Improved hydrogen storage performance of Sm-Mg composites by adding nano-graphite

Abstract

Sm5Mg41 + x wt% nano-graphite (NG) (x = 0, 2, 4, 8, 12) composites are synthesized via ball-milling. The influences of NG content on microstructure and hydrogen storage properties of the composites are researched systematically. The composites consist of Mg, Sm5Mg41, SmMg3, and NG. Nano scale Sm3H7 and MgH2 phases are observed after the hydrogen absorbing, and the newly formed Mg phase and the remaining Sm3H7 phase appear after hydrogen desorbing. NG is embedded in the surface of the composites in the form of nanosheets. Nanocrystalline Mg and some microscopic defects, including twin crystal, grain boundaries, and dislocations can be observed in the dehydrogenated sample. Such microscopic defects and nanocrystalline boundaries can reduce the total potential barrier needed to overcome during the hydrogen absorption/desorption reactions and thus the hydrogen storage kinetics of the alloy are improved. For the composites with x = 2, 4, 8 and 12, the activation energies of hydrogen desorption are 132.08, 128.06, 112.90, and 112.35 kJ/mol H2, respectively. The enthalpy changes for the hydrogen absorption are evaluated to be − 79.18, − 78.09, − 76.30, − 76.10 kJ/mol H2, indicating that the addition of NG had no obvious effect on the thermodynamic properties of the composites.