Hydrogen absorption–desorption characteristics of a Gd2Co7-type Sm1.6Mg0.4Ni7 compound

Abstract

In this paper, we report a new Gd₂Co₇-type Sm_1.6Mg_0.4Ni₇ compound as a hydrogen storage material with a special hydrogen absorption/desorption process and good hydrogen storage ability. The Gd₂Co₇-type Sm_1.6Mg_0.4Ni₇ compound absorbs 1.88 wt% H₂ within 17 min at 298 K under 10 MPa H₂. Meanwhile, the hydrogen absorption speed accelerates to 3.4 min after 20 hydrogenation/dehydrogenation cycles with a 1.44 wt% H₂ under 3 MPa H₂. Especially, the capacity retention of the compound is 99.3% at the 100^th cycle. We found the hydrogen absorption/desorption of the compound undergoes two equilibrium stages, relating to the transformation of H₂ between H-solid solution phase and hydride phase with a lower rate and higher enthalpy change at the lower concentration H₂ stage, and the direct conversion between H₂ and the hydride phase with a higher rate and lower enthalpy change at the higher concentration H₂ stage. The two step mode lowers the inner-molecular strain and mismatch in subunit volumes of the compound in hydrogen absorption/desorption, caused by the transformation of H₂ at the lower concentration of H₂ stage, thus leading to good structural stability and excellent cycling stability. The new insights are expected to provide viable intermetallic materials as high-pressure tank materials for hydrogen storage with nice hydrogen storage properties.