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% H2 at desorption temperatures less than 100 °C. The decomposition kinetics, measured by isothermal hydrogen desorption between 30 and 140 °C, suggest that the rate of H2 evolution is limited by nucleation and growth of the aluminum phase. The H2 evolution rates for small crystallites of α and γ-AlH3 (undoped) meet the DOE full flow target for a 50 kW fuel cell (1 gH2/s) above 114 °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, γ-AlH3, occurs by an exothermic transformation to the α phase (∼100 °C) followed by the decomposition of α-AlH3. A formation enthalpy of approximately −10 kJ/mol AlH3 was measured for α-AlH3, which is in good agreement with previous experimental and calculated results.