Ti替代La对AB3型储氢合金的结构与电化学性能的影响

研究(La1-xTix)2MgNi8.25Co0.75(x=0、0.1、0.2)合金的微观结构与电化学性能。相测试结果显示:所有合金都是由(La,Mg)Ni3和LaNi52个主相所构成的,晶胞参数随着Ti的替代而逐渐减小,这是因为Ti的共价键半径(0.132 nm)小于La(0.169 nm)所引起的。电化学测试结果表明:所有的合金电极经过4次活化后都能够达到最大放电容量,且放电容量随着Ti含量的增加而减少,从x=0时的384.6 mAh/g降低到x=0.2时的321.9 mAh/g,合金电极的循环寿命则从x=0时的53.1%提高到x=0.2时的67.8%,合金在1 200 mA/g时的高倍率放电性能先从x=0时的59.3%升高到x=0.1时的66.5%,然后又降低到x=0.2时的63.1%。此外,电化学动力学也显示出先增大后减小的特点。造成以上电化学性能变化的原因是Ti的加入一方面起到了脱氢催化的作用,另一方面使合金表面形成了致密氧化层,虽然阻止了合金进一步的腐蚀,但也降低了合金电极的动力学性能。

Effects of substituting La with Ti on microstructure and electrochemical properties of AB3-type hydrogen storage alloys

The crystalline structures and electrochemical properties of(La1-xTix)2 MgNi8.25Co0.25(x=0,0.1 and 0.2) alloys were investigated.The structural analysis results showed that all the alloys mainly consisted of(La,Mg) Ni3 and LaNi5 phases.The lattice parameters and cell volumes of the main phases decreased with increasing x.It was mainly caused by that the covalent radius of Ti(1.32 nm) was smaller than that of La(1.69 nm).The electrochemical measurement results showed that all alloy electrodes could be activated to their maximum discharge capacities within four cycles,with the maximum discharge capacity decreased from 384.6 mAh/g(x=0) to 321.9 mAh/g(x=0.2),and the cycle stability increased from 53.1%(x=0) to 67.8%(x=0.2),but the high rate dischargeability firstly increased from 59.3%(x=0) to 66.5%(x= 0.1),then decreased to 63.1%(x=0.2) on the discharge current density of 1 200 mA/g,and the electrochemical kinetics of the alloy electrodes also firstly increased and then decreased.All the results indicated that substitution of La with Ti in AB3-type hydrogen storage alloys was effective for improving the overall electrochemical properties due to electro-catalyst and passive oxides.