Complex metal hydrides have gained much interest as hydrogen storage materials for practical applications due to their relative high theoretical gravimetric hydrogen capacity. Unfortunately, many complex metal hydrides require high pressures and temperatures for their hydrogenation and high temperature for dehydrogenation. Researchers have therefore focused on altering the thermodynamics for dehydrogenation of these complex metal hydrides to lower the temperature of hydrogen release and improving the kinetics for the hydrogenation and dehydrogenation reactions [
1,
2]. Among the complex metal hydrides, sodium alanate, NaAlH
4, has acquired much attention as a potential hydrogen storage material since it was reported to be reversible through doping with small amounts of Ti-catalysts [
3]. NaAlH
4 has a theoretical reversible hydrogen storage capacity of 5.6 wt% and releases its hydrogen according to Eqs. (
1) and (
2).
$$ {\text{NaAlH}}_{4} \rightleftharpoons 1 /3\,{\text{Na}}_{3} {\text{AlH}}_{6} + \, 2 /3{\text{ Al }} + {\text{ H}}_{2} $$
(1)
$$ 1 /3\,{\text{Na}}_{3} {\text{AlH}}_{6} \rightleftharpoons {\text{NaH }} + \, 1 /3\,{\text{Al }} + \, 1 /2\,{\text{H}}_{2} $$
(2)
Despite its lower hydrogen capacity of 3.0 wt%, Na
3AlH
6 has attracted also attention as a potential hydrogen storage material, because of much lower dissociation pressure than NaAlH
4 (6 vs. 66 bar at 150 °C, respectively) and therefore making it more suitable for practical applications [
3]. Formed in the first decomposition step of NaAlH
4 (1), Na
3AlH
6 can also be directly synthesized through the reaction of 2 NaH and NaAlH
4 in heptane at 160 °C [
4] or through hydrogenation of Na and Al in toluene at 165 °C [
5]. More recently, direct synthesis of β-Na
3AlH
6 was also achieved through mechanical milling of NaAlH
4 and 2 NaH [
6] and from NaH and Al in the presence of 4 mol% TiF
3 under hydrogen pressure [
7]. Furthermore, it was found that Na
3AlH
6 synthesized through mechanical milling exhibited faster kinetics than Na
3AlH
6 obtained from the decomposition of NaAlH
4 [
8]. Na
3AlH
6 proved to be reversible through the addition of Ti compounds [
3]; however, doping with Zr compounds was reported to have better effect on the dehydrogenation of Na
3AlH
6 than doping with Ti [
8,
9]. Although a large number of experimental results have been described on NaAlH
4, a small number of studies have been reported on Na
3AlH
6 and its use as a hydrogen storage material. So far the studies on Na
3AlH
6 deal with theoretical calculations of structural parameters, effect of Ti doping, and hydrogen diffusion in pure and Ti-doped Na
3AlH
6. To our knowledge, no extensive study on the direct synthesis of Na
3AlH
6 through mechanochemical processes and the effect of various additives on the hydrogenation and dehydrogenation behavior has been reported so far. In this work, we investigate the synthesis of Na
3AlH
6 starting from NaAlH
4 and NaH in the presence of TiCl
3, combined with the addition of Al and AC. It has been reported that addition of AC to Ti-doped NaAlH
4 not only resulted in improved dehydrogenation and hydrogenation kinetics but also enhanced the cycle life [
10] and ability to conduct heat [
11]. We also investigate whether the additives are best added during or after the Na
3AlH
6 synthesis in order to obtain the optimum performance of the material. The long-term cycle stability measurements are performed at 170 °C, which allows using this material in combination with HT-PEM fuel cells.