Spontaneous hydrolysis of sodium borohydride in harsh conditions

Abstract The present study reports fundamental results about the spontaneous hydrolysis of sodium borohydride NaBH 4 , 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 °C, initial NaBH 4 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 BH 4− z (OH) z − , by XRD, IR and 11 B 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 NaBH 4 concentration was 0; (ii) all of the reactions BH 4 −  → BH 3 (OH) − , BH 3 (OH) −  → BH 2 (OH) 2 − , BH 2 (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 BH 4 − were found at 25% of conversion; (iii) the hydroxyborates are very short-lived intermediates and only traces of BH 3 (OH) − were detected.