Mechanistic studies of combustion-stimulated hydrogen generation from sodium borohydride

The mechanism of hydrogen evolution from sodium borohydride reaction with water is investigated for the case where the process is stimulated by metal (magnesium or aluminum) combustion in water, with H-2 yield similar to 7 wt%. With this goal, process temperature, combustion velocity, reaction rates and product composition were studied experimentally using direct thermocouple measurements, video recording, TGA and XRD analysis. Both combustion temperature (950-1600 degrees C) and front velocity (0.05-0.4 cm/s) were found to increase with the amount of metal fuel in the mixture, correlating with the overall reaction heat effect. Kinetic parameters were estimated based on TGA measurements for temperatures 400-700 degrees C. Experiments demonstrate that mixtures of Mg/NaBH4/H2O exhibit higher conversion than Al/NaBH4/H2O. Using deuterated water and mass-spectral analysis of the HD to DD ratio in the evolved hydrogen, it was determined that combustion favors borohydride pyrolysis over hydrolysis. (c) 2006 Elsevier Ltd. All rights reserved.