La-Mg-Ni系AB3型贮氢电极合金的相结构与电化学性能

XRDRietveld分析显示,LaxMg3-xNi9(x=1.0-2.3)均由六方PuNi3型结构的主相及少量LaNi5及MgNi2杂相组成,主相的晶胞参数随x的增加而线性增大.合金的氢化物仍保持PuNi3型结构,但其晶胞体积有较大的膨胀.电化学测试表明,随x增加,合金的最大放电容量由88.3(x=1.0)逐渐增大到397.5mA·h/g(x=2.0),然后又降低到230mA·h/g(x=2.3).对放电容量超过348mA·h/g的合金(x=1.7-2.2),在放电电流i=400-1200mA/g的条件下,合金的高倍率放电性能(HRD)均随x增加而有不同程度的降低.HRD的缓慢降低主要与合金电极进行电荷迁移反应时的电催化活性的逐渐降低有关,而在x＞2.0时,HRD的快速降低与氢在合金中的扩散速率明显降低有关,上述合金经100次循环后合金的容量保持率为55.7%-62.9%,容量衰退较快与循环过程中La和Mg的氧化腐蚀以及合金较大的吸氢体积膨胀率有关.

Phase Structure and Electrochemical Properties of La--Mg--Ni System AB3 Type Hydrogen Storage Electrode Alloys

XRD Rietveld analysis shows that LaxMg3-xNi9 (x=1.0---2.3) alloys consist of a main phase with hexagonal PuNi3-type structure and a few impurity phases (mainly LaNi5 and MgNi2, increasing x leads to an increase in both the lattice parameters and the unit cell volume of the main phase. The hydride of the alloys preserves the PuNi3 type structure, but shows a large unit cell volume expansion. The electrochemical measures indicate that the desorption plateau pressure Prm of the alloys decreases noticeably as x increases, while the maximum discharge capacity Cmax increases from 88.3 (x=1.0) to 397.5 (x=2.0), and then decreases to 230 mA.h/g (x=2.3). For the alloys with Cmax>348 mA.h/g (x=1.7-2.2), the high-rate dischargeability (HRD) of the electrodes at i=400-1200 mA/g decreases with increaseing x. The slower decrease of HRD is mainly attributed to the decrease of eletrocatalytic activity due to the charge--transfer reaction, and the more rapid decrease of HRD of the alloys with $x>$2.0 is related to the lower hydrogen diffusion rate in the bulk of alloy. The rate of capacity retention (S100) of the alloys after 100 charge/ discharge cycles is around 55.7%-62.9%, the rather fast cycling capacity degradation is mainly due to the corrosion of La and Mg and the large unit cell volume expansion in the hydride phase.