Effects of rare earth elements substitution for Ti on the structure and electrochemical properties of a Fe-doped Ti–V-based hydrogen storage alloy

Effects of a partial substitution of rare-earth elements of Y, La, Ce, Pr, Nd in an atom fraction of 1/8 for Ti on the structure and electrochemical property of a Fe-doped Ti–V-based hydrogen storage alloy, Ti0.8Zr0.2V2.7Mn0.5Cr0.6Ni1.25Fe0.2, have been investigated systematically using X-ray diffraction, scanning electron microscope, energy dispersive spectroscope and electrochemical tests including charge/discharge, high rate dischargeability, polarization, etc. The results indicate that all the alloys mainly consist of a C14 Laves phase in a three-dimensional (3D) network and a dendritic V-based BCC phase. The lattice parameters and the unit cell volumes of the two main phases are almost unchanged with the substitution of the rare earth elements for Ti except for Y, which increase slightly. Additionally, the La, Ce, Pr and Nd element added mostly formed a new phase with an approximate composition of R2Ni3 (R = La, Ce, Pr, Nd). The formation of the R2Ni3 phase favors the activation property of the alloy. As a result, with the substitution of the rare earth elements for Ti, the maximum discharge capacities (Cmax) of the alloy electrodes increase, and a maximum value of 360 mAh/g is obtained for the substitution of Y. The HRD600 of the alloy electrodes increases from 63% to 64–70% depending on the different substituent elements, but the cycle stability slightly decreases. In addition, the exchange current density, the limiting current density and the hydrogen diffusion coefficient of the alloy electrodes further indicate that the electrochemical kinetics of the alloy electrodes is improved by the increase in both the proximity effect between C14/BCC and the catalytic effect from R2Ni3 phase.