Synthesis and hydriding/dehydriding properties of nanosized sodium alanates prepared by reactive ball-milling

TiF3-doped NaH/Al mixture was hydrogenated into Na3AlH6 and NaAlH4 complex hydrides by reactive ball-milling at room temperature through the optimization of milling duration and hydrogen pressure. The analysis of the preparation of NaAlH4 samples during reactive ball-milling process has been performed by XRD and TG/DSC. It has been found that Na3AlH6 was formed under 0.5 MPa hydrogen pressure and 30 h milling duration, while NaAlH4 was formed under 0.8 MPa hydrogen pressure and 45 h milling duration. The process of preparing NaAlH4 by ball-milling was found accomplished via two reaction steps, namely: (1) NaH + Al + H2 → Na3AlH6 and (2) Na3AlH6 + Al + H2 → NaAlH4. As the hydrogen pressure and milling duration increase, the synthetic yield of NaAlH4 and its corresponding dehydriding capacity are both increased. With increased hydrogen pressure (0.8–3 MPa) and milling duration (45–60 h), the cell volume of Na3AlH6 decreases while that of NaAlH4 increases gradually. The abundance of Na3AlH6 phase decreases from 57.76 (x = 0.8, y = 45) to 8.69 wt.% (x = 3, y = 60), and the abundance of NaAlH4 phase increases from 20.63 (x = 0.8, y = 45) to 86.50 wt.% (x = 3, y = 60). All the samples prepared in this way have fairly good activation behavior and fast hydriding/dehydriding reaction kinetics, which are capable of absorbing 4.26 wt.% hydrogen at 120 °C and desorbing 4.12 wt.% hydrogen at 150 °C, respectively. The improvement of hydriding/dehydriding properties is ascribed to the favorable microstructure and ultrafine particle features of nanosized NaAlH4 formed during ball-milling at the optimum synthetic condition.