Computational Screening of Dual Cation Metal Ammine Borohydrides

Abstract

Hydrogen is one of the promising alternatives for the replacement of fossil-fuels. One of the major bottlenecks preventing its widespread commercialization for on-board applications is to find the most suitable storage medium. Metal borohydrides are one of the classes of solid materials studied intensively to store hydrogen due to their high theoretical hydrogen capacities. However, their high thermodynamic stability is one of the major problems limiting their usage. The requirement of high decomposition temperature can be lowered by the inclusion of ammonia. The resulting new complex containing both borohydrides and ammines is called as Ammine Metal Borohydrides (AMBs). However, some of the AMBs have insuppressible release of ammonia during the dehydrogenation. This can be solved by the inclusion of a second metal atom into AMBs leading to dual-cation AMBs with a general formula of M1M2(BH4)y(NH3)x. Experimental studies about one dual-cation AMBs (LiMg(BH4)3(NH3)2) indicate desired properties e.g., decomposition occurs below 200 °C and no release of ammonia. In this study, a computational screening using periodic density functional theory was performed to find promising dual-cation AMBs (M1M2(BH4)y(NH3)x with M1 = Li, Na, K, M2 = Mg, Ca, Sr, Zn, Mn, Ni, y = 3, x = 2, 3, 4, 5, 6). The screening has been accomplished using template crystal structures generated by Crystal Structure Prediction via Simulated Annealing (CASPESA) method. Formation, alloying and simple decomposition reactions were employed for the evaluation of the complexes.