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 A(2)B(7)-type R0.3Y0.7Ni3.25Mn0.15Al10.1 (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%similar to 1.48% (w/w) and 0.037 similar to 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 S-100=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%similar to 86.94% and the control step of the reaction kinetics was mainly controlled by the diffusion rate of the hydrogen atom in the alloys.