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.