The Role of Intermediates in the Transformation of Complex Hydrides

Maximilian Fichtner, Johannes Kostka, Wiebke Lohstroh

Research Center Karlsruhe, Institute of Nanotechnology,

Germany

Albeit sometimes hard to detect, intermediates may play an important role in the transformation of complex hydrides. The formation of highly mobile species may - at least in the case of alanates - explain and link the obvious mismatch of theoretical and experimental results concerning the materials transformation and role of the Ti dopant in the material. According to recent experimental findings a transport reaction is likely which is able to take on the enormous mass transport at the low operating temperatures of the storage.

Ammonia (NH₃) is believed to be a mobile intermediate and a reaction partner in the transformation of amides, Mx(NH2)y. While there are several experimental indications for this assumption, there are also observations such as the degradation of the reversible properties under cycling which can not be explained by the current model description.

A particular challenge is the question whether and how intermediates are formed in the transformation of boranate (Mx(BH4)y) based systems, which promise the highest gravimetric storage capacities at the moment. A good reversibility of thermodynamically viable systems was observed in a few cases only - even though the working temperatures were high and kinetic barriers should be low compared to the alanates. In particular, the role of volatile intermediates is unclear at the moment, although they may be crucial for the process. Here, the formation of BH3 may cause problems if volatile species are desorbed which are toxic for both technical and natural environment of the storage. The situation is discussed based on own results and the literature and strategies will be outlined how to proceed in the future.