Azobenzene/graphene hybrid for high-density solar thermal storage by optimizing molecular structure

A large capacity storing solar energy as latent heat in a close-cycle is essentially important for solar thermal fuels. This paper presents a solar thermal molecule model of a photo-isomerizable azobenzene (Azo) molecule covalently bound to graphene. The storage capacity of the Azo depending on isomerization enthalpy (ΔH) is calculated based on density functional theory. The result indicates that the ΔH of Azo molecules on the graphene can be tuned by electronic interaction, steric hindrance and molecular hydrogen bonds (H-bonds). Azo with the withdrawing group on the ortho-position of the free benzene shows a relatively high ΔH due to resonance effect. Moreover, the H-bonds on the trans-isomer largely increase ΔH because they stabilize the trans-isomer at a low energy. 2-hydroxy-4-carboxyl-2′,6′,-dimethylamino-Azo/graphene shows the maximum ΔH up to 1.871 eV (107.14 Wh kg-1), which is 125.4% higher than Azo without functional groups. The Azo/graphene model can be used for developing high-density solar thermal storage materials by controlling molecular interaction.