Improvement of hydrogen storage properties of Ammonia Borane in ionic liquids

Figure 1. Glass pressure reactor

The use of hydrogen as an energy vector stands as an interesting alternative to traditional energy sources, but it is necessary to find an efficient way of hydrogen storage for mobile devices. A possible solution is the use of solid materials containing hydrogen which would desorb it through thermal treatment on-board. Many solid hydrides that can reversibly store hydrogen by absorption mechanisms have been tested to this aim and, in this context, ammonia borane (NH3BH3, AB) has shown to be very promising. Due to its solid nature, its performance in terms of rates and extent of H2 release can be improved by the use of liquid solvents, particularly ionic liquids: their low vapor pressure makes them suitable to replace volatile organic solvents for dissolving AB and their stability to high temperature allows operating at fuel cells working conditions.

To further understand the differences and particular decomposition mechanisms, the release of hydrogen by thermolysis of AB dissolved in a big variety of ionic liquids has been investigated. For each hydrogen release measurement, the ionic liquid under study was introduced in a glass pressure reactor (see Figure 1) and dried at 90ºC in a vacuum oven for at least 24 h prior to the measurement. 10 % wt of AB was then added and the glass reactor sealed and subjected to vacuum in an oven set at 85ºC. The evolution of pressure inside the reactor was recorded and provides the equivalents of released H2. The results for most of ionic liquids share similar characteristics: a considerable reduction of the induction time, with an accelerated release of hydrogen that in many cases allows reaching a higher yield than from neat AB.

However, considerable differences in H2 release yield and rate were observed depending on the IL used. These results together with a characterization of the foaming and solubilization behavior of the solutions observed during hydrogen release experiments in the high pressure glass tubes will systematically be studied as it would be highly desirable to obtain soluble residues of AB decomposition. Next step would be the study of the residues composition by spectroscopic techniques to set the different decomposition mechanisms leading to such variety of results.


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