Hydrogen discharging and recharging of NaAIH0subscript 4] and HfCI[subscript 4]-NaAIH

Abstract

Mechanisms of hydrogen desorption and absorption were investigated by the X-ray diffraction technique. NaH and AIH which play a major role in both dehydrogenation and rehydrogenation, were found. In addition, HfH , O,x2<, can be observed in XRD patterns of the doped NaAIH . Attempts were also made to reduce the formation of Hf-AI alloys during the hydrogen desorption/absorption of HfCI-NaAIH by adding a chelating complex (H PP or HfCI TPP) as a co-catalyst. The desorption temperature of HfCI-NaAIH doped with the co-catalyst is lower (75 C) than that of the undoped co-catalyst one (90 C). Moreover, there was no Hf-AI alloys detected. However, the amount of reabsorbed hydrogen in subsequent cycles reduced because of the incomplete rehydrogenation from Na AIH to NaAIH. When using CuCrO as another co-catalyst, the hydrogen desorption took place at the temperature range of 62-270 C. The temperature of the first hydrogen desorption decreased much further to 62 C and the hydrogen released was 5.9 wt%, which was higher than the theoretical value of 5.6 wt%. The equivalent mole of H released per one equivalent mole of NaAIH was increased to 1.65, compared to 1.5 of NaAIH doped with HfCI alone.