Phase transformation and electrochemical hydrogen storage performances of La3RMgNi19 (RÂ =Â La, Pr, Nd, Sm, Gd and Y) alloys

Adjusting the rare earth (RE) compositions in the RE-Mg-Ni alloys can effectively improve the electrochemical hydrogen storage performances of the alloy electrodes. Herein, A5B19 type hydrogen storage alloys with the elemental composition of La3RMgNi19 (R = La, Pr, Nd, Sm, Gd and Y) were prepared by induction melting and subsequent annealing. The phase transformation and electrochemical hydrogen storage performances of La3RMgNi19 alloys were investigated in detail. X-ray diffraction analysis shows that La3RMgNi19 alloys contains AB5, A2B7 (Ce2Ni7 and Gd2Co7) and A5B19 (Pr5Co19 and Ce5Co19) phases, and the increase of annealing temperature obviously reduces the phase abundance of LaNi5 phase. Sm, Gd and Y contribute to the formation of A5B19 phase, especially Ce5Co19, and Pr and Nd promote the formation of A2B7 phase for La3RMgNi19 alloys. With increasing annealing temperature, the maximum discharge capacity (Cmax) of La3RMgNi19 alloy electrodes first increases and then decreases, and the highest value of Cmax is achieved as the annealing temperature is 1223 K. This evolution trend of the Cmax is inversely proportional to that of LaNi5 phase abundance. The substation of La by Pr, Nd, Sm, Gd or Y causes the decrease of Cmax, which is mainly ascribed to the decrease of cell volume. Due to the decrement of LaNi5 phase, the cycling stability increases at first when the annealing temperature is below 1223 K. However, when annealing temperate further increases to 1273 K, the cycling stability decreases, which is caused by the increment of LaNi5 phase. It is worth noting that the phase composition (LaNi5 phase abundance) plays more important role than other factor. The slight decrement of high-rate dischargeability resulted from the substitution of La by Pr, Nd, Sm, Gd or Y should be attributed to the combined effect of advantageous and disadvantageous factors.