Spontaneous hydrolysis of sodium borohydride in harsh conditions

The present study reports fundamental results about the spontaneous hydrolysis of sodium borohydride NaBH4, which is a potential hydrogen storage material for small, portable applications. The reaction (without stabilizing agent or catalyst) was carried out at temperatures of 30-80 degrees C, initial NaBH4 concentrations of 0.63-6.19 mol L-1 (2.3-18.9 wt%), and for unbuffered solutions, which are harsher experimental conditions than those reported so far. The H-2 evolution and the subsequent pH variation were observed to determine the reaction kinetic parameters and characterize the hydrolysis intermediates, i.e. the hydroxyborates BH4-x(OH)(z)(-), by XRD, IR and B-11 NMR. It was found that: (i) the apparent activation energy of the reaction was 98 +/- 10 kJ mol(-1) and the reaction order versus the initial NaBH4 concentration was 0; (ii) all of the reactions BH4- -> BH3(OH)(-), BH3(OH)(-) -> BH2(OH)(2)(-), BH2(OH)(2)(-) -> BH(OH)(3)(-) -> and BH(OH)(3)(-) B(OH)(4)(-) took place simultaneously; (iii) only 25 mol% of B(OH)(4)(-) and 75 mol% of BH4- were found at 25% of conversion; (iii) the hydroxyborates are very short-lived intermediates and only traces of BH3(OH)(-) were detected. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.