Ca(BH4)2–LiBH4–MgH2: a novel ternary hydrogen storage system with superior long-term cycling performance

Abstract

A ternary hydrogen storage system, of superior cyclic stability and high capacity, was developed from a mixture of Ca(BH₄)₂, LiBH₄ and MgH₂ in molar ratios of 1 : 2 : 2. Investigation on both non-isothermal and isothermal hydrogen desorption/absorption properties shows that the hydrogen desorption starts from 320 °C and completes at 370 °C under a heating rate of 2 °C min⁻¹, releasing ca. 8.1 wt% H₂. The finishing temperature of desorption is much lower and the capacity much higher than any of the two-hydride mixtures in the ternary system. In particular, hydrogenation of the ternary system initiates at an extremely low temperature of ca. 75 °C and the onset dehydrogenation temperature is significantly reduced by 90 °C after the initial dehydrogenation/hydrogenation cycle, which is ascribed to the formation of an active dual-cation hydride of CaMgH_3.72 for dehydrogenation in the hydrogenation process. There is ca. 7.6 wt% H₂ absorbed at 350 °C and 90 bar H₂ for 18 h for the system post-dehydrogenated at 370 °C for 30 min, demonstrating a reversibility of over 94%. The capacity seems to fade mainly in the initial few cycles and stabilizes after further cycling. The reversibility is as high as 97% and a dehydrogenation capacity of ca. 6.2 wt% H₂ at the 10^th cycle. Favourable kinetics and thermodynamics of hydrogen desorption/absorption are achieved, which are responsible for the low completion temperature and the superior cycling performance. Mechanisms of the improved dehydrogenation/hydrogenation properties including the cyclic behaviour of the system are also proposed in relation to microstructural analyses.