Solar thermal fuels (STFs), also called molecular solar thermal (MOST) energy storage system, has the advantages of clean, pollution-free, rechargeability, controllable heat release, long thermal storage life compared with phase change materials (PCMs) and sensible heat storage materials. Azobenzene-based STFs have been widely explored for their simple synthesis, low cost, and extraordinary cycling stability through repeated photoisomerization and reverse isomerization between the trans and cis forms. High energy storage and controllable energy output are the key to determine the efficiency of photothermal cycle system. The photothermal capacity of azobenzene derivative molecule mainly depends on the energy level difference between trans and cis isomers. High energy storage and controllable energy output has always been a highlighted scientific issue to determine the efficiency of photothermal cycle system. Azobenzene-based solar thermal fuels with reversible photoisomerization is one of the ideal materials for solar energy conversion, storage and utilization in the future. However, the low thermal energy density hinders its application in photothermal conversion and storage technology due to the limitation of its molecular structure, while the photothermal capacity of azobenzene derivative molecule mainly depends on the energy level difference between trans and cis isomers. Although it can be improved by intermolecular forces and nanocarbon-templated. Herein, introducing reversible dynamic bond into graphene-templated azobenzene hybrid materials as a way to improve the energy level difference between trans and cis isomers and to enhance the thermal energy density in the materials. The graphene-azobenzene-Mg hybrid materials are prepared using photo-responsive Mg-sulfonate coordination bonds as reversible crosslinks. Due to the photo-responsibility of azobenzene, the Mg-sulfonate coordination bonds are dissociated under UV light irradiation and reformed reversibly in the dark or under blue light irradiation, which results in an extremely large energy level difference between trans and cis isomers. The thermal energy density can reach 719.9 J g<sup>-1</sup>. Based on these features, the graphene-azobenzene-Mg film can store a large amount of solar energy and serve as a heat source providing heat and energy controllable to other systems. This approach unlocks opportunities for the micro and portable thermal energy storage systems.

Graphene-templated azobenzene hybrids based on photo-controlled reversible dynamic bond for high-energy solar thermal fuels

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.