稀土元素对R-Y-Ni系A2B7型无镁储氢合金微观结构和电化学性能的影响

采用真空电弧熔炼及退火处理制备R-Y-Ni系A_2B_7型R0.3Y0.7Ni3.25Mn0.15Al0.1(R=Y,La,Pr,Ce,Nd,Gd,Sm)储氢合金,系统研究稀土元素R对合金微观组织与结构、储氢和电化学性能的影响。XRD和SEM-EDS分析表明,合金退火组织由Ce2Ni7型主相、PuNi3型及少量Ca Cu5型相组成,Ce2Ni7型主相的晶格常数a、c及晶胞体积V均随稀土R原子半径的减小而依次降低。该合金均具有明显的吸放氢平台,常温下最大吸氢容量为1.17%~1.48%(w/w),吸氢平台压Peq为0.037~0.194 MPa。电化学分析表明,退火合金电极的电化学活化性能优良,R=La合金具有最高的放电容量(389.2 mAh·g-1)和较佳的容量保持率(充放电循环100次后的S100=85.7%),其中合金微观组织的不均匀性及稀土元素的电化学腐蚀是影响电极循环稳定性的主要原因。合金电极的高倍率放电性能(电流密度为900 m A·g-1)HRD900=71.05%~86.94%,其电极反应动力学控制步骤主要由氢原子在合金体相中的扩散速率所控制。

Effect of Rare Earth Elements on the Microstructure and Electrochemical Properties of Mg-Free R-Y-Ni Based A2B7-Type Hydrogen Storage Alloys

The R-Y-Ni based A₂B₇-type R_0.3Y_0.7Ni_3.25Mn_0.15Al_0.1 (R=Y, La, Pr, Ce, Nd, Gd, Sm) alloys were prepared by vacuum arc melting and followed by annealing treatment. The effects of rare earth element R on the microstructure, hydrogen storage and electrochemical properties of the alloys were investigated systematically. XRD and SEM-EDS analysis showed that the annealed alloys mainly composed of Ce₂Ni₇-type main phases, minor PuNi₃-type and CaCu₅-type phases. The lattice constants a, c and cell volume V of the Ce₂Ni₇-type main phases decreased in sequence with the decreasing radius of R atoms. The annealed alloys obviously presented a hydrogen absorption/desorption platform, the maximum hydrogen storage capacities and the equilibrium hydrogen absorption pressures (P_eq) at room temperature were in the range of 1.17%~1.48% (w/w) and 0.037~0.194 MPa, respectively. Electrochemical analysis showed that the electrochemical activation performance was excellent. The heterogeneity of microstructure and electrochemical oxidation corrosion of rare earth elements were main reason that affected the cycling stability of alloy electrodes. For all the alloys, the R=La alloys exhibited the highest electrochemical discharge capacity of 389.2 mAh·g^-1 and the best cyclic stability S₁₀₀=85.7% (charge/discharge after 100 cycles). The high rate discharge ability HRD₉₀₀ (current density was 900 mA·g^-1) of the alloys was 71.05%~86.94% and the control step of the reaction kinetics was mainly controlled by the diffusion rate of the hydrogen atom in the alloys.