The TiCl3 catalyst in NaAlH4 for hydrogen storage induces grain refinement and impacts on hydrogen vacancy formation Original Research Article

TiCl3 acts as an efficient catalyst for NaAlH4 (sodium alanate), altering its hydrogen sorption kinetics and reversibility considerably. In order to clarify its role, we performed in situ neutron diffraction experiments on protonated catalysed and uncatalysed NaAlH4. The phase transformations were monitored in the first two reaction steps during hydrogen release and in the second step during reloading. Our study for the first time provides clear indications that both TixAl1−x and NaCl formed act as grain refiner for Al and NaH, respectively, preventing particle growth. Particle sizes generally stay small upon desorption and reloading of TiCl3 catalysed NaAlH4, while significant particle growth is observed for uncatalysed NaAlH4. The small crystallite sizes and observed hydrogen vacancy formation greatly facilitate the mass transfer during loading and unloading. This study underlines the importance of grain refining for achieving reversibility and faster kinetics of the hydrogen sorption processes, with a crucial double role played by the catalyst.