Covalent functionalization of graphene by azobenzene with molecular hydrogen bonds for long-term solar thermal storage

Abstract

Reduced graphene oxide-azobenzene (RGO-AZO) hybrids were prepared via covalent functionalization for long-term solar thermal storage. Thermal barrier (Δ E _a ) of cis to tran reversion and thermal storage (Δ H) were improved by molecular hydrogen bonds (H-bonds) through ortho- or para-substitution of AZO. Intramolecular H-bonds thermally stabilized cis- ortho-AZO on RGO with a long-term half-life of 5400 h (Δ E _a = 1.2 eV), which was much longer than that of RGO- para-AZO (116 h). RGO- para-AZO with one intermolecular H-bond showed a high density of thermal storage up to 269.8 kJ kg ⁻¹ compared with RGO- ortho-AZO (149.6 kJ kg ⁻¹) with multiple intra- and intermolecular H-bonds of AZO according to relaxed stable structures. Thermal storage in experiment was the same order magnitude to theoretical data based on Δ H calculated by density functional theory and packing density. Photoactive RGO-AZO hybrid can be developed for high-performance solar thermal storage by optimizing molecular H-bonds.