The addition of NaH by ball milling is shown to greatly improve the hydrogen storage properties and the hydrolysis properties of MgH 2 , which is related to the formation of ternary hydride NaMgH 3 with specific perovskite structure. The MgH 2 –10%NaH mixture exhibits better hydriding and dehydriding kinetics than the MgH 2 –10%LiH mixture, in which the LiMgH 3 with perovskite structure could not be formed. The catalytic role of NaMgH 3 is attributed to fast hydrogen mobility in the perovskite structure, which provides fast hydrogen diffusion pathways for the hydriding and dehydriding of MgH 2 . The NaMgH 3 also shows fast hydrolysis reaction kinetics without any passivation. Our work shows that such perovskite-type hydride demonstrates great potential as efficient catalysts for the high-capacity hydrides for whether reversible or irreversible hydrogen storage.
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 Na0.46TiO2 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 °C, and the first- and second-step decomposition peak temperatures are decreased to 346.3 and 355.8 °C, respectively. The TiO2 NT-doped sample desorbs approximately 3.4 wt % H2 at 350 °C within 10 min, while the pure NaMgH3 sample releases only 0.2 wt % H2 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.
Metal nitrogen hydride (Li-Mg-N-H) is regarded as an ideal hydrogen storage material for automotive applications due to its advantages of high hydrogen storage capacity (5.6 wt%), low operating temperature, and good cycling performance, but the comprehensive hydrogen storage performance of this material still could be improved. For this reason, this paper constructed a Li-Na-Mg-N-H composite system by synthesizing NaMgH 3 to replace MgH 2 in the Li-Mg-N-H system. The improvement of the hydrogen absorption and desorption properties of the composite system after the replacement of MgH 2 by NaMgH 3 was investigated. Various properties and test results showed that the composite system after the replacement of MgH 2 by NaMgH 3 showed better hydrogen absorption and desorption properties. The initial/peak hydrogen release temperatures of Li-Na-Mg-N-H decreased from 135 ˚C/245 ˚C to 105 ˚C/145 ˚C, and the apparent activation energy of hydrogen release was reduced from 114.90 to 104.05 kJ/mol which means the kinetics of hydrogen absorption and desorption was greatly improved. The mechanism study shows that NaMgH 3 will react with LiNH 2 and forms the intermediate product Li 3 Na(NH 2 ) 4 during the hydrogen release process, which changes the hydrogen desorption pathway of the composite system and thus the hydrogen absorption and desorption kinetics were improved.
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