Ca-intercalated graphite as a hydrogen storage material: stability against decomposition into calcium hydride and graphite

TL;DR

This study uses density functional theory to evaluate calcium-intercalated graphite's potential for hydrogen storage, finding it energetically favorable but unstable against decomposition into calcium hydride and graphite.

Contribution

The paper provides a detailed theoretical analysis of hydrogen absorption energetics and stability in calcium-intercalated graphite, highlighting its limitations for practical hydrogen storage.

Findings

Absorption energy is approximately -0.2 to -0.4 eV, favorable for storage.

Hydrogenated calcium-intercalated graphite is unstable against decomposition.

Experimental observations of hydride formation are explained by the instability.

Abstract

We have used calculations based on density functional theory to investigate the energetics of hydrogen absorption in calcium-intercalated graphites. We focus particularly on the absorption energy and the stability of the hydrogenated material with respect to decomposition into graphite and calcium hydride, which is essential if this material is to be useful for practical H2 storage. The calculations are performed with two commonly used approximations for the exchange-correlation energies. Our calculations confirm earlier predictions that the absorption energy is approximately -0.2 to -0.4 eV, which is favourable for practical use of Ca-intercalated graphite as a hydrogen storage medium. However, we find that the hydrogenated material is strongly unstable against decomposition. Our results therefore explain recent experiments which show that H2 does not remain stable in CaC6 but instead forms a hydride plus graphite.