The effects of Pd and/or Zr additives on the structures and cyclic stabilities of Mg₅₀Ni₅₀-based electrode alloys

The Mg₅₀Ni₅₀-based ternary and quaternary alloys partially substituted by Pd and/or Zr have been intensively studied on their structures and electrochemical hydrogen storage performances. The Mg₄₅Pd₅Ni₅₀ and Mg₄₅Pd₅Ni₄₅Zr₅ alloys contained both amorphous phase and body-centered cubic phase, differing from most previously studied amorphous Mg₅₀Ni₅₀-based alloys. The detected lattice parameters of nano-crystalline BCC phase in Mg₄₅Pd₅Ni₅₀ and Mg₄₅Pd₅Ni₄₅Zr₅ alloys were 0.2916 nm and 0.2933 nm, respectively. Charge-discharge tests indicated that Pd facilitates lifting the cyclic retention rate in 20 cycles (C₂₀/C_max) from 13.2% of Mg₅₀Ni₅₀ alloy to 47.3% of Mg₄₅Pd₅Ni₅₀ alloy. The partial substitution of Zr for Ni in Mg₅₀Ni₅₀ alloy improved the initial capacity by 12% from that of Mg₅₀Ni₅₀ alloy (445 mAh g^-1). Synergetic substitution of Pd and Zr in Mg₅₀Ni₅₀ can greatly inhibit the corrosion of Mg₄₅Pd₅Ni₄₅Zr₅ alloy. According to XPS study, the Mg, Pd, Ni and Zr on the surface of Mg₄₅Pd₅Ni₄₅Zr₅ alloy would be oxidized into MgO, PdO₂, Ni₂O₃ and ZrO₂ during charge/discharge cycles. They formed a passive composite film and therefore improved the cyclic stability.