Zirconium tetrachloride (ZrCl4) is one of the best catalysts for improving the reversible hydrogen storage performance of the MgH2 Mg system. MgH2 catalyzed by ZrCl4 shows a remarkably reduced apparent activation energy, which leads to improved dehydrogenation and rehydrogenation kinetics. X-ray photoelectron spectroscopy (XPS) revealed the chemical state of ZrCl4 as ZrCl3 and metallic Zr after ball milling with MgH2. The in situ-formed ZrCl3 and metallic Zr showed good catalytic effect on MgH2, which substantially lowered the dehydrogenation and rehydrogenation temperatures. The scanning electron microstructure analysis revealed the excellent grain refinement property of the catalyst to reduce the crystallite size of MgH2 during ball-milling. The decreased crystallite size reduces the diffusion path length of hydrogen and increases the active surface area of MgH2 Mg, which eventually enhances the dehydrogenation and rehydrogenation kinetics. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.