Asymmetric Reaction Paths and Hydrogen Sorption Mechanism in Mechanochemically Synthesized Potassium Alanate (KAlH 4 )
Résumé
KAlH 4 is the unique monoalkali alanate that absorbs hydrogen reversibly with reasonable kinetics at moderate pressures without doping, offering an exciting playground to understand the role of additives. KAlH 4 was successfully synthesized by reactive ball milling (RBM) of a potassium hydride and aluminum 1:1 powder mixture under a hydrogen pressure of 8.5 MPa and low temperature (<65 °C). The synthesis without additives entails two consecutive hydrogenation steps through the formation of K 3 AlH 6 as intermediate. Among the different additives investigated (TiCl 3 , ScCl 3 , and LaCl 3), TiCl 3 is the most effective by reducing the hydrogenation time by a factor of 2. Moreover, it promotes the simultaneous formation of K 3 AlH 6 and KAlH 4 during mechanochemical synthesis before all KH is consumed. X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) analysis indicate that the additives do not modify the structural properties of the formed alanate. Independently of TiCl 3 addition, KAlH 4 is decomposed by three consecutive steps with K 3 AlH 6 and KH as intermediates. Thus, the absorption and desorption reaction paths are asymmetric when TiCl 3 is used as an additive. Moreover, the TiCl 3 additive efficiently reduces desorption temperatures and activation energies for KAlH 4 and K 3 AlH 6 decompositions, with that for KH remaining unchanged. These results suggest that TiCl 3 enhances the mobility of Al species during desorption, likely through the formation of species at the interphase and grain boundaries as NMR measurements suggest. Reaction kinetics is limited by nucleation and growth during absorption and by diffusion processes during desorption.