Study of the effect of praseodymium substitution on hydrogen storage and electrochemical properties of cerium-rich AB5 alloys

In order to investigate the hydrogen storage properties of cerium-rich RE-based alloys, series of MmNi5 and Mm(1−x)PrxNi3.55Co0.75Mn0.4Al0.3 (x = 0, 0.2, 0.5) have been produced by vacuum induction melting and vacuum arc remelting methods. X-ray diffraction results of the alloys showed CaCu5-type hexagonal phase as the main structure, but Ce2Ni7-type structure is also observed in some cases. Pressure–composition–temperature curve at room temperature for MmNi5 showed an extremely large hysteresis loop without any notable plateau pressure. This leads to 12 mAh g−1 capacity after eight charge–discharge cycles; In contrast Mm(1−x)PrxNi3.55Co0.75Mn0.4Al0.3 (x = 0, 0.2, 0.5) alloys exhibited significant reduction in hysteresis loop and wider plateau pressure region. As a result, 156, 217 and 218 mAh g−1 capacities were obtained for x = 0, 0.2 and 0.5, respectively. Kinetic studies on the negative electrode consist of Mm0.5Pr0.5Ni3.55Co0.75Mn0.4Al0.3 showed that diffusion-controlled models are well-fitted on the discharge curves. The values of calculated activation energies (27–28 kJ mol−1) also guaranteed this hypothesis.