Effect of long-term hydrogen absorption/desorption cycling on hydrogen storage properties of MmNi3.55Co0.75Mn0.4Al0.3

The effect of a long-term hydrogen absorption/desorption cycling up to 2000 cycles on the hydrogen storage properties of MmNi3.55Co0.75Mn0.4Al0.3 alloy was investigated. The pressure–composition (PC) isotherms for absorption/desorption and absorption kinetics were measured at 338 K, 353 K and 368 K both after initial activation and 2000 cycles. X-ray diffraction analysis revealed that the alloy had a homogeneous hexagonal CaCu5 type structure and kept this structure even after 2000 hydrogen absorption/desorption cycles. It is found that the absorption/desorption plateau pressures were lowered, the storage capacity and the absorption kinetics were slightly degraded and the hysteresis loss was increased at all the investigated temperatures after 2000 cycles. It is also found that the particle size after 2000 cycles was much smaller compared to that after initial activation. The change of the hydrogen absorption/desorption properties of the alloy after 2000 cycles has been explained by considering the crystal structure, disproportionation property, pulverization of the sample and the impurities in the charging hydrogen employed in cycling.