混合稀土对A_2B_7型储氢合金结构和电化学性能的影响

采用感应熔炼方法制备了A2B7型La0.83-0.5x(Pr0.1Nd0.1Sm0.1Gd0.2)xMg0.17Ni3.1Co0.3Al0.1(x=0~1.66)储氢合金,并在He+Ar气氛和1 173 K下进行退火处理。通过X射线衍射(XRD)、扫描电子显微镜(SEM)和电化学方法,研究了混合稀土(Pr,Nd,Sm,Gd)替代La元素对合金物相结构和电化学性能的影响。合金相结构分析表明,混合稀土含量对合金组成和相结构有重要的影响,随混合稀土含量x的增加,合金中主相A2B7型(2H-Ce2Ni7型+3R-Gd2Co7型)相丰度逐渐增多,其中2H-Ce2Ni7型相丰度先增多后减少,3RGd2Co7型相丰度则逐渐增加,主相晶胞参数随x增加而减小。电化学结果表明,随混合稀土含量增加,放氢平台压逐渐升高,合金电极的最大放电容量和循环稳定性均呈先增大后减小的规律,其中x=0.4合金电极具有最高的电化学放电容量(389.8 mAh·g-1)和最佳的循环寿命(S100=91.30%);合金电极的高倍率放电性能(HRD)则随x的增加获得显著提高。适量的混合稀土替代量可显著改善合金电极的综合电化学性能。

Effect of Mixed Rare Earth Elements on Structural and Electrochemical Properties of A2B7-Type Hydrogen Storage Alloys

A₂B₇-type La_0.83-0.5x(Pr_0.1Nd_0.1Sm_0.1Gd_0.2)_xMg_0.17Ni_3.1Co_0.3Al_0.1(x=0~1.66) hydrogen storage alloys were prepared by induction melting method and annealed at 1 173 K in He+Ar atmosphere. The effect of the partial substitution of mixed rare earth for La on the structure and electrochemical properties of the alloys was investigated by XRD, SEM and electrochemical measurement. Phase structure analyses showed that the annealed alloys mainly consisted of Ce₂Ni₇-type and Gd₂Co₇-type phases accompanied by minor CaCu₅-type and PuNi₃-type phases and the influence of the substituted mixed rare earth content x in alloys was great on the phase constitution and structure. With the increase of x value, the main phase abundance of A₂B₇-type (Ce₂Ni₇-type and Gd₂Co₇-type) gradually increased and the unit cell parameters of the main phase decreased. The partial substitution of the mixed rare earth for La beneted the formation of Gd₂Co₇-type phase, indicating that the atoms with smaller radius favored the formation of the Gd₂Co₇-type phase. The electrochemical measurement results showed that the activation performance was influenced little but the discharge capacity and the cycle stability of the alloys were all first increased and then decreased with the increase of x value. When x value was equal to 0.4, the system exhibited the maximum electrochemical discharge capacity (389.8 mAh·g^-1) and the best cyclic stability (S₁₀₀=92.82%). The high rate discharge ability of alloys presented an increasing trend with the increase of x value. The appropriate substitution of the mixed rare earth for La improved obviously comprehensive electrochemical properties of the alloy electrodes.