Structure and Thermodynamics of the Mixed Alkali Alanates

TL;DR

This study investigates the structure and thermodynamics of mixed alkali alanates, revealing how cation substitutions influence hydrogen absorption/desorption properties and thermodynamic stability.

Contribution

It provides new insights into how cation substitutions affect the stability and thermodynamics of mixed alkali alanates for hydrogen storage.

Findings

Mixed alkali alanates absorb and desorb hydrogen reversibly without catalysts.

Cation substitutions can stabilize or destabilize the material.

Structural analysis shows preference for Fm-3m space group with smaller ions in octahedral sites.

Abstract

The thermodynamics and structural properties of the hexahydride alanates (M2M'AlH6) with the elpasolite structure have been investigated. A series of mixed alkali alanates (Na2LiAlH6, K2LiAlH6 and K2NaAlH6) were synthesized and found to reversibly absorb and desorb hydrogen without the need for a catalyst. Pressure-composition isotherms were measured to investigate the thermodynamics of the absorption and desorption reactions with hydrogen. Isotherms for catalyzed (4 mol% TiCl3) and uncatalyzed Na2LiAlH6 exhibited an increase in kinetics, but no change in the bulk thermodynamics with the addition of a dopant. A structural analysis using synchrotron x-ray diffraction showed that these compounds favor the Fm-3m space group with the smaller ion (M') occupying an octahedral site. These results demonstrate that appropriate cation substitutions can be used to stabilize or destabilize the material and may provide an avenue to improving the unfavorable thermodynamics of a number of materials with promising gravimetric hydrogen densities.