Hydrogen storage properties and phase structures of RMg2Ni (R = La, Ce, Pr, Nd) alloys

RMg2Ni alloys were prepared by inductive melting where R is rare earth (R = La, Ce, Pr, Nd). X-ray diffraction (XRD) patterns revealed a single-phase composition of RMg2Ni phase when R was one of the three elements (La, Pr, Nd), and a double-phase composition of CeMg2Ni and CeMg3 phases when R was Ce. In the hydriding process, RMg2Ni phases transformed to rare earth hydrides (R-H) and Mg2NiH4 phase, and for CeMg3 phase, it is decomposed to CeH2.74 and MgH2 phases. The enthalpy change of Mg2Ni phase in RMg2Ni alloys during the hydriding/dehydriding process was smaller compared with that of pristine Mg2Ni alloy, which could be attributed to the existence of R-H. The hydrogen storage properties of RMg2Ni alloys changed with different R compositions in R-H. At 573 K, the NdMg2Ni alloy had the highest hydrogen storage capacity and dehydriding plateau, and the descending order of hysteresis was PrMg2Ni < NdMg2Ni < CeMg2Ni < LaMg2Ni, which suggested that the PrMg2Ni alloy exhibited a better cycling stability and reversibility than the other three alloys. At 523 K, the uptake time of RMg2Ni alloys to reach 90% of the maximum hydrogen storage capacity was 75 s, 34 s, 65 s and 52 s, respectively, compared with 110 s of pristine Mg2Ni alloy. Therefore, we believed the R-H in the alloys not only improved their thermodynamic properties but also accelerated their hydriding kinetics.