Dehydriding and rehydriding processes of well-crystallized Mg(BH4)2 accompanying with formation of intermediate compounds

Dehydriding and rehydriding properties of well-crystallized Mg(BH4)2 were systematically investigated by thermogravimetry (TG) and pressure–composition–temperature (PCT) measurements. The dehydriding reaction of Mg(BH4)2 starts at approximately 500 K, and about 14.4 mass% of hydrogen is desorbed according to the following multi-step reaction:Mg(BH4)2 → some intermediate compounds → MgH2 + 2B + 3H2 → Mg + 2B + 4H2The apparent enthalpy change in the dehydriding reaction from Mg(BH4)2 to MgH2 is estimated to be 57 ± 5 kJ mol−1 H2 based on the result of the PCT measurement. It is proved that approximately 6.1 mass% of hydrogen can be reversibly stored for the sample of Mg(BH4)2 after the dehydriding reaction, through the formation of a possible intermediate compound such as MgB12H12.