Effect of Ni content on the structure, thermodynamic and electrochemical properties of the non-stoichiometric hydrogen storage alloys

Commercial MmNixCo0.75Mn0.40Al0.30 hydrogen storage alloys have been investigated to examine the effect of non-stoichiometry on the microstructure, thermodynamic and electrochemical properties, where Ni content, x, is tuned from 3.05 to 4.05. It is found that for the stoichiometric and B-rich compound, only single phase with CaCu5-type exists. However, for B-poor compounds, there is principal a CaCu5-type phase with small amount of Ln2Ni7 type phase (Ln: rear earth elements) and the amount of Ln2Ni7 phase reduces with Ni. With the increase of Ni content, the lattice parameter along a and c axis, thus the cell volume, of the hexagonal CaCu5-type phase decrease monotonously. The hydrogen desorption pressure and enthalpy of hydride increase with Ni increasing. The electrochemical properties of the non-stoichiometric compounds with different Ni content are well related to the structure and thermodynamics of compound.