Reni5 Hydrogen Storage Alloy And Electrochemical Performance

On the basis of LaNi₅ hydrogen storage alloy, Mi_α Mm_1-αNi₅ (α=1, 0.8, 0.6, 0.4,0.2, 0) alloys were Wared by means of substitution for La with Lthe-rich(Ml) and Cerium-rich(Mm)mishchmetal. The phase structure, crystal StructUe,microstrucfore, hydrogen storage property and the influence of rare earthcomposition on them were systematically investigated by means Qf X-ray diffiaction,light and SEM metallographic examination, energy spectrum analysis,electTochemical method and so on. The results Showed that:The main phases of Ml_αMm_1-αNi₅ alloys as cast were hp6-CaCu₅ tyPe structurewith occasional traces of rear earth oxide and Ni. The calculation of crystalparameter showed that, with the decrease of α, parameter 'a' and cell volumedecreased, 'c' had sligh variation, and thus polyhedron interstice in the alloysreduced. The unit cell volume (V) was linear function to the Ml content α in Ml_αMm(1-α)Ni₅ alloys, Which can be expressed as V= 83.8l474 + 1.96l28 α.Nonequilibrium eutectic led to small amount of Ni segregation at grain boundary andintracrystal during solidification process. The segregation of alloy elemellt formedprecipitate phase and the composition of them at grain boundary and intracryStal wasas same as corresponding grain boundarypartial substitutions for La with Ce, Pr and Nd brought about increase in plateaupressures of alloys. The order of increasing degree from high to low was Ce, Nd andPrin truns. La, Pr and Nd reduced hysteresis of alloys. Its order was that Nd>La>Pr,whereas Ce enhanced hysteresis. The thermodynamic stability was deteriorated withpartial substituions for La with Ce, Pr and Nd. Plateau pressure of alloys can beadjusted by means of change the ratio of different radium element, which led to thevariation of cell volume.The order of activation rate (n_a) of Ml_αMm_1-αNi₅(α=l, 0.6, 0) alloys can beexpressed as Ml_0.6Mm_0.4Ni₅ (6 cycles) < MlNi₅ (8 cycles)< MmNi₅(l0 cycles). Thedischarge caPacity of Lanthanum-rich class hydrogen storage alloys was higher thanthat of Cerium-rich class alloys. The discharge capacity of Ml_αMm(1-α)Ni₅ alloysincreased first and then decreased. The max value (228mAh/g) was gained at α= 0.6.During the process of first 30 cycles' charge-discharge, MlNi₅ alloys decayed raPidly,while MmNi₅, alloys decayed slowly. High rate discharge led to drop of dischargecaPacity and voltage due tO stongly polarization.The electrochemical polarization determined the decay of potential at beginningof discharge. Then, the diffession polarization ialluened the polarization processand at lat becarne controIling step. OhInic polariZation eallibits a slight change.