Hydrogen Storage Properties of the Destabilized 4NaBH4/5Mg2NiH4 Composite System

A composite mixture of NaBH4 and Mg2NiH4 (4:5, M/M) was prepared by high-energy ball milling and was investigated for solid-state storage of hydrogen. Addition of Mg2NiH4 to NaBH4 leads to formation of the stable ternary boride phase MgNi2.5B2 and lowers the enthalpy of hydrogen desorption for NaBH4 from 110 to 76 +/- 5 kJ mol(H2)(-1). As a consequence of this addition, the onset temperature of hydrogen desorption decreases from about 500 degrees C in NaBH4 to 360 degrees C in the NaBH4/Mg2NiH4 composite mixture. Furthermore, the system shows reversibility and is able to store 2.5 wt % hydrogen for a number of cycles. The impact of the ball milling time, the hydrogen desorption back-pressure, and cycling on the hydrogen desorption/absorption properties and crystallographic phases is discussed in detail.