Understanding the effect of TiO2, VCl3, and HfCl4 on hydrogen desorption/absorption of NaAlH4

The main objective of this work was to investigate the different effects of transition metals (TiO2, VCl3, HfCl4) on the hydrogen desorption/absorption of NaAlH4. The HfCl4 doped NaAlH4 showed the lowest temperature of the first desorption at 85 °C, while the one doped with VCl3 or TiO2 desorbed at 135 °C and 155 °C, respectively. Interestingly, the temperature of desorption in subsequent cycles of the NaAlH4 doped with TiO2 reduced to 140 °C. On the contrary, in the case of NaAlH4 doped with HfCl4 or VCl3, the temperature of desorption increased to 150 °C and 175 °C, respectively. This may be because Ti can disperse in NaAlH4 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 TiO2, while the amount of restored hydrogen was lower for HfCl4 and VCl3 doped samples. XRD analysis demonstrated that no Ti-compound was observed for the TiO2 doped samples. In contrast, there was evidence of Al–V alloy in the VCl3 doped sample and Al–Hf alloy in the HfCl4 doped sample after subsequent desorption/absorption. As a result, the V- or Hf-doped NaAlH4 showed the lower ability to reabsorb hydrogen and required higher temperature in the subsequent desorptions.