Kinetics and thermodynamics of the aluminum hydride polymorphs

Polymorphs of AlH3 were prepared by organometallic synthesis. We demonstrate that freshly synthesized, nonsolvated AlH3 releases approximately 10 wt% H-2 at desorption temperatures less than 100 degrees C. The decomposition kinetics, measured by isothermal hydrogen desorption between 30 and 140 degrees C, suggest that the rate of H-2 evolution is limited by nucleation and growth of the aluminum phase. The H-2 evolution rates for small crystallites of alpha and gamma-AlH3 (undoped) meet the DOE full flow target for a 50 kW fuel cell (1 gH(2)/s) above 114 degrees C (based on 100 kg AlH3). The decomposition thermodynamics were measured using differential scanning calorimetry and ex situ X-ray diffraction. The decomposition of the less stable polymorph, gamma-AlH3, occurs by an exothermic transformation to the a phase (similar to 100 degrees C followed by the decomposition of alpha-AlH3. A formation enthalpy of approximately -10 kJ/mol AIH(3) was measured for alpha-AlH3, which is in good agreement with previous experimental and calculated results. (C) 2006 Elsevier B.V. All rights reserved.