NOVELTY - Synthesis (100) of magnesium-cobalt pentahydride (MgCoH5) comprises melting (101) magnesium (Mg) metal and cobalt (Co) metal at a first temperature to form a molten metal; cooling the molten metal to a second temperature, isothermally heating for forming a magnesium-cobalt (MgCo2) alloy, and removing (103) unreacted pure Mg; cooling down (105) the MgCo2 alloy; ball-milling (107) the MgCo2 alloy and another Mg metal to form metal mixture powder; and hydrogenating (109) for less than or equal to 15 hours at 100-600 degrees C, under a hydrogen pressure of 6-120 atm.    USE - Method for the synthesis of magnesium-cobalt pentahydride (Mg2CoH5) (claimed) used on a fuel cell system.    ADVANTAGE - The method reduces overall process time. The Mg2CoH5 has no residual pure Co and pure Mg left, and has excellent hydrogen absorption-desorption rate.    DETAILED DESCRIPTION - Synthesis (100) of magnesium-cobalt pentahydride (Mg2CoH5) comprises:    (A) melting (101) Mg metal and 30 at.% Co metal at a first temperature, where the first temperature is between the melting point of the Mg metal and the melting point of the Co metal, thus forming a molten metal;    (B) cooling the molten metal to a second temperature, isothermally heating the molten metal for forming a MgCo2 alloy, and removing (103) unreacted pure magnesium, where the second temperature is between the first temperature and the melting point of the magnesium metal;    (C) cooling down (105) the MgCo2 alloy;    (D) ball-milling (107) the MgCo2 alloy and another Mg metal to form metal mixture powder, where the mole ratio of the MgCo2 alloy to the other Mg metal is substantially 1:3; and    (E) hydrogenating (109) the metal mixture powder for less than or equal to 15 hours at 100-600 degrees C, under a hydrogen pressure of 6-120 atm to form Mg2CoH5 without leaving residual pure Co, where the hydrogen absorption amount of the Mg2CoH5 is 2.5 mass % at 350 degrees C, and the hydrogen desorption amount of the Mg2CoH5 is 2 mass % at 350 degrees C.    The Mg metal and the Co metal are heated, isothermally heated, and ball-milled under vacuum or protection gas. The first temperature is higher than the second temperature by 150-200 degrees C. Hydrogenation is carried out at 400 degrees C and less than 50 atm.    DESCRIPTION OF DRAWING(S) - The drawing is a flow chart of the synthesis of magnesium-cobalt pentahydride.    Synthesis method (100)    Melting metals (101)    Removing unreacted pure magnesium (103)    Cooling (105)    Ball-milling (107)    Hydrogenating (109)