The structural and electrochemical properties of La2Mg(Ni0.8−xCo0.2Alx)9 (x = 0–0.03) hydrogen storage electrode alloys

The effect of Al substitution for Ni on the structure and electrochemical properties of La2Mg(Ni0.8−xCo0.2Alx)9 (x = 0–0.03) quinary alloys was investigated. All alloys consisted of a main phase with hexagonal PuNi3-type structure and a small quantity of the impurity phase (La,Mg)2Ni7. The increase of Al substitution in the alloys leads to an increase in both the cell volume and the hydride stability but a decrease in unit cell volume expansion rate. The increase of Al content leads to some decrease in both the discharge capacity and the high-rate dischargeability (HRD) but leads to a significant improvement in cycling stability of the alloys. Among the alloys studied, the alloy with x = 0.02 showed a high discharge capacity of 375.9 mAh/g and the best cycling stability (S150 = 77.8%) but a poor high-rate dischargeability (HRD800 = 8.3%). The decrease of HRD of the alloys is ascribed to the decrease of electrocatalytic activity for the hydrogen electrode reaction and the lower diffusion rate of hydrogen in the bulk of the alloys, while the improvement in cycling stability is mainly attributed to the decrease of VH of the hydride phase.