Physicochemical Properties of Alkaline Aqueous Sodium Metaborate Solutions

Background: The transition to a hydrogen fuel economy is hindered by the lack of a practical storage method and concerns associated with its safe handling. Chemical hydrides have the potential to address these concerns. Sodium borohydride (sodium tetrahydroborate, $NaBH4$⁠), is the most attractive chemical hydride for $H2$ generation and storage in automotive fuel cell applications, but recycling from sodium metaborate $(NaBO2)$⁠, is difficult and costly. An electrochemical regeneration process could represent an economically feasible and environmentally friendly solution. Method of Approach: We report a study of the properties of concentrated $NaBO2$ alkaline aqueous solutions that are necessary to the development of electrochemical recycling methods. The solubility, pH, density, conductivity, and viscosity of aqueous $NaBO2$ solutions containing varying weight percentages (1, 2, 3, 5, 7.5, and $10wt.%$⁠) of alkali hydroxides (NaOH, KOH, and LiOH) were evaluated at $25°C$⁠. The precipitates formed in supersaturated solutions were characterized by x-ray diffraction and scanning electron microscopy. Results: All $NaBO2$ physicochemical properties investigated, except solubility, increased with increased hydroxide ion concentration. The solubility of $NaBO2$ was enhanced by the addition of KOH to the saturated solution, but decreased when LiOH and NaOH were used. The highest ionic conductivity $(198.27S∕m)$ was obtained from the filtrate of saturated aqueous solutions containing more than $30wt.%$$NaBO2$ and $10wt.%$ NaOH prior to filtration. At $10wt.%$ hydroxide, the viscosity of the $NaBO2$ solution was the highest in the case of LiOH (11.38 cP) and lowest for those containing NaOH (6.37 cP). The precipitate was hydrated, $NaBO2$ for all hydroxides, but its hydration level was unclear. Conclusions: The use of KOH as the electrolyte was found to be more advantageous for the $H2$ storage and generation system based on $NaBO2$ solubility and solution half-life. However, the addition of NaOH led to the highest ionic conductivity, and its use seems more suitable for the electroreduction of $NaBO2$⁠. Further investigations on the impact of KOH and NaOH on the electroreduction of $NaBO2$ in aqueous media have the potential to enhance the commercial viability of $NaBH4$⁠.