Effect Of Rare Earth Oxide On The Electrode Propetries Of La_0.65Gd_0.2Mg_0.15（NiCoAl）_3.5 Hydrogen Storage Alloy

A₂B₇-type rare earth based alloys have been paid much attention due to theircomparatively higher capacity. However, the currently commercialized A₂B₇-type electrodealloys can not meet the demand of practical application. In order to improve electrochemicalproperties of the A₂B₇-type alloy electrodes, some rare earth oxides （Yb₂O₃、Sm₂O₃、Er₂O₃、Eu₂O₃、Sc₂O₃、CeO₂） were added to the electrode. Effects of these oxides on electrochemicalproperties of A₂B₇-type alloy electrodes were investigated.Firstly, microsized and nanosized ytterbium oxides were added separately to A₂B₇-typehydrogen storage alloys and their effects on electrode properties were investigatedsystematically. The results showed that ytterbium oxides additives suppressed the formationof La（OH）₃and Mg（OH）2during charge/discharge process of alloy electrode and therefore thecyclic stability was improved. With the increase content of ytterbium oxides, cyclic stabilityof alloy electrode was ameliorated, however, discharge capacity was reduced and reactionresistance was increased. Particle size and aggregating state of ytterbium oxides hadsignificant effect on electrochemical properties of alloy electrode. Alloy electrodes with highdispersive microsized ytterbium oxides had better cyclic stability. The cut-off potential ofalloy electrodes in discharge process as well as redox reaction of ytterbium oxides in alkalisolution were both closely associated with cyclic stability of alloy electrodes.For further improving the performances of the A₂B₇-type alloy electrodes, the effect ofother nanosized rare earth oxides （Sm₂O₃、Er₂O₃、Eu₂O₃、Sc₂O₃、CeO₂） additives on theelectrochemical properties of A₂B₇-type hydrogen storage alloy electrode was systematicallystudied. With the addition of rare earth oxides, cyclic stability and anti-corrosion ability ofalloy electrodes were improved, but reaction resistance was increased. And discharge capacityof alloy electrodes were reduced except the addition of nanosized CeO₂. Different rare earthoxide had disparate effect on alloy electrode because of independent particle size andaggregating state. The discharge capacity retention at100th cycle increased from84.69%（blank） to86.59（Sm₂O₃）、90.62%（Er₂O₃）、91.28%（Eu₂O₃）、91.44%（Sc₂O₃）and85.35%（CeO₂）. Nanosized rare earth oxides can partly react in alkali solution, which was closelyassociated with cyclic stability of alloy electrodes. The discharge efficiency, especially at highdischarge current density was greatly improved, and the high rate discharge capacity at900mA/g increased from65.23%（blank） to71.2（Sm₂O₃）、71.07%（Er₂O₃）、68.43%（Sc₂O₃）and75.93%（CeO₂）. Finally, in order to improve the cyclic stability of A₂B₇-type alloy electrodes, the mixedoxides of nanosized Yb₂O₃and Co₃O₄were added to the alloy electrodes and their effects onelectrode properties were investigated systematically. With the addition of pure Co₃O₄, thedischarge capacity of alloy electrodes were increased, but cyclic stability were decreased.With the mixed oxides, the discharge capacity of alloy electrodes were lower, but cyclicstability were increased to some extend. The discharge curves of the electrodes with Co₃O₄and Yb₂O₃present a second discharge platform, indicating the occurrence of another electrodereaction. The higher the content of Co₃O₄added in electrode is, the more obvious the seconddischarge platform.