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.