Effect of Mn Element on the Structures and Properties of A2B7-Type La-Y-Ni-Based Hydrogen Storage Alloys

The structures, hydrogen storage behaviors and electrochemical properties of Y0.75La0.25Ni3.5-xMnx (x = 0-0.3) alloys were analyzed by X-ray diffraction, Neutron powder diffraction, pressure-composition isotherms and electrochemical tests. All alloys have a multiphase structure. With the increase in Mn content, the Gd2Co7-type phase of the alloys gradually transforms into the Ce2Ni7-type phase; the Mn atom mainly occupies the Ni sites in the [AB(5)] subunit and the interface between the [AB(5)] and [A(2)B(4)] subunits; the V-[A2B4]/V-[AB5] continuously decreases from 1.045 (x = 0) to 1.019 (x = 0.3), which reduces the volume mismatch between [A(2)B(4)] and [AB(5)] subunits. The maximum hydrogen absorption of the series alloys increases first and then decreases, and the addition of Mn effectively promotes the hydrogen absorption/desorption performance of the alloys. The maximum discharge capacity of the alloy electrodes is closely related to their hydrogen storage capacity at 0.1 MPa and hydrogen absorption/desorption plateau pressure. The cyclic stability of all the Mn-containing alloy electrodes is improved clearly compared to that of Mn-free alloy electrodes, because the volume mismatch between the [AB(5)] and [A(2)B(4)] subunits of the alloys becomes smaller after the addition of Mn, which can improve the structural stability and reduce the corrosion of alloys during hydrogen absorption/desorption cycles. When the Mn content is between 0.1 and 0.15, the Ce2Ni7-type phase of the alloys has high abundance and the alloy electrodes exhibit excellent overall performance.