One of the major limitations to the use of fuel cell systems in vehicular transportation is the lack of hydrogen storage systems that have the required hydrogen storage density and moderate enthalpy of dehydrogenation. Organic liquid H_2 carriers that release H_2 endothermically are easier to handle with existing infrastructure because they are liquids, but they have low storage densities and their endothermicity consumes energy in the vehicle. On the other hand, inorganic solid H_2 carriers that release H_2 exothermically have greater storage densities but are unpumpable solids. This paper explores combinations of an endothermic carrier and an exothermic carrier, where the exothermic carrier provides some or all of the necessary heat required for dehydrogenation to the endothermic system, and the endothermic carrier serves as a solvent for the exothermic carrier. The two carriers can be either physically mixed or actually bonded to each other. To test the latter strategy, a number of chemically bound N-heterocycle:BH_3 adducts were synthesized and in turn tested for their ability to release H_2 by tandem hydrolysis of the BH_3 moiety and dehydrogenation of the heterocycle. To test the strategy of physically mixing two carriers, the hydrolysis of a variety of amine-boranes (H_3N:BH_3, Me_2HN:BH_3, Et_3N:BH_3) and the catalytic dehydrogenation of indoline were carried out together.
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