An Azobenzene-Based Liquid Molecular Solar Thermal (MOST) Storage System–Energy Carrier and Solvent

A molecular solar thermal (MOST) storage systems is based on capturing solar energy via photoisomerization, which can be released later as thermal energy. Herein, the low viscosity, green light active, 2,6-difluoroazobenzene is introduced, which can be efficiently irradiated, pumped, and handled in its neat state. Synthesis as well as isomerization can be done conveniently in a continuous flow setup. Storage densities of 218 kJ kg⁻¹ for 100% (Z)-isomer (137 kJ kg⁻¹ after green light irradiation) are the highest compared to other liquid azobenzenes (ABs). Additionally, the irradiation with green light and the processibility in the neat state make this compound a promising candidate for energy storage applications. Furthermore, the liquid AB can be employed as a MOST-active solvent. For example, the solvation of an electrolyte is demonstrated to induce a measurable conductivity, which then allows for complete electron-catalyzed back-isomerization. Alternatively, it can act as a solvent for a higher energy MOST material. As a proof-of-concept a norbornadiene (NBD) is dissolved in the AB solvent allowing to utilize the energy of the NBD as well as the AB solvent. Further optimization of the solute-solvents systems is required to fully harvest the potential of this new concept for efficient energy storage.