De/hydrogenation kinetics against air exposure and microstructure evolution during hydrogen absorption/desorption of Mg-Ni-Ce alloys

Aiming at elucidating the positive effects of Ce element on the oxidation resistance of Mg-based hydrogen storage alloys, Mg-Ni-Ce alloys with different Ce contents have been prepared in a resistance furnace with furnace cooling. The mass fraction of Mg in each sample is 80% to maintain high and consistent hydrogen storage capacity. An 18R-type long-period stacking ordered phase (LPSO) is observed within the Mg₁₂Ce matrix in Mg-Ni-Ce alloys. A two-step activation process is observed in Ce containing alloys. Detailed microstructural characterization of activated samples during air exposure and in-depth analysis of absorption/desorption kinetics on air-exposed samples based on Johnson-Mehl-Avrami (JMA) model are performed to discuss the mechanism underlying improved anti-oxidation properties. CeH_2.73 forms after activation and transforms to CeO₂ during air exposure, which is earlier and faster than the MgO formation during initial air contact preventing forming a compact and uniform MgO layer on the surface. The formed CeH_2.73/CeO₂ particles with average particle size less than 60 nm act as catalysts accelerating the hydrogen dissociation and nucleation sites for the MgH₂ formation during hydrogenation. After de-/hydrogenation cycles, CeO₂ turns back to CeH_2.73, which can react with oxygen again when the sample is exposed to air.