Effect of hydrogen absorption/desorption cycling on hydrogen storage properties of a LaNi3.8Al1.0Mn0.2 alloy

The effect of long-term hydrogen absorption/desorption cycling up to 3500 cycles on the hydrogen storage properties of LaNi_3.8Al_1.0Mn_0.2 alloy was investigated. The pressure-composition (PC) isotherms for absorption/desorption and the absorption kinetics were measured at 433 K, 453 K and 473 K. X-ray diffraction analysis revealed that the alloy had a homogeneous hexagonal CaCu₅ type structure and kept this structure even after 3500 cycles, but the diffraction peaks were broadened. The degree of peak broadening was increased with increase of the cycle number, but exhibiting a maximum after initial activation. The shapes of PCT curves after 300, 2000 and 3500 cycles were similar to that after initial activation. It was found that the alloy subjected to 300 cycles did not exhibit significant changes in hydrogen storage capacity, but the long-term cycling up to 2000 and 3500 cycles resulted in obvious decrease in hydrogen storage capacity. The degradation of the hydrogen capacity might be resulted from the formation of the irreversible sites and more stable hydride phase, though no new phase was found after absorption/desorption cycling from XRD pattern as shown in Fig. 6 because of the limitation of XRD analysis sensibility. The hydrogen absorption kinetics after 300 cycles was deteriorated but improved again after 2000 and 3500 cycles compared with that of after initial activation. The changes in hydrogenation properties of the alloy induced by cycling were discussed by considering the crystal structure, lattice strain and pulverization of the sample.