Kinetic measurement and prediction of the hydrogen outgassing from the polycrystalline LiH/Li2O/LiOH system

Due to the exothermic reaction of lithium hydride (LiH) salt with water during transportation and handling, there is always a thin film of lithium hydroxide (LiOH) present on the LiH surface. In dry or vacuum storage, this thin LiOH film slowly decomposes. The technique of temperature-programmed reaction/decomposition (TPR) was employed in combination with the isoconversion method of thermal analysis to determine the outgassing kinetics of H2O from pure LiOH and H2 and H2O from this thin LiOH film. H2 production via the reaction of LiH with LiOH, forming a lithium oxide (Li2O) interlayer, is thermodynamically favored, with the rate of further reaction limited by diffusion through the Li2O and the stability of the decomposing LiOH. Lithium hydroxide at the LiOH/vacuum interface also decomposes easily to Li2O, releasing H2O which subsequently reacts with LiH in a closed system to form H2. At the onset of dry decomposition, where H2 is the predominant product, the activation energy for outgassing from a thin LiOH film is lower than that for bulk LiOH. However, as the reactions at the LiH/Li2O/LiOH and at the LiOH/vacuum interfaces proceed, the overall activation energy barrier for the outgassing approaches that of bulk LiOH decomposition. The kinetics developed here predict a hydrogen evolution profile in good agreement with hydrogen release observed during long term isothermal storage.