Hydriding-dehydriding kinetics and the microstructure of La- and Sm-doped NaAlH4 prepared via direct synthesis method

By directly introducing LaCl3, La3Al11, SmCl3, SmAl3 into NaAlH4 system using one-step synthesis method, the effects of these additives on NaAlH4 were systematically investigated with regard to hydriding and dehydriding properties. Results showed that the materials doped with aluminide exhibit similar kinetics to the chloride-doped NaAlH4. The apparent activation energy Ea of doped NaAlH4 were calculated to be 86.4–93.0 kJ/mol and 96.1–99.3 kJ/mol for the first and second dehydrogenation step respectively by using Kissinger’s approach, much lower than those of pristine NaAlH4. A reversible hydrogen capacity of 4.8 wt% can be achieved for the La3Al11- and SmAl3-doped NaAlH4, which is 10–20% higher than chloride-doped NaAlH4. Investigations on the phase evolvement and microstructure in the cycling in LaCl3- and La3Al11-doped NaAlH4 clearly demonstrate that La species is presented as the form of La-Al nanoclusters in the materials. The combination of hydrogen storage properties and the microstructures unequivocally reveal that the in situ formed rare-earth-Al species play a crucial rule in catalyzing the chloride-doped NaAlH4.