Solid-State Solar Thermal Fuels for Heat Release Applications

but remain prohibitively expensive for wide adoption. With the availability of accurate computational tools, new approaches have leveraged carbon nanostructures and insights into the steric interaction of STFs to increase energy density employing highly cyclable and modest energy density (60–70 Wh kg −1 ) azobenzene derivatives. [ 8,9 ] While demonstrating a per-molecule increase in energy density via templating, [ 10 ] these approaches require complex multistep reactions, have low yields, and the resulting material has low solubility in most organic solvents (<1 g L −1 ). More recently, it was possible to develop liquid azobenzene fuels at room temperature by attaching bulky ligands to the molecule, [ 11 ] and with several computational works detailing the possibility of increasing its energy density through functionalization of the benzene rings, [ 12 ] this platform holds much promise for future STF developments. With such rapid progress in STF materials, it is perhaps surprising that the solid-state platform and related applications have remained largely unexplored, with only recent studies on semi-solid photoliquefi able ionic crystals reaching energy densities of 35 Wh kg −1 . [ 13 ] Transitioning fully to the solid-state offers the possibility of integrating STF materials into a multitude of existing solid-state devices such as coatings for deicing, or novel applications such as solar blankets and other consumer oriented heating equipment. We took the view that if properly engineered on the molecular level, STF materials could be controllably tailored within the solid-state, and that until now, there has not been an effi cient method to accomplish this. For one, the most recent STF reports have relied on carbon scaffolds [ 10,14 ] that simultaneously increase synthesis complexity, cannot be deposited into uniform fi lms, contribute to the optical density without resulting in photocharging, and introduce uncontrollable morphological effects that may limit charging and reversible switching in the solid-state. [ 15 ] Similarly, single-molecule thin fi lms do not make homogenous layers, can often result in crystallization, and melt at low temperatures (≈70 °C for azobenzene) thus limiting their utility in the solid-state. Fortunately, a wealth of literature exists on azobenzene-based materials in solid-state applications for microswitches, microactuators, and sensors. [ 16–21 ] We postulated that the ideal material class to form solid-state STF coatings would need to (1) form smooth fi lms with controllable thickness, (2) be resilient at high temperatures, (3) preserve the heat release properties of Closed cycle systems offer an opportunity for solar energy harvesting and storage all within the same material. Photon energy is stored within the chemical conformations of molecules and is retrieved by a triggered release in the form of heat. Until now, such solar thermal fuels (STFs) have been largely unavailable in the solid-state, which would enable them to be utilized for a multitude of applications. A polymer STF storage platform is synthesized employing STFs in the solid-state. This approach enables uniform fi lms capable of appreciable heat storage of up to 30 Wh kg −1 and that can withstand temperature of up to 180 °C. For the fi rst time a macroscopic energy release is demonstrated using spatial infrared heat maps with up to a 10 °C temperature change. These fi ndings pave the way for developing highly effi cient and high energy density STFs for applications in the solid-state.