Discovery of new dual cation ammine borohydrides: A computational screening study

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 widespread 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)x(NH3)y, x=3-5 and y=2-6. Until now, there are only a few synthesized dual cation AMBs reported in the literature. Therefore, by conducting a computational screening study we aim to find new AMBs with desired properties. In this respect, M1 was selected as an alkali metal (Li, Na or K) and M2 was assumed to be one of the following species: Mg, Ca, Ni, Mn, Sr, Zn, Al, Y, Sc, Ti, Zr and Co. The ideal case in a screening study is to use known crystal structures of the studied system. However, this is not an easy task and in general our target is to design new materials which are not seen on the literature. Therefore, employment of template structures is a well accepted strategy in such screening studies. A template structure refers that it is a prototype structure designed using the properties one of the system among the scope of the study. This template structure can be used for the other systems by just doing the proper replacements. For example, if a template structure generated for a system including Mg atom and the same structure can be invoked for a system including Zn by replacing Mg with Zn. Similar to the general situation depicted above, there is very limited information about the crystal structures of AMBs in the literature. Therefore, we found first template structures using a crystal structure prediction algorithm called as CASPESA. Subsequently, these structures were further relaxed at the DFT level. AMBs were evaluated with the help of some alloying and decomposition reactions. The results obtained so far indicate that many new AMBs were quite promising. This work is a good example showing how supercomputers can be utilized to design new materials. In this case, the target is an energy material, however, the scope of design can easily be broadened e.g. to batteries or gas sensing materials. All these computational efforts allow a fast, economic and less expensive (in terms of time compared to experiment) way of strategy in both chemical and physical sciences.