Excellent Catalysis of Various TiO2 Dopants with Na0.46TiO2 in Situ Formed on the Enhanced Dehydrogenation Properties of NaMgH3

NaMgH3 has been considered to be a potential candidate for solid-state hydrogen storage due to its considerable hydrogen gravimetric (6.0 wt %) and volumetric (88.0 g/L) densities. Meanwhile, NaMgH3 possesses an outstanding theoretical thermal storage density of 2881 kJ/kg, which makes it one of the most promising thermal energy storage materials. However, the sluggish dehydrogenation kinetics of NaMgH3 embarrasses further practical application. Doping a nanosize Ti-based catalyst is treated to be one of the most effective methods to settle the poor dehydriding kinetics. In this work, different kinds of TiO2 catalysts, the 5 wt % TiO2 microparticle (MP) (100 nm), TiO2 nanoparticle (NP) (5-10 nm), and TiO2 nanotube (NT) (5-10 nm), were doped into NaMgH3 in the process of ball milling and heat treatment, which in situ formed Na(0.4)6TiO(2) significantly promoting the full hydrogen desorption kinetics of NaMgH3. Among all samples, the TiO2 NT doped sample shows the best performance of which the onset decomposition temperature is reduced to 300 degrees C, and the first-and second-step decomposition peak temperatures are decreased to 346.3 and 355.8 degrees C, respectively. The TiO2 NT-doped sample desorbs approximately 3.4 wt % H-2 at 350 degrees C within 10 min, while the pure NaMgH3 sample releases only 0.2 wt % H-2 in 10 min. The significant improvement in both two decomposition reactions kinetics of NaMgH3 can be attributed to the tubular morphology of the TiO2 NT and the in situ formation of multivalence Ti species (Na0.46TiO2). These two reasons can change the kinetic models of NaMgH3 from A2 to R2 and further dramatically decrease the activation energies of first-and second-step decomposition reactions of NaMgH3 to 91.7 and 142.1 kJ/mol, respectively. In particular, the in situ formed Na0.46TiO2 can benefit the e-transfers among Na+, Mg2+, and H-, tremendously enhancing dehydrogenation properties.