Electrochemical study of cobalt-free AB5-type hydrogen storage alloys

Cobalt-free AB5-type hydrogen storage alloys have been examined for the purpose of lowering MH raw material costs. The electrochemical measurements showed that the cobalt-containing LaNi3.55Mn0.4Al0.3Co0.75 alloy had a maximum capacity of 300 mA h/g at a rate of C/6. The capacity decrease was calculated by 5% after 20 cycles at C/6 rate with 100% depth of discharge (DOD). The cobalt-free LaNi3.55Mn0.4Al0.3Fe0.75 alloy showed a very good cycling stability, although the initial capacity (250 mA h/g at C/6 rate) was lower than that of the cobalt containing alloy. The capacity decrease of the cobalt-free alloy amounted to 1% after 20 cycles under the same rate and DOD. The mechanism of hydriding/dehydriding reaction on the alloy electrodes was also investigated using a constant potential discharge (CPD) and cyclic voltammetry (CV) techniques. The obtained values of the hydrogen diffusion coefficient DH, were, 2×10−8 and 1.1×10−10 cm2 s−1 for LaNi3.55Mn0.4Al0.3Co0.75 and 8×10−9 and 1.0×10−10 cm2 s−1 for LaNi3.55Mn0.4Al0.3Fe0.75, by CV and by CPD, respectively.