Then, we highlight the recent advances in diverse azobenzene materials for solar thermal fuels such as pure azobenzene derivatives, nanocarbon-templated azobenzene, and polymer-templated azobenzene. The basic design concepts of these advanced solar energy storage materials are discussed, and their promising applications are highlighted.
In 1983, Olmsted et al. studied photochemical storage potential of azobenzene compounds. They concluded that azobenzene compounds were not favorable for photochemical solar energy storage because of limited solubility and rapid thermal reversion rates in polar solvents.
These efforts involve the introducing of covalent bonds or designing novel azobenzene derivatives to increase energy densities, exploring diverse heat release induction modes, engineering azobenzene switches for energy storage in the visible light range and developing integrated devices to cover the full solar spectrum.
Azobenzene materials are the most commercially promising photothermal energy storage PCMs, and while their energy storage performance is gradually improved, their controllable energy release deserves more attention. The discharge of Azo-STFs can be effectively controlled using catalysts.
In 1987, Taoda et al. reported their study on photochemical conversion and storage of solar energy by azobenzene. They suggested keeping the storage tank of azobenzene solutions in a dark, cool room because azobenzene is apt to convert into form at high temperatures.
Because the energy level of azobenzene is ≈50 kJ mol solar energy is stored in the metastable isomer. The stored solar energy in azobenzene can be released as heat spontaneously, by heating or catalysis (Figure c). Upon energy release, azobenzene is switched back to the isomer that is ready for the next charging cycle (Figure