Doping of AlH$_3$ with alkali metal hydrides for enhanced decomposition kinetics

Aluminum hydride, AlH$_{3}$, has inherently high gravimetric and volumetric properties for onboard vehiclular hydrogen storage (10 wt{\%} H$_{2}$ and 0.148 kg H$_{2}$/L). Yet it has been widely neglected because of its kinetic limitations for low-temperature H$_{2}$ desorption and the thermodynamic difficulties associated with recharging. This paper considers a scenario whereby doped AlH$_{3}$ is decomposed onboard and recharged offboard. In particular, we show that particle size control and doping with small levels of alkali metal hydrides (e.g., LiH) results in accelerated H$_{2}$ desorption rates nearly high enough to supply fuel-cell and ICE vehicles. The mechanism of enhanced H$_{2}$ desorption is associated with the formation of alanate windows (e.g., LiAlH$_{4})$ between the AlH$_{3}$ particles and the external gas phase. These alanate windows can be doped with Ti to further enhance transparency, even to the point of accomplishing slow decomposition of AlH$_{3}$ at room temperature. It is highly likely 2010 gravimetric and volumetric vehicular system targets (6 wt{\%} H$_{2}$ and 0.045 kg/L) can be met with AlH$_{3}$. But a new, low-cost method of offboard regeneration of spent Al back to AlH$_{3}$ is yet needed.