A reality check on using NaAlH4 as a hydrogen storage material

Sodium aluminum hydride or sodium alanate (NaAlH₄) has been considered as a potential material for hydrogen storage. Although its theoretical hydrogen storage capacity is 5.5 wt.% at 250 °C, the material still has its drawback in the regeneration issue. With the use of certain catalysts, the regeneration problem can somewhat be alleviated with added benefits in the decrease in the hydrogen decomposition temperature and the increase in the decomposition rate. This work summarizes what we have learned from the decomposition of NaAlH₄ with/without catalysts and co-dopants. The decomposition was carried out using a thermovolumetric apparatus. For the tested catalysts—HfCl₄, VCl₃, TiO₂, TiCl₃, and Ti—the decomposition temperature of the hydride decreases; however, they affect the temperature in the subsequent cycles differently and TiO₂ appears to have the most positive effect on the temperature. Sample segregation and the morphological change are postulated to hinder the reversibility of the hydride. To prevent the problems, co-dopants—activated carbon, graphite, and MCM-41—were loaded. Results show that the hydrogen reabsorption capacity of HfCl₄- and TiO₂-doped NaAlH₄ added with the co-dopants increases 10–50% compared with that without a co-dopant, and graphite is the best co-dopant in terms of reabsorption capacity. In addition, the decomposition temperature in the subsequent cycles of the co-dopant doped samples decreases about 10–15 °C as compared to the sample without a co-dopant. Porosity and large surface area of the co-dopant may decrease the segregation of bulk aluminum after the desorption and improve hydrogen diffusion in/out bulk of desorbed/reabsorbed samples.