Microstructure and absorption/desorption kinetics evolutions of Mg[sbnd]Ni[sbnd]Ce alloys during hydrogenation and dehydrogenation cycles

Abstract

Poor cycling stability limits the sustainable and long-life application of Mg as a hydrogen storage material. In the light of catalytic effects of rare earth elements and transition metals on de-/hydrogenation kinetics of Mg, binary Mg20Ce, Mg20Ni and ternary Mg5Ni15Ce (wt%) alloys have been prepared with the aim of investigating the absorption and desorption cycling performance. Special attention is paid to the microstructure evolution during cycling and its interrelation with hydrogen storage properties. It is found that CeH2.73 facilitates hydrogenation of Mg better than Mg2Ni. Conversely, Mg2Ni is more conducive to desorption. A synergistic catalytic effect between Mg2Ni and CeH2.73 is observed in Mg5Ni15Ce alloy, the hydriding rate of which is faster than that of MgNi and MgCe binary alloys. The average particle size of each experimental alloy decreases significantly after 100 absorption/desorption cycles at 320 °C. Moreover, the particle sizes of CeH2.73 and Mg2Ni phases remain stable during cycling. The grain size of Mg increases significantly for binary alloys, while introducing Mg2Ni and CeH2.73 simultaneously can effectively inhibit the growth of Mg grains. The hydrogen absorption capacities increase gradually for Mg20Ce and Mg5Ni15Ce alloys due to reduced diffusion distance by particle pulverization and freshly exposed CeH2.73 on newly formed particle surface. However, the capacity of Mg20Ni alloy decreases slightly considering the facts that Mg grains agglomerate during cycling and Mg2Ni possesses weak catalytic effect on absorption. Both the absorption and desorption kinetics keep stable during cycling for Mg5Ni15Ce alloy.