The study on synergetic catalytic mechanism of (V-CsF) additives in Mg-V-CsF H-storage systems

Abstract

In situ reactive high-energy ball milling was employed to prepare nano Mg-V-CsF hydrogen storage materials. The morphology, crystal structure, isothermal hydrogen ab-/desorption properties of the composites and possible catalytic mechanisms were investigated. The as-synthesized Mg-V-CsF nanocomposites are comprised of β-MgH2, γ-MgH2, VH0.81, MgF2 and CsH phases as confirmed by X-ray Diffraction analysis. Among as synthesized materials, the Mg-5 V-3CsF powders possess excellent de-/absorption kinetic property, indicated by the shorter time of 35 min required to desorb 7.0 wt% H2 and higher hydrogen storage capacity (7.0 wt% H2). Besides, the dehydrogenation enthalpy of as milled Mg-5 V-3CsF composite is determined to be the 68.98 kJ/(mol·H2). XRD and XPS analysis provide the evidences that MgF2 and CsH phases co-existed in the Mg-5 V-3CsF composite. The superior hydrogen absorption properties of the Mg-V-CsF composites can be attributed to the in situ formation of CsH and MgF2. Whereas, it suffers the sluggish dehydrogenation kinetics, caused by the H-H exchange effect between CsH and MgH2. In this regard, this paper sheds light on catalytic mechanisms of two different types of catalysts (V/CsF) and put on the ways to achieve optimal hydrogen storage properties of Mg/MgH2 systems by “synergetic” catalytic effect between VHx and in situ formation of CsH/CsF/MgF2.