Mechanistic insights into Ti3CN MXene-modified NaAlH4: Boosting hydrogen storage kinetics and cyclability

Sodium alanate has a reversible hydrogen storage capacity of 5.6 wt% at relatively mild conditions, but it is limited in practical use because of low reaction rates and the phase separation of the reaction product. In this study, to enhance the hydrogen storage ability of sodium alanate, a one-step hydrogen pressure ball milling process was used to introduce the MXene-based carbonitride, Ti 3 CN, and its catalytic modification ability was compared to that of Ti 3 C 2 on sodium alanate. The onset and peak hydrogen desorption temperatures of NaAlH 4 decreased significantly from 176.9 /203.3 °C to 76.8/129.3 °C, and the activation energies for the two-step dehydrogenation process were reduced from 118/159 kJ/mol (pure NaAlH 4 ) to 42/72 kJ/mol. In an isothermal condition at 100 °C, the composite material desorbed 3.30 wt% hydrogen within one hour, and the capacity retention rate kept as high as 93.7% after ten cycles of hydrogen absorption and desorption. It can be obtained that Ti 3 CN maintained its layered structure during the process of ball milling, and there possessed multiple active sites, such as Ti 3 CN, Ti 0 and TiAl. Moreover, TiAl nanointermetallic phases were found to be in situ generated during the cycling process. TiAl displayed a reversible “hydrogen pump” property, acting as nucleation sites to promote the adsorption and desorption of hydrogen. At the same time, the MXene lattice promoted the dispersion of aluminum and the prevention of phase separation. The support stability provided by Ti 3 CN and the coupled catalytic ability of TiAl and Ti 0 phases significantly improved the hydrogen desorption rate and cycling performance of NaAlH 4 at low temperature. These results are valuable to the construction of an efficient solid hydrogen storage system.