Electron-microscopy studies of NaAlH4 with TiF3 additive:: hydrogen-cycling effects

NaAlH4 is a promising candidate material for hydrogen storage. Ti additives are effective in reducing the reaction temperatures and improving kinetics. In this work, the microstructure of NaAlH4 with 2% TiF3 has been studied in different conditions using a combination of transmission electron microscopy and scanning electron microscopy, both with energy-dispersive spectroscopic X-ray analysis. The effect of the additive on particle and grain size was examined after the initial ball-milling process and after 15 cycles. The additive has an uneven distribution in the sample after ball milling. Selected-area diffraction and high-resolution imaging confirmed the presence of TiF3. This phase accounts for most of the Ti in the material at this stage and showed limited mixing with the alanate. The grain size within particles for TiF3 is larger than for the alanate particles. Diffraction from the latter was dominated by metallic aluminium. After cycling, the TiF3 has decomposed and energy-dispersive spectroscopic X-ray analysis maps showed some combination of Ti with the alanate phase. There is no significant change in the measurable grain size of the Al-containing alanate particles between the ball-milled and the 15-cycled samples, but more cycles result in agglomeration of the material.