NaBH4) over efficient metal catalysts is a promising approach to hydrogen storage. An alkali such as NaOH is often added to stabilise the system in practical applications. The concentration of the NaBH4 solution should be as high as possible to improve energy density of the system. However, the byproduct sodium metaborate (NaB02) would become saturated and precipitate from the solution when the concentration of sodium borohydride is over a limit, resulting in piping blockage and the decrease of the catalyst efficiency. The theme of this thesis was to investigate the maximum NaB~ concentration. Below the maximum concentration, the precipitation of the by-product will not occur, and above the maximum concentration, the by-product tends to precipitate from the solution. Hydrogen generation rate was then investigated up to high concentration. The maximum concentration was studied using a thermodynamic approach. The relationship between the solubility and the temperature was derived based on the equality of the chemical potential of the solute in solution and in its solid state. The solubility data of NaBH4 and NaB02 were obtained by analysing the phase diagrams of NaBH4-NaOHH20 and NaB02-NaOH-H20 respectively. The model parameters were then determined by regression of the solubility data and the temperature. Activity coefficients of NaBH4 and NaB02 were needed during the regression and these were achieved by hydration analysis of the phase diagrams. The maximum concentration of NaBH4 was obtained by taking the maximum between the water in saturated NaBH4 solution and the sum of the water in saturated NaB02 solution and the water consumed for hydrolysis. The maximum concentration ofNaBH4 is mainly determined by the solubility of NaB02. The modelling of the maximum concentration was then validated experimentally. The rate of hydrogen generation from NaBH4 hydrolysis was then investigated over carbon supported ruthenium catalyst over a wide range of concentrations. The intrinsic hydrolysis rate is zero-order to NaBH4 concentration, and has a linear relationship with the basicity of the solution (-ln[OH]). The overall kinetics was modelled by building diffusion and heat effect into the intrinsic rate expression. Experimental results agree well with model prediction.