The Hydrolysis and Oxidation Behavior of Lithium Borohydride and Magnesium Hydride Determined by Calorimetry
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1098-HH03-14
The Hydrolysis and Oxidation Behavior of Lithium Borohydride and Magnesium Hydride Determined by Calorimetry Kyle S Brinkman1,2, Joshua R. Gray1, Bruce Hardy3, and Donald L. Anton1 1 Energy Security, Savannah River National Laboratory (SRNL), Aiken, SC, 29808 2 Materials Science and Technology, Savannah River National Lab (SRNL), Aiken, SC, 29808 3 Engineering Modeling and Simulation, Savannah River National Laboratory (SRNL), Aiken, SC, 29808 ABSTRACT Lithium borohydride, magnesium hydride and the 2:1 “destabilized” ball milled mixtures (2LiBH4:MgH2) underwent liquid phase hydrolysis, gas phase hydrolysis and air oxidation reactions monitored by isothermal calorimetry. The experimentally determined heats of reaction and resulting products were compared with those theoretically predicted using thermodynamic databases. Results showed a discrepancy between the predicted and observed hydrolysis and oxidation products due to both kinetic limitations and to the significant amorphous character of observed reaction products. Gas phase and liquid phase hydrolysis were the dominant reactions in 2LiBH4:MgH2 with approximately the same total energy release and reaction products; liquid phase hydrolysis displayed the maximum heat flow for likely environmental exposure with a peak energy release of 6 (mW/mg). INTRODUCTION In order to design commercially viable solid state hydrogen storage systems, it is important to understand and quantify the environmental reactivity of the active species in possible environmental exposure scenarios. The “destabilized” mixture of 2LiBH4:MgH2 has been reported to exhibit >10wt% H capacity while being rechargeable under the constraints of reasonable pressure and temperature (1 to 10 atm and 20 to 100oC) [1]. This material system is currently one of a number of leading candidate materials for solid state hydrogen storage [2]. The hydrolysis of compounds such as sodium borohydride has been a topic of intense focus for hydrogen generation, however much less is understood about the hydrolysis and oxidation under environmental exposure scenarios [3]. This report aims to fill that gap by presenting a summary of thermodynamic calculations and substantiating calorimetric experiments performed in order to quantify both the rate and the amount of the energy released; as well as to characterize the reaction products resulting from water and air exposure of 2LiBH4:MgH2. EXPERIMENTAL LiBH4 and MgH2 (Aldrich) were studied individually in the as-received forms as well as the 2:1 “destabilized” ball milled mixture [1]. Materials were characterized in the fully hydride state and hydrolysis was performed in a Calvet calorimeter (Setaram C-80)
equipped with a mixing cell using both neutral and acid water to react nominally 5-10 mg of solid with 1ml of liquid. Gas phase reactivity examining oxidation and gas phase
hydrolysis was performed at varying relative humidity levels and temperatures using the calorimeter equipped with a flow cell using argon or air as the carrier gas with a flow rate of 10
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