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

The concept of molecular solar thermal (MOST) storage systems is based on capturing solar energy via photoisomerization, which can be released later as thermal energy. Generally, suitable compounds are irradiated and analyzed as a solution, as this facilitates isomerization and handling in general. Energy densities, though, are calculated for the neat compound. Herein, we want to introduce the low viscous, green light active, 2,6-difluoroazobenzene (AB), which can be efficiently irradiated, pumped and handled in its neat state. Synthesis as well as isomerization can be done con¬veniently in continuous flow set-up. Storage densities of 218 kJ kg-1 are the highest compared to other liquid azobenzenes. 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, we demonstrated the solvation of an electrolyte to induce a measurable conductivity, which then allows for electron-catalyzed back-isomerization to harvest the stored energy. Alternatively, it can act as a solvent for a higher energy MOST material. As a proof-of-concept a norbornadiene (NBD) has been dissolved in our 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.