Effect of Co substitution for Ni on the structural and electrochemical properties of La2Mg(Ni1−xCox)9 (x = 0.1-0.5) hydrogen storage electrode alloys

The effect of partial substitution of Co for Ni on the structure and electrochemical properties of the thus formed La₂Mg(Ni_1−xCo_x)₉ (x = 0.1-0.5) quaternary alloys was investigated. All alloys are consisted of a main phase with hexagonal PuNi₃-type structure and a few impurity phases (mainly La₂Ni₇ and LaNi). The increase of Co content in the alloys leads to an increase in both the cell volume and the hydride stability, and leads to a noticeable decrease in cell volume expansion rate (ΔV/V) on hydriding. The discharge capacity of the alloys at 50 mA/g increases slightly with the increase of Co content and passes though a maximum of 404.5 mAh/g at x = 0.2. As the Co content increases, the high-rate dischargeability of the alloy electrodes at 800 mA/g (HRD₈₀₀) decreases sharply from 72.8 (x = 0) to 24.5% (x = 0.5), yet the decrease of HRD₈₀₀ of the alloy electrodes with lower Co substitution (with x ≤ 0.2) is much milder. The slower decrease of HRD₈₀₀ (from 72.8 to 64.2%) of the alloys with x from 0 to 0.2 is mainly attributed to the decrease of eletrocatalytic activity for charge-transfer reaction, the more rapid decrease of the alloys with x > 0.2 is mainly attributed to the lowering of the hydrogen diffusion rate in the bulk of alloy. The cycling capacity retention rate (S₁₀₀) of the alloys increase greatly with increasing of Co content, increasing from 60.2% for the alloy with x = 0 to a much higher value of 87.9% for the alloy with x = 0.5. The improvement in cycling stability is attributed to the lower cell volume expansion on hydriding.