Stimuli-Responsive Organic Phase Change Materials: Molecular Designs and Applications in Energy Storage

Achieving a stable latent heat storage over a wide temperature range and a long period of time as well as accomplishing a controlled heat release from conventional phase change materials have remained prominent challenges in thermal energy control. Because the conventional phase change materials have the fixed phase transition temperatures under the standard ambient pressure, the latent heat storage is only viable at temperatures higher than their melting and crystallization points. Once the latent heat is stored in the liquid phase of materials, it is imperative to maintain the temperature of the system above its crystallization point to prolong the heat storage. Without the continuous energy input or extensive insulation, the crystallization of liquid occurs spontaneously, leading to the loss of latent heat and a short heat storage time. Thus, there exists a critical need to develop methods that enable the control over the phase transition of materials, unrestricted by the temperature of the surroundings. The successful development of such methods and materials in recent years has been viable through the incorporation of molecular switches that respond to external stimuli, notably light, electrochemical bias, and ions, by reversible structural changes. We have discovered novel functional organic material systems that exhibit solid-to-liquid phase transitions controlled by optical and electrical stimulation, along with other research groups. The development and optimization of such materials are enabled by the judicious molecular designs and syntheses. In this Account, we will introduce the cutting-edge design principles of controllable phase change materials that have demonstrated the storage of thermal energy for up to a couple of months without crystallization over a wide temperature range, from subzero to over 100 degrees C, which addresses the major weakness of conventional phase change materials. In particular, materials that are controlled by solar irradiation and compounds that exhibit substantial gravimetric energy densities are presented, which will guide the further development of functional materials for practical energy storage applications. The controllable heat storage materials will not only recycle the wasted solar thermal energy but also ameliorate massive energy loss in industrial processes. In particular, the waste heat released at temperatures below 100 degrees C can be effectively absorbed and stored in organic phase change materials, which cannot be easily reclaimed by other energy conversion and storage systems, including thermoelectrics and electrical batteries. This complementary storage method will harness various sources of waste heat and reduce the consumption of fossil fuels by recycling the wasted energy, which will contribute to mitigating climate change.