Photoswitchable phase change materials for unconventional thermal energy storage and upgrade

Conventional thermophysical latent heat storage based on solid-liquid phase change materials (PCMs) has been suffering three long-standing bottlenecks—i.e., relatively low storage density, short storage duration, and heat degradation during one charging/discharging cycle. In this perspective, we introduce a class of recently developed photoswitchable PCMs that have the potential to address these issues. Such PCMs present simultaneously the photon-induced molecule isomerization and thermally induced solid-liquid phase change, which endows them with dual and switchable phase change behaviors. This unique characteristic opens up new paths for exploring the unconventional thermal energy storage and upgrade technologies and even developing novel solar thermal fuels. However, there is still a long way to go for the practical applications of these materials and the derived technologies. Tuning the thermodynamic parameters, enhancing kinetics of isomerization and phase change, improving energy conversion efficiency, and modulating the absorption spectrum are four key issues that need further consideration. This article systematically summarizes the progress in photoswitchable PCMs, provides a deep insight into their potential applications, analyzes the challenges ahead, and proposes future research directions and solutions to continue the path.