The global energy demand continues to grow both due to the increasing population and wealth. As one of the potential solutions, renewable energy resources can relieve the pressure on conventional energy sources. However, due to fluctuations in both supply and demand, they need to be complemented with load-leveling technologies.
Since the sun is considered one of the most abundant renewable energy resources, solar energy storage solutions based on battery technologies or power-to-X technologies have attracted increasing attention. Such technologies offer emission-free energy on demand; they also require, in most cases, rare and depletable raw materials. In this context, it is stimulating to imagine alternative technologies that directly capture and store the energy from the sun, based entirely on earth-abundant raw materials, specifically organic molecules.
Some molecular photoswitches can absorb and transform sunlight into chemical energy, available for later release in the form of heat without any emission. We define their common properties as an innovative molecular system that can store solar energy into chemical bond strain and later release it on demand. Such photoisomers are referred to as molecular solar thermal energy storage systems (MOST), also known as solar thermal fuels (STF). In this review, we introduce the functional principles and criteria of a general MOST system, then introduce to a broad audience three key candidates and show the efforts made so far to achieve recent advances in results. Finally, we discuss the challenges and future opportunities that these dynamic molecular systems offer.