A study on the structure and electrochemical properties of La2Mg(Ni0.95M0.05)9 (M = Co, Mn, Fe, Al, Cu, Sn) hydrogen storage electrode alloys

The effect of replacing part of the Ni by a metallic element in La2MgNi9 on the structure and electrochemical properties of the thus formed La2Mg(Ni0.95M0.05)(9) quaternary alloys (M = Co, Mn, Fe, Al, Cu, Sn) was investigated. The substitutions did not change the main phase structure (the hexagonal PuNi3-type structure), but all increased the unit cell volume of the alloys except the Sn substituted one, in which some LaNiSn second phase was formed. All hydrides of the alloys preserved the PuNi3-type structure, while the amorphization of a portion of the hydride of the Al substituted alloy was observed. The substitution led to some decrease in hydrogen capacity, and an increase in hydride stability except for the Sn substituted one, and led to some decrease in both the discharge capacity and in the high-rate dischargeability (HRD), but led to a noticeable improvement in cycling stability for most of the substituted alloys (except for the Sn substituted one). The decrease of the high-rate dischargeability was due to the decrease of the electrocatalytic activity of the alloy electrodes and the lower diffusibility of hydrogen in the bulk of alloys as a result of the relatively low stability of the hydrides. The improvement in cycling stability was ascribed to the lower unit volume change on hydriding and the formation of a corrosion resistant layer on the alloy surface. (C) 2004 Elsevier B.V. All rights reserved.