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TECHNICAL PAPERS

Physicochemical Properties of Alkaline Aqueous Sodium Metaborate Solutions

[+] Author and Article Information
Caroline R. Cloutier

Department of Material Engineering,  University of British Columbia, 309-6335 Stores Road, Vancouver, B.C., V6T 1Z4, Canadaccloutier@chml.ubc.ca

Akram Alfantazi

Department of Material Engineering,  University of British Columbia, 309-6335 Stores Road, Vancouver, B.C., V6T 1Z4, Canada

Elod Gyenge

Department of Chemical and Biological Engineering,  University of British Columbia, 207-2360 East Mall, Vancouver, B.C., V6T 1Z3, Canada

J. Fuel Cell Sci. Technol 4(1), 88-98 (Apr 17, 2006) (11 pages) doi:10.1115/1.2393310 History: Received November 30, 2005; Revised April 17, 2006

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 (198.27Sm) 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.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

NaBO2 solubility as a function of hydroxide content at 25°C±3°C

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Figure 2

Filtrate pH as a function of hydroxide content at 25°C±3°C

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Figure 3

Estimated half-life of alkaline aqueous solutions of NaBH4 as a function pH at 25°C±3°C using Eq. 5

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Figure 4

Filtrate ionic conductivity as a function of hydroxide content of the saturated NaBO2 solution at 25°C±3°C

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Figure 5

Filtrate ionic conductivity as a function of hydroxide content of the unsaturated 5wt.%NaBO2 aqueous solution at 25°C±3°C

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Figure 6

Molar conductivity as a function of hydroxide content of the unsaturated 5wt.%NaBO2 aqueous solution at 25°C±3°C

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Figure 7

Dynamic viscosity of saturated NaBO2 solution as a function of hydroxide content at 25°C±3°C

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Figure 8

Walden product as a function of hydroxide content at 25°C±3°C

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Figure 9

Specific gravity as a function of hydroxide content at 25°C±3°C

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Figure 10

Intensity as a function of diffraction angle for pure NaBO2.2H2O and Precipitates of 10wt.% hydroxide NaBO2.2H2O supersaturated aqueous solutions

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Figure 11

Scanning electron microscope precipitate crystal images. (a) Pure NaBO2.2H2O flakes (SE, WD14.1mm, ×45, 1mm), (b) NaBO2.2H2O Crystal (SE, WD15.8mm, ×10.0k, 5μm), (c) 10wt.% NaOH, Saturated NaBO2.2H2O Aqueous Solution Precipitate (BSE2, WD14mm, ×2.0k, 20μm), (d) 10wt.% KOH, Saturated NaBO2.2H2O Aqueous Solution Precipitate (BSE2, WD14mm, ×500, 100μm), (e) 10wt.% LiOH, Saturated NaBO2.2H2O Aqueous Solution Precipitate (BSE2, WD13.4mm, ×1.0k, 50μm), (f) 10wt.% LiOH Saturated NaBO2.2H2O Aqueous Solution Precipitate (BSE2, WD13.3mm, ×1.0k, 50μm).

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