Crucial role of Mg on phase transformation and stability of Sm-Mg-Ni-based AB2 type hydrogen storage alloy

Rare earth-Mg-Ni-based alloys are promising candidate for high-capacity hydrogen storage materials, and Mg is the key elements to maintain its crystal structure stable. In order to understand the role of Mg element in rare earth-Mg-Ni based alloys, the alloys with the composition of RE1-xMgxNi1.97Co0.03Al0.07 (RE = Sm0.80Y0.20; x = 0.35, 0.44, 0.52) were synthesized by powder sintering treatment. The increasing Mg content promotes the phase transformation from SmNi2 (Fd-3 m group) phase to MgCu4Sn-type (F-43 m group) phase. First-principle cal-culations and XRD simulations show that Mg element is reasonably approximated as the para-position substi-tution of rare earth elements at 4c sites when it enters into the unit cell of SmNi2 phase. The Sm0.38Y0.10Mg0.52Ni1.93Co0.03Al0.05 alloy with Mg fully occupying the 4c sites forms a stable structure, and it shows flat hydrogen absorption/desorption platforms, with a hydrogen absorption capacity of 1.12 wt%. Meanwhile, it has more excellent cycling stability. After 20 cycles of hydrogen absorption, the crystal structure of the alloy still maintains MgCu4Sn-type phase, and the capacity retention rate can reach 94.42 %.

Automated summary

Rare earth-Mg-Ni based alloys with increased Mg content show phase transformation and improved stability and hydrogen absorption properties.