Azobenzene-based solar thermal energy storage enhanced by gold nanoparticles for rapid, optically-triggered heat release at room temperature

Solar thermal fuel (STF) technology based on azobenzene (Azo) compounds represents a novel approach for the capture, conversion, and storage of solar energy. Azos can store energy by isomerization between their thermodynamically stable trans -isomers and higher energy, metastable cis -isomers. The energy barrier to Azo isomerization must be overcome in order to store and release energy using these materials. Generally, this is achieved using heat, light, or catalysis. Heating is the most frequently used strategy to overcome the energy barrier and release the stored energy in Azo-based STFs (Azo-STFs). However, heating Azos is not economical in practical STF applications. A notable target is the development of strategies for the construction of STFs that operate at room temperature. In this study, polyvinyl alcohol (PVA) polymer-templated, gold nanoparticle (AuNP) doped, 1,3,5-tris(arylazo)benzene (tri-Azo)-based STFs (PVA/Azo@AuNP STFs) are presented. The heat storage and release behavior of these composites can be controlled completely using light alone at room temperature. Light at a wavelength of 365 nm is effective for charging the PVA/Azo@AuNP STFs, while light at 520 nm induces the release of energy from the samples. Temperature increases of up to 8.8 °C were observed during the discharge of the PVA/Azo@AuNP STFs. The significant heat output can be attributed to three cooperative effects. First, irradiation at 520 nm irradiation triggers cis -to- trans isomerization of tri-Azo. Second, light irradiation at 520 nm stimulates the plasmonic photothermal effect in the AuNPs, releasing heat, which stimulates the cis -to- trans isomerization reaction. Finally, the AuNPs act as catalysts, accelerating the cis -to- trans isomerization reaction. Through these multiple stimulation modes, heat is released rapidly from the charged PVA/Azo@AuNP STFs. The mechanism and design principles for such novel Azo-STFs are discussed. This work provides a new strategy for the design of efficient Azo-STFs that operate at room temperature. The energy storage and heat release of this STF sample can be controlled completely using light alone at room temperature.