Functionalized azobenzenes for micellar solar thermal energy storage as a next-generation MOST system

Despite being the most abundant sustainable energy resource, solar energy still faces major challenges in efficient capture and long-term storage. Molecular Solar Thermal Energy Storage (MOST) systems address this issue by employing photoswitchable molecules that absorb sunlight and store energy through reversible isomerization, cyclization or other intramolecular rearrangements. Azobenzenes are attractive due to their well-characterized photoresponsive behavior; however, conventional systems are hindered by low energy density, limited energy storage duration, and a reliance on organic solvents. Here, we present the Micellar Solar Thermal Energy Storage system (MIST) approach based on micellar aggregates that operate effectively across aqueous dispersions and gel states. These systems exhibit progressively enhanced energy storage lifetimes with increasing degrees of self-assembly, while delivering competitive energy densities. The thermal stability arises from restricted molecular mobility within the self-assembled structures and is enhanced on gelation, extending the calculated thermal half-life of the cis isomer from 148 days in dimethyl sulfoxide (DMSO), to 233 days in water, and to 12.8 years in the gel state. Compared to previous azobenzene-based MOST systems, our MIST approach offers significantly extended energy storage durations and improved material processability, including water-compatible formulations and, macroscopic heat release in the gel state (up to 5.7 °C).