Graphene Based Phase Change Composites for Heat Storage

The transition to renewable energy systems requires effective and scalable thermal energy storage (TES) technologies to bridge the gap between intermittent energy supply and continuous demand. Phase Change Materials (PCMs), especially organic materials such as paraffins, have emerged as promising candidates for heat storage due to their high energy density, low cost, non-toxicity, and ability to store latent heat. However, their practical implementation is significantly limited by two main drawbacks: low thermal conductivity and limited shape stability. This thesis focuses on improving the performance of paraffin-based PCMs through the integration of high thermally conductive additives, with an emphasis on understanding and optimizing the network of graphene nanoplatelets (GNPs). The role of filler topology, aspect ratio, and dispersion was studied to improve both thermal conductivity and long-term performance. First, from a colloidal perspective, we conducted an in-depth study of GNP networks in paraffin PCMs. We investigate how GNP size, aspect ratio, and network topology determine thermal conductivity and cyclic stability of the composite. This was done by using different aspect ration GNPs (M5, M25, and H25) and different weight percentages (2-10 wt%) to form paraffin-based composites. The orientation of the GNPs in the matrix was found to have a significant effect on the shape stability as well as the thermal conductive behavior of the composites. Secondly, in a bottom-up approach, graphene foams were synthesized from graphene oxide, used as high thermally conductive filler, and combined with paraffin to form structured thermal networks. A design of experiments (DoE) methodology was used to systematically analyze the effect of synthesis parameters (sonication time, GNP surface area, Graphene oxide concentration) on the foam morphology and performance. The resulting foams demonstrated controllable pore sizes and network topology, enabling tailored material properties such as thermal conductivity. Thirdly, to avoid the limitations of low-conductivity graphene oxide, used in the foam formation, another approach was developed. This involved a self-assembly method to coat paraffin with GNP without the use of surfactants, based on the observation that the GNP will assemble at the water-oil interphase, and that this will also happen on the solid paraffin surface. Remarkably, this method produced composites with thermal conductivity as high as 8.3 W/m·K, representing a substantial improvement over conventional PCM systems. Microscopic analysis revealed that GNPs were embedded mainly on the particle surface. Finally, recognizing the importance of sustainable material sourcing, a life cycle assessment (LCA) was conducted to evaluate the environmental impact of various PCM feedstocks. The study concluded that waste streams from cooking oil and animal fat offer the most sustainable options when climate resilience and resource scarcity are prioritized. These material sources were then experimentally mimicked by using commercially pure materials. These materials resulted in similar thermophysical properties to paraffin, supporting their potential use in future sustainable TES systems.