The study of La and Mg elements on the improved structure and hydrogen storage performance of Ca2MgNi9 alloy

In order to improve the comprehensive hydrogen storage performance of AB3-type (RE/Ca)-Mg-Ni alloys, a new series of Ca(Ca0.75La0.25)2-xMgxNi9 (x = 0.8, 0.9, 1.0, 1.1, 1.2) alloys is designed and prepared. Based on the as-cast Ca2MgNi9 alloy, La is introduced, and the ratio of rare earth (RE) to Mg is adjusted using the induction melting method. This alteration modifies the microstructure of the alloy and significantly enhances its hydrogen storage performance. The effects of microstructure on hydrogen storage performance and mechanism are analyzed. The phase composition and lattice parameters of the main phase are regularly regulated by appropriate element substitution. The lattice parameter of the main phase is increased, the capacity and kinetics of hydrogen storage are significantly improved. The initial hydrogen absorption capacity of the alloy increases from 1.744 wt% for Ca2MgNi9 to 1.975 wt% for Ca(Ca0.75La0.25)1.2Mg0.8Ni9 alloy. Furthermore, the synergistic regulation of La and Mg elements can increase the hydrogen desorption ratio and crystal structure stability of the alloy. This leads to the concurrent enhancement of effective hydrogen storage and cycle performance of the Ca2MgNi9 hydrogen storage alloy. The retention rate of hydrogen absorption capacity for the 50th cycle is increased from 77.13% for Ca2MgNi9 to 84.26% for Ca(Ca0.75La0.25)0.8Mg1.2Ni9. At the same time, owing to the adjustment of the cell volume and phase composition, the platform pressures of hydrogen absorption and desorption of the alloy have also experienced a multi-fold increase between 0.002 and 0.5 MPa at 303 K. This enhancement is advantageous for expanding the potential applications of this type of alloy. This work presents new ideas for enhancing the hydrogen storage performance of Ca2MgNi9 hydrogen storage alloys. •A new series of Ca(Ca0.75La0.25)2-xMgxNi9 (x = 0.8, 0.9, 1.0, 1.1, 1.2) hydrogen storage alloys is designed.•The activation and the kinetics performance are improved after replacing Ca with a small amount of La.•The effective hydrogen storage and cycling performance are improved by optimizing the ratio of (Ca0.75La0.25)/Mg in the [A2B4] subunit.