Ca(BH<inf>4</inf>)<inf>2</inf>-LiBH<inf>4</inf>-MgH<inf>2</inf>: 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, LiBH4and MgH2in 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 CaMgH3.72for dehydrogenation in the hydrogenation process. There is ca. 7.6 wt% H2absorbed at 350 °C and 90 bar H2for 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% H2at the 10thcycle. 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. © 2013 The Royal Society of Chemistry.