Hydrogen sorption behaviour of Mg-5wt.%La alloys after the initial hydrogen absorption process

In our earlier study, it has been shown that trace Na additions can improve the reaction kinetics of Mg–5%La (wt.%) alloys during the first absorption. However, the subsequent hydrogen desorption/absorption process of the Mg–5%La after the first absorption has not been investigated. In this study, we have investigated the hydrogen sorption behaviour of the Mg–5%La alloy after the first absorption in terms of phase evolution, and lattice expansion properties during desorption as function of temperature using in-situ synchrotron Powder X-ray Diffraction (PXRD) and in-situ High Voltage Transmission Electron Microscopy (HVTEM). Two distinct phase evolutions, a continuous phase transformation of LaH₃ → LaH₂ + ½ H₂ (from 250 °C) and decomposition of MgH₂ → Mg + H₂ (between 440 and 460 °C) were identified during the desorption. It is determined that this alloy is cyclable in the absence of Mg₁₂La intermetallic during the subsequent absorption/desorption cycling after the first hydrogen absorption.