Alane hydrogen storage for automotive fuel cells – Off-board regeneration processes and efficiencies

Alane is considered an attractive carrier of hydrogen for on-board light-duty vehicle hydrogen storage systems because of its high intrinsic capacity (10.1 wt% H2), small heat of formation (∼7 kJ/mol H2), and fast apparent decomposition kinetics. Regeneration of spent Al by direct hydrogenation is impractical due to the extremely high hydrogen equilibrium pressure required (∼7000 bar). This paper examines the off-board regeneration of alane using a three-step organometallic process. In the first step, a relatively stable adduct of a tertiary amine and alane is formed from elemental aluminum and hydrogen gas under moderate conditions of temperature and pressure. The second step involves transamination of the adduct by a second tertiary amine to form a secondary tertiary amine-alane adduct that is less stable than the first adduct. This secondary amine alane adduct is thermally decomposed in the final step to yield alane and the secondary amine for reuse in the process. All reagents, except aluminum and hydrogen, are recovered and recycled. Two process flowsheets have been constructed, and energy consumption in each step of the regeneration process has been calculated. Additionally, total energy requirements across the entire chain of production, delivery, storage, recovery, and regeneration has been evaluated to determine the overall well-to-tank efficiency and greenhouse gas emissions. In one flowsheet, the well-to-tank efficiency is ∼24.2% which improves to ∼42.1% if waste heat is freely available from industrial sources. The estimated greenhouse gas emissions are 31.6 kg CO2 (eq) per kg H2 delivered to the vehicle and reduce to 20.6 kg/kg-H2 if free waste heat is readily available.