Catalytic effect of rare earth oxides with variable valences on hydrogen storage properties of Mg₂Ni alloy

Abstract

Mg2Ni-Ni-5%RExOy(CeO2, Nd2O3, Tb4O7) composites were synthesized via ball milling method. The microstructure and hydrogen storage properties of the materials were analyzed systematically by XRD, SEM, EDS, the electrochemical and kinetic measurements. The results show that the crystallinities of the composites with rare earth oxides decrease, and the distribution of the rare earth oxide catalysts on the surface of alloys is uniform. The maximum discharge capacities of the composites with rare earth oxides rise evidently,the discharge capacity of the sample containing Tb4O7 reaches to 871mAh·g-1 at room temperature, and the composites can also keep higher cycling stability. The CeO2 and Tb4O7 catalysts can enhance the abilities of charge transfer on the surface of alloys, and improve the transmission rate of H atoms in the bulk of Mg2Ni alloy obviously. The rare earth oxides catalysts can also increase the gaseous hydrogen absorption capacity. The hydrogen storage capacity of the sample with Tb4O7 reaches to 2.02%(mass fraction) at 250℃, which is the maximum hydrogen absorption capacity among these samples, but the hydriding rate is relatively slow at lower temperature. The catalytic effect of the rare earth oxides is mainly related to the changeable valences of the rare earth ions, that is, the more likely of the valences change of the ions, the better the catalytic activities exhibit. The catalytic activities in descending order are Tb4O7, CeO2 and Nd2O3.

Keywords

• rare earth oxide
• mg₂ni alloy
• electrochemical property
• kinetic property
• catalytic effect