Understanding the effect of TiO 2, VCl 3, and HfCl 4 on hydrogen desorption/absorption of NaAlH 4

Abstract

The main objective of this work was to investigate the different effects of transition metals (TiO 2, VCl 3, HfCl 4) on the hydrogen desorption/absorption of NaAlH 4. The HfCl 4 doped NaAlH 4 showed the lowest temperature of the first desorption at 85 °C, while the one doped with VCl 3 or TiO 2 desorbed at 135 °C and 155 °C, respectively. Interestingly, the temperature of desorption in subsequent cycles of the NaAlH 4 doped with TiO 2 reduced to 140 °C. On the contrary, in the case of NaAlH 4 doped with HfCl 4 or VCl 3, the temperature of desorption increased to 150 °C and 175 °C, respectively. This may be because Ti can disperse in NaAlH 4 better than Hf and V; therefore, this affected segregation of the sample after the desorption. The maximum hydrogen absorption capacity can be restored up to 3.5 wt% by doping with TiO 2, while the amount of restored hydrogen was lower for HfCl 4 and VCl 3 doped samples. XRD analysis demonstrated that no Ti-compound was observed for the TiO 2 doped samples. In contrast, there was evidence of Al–V alloy in the VCl 3 doped sample and Al–Hf alloy in the HfCl 4 doped sample after subsequent desorption/absorption. As a result, the V- or Hf-doped NaAlH 4 showed the lower ability to reabsorb hydrogen and required higher temperature in the subsequent desorptions.