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 for 100% (Z)-isomer (137 kJ kg 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.