A study of the Al content impact on the properties of MmNi4.4−xCo0.6Alx alloys as precursors for negative electrodes in NiMH batteries

AB5-type hydrogen storage alloys with MmNi4.4−xCo0.6Alx (Mm-mischmetal) composition are synthesized, structurally and thermodynamically characterized, and electrochemically tested in 6 M KOH electrolyte. It is shown that an increase of the Al content in the alloy results in expansion of both the alloy lattice cell size and the unit cell volume. These structural changes lead to decrease of the plateau pressure and increase of the plateau width in the pressure-composition-temperature desorption isotherms. Improvement of the specific electrode capacity is also registered with the increase of the cell parameters. In addition to that the higher Al content is found to enhance the stability of the alloy components’ hydrides. Maximum discharge capacity of 278 mAh g−1 is measured with an electrode made from a MmNi3.6Co0.6Al0.8 alloy. Cycle life tests of the accordingly prepared electrodes suggest a stability that is comparable to the stability of commercially available hydrogen storage electrodes.