Capacity enhancement of aqueous borohydride fuels for hydrogen storage in liquids



Publication Details

Schubert, D., Neiner, D., Bowden, M., Whittemore, S., Holladay, J., Huang, Z. & Autrey, T. (2015). Capacity enhancement of aqueous borohydride fuels for hydrogen storage in liquids. Journal of Alloys and Compounds, 645 (S1), S196-S199.


In this work we demonstrate enhanced hydrogen storage capacities through increased solubility of sodium borate product species in aqueous media achieved by adjusting the sodium (NaOH) to boron (B(OH)3) ratio, i.e., M/B, to obtain a distribution of polyborate anions. For a 1:1mol ratio of NaOH to B(OH)3, M/B=1, the ratio of the hydrolysis product formed from NaBH4 hydrolysis, the sole borate species formed and observed by 11B NMR is sodium metaborate, NaB(OH)4. When the ratio is 1:3 NaOH to B(OH)3, M/B=0.33, a mixture of borate anions is formed and observed as a broad peak in the 11B NMR spectrum. The complex polyborate mixture yields a metastable solution that is difficult to crystallize. Given the enhanced solubility of the polyborate mixture formed when M/B=0.33 it should follow that the hydrolysis of sodium octahydrotriborate, NaB3H8, can provide a greater storage capacity of hydrogen for fuel cell applications compared to sodium borohydride while maintaining a single phase. Accordingly, the hydrolysis of a 23wt.% NaB3H8 solution in water yields a solution having the same complex polyborate mixture as formed by mixing a 1:3M ratio of NaOH and B(OH)3 and releases >8eq of H2. By optimizing the M/B ratio a complex mixture of soluble products, including B3O3(OH)5 2-, B4O5(OH)4 2-, B3O3(OH)4 -, B5O6(OH)4 - and B(OH)3, can be maintained as a single liquid phase throughout the hydrogen release process. Consequently, hydrolysis of NaB3H8 can provide a 40% increase in H2 storage density compared to the hydrolysis of NaBH4 given the decreased solubility of sodium metaborate.

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