Dual-tuning of de/hydrogenation kinetic properties of Mg-based hydrogen storage alloy by building a Ni-/Co-multi-platform collaborative system

Abstract

The Mg-based hydrogen storage alloy with multiple platforms is successfully prepared by ball milling Co powder and Mg-RE-Ni precursor alloy, and its hydrogen storage behavior was investigated in detail by XRD, EDS, TEM, PCI, and DSC methods. The ball-milled alloy consists of the main phase Mg, the catalytic phases Mg2Ni, Mg2Co as well as a small amount of Mg12Ce, and convert into the MgH2–CeH2.73-Mg2NiH4–Mg2CoH5 composite after hydrogenation. The composite has three PCI platforms corresponding to the reversible de/hydrogenation reaction of Mg/MgH2, Mg2Ni/Mg2NiH4 and Mg6Co2H11/Mg2CoH5. Among them, the transformation between Mg2Ni and Mg2NiH4 triggers the “spill-over” effect which promote the decomposition of MgH2 phases and enhances the hydrogen desorption kinetics. Meanwhile, the conversion of the Mg6Co2H11 to Mg2CoH5 phase induces the “chain reaction” effect, which leads to preferential nucleation of Mg phase and improves the hydrogen absorption kinetics. Therefore, the Mg-RE-Ni-Co alloy has a double improvement on hydrogen absorption and desorption kinetics. Concretely, the alloy has an optimal hydrogen absorption temperature of 200 °C, at which it can absorb 5.5 wt. % H2 within 40 s. Under the conditions, the capacity of absorption almost reaches the maximum reversible value (about 5.6 wt. %). Besides, the alloy has a dehydrogenation activation energy of 67.9 kJ/mol and can desorb 5.0 wt. % H2 within 60 min at the temperature of 260 °C.