Synthesis and Characterisation of Phase Change Materials for Molecular Solar Thermal Systems

Licentiate thesis, 2024

Demand for energy has steadily increased over the last decade and several technologies have been developed to combat the resulting global impact. Solar power systems are an attractive option for a cleaner and more sustainable source of energy owing to the magnitude of energy the sun provides as well as its availability and consistency. However, modern versions of these technologies still face challenges in effectively storing energy, particularly concerning storage capacity, lifespan, and disposal of the devices. The Molecular Solar Thermal Systems (MOST) are a class of organic materials which can undergo a reversible photochemical reaction allowing for storage of solar energy as chemical energy which ultimately can be released as heat. One such system is based on the norbornadiene (NBD) derivatives that can isomerise to form the metastable quadricyclane (QC). The QC to NBD conversion can be initiated with the use of a catalyst, light, or heat to release thermal energy. However, the best performing NBDs struggle to be integrated into devices due to properties such as poor solubility and short storage life. Phase change materials (PCMs) can also store energy via a phase transition from solid-to-liquid from latent heat, although the energy density stored is limited. Thus, a material that combines the function of MOST and PCM was synthesised to tackle the drawbacks of the single function materials.

In this thesis, a series of NBDs engineered with varying substituents in order to explore improvements to solubility of the systems. These compounds were synthesised via Diels-Alder reactions in a comparative study of thermal and microwave-assisted synthesis. Furthermore, evaluation of different purification methods (distillation, HPLC, flash chromatography) of the products were explored to determine which method yields pure products for photocharacterisation, solubility, and stability studies. It was revealed that the chosen substituents had a positive effect on the solubility of the molecules, though did not improve on the degradation observed in similar derivatives.

Another class of MOST compounds were studied to improve the phase change transition properties for future MOST-PCM applications. By varying the alkyl chain length on an azobenzene moiety, the effects on the phase-transition temperatures and energy storage were evaluated. This led to the observation that the azobenzene with six carbons in the alkoxy chain are the best performing material for MOST-PCM applications with the highest energy storage density, achieving the target suggested for solar thermal storage systems.