Formation and migration of native defects in NaAlH$_4$

TL;DR

This study uses first-principles calculations to analyze native defects in NaAlH4, estimating their migration barriers and proposing which defects facilitate hydrogen desorption, aligning with experimental activation energies.

Contribution

It provides a detailed first-principles analysis of native defect structures, energetics, and migration barriers in NaAlH4, linking them to hydrogen desorption mechanisms.

Findings

Identified key defect types involved in diffusion.

Calculated migration barriers consistent with experimental activation energies.

Suggested defect pairs responsible for hydrogen desorption.

Abstract

We present a first-principles study of native defects in NaAlH$_4$. Our analysis indicates that the structure and energetics of these defects can be interpreted in terms of elementary building blocks, which include $V_{\rm{AlH_4}}^+$, $V_{\rm{Na}}^-$, $V_{\rm{H}}^+$, H$_i^-$, and (H$_2$)$_i$. We also calculate migration barriers for several key defects, in order to compare enthalpies of diffusion to experimentally measured activation energies of desorption. From this, we estimate activation energies for diffusion of defects and defect pairs. We suggest that $V_{\rm{AlH_4}}^+$ and H$_i^-$, or $V_{\rm{Na}}^-$ and $V_{\rm{H}}^+$, may be responsible for diffusion necessary for desorption. We discuss the possible role of $V_{\rm{H}}^+$-H$_i^-$ complex formation. The values we find are in the range of activation energies reported for catalyzed desorption.