Predicting formation enthalpies of metal hydrides

In order for the hydrogen based society viz. a society in which hydrogen is the primary energy carrier to become realizable an efficient way of storing hydrogen is required. For this purpose metal hydrides are serious candidates. Metal hydrides are formed by chemical reaction between hydrogen and metal and for the stable hydrides this is associated with release of heat (∆Hf ). The more thermodynamically stable the hydride, the larger ∆Hf, and the higher temperature is needed in order to desorp hydrogen (reverse reaction) and vice versa. For practical application the temperature needed for desorption should not be too high i.e. ∆Hf should not be too large. If hydrogen desorption is to be possible below 100°C (which is the ultimate goal if hydrogen storage in metal hydrides should be used in conjunction with a PEM fuel cell), ∆Hf should not exceed -48 kJ/mol. Until recently only intermetallic metal hydrides with a storage capacity less than 2 wt.% H2 have met this criterion. However, discovering reversible hydrogen storage in complex metal hydrides such as NaAlH4 (5.5 wt. % reversible hydrogen capacity) have revealed a new group of potential candiates. However, still many combination of elements from the periodic table are yet to be explored. Since experimental determination of thermodynamic properties of the vast combinations of elements is tedious it may be advantagous to have a predictive tool for this task. In this report different ways of predicting ∆Hf for binary and ternary metal hydrides are reviewed. Main focus will be on how well these methods perform numerically i.e. how well experimental results are resembled by the model. The theoretical background of the different methods is only briefly reviewed.