Abstract: The structures and electronic properties of designed alloys La_0.75Mg_0.25Ni_3.5-xCo_x (x=0, 0.25, 0.5, 0.75) were theoretically investigated by using density functional theory (DFT) of plane wave pseudopotential method. Calculation results showed that, with the increasing Co content, the charge transfer on La atom and the number of states at Fermi level (N(EF)) first increase and then slightly decrease, reaching maximum values at x=0.5. Meanwhile, La_0.75Mg_0.25Ni_3.5-xCo_x　(x=0, 0.3, 0.5, 0.7) alloys were prepared through induced melting method. The effects of substitution Co for Ni on the structure and properties of the alloys were systematically investigated. X-ray diffraction results show that the majority phase in the alloys is Ce₂Ni₇-type AB_3.5 phase. The electrochemical analysis shows that both the discharge capacity and the cycling stability of the alloys reach the maximum values at x=0.5. It is observed from PCT measurements that the alloys can absorb hydrogen under 0.04–0.09 MPa hydrogen pressure at room temperature. Both the lowest absorption pressure of 0.04 MPa and the highest storage capacity of 1.587wt% were obtained with Co content of 0.5. The results clearly show that the property change of AB_3.5 alloys with Co content can be predicted by first principle based theoretical calculations.

Key words: hydrogen storage alloy, the first principle calculation, Ce₂Ni₇-type phase, electrochemical properties, PCT measurement