Improvement of the LiAlH4-NaBH4 System for Reversible Hydrogen Storage

The hydrogen storage properties of a combined LiAlH4-NaBH4 system have been investigated. It was found that there is a mutual destabilization between the LiAlH4 and the NaBH4. Two major dehydrogenation steps with hydrogen capacities of 3.1 wt % and 5.2 wt % were observed for the system with a molar ratio of LiAlH4:NaBH4 = 1: 1 at around 250 and 600 degrees C, respectively. The onset dehydrogenation temperatures for the first and the second steps in the LiAlH4-NaBH4 mixture were decreased to 95 and 450 degrees C, respectively, which are 30 and 50 degrees C lower than those of bare LiAlH4 and NaBH4, respectively. Furthermore, doping with TiF3 significantly decreases the hydrogen release temperature compared to the undoped system. The TiF3 doping can further decrease the onset temperatures of the first and second steps to 60 and 300 degrees C. The presence of TiF3 also destabilizes the NaH phase, which leads to an increased desorption capacity. Pressure-composition temperature (PCT) and van't Hoff plots illustrate that the decomposition enthalpy of NaBH4 in the TiF3-doped LiAlH4-NaBH4 system is decreased from 106.8 kJ/(mol of H-2) for pure NaBH4 to 68.16 kJ/(mol of H-2). In addition, about 4 wt % hydrogen can be reversibly stored by the dehydrogenated product, and the formation of NaBH4 was detected by X-ray diffraction in the rehydrogenated sample.