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Electrochemical recycling of sodium borohydride for hydrogen storage : physicochemical properties of sodium metaborate solutions Cloutier, Caroliine R.

Abstract

The large-scale adoption of a "hydrogen economy" is hindered by the lack of a practical storage method and concerns associated with its safe handling. Chemical hydndes 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 (NaB0₂) is difficult and costly. An electrochemical regeneration process could represent an economically feasible and environmentally friendly solution. In this thesis, the properties of diluted NaB0₂ aqueous solutions and concentrated NaB0₂ alkaline aqueous solutions that are necessary for the development of electrochemical recycling methods have been studied. The conductivity and viscosity of dilute aqueous solutions of NaB0₂ were measured as a function of concentration at 25°C. Also, the solubility, pH, density, conductivity and viscosity of the filtrate of saturated aqueous NaB0₂ solutions containing varying weight percentages (1, 2, 3, 5, 7.5 and 10 wt%) of alkali hydroxides (NaOH, KOH and LiOH) were evaluated at 25°C. Selected experiments were repeated at 50 and 75°C to investigate the effect of temperature on the NaB0₂ alkaline aqueous solution solubility and physicochemical properties. Preliminary experiments to investigate the effect of glycine (C₂H₅N0₂), the smallest amino acid, on the solubility and physicochemical properties of NaB0₂ alkaline aqueous solutions were conducted at 25°C. Furthermore, the precipitates formed in the supersaturated 10 wt% alkaline aqueous NaB0₂ solutions at 25°C were characterized by X-Ray Diffraction and Scanning Electron Microscopy. 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 NaBH₄ half-life due to the pH effect. However, the addition of NaOH led to the highest ionic conductivity, and its use seems more suitable for the electroreduction of NaB0₂. Further investigations on the impact of KOH and NaOH on the electroreduction of NaB0₂ in aqueous media have the potential to enhance the commercial viability of this H₂ generation and storage system.

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