Synergistically thermodynamic and kinetic tailoring of the hydrogen desorption properties of MgH2 by co-addition of AlH3 and CeF3

Abstract

MgH₂ possesses a high hydrogen capacity and excellent reversibility. However, the high thermal stability and slow sorption kinetics retard its practical application as an on-board hydrogen storage material. In this work, AlH₃ and CeF₃ were introduced into Mg-based materials for the purpose of improving both the thermodynamic and the kinetic properties of MgH₂. DSC-TG analysis shows that the onset hydrogen desorption temperature of MgH₂ can be synergistically reduced by 86 °C through the co-addition of 0.25AlH₃ and 0.01CeF₃. Isothermal desorption measurements demonstrate that the co-addition of AlH₃ and CeF₃ significantly enhances the hydrogen desorption kinetics of MgH₂ with the absence of the induction period in the initial stage and the acceleration of the hydrogen desorption process. In addition, this co-doped MgH₂ shows very good cycling stability at 300 °C with a 1 h capacity of 3.5 wt% and a 3 h capacity of 4.5 wt%. Structural analysis by XRD measurements indicates that during the hydrogen desorption process, MgH₂ may react with Al (generated from the in situ decomposition of AlH₃) to form Mg solid solution and Mg₁₇Al₁₂, which contribute to the thermodynamic improvement of the Mg-based material. In addition, MgH₂ may also react with CeF₃ to form MgF₂ and CeH_2–3, which act both as hydrogen diffusion gateways and as an impediment to the grain growth of MgH₂ during hydrogen sorption cycling, thus improving the hydrogen desorption kinetics and the cycling stability of MgH₂. Finally, it was found that the presence of AlH₃ kinetically helps CeF₃ to exert its positive effect on the hydrogen desorption properties of MgH₂. This work provides a method for simultaneously tailoring the thermodynamic and kinetic properties of MgH₂ by the synergistic addition of metal hydride and rare earth fluoride.