Study on mechanisms of two-step hydrogen sorption in a MgH2–TiCrMnFeZr high-entropy alloy composite†

High-entropy alloys (HEAs) featuring multi-element active sites show exclusive catalytic ability, as well as great potential to store hydrogen (H₂) at room temperature by adjusting the electronic and geometrical factors. Herein, a TiCrMnFeZr HEA was adopted to improve the hydrogen storage properties of magnesium hydride (MgH₂). It was demonstrated that MgH₂ provides substantial cushion protection for the crystal structure and hydrogen storage capacity of the TiCrMnFeZr HEA during ball milling, while the TiCrMnFeZr HEA exhibits superior catalytic ability for the dissociation of H–H and Mg–H bonds. In particular, the MgH₂-40 wt% TiCrMnFeZr composite can absorb and desorb 0.72 wt% H₂ even at room temperature. Moreover, MgH₂ starts to release hydrogen at 162 °C, and 90% of stored H₂ (∼3.7 wt%) can be released at 230 °C within 60 min. There is no capacity fading after 20 cycles at 300 °C for both TiCrMnFeZr HEA and Mg/MgH₂ phases in the composite, showing an outstanding cycling performance. Microstructure investigations reveal that the well-protected TiCrMnFeZr HEA particle surfaces act as the catalytic sites for the dissociation of Mg–H bonds because of their intrinsic multivalent electronic configuration, and also serve as the channels for hydrogen sorption in Mg/MgH₂. Such a method to design and synthesize high-performance Mg-based hydrogen storage composites and to provide H₂ in two steps paves a new way to realize their practical applications in the hydrogen energy field.