A boron-11 NMR study of the stability of the alkaline aqueous solution of sodium borohydride that is both an indirect fuel and a direct fuel for low-temperature fuel cells

Alkaline aqueous solution of sodium borohydride NaBH4 (denoted SB-fuel) is an indirect fuel when it is used to generate H2 by hydrolysis, with the as-generated H2 feeding a fuel cell, and it is a direct fuel when it is an anodic fuel of a direct fuel cell. However, SB-fuel suffers from a major drawback: NaBH4 spontaneously hydrolyzes. Our study falls within this context. We studied the instability, at the NMR scale and over 12 weeks, of a series of SB-fuels (initial NaBH4 concentration from 3.65 to 31.22 wt%, NaOH concentration from 1 to 16 M, and temperature between -15 and 60 degrees C) to find the conditions at which SB-fuel can be stored for weeks in relative safety. We found that SB-fuel with a NaOH concentration of >= 8 M is relatively stable under cold conditions (-15 and 4 degrees C). In these conditions, NaBH4 is not prevented from hydrolyzing, but the reaction is significantly mitigated. Otherwise, our study highlights the gaps in our understanding of the SB-fuel, emphasizes SB-fuel is a new concept of fuel (it should not be seen as any current fuel) , points out the challenges for attaining higher technology readiness levels.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the instability of alkaline aqueous solution of sodium borohydride (SB-fuel), finding that it can be relatively stable under cold conditions (-15 and 4 degrees C) when the NaOH concentration is >= 8M, mitigating the spontaneous hydrolysis of NaBH4.