Understanding and engineering azobenzene for thermal energy storage

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017.Cataloged from PDF version of thesis.Includes bibliographical references (pages 135-146).This thesis focuses on the understanding and engineering of a molecule known as azobenzene which holds unique properties for thermal storage applications. The azobenzene molecule undergoes structural change into a metastable state which has the ability to store energy. This thesis utilizes the energy storage and structural change properties of this molecule to develop new materials for thermal energy storage. The first is through a concept called solar thermal fuel which is storing the solar energy in rearranged bonds of the azobenzene and later releasing that energy in the form of heat. The second approach is through the structural property difference of its two states in order to moderate the phase change temperature of organic phase change materials. Essentially, the molecule azobenzene was modified and engineered to be used as a thermal battery as well as to mediate thermal energy storage in other materials. The first chapter will give a brief introduction on the concept and past examples of solar thermal fuel. Chapter 2, 3, 4 will discuss about the development of solar thermal fuel while chapter 5 discusses about a recently developed concept of using azobenzene to moderate phase change temperature. Chapter 2 shows the first demonstration of using solar thermal fuel in the solid state through functionalizing azobenzene on a polymer template. The polymer platform allows fabrication of a thin film of this material which enabled charging, discharging, and heat release using optically chargeable molecules all within the solid-state. A demonstration of solid state application was shown by constructing a macroscopic device which resulted in heat release bringing a temperature increase of as high as 10 OC. Next in chapter 3, azobenzene was engineered on the molecular lever with bulky aromatic groups (phenyl, biphenyl, and tert-butyl phenyl groups). The molecules were designed and synthesized for the purpose of increasing energy stored while promoting solid state solar thermal fuels. The design allowed fabrication of molecular based thin film, which was able to be charged with light, a great improvement from the original azobenzene, which crystallized preventing switching in the solid state. Molecular engineering proved to be a powerful and effective method in improving other solar thermal fuel properties, such as energy storage in STFs, chargeability, and also the thermal stability of the molecular thin film. In chapter 4, new diacetylene derivatives with azobenzene moieties and with varied alkyl spacers and linkers were synthesized to show photocontrolled self-assembly and disassembly of photon energy storage materials. This azobenzene decorated diacetylenes not only allowed solar energy storage but also demonstrated phase change characteristic of organic materials can be a parameter to consider in terms of designing high energy density photon energy storage materials. Chapter 5 discusses azobenzene based dopants in organic phase change material to photomoderate the phase change temperature. Three different types, 8 in total, organic phase change materials were tested to show the possibilty of this concept in a wide variety of phase change materials. A deep understanding was developed giving parameters to achieve a large phase change temperature difference in the organic phase change materials using the structual difference of the trans and the cis state of azobenzene.by Eugene N. Cho.Ph. D