Azobenzene-containing polymer for solar thermal energy storage and release: Advances, challenges, and opportunities

Molecular solar thermal (MOST) fuels have attracted enormous research enthusiasm in solar energy conversion and storage, which can generate high-energy isomers upon harvesting photon energy and release heat on demand through reversible isomerization of molecular photo-switches such as azobenzene. However, the pristine azobenzene suffers from limitations like low energy density, short half-life and narrow absorption waveband. Recently, numerous azobenzene-based MOST fuels have been developed by various strategies including molecular engineering and template self-assembly to enhance the storage capacities, among which azobenzene-containing polymers (i.e., ‘azopolymers’) are the most promising materials for the development of MOST fuels. In this review, the state-of-the-art advances in azopolymer MOST fuels are systematically summarized. The critical parameters of azobenzene-based MOST fuels are highlighted. Various kinds of azopolymers for solar thermal energy storage and release such as azobenzene compound/polymer composites, linear azopolymers, dendrimer azopolymers, and other types of azopolymers are addressed. The most promising advantages and challenges of azopolymers for MOST fuels are analyzed, and emerging strategies as well as opportunities for future development are discussed with the goal to promote future development of MOST fuels towards innovative applications.