Enhanced hydrolysis properties and energy efficiency of MgH2-base hydrides

• The maximum energy efficiency of HeLa2Mg17 hydrolysis cycle is 41.1%.
• La2Mg17eNi hydrides can generate up to 1208 ml/g hydrogen in 40 min at 25 °C.
• A convenient method for La2Mg17 hydrogenation is achieved by ball milling under 7 MPa.

MgH2 hydrolysis is an efficient and convenient method for hydrogen generation. By introducing rare earth hydrides, the hydrolysis properties of hydrogenated Mg3La hydrides (HMg3La) and hydrogenated La2Mg17 hydrides (HeLa2Mg17) are enhanced compared with MgH2 hydrolysis, but the energy efficiency of the three hydrolysis cycles is unknown and a relatively cheap MgH2-based hydride synthesis method still needs to be developed. We design and calculate for the first time the energy efficiency of such three hydrolysis cycles based on thermodynamics. Maximum energy efficiencies of 45.3%, 40.1% and 41.1% are obtained, which indicates that hydrogen generation from the three methods is feasible and energy favorable. A convenient method for La2Mg17 hydrogenation is achieved by ball milling La2Mg17 and Ni under 7 MPa hydrogen pressure at 25 °C. The ball–milled products, MgH2 + LaH3 + Ni hydrides composite, show excellent hydrolysis performance, and produce 1208 ml/g hydrogen in 40 min at 25 °C, with a reduced activation energy of 52.9 kJ/mol. These benefits result from the particle size refinement from ball milling and catalysis of LaH3 and Ni.