New Reversible Complex Metal Hydrides

Novel, reversible complex metal hydrides with the general formula $A_{3-x}A'_x$AlH$_6$ ($A$, $A' =$ Na, Li, K, Mg, Ca) were synthesized and structurally characterized using synchrotron x-ray diffraction. The hydrogen absorption/desorption characteristics and thermodynamic properties were studied using pressure-composition isotherms. These results demonstrate that the partial substitution of the alkali metal can change the equilibrium pressures substantially. As an example, the substitution of one Li for Na in Na$_3$AlH$_6$ cryolite to form Na$_2$LiAlH$_6$ elpasolite increases the dissociation enthalpy by $6.5 \pm1.6$ kJ/mol H$_2$. This thermodynamic change lowers the plateau pressure by 30 bar at 518 K. Similar trends were observed in the potassium cryolite and elpasolite phases. This form of thermodynamic tuning may be applied to other, high capacity alanates (e.~g. LiAlH$_4$ and Mg(AlH$_4$)$_2$), which are currently hindered by reaction enthalpies that are largely unfavorable for PEM fuel cell applications.