Remarkable enhancement and electronic mechanism for hydrogen storage kinetics of Mg nano-composite by a multi-valence Co-based catalyst

Abstract

Transition metals are the traditional catalysts to improve the hydrogen storage performance of Mg. In the present article, the multi-valence Co catalyst (Co@CNTs) is prepared to improve the kinetics of Mg. The dehydrogenation temperature decreases, and desorption kinetics of MgH2 is significantly enhanced by the addition of the catalyst. The dehydrogenation of MgH2–Co@CNTs composite is a random bulk nucleation/surface and three-dimensional growth with constant interface velocity reaction. Although the dehydrogenation activation energy of MgH2–Co@CNTs composite is reduced, the enthalpy change and entropy change of the composite are similar to the raw MgH2, which means the dehydrogenation is mainly dissociation of MgH2. Besides, the cycle performance of MgH2–Co@CNTs composite is stable. The damping of the performance for the composite is negligible after 50 cycles, which benefits from the in-site formed Co3MgC0.5 in the composite. The Co3MgC0.5 can act as a ‘hydrogen pump’ to adjust the hydrogen storage performance of Mg. Theoretical studies show that multi-valence Co can trap H2 molecules and weaken their σ-bonds. In the first cycle, the bridging effect of Co plays an important role in the improvement of H2 dissociation and release. In the subsequent cycles, Co6C1 becomes the active site to assist the hydrogenation and dehydrogenation of the composite.