Title : Stability optimization of NaBH4 via pH and H2O:NaBH4 ratios for large scale hydrogen production
Abstract:
There is an increasing need for alternative clean fuels, and hydrogen (H2) has long been considered as promising solution with high calorific value (142MJ/kg). However, storage of H2 and expensive processes for it generation have hindered its usage. Sodium borohydride (NaBH4) can potentially be used as an economically viable means of H2 storage. Thus far, there have been attempts to optimize the life of NaBH4 (half-life) in aqueous media by stabilizing it with sodium hydroxide (NaOH) for various pH values [1]. Other reports have shown that H2 yield and reaction kinetics remained constant for all ratios of H2O to NaBH4 > 30:1, without any acidic catalysts [2]. Here we highlight the importance of pH and H2O: NaBH4 ratio (80:1, 40:1, 20:1 and 10:1 by weight), for NaBH4 stabilization (half-life reaction time at room temperature) and corrosion minimization of H2 reactor components. It is interesting to observe that at any particular pH>10 (e.g., pH = 10, 11 and 12), the H2O: NaBH4 ratio does not have the expected linear dependence with stability. On the contrary, high stability was observed at the ratio of 10:1 H2O: NaBH4 across all pH>10. When the H2O: NaBH4 ratio is increased from 10:1 to 20:1 and beyond (till 80:1), a constant stability (% degradation) is observed with respect to time. For practical usage (consumption within 6 hours of making NaBH4 solution), 15% degradation at pH 11 and NaBH4: H2O ratio of 10:1 is recommended. Increasing this ratio, demands higher NaOH concentration at the same pH, thus requiring higher concentration or volume of acid (e.g., HCl) for H2 generation. The reactions are done with tap-water to render the results useful from an industrial standpoint. The observed stability regimes are rationalized based on complexes associated with NaBH4 when solvated in water, which depend sensitively on both pH and NaBH4: H2O ratio [3].
