Thermal management is a critical challenge in electronics, aerospace, energy storage, and automotive applications, where inadequate heat dissipation can lead to overheating and reduced reliability. Hexagonal boron nitride (h-BN)-based polymer nanocomposites offer a promising solution due to their exceptional thermal properties. This thesis focuses on developing multifunctional h-BN composites for thermal switching, high-voltage insulation, and thermochemical energy storage, characterized using Differential Scanning Calorimetry (DSC), Laser Flash Analysis (LFA), and Thermomechanical Analysis (TMA). Thermal interface materials (TIMs) incorporating mixed-size h-BN particles (5 µm + 30 µm) achieved an optimal 40 wt% composition, reaching a thermal conductivity of 1.157 W/m·K. While higher h-BN content reduced thermal expansion (270.6 ppm/°C), adjusting the TIM’s thickness ensured stable thermal switching over multiple cycles, making these materials suitable for battery thermal management and power electronics. Additionally, incorporating h-BN, silver, and multi-walled carbon nanotube fillers into polytetrafluoroethylene (PTFE) matrices enhanced thermal dissipation while maintaining high-voltage insulation. A 30% CNT (3-layer) composite achieved a thermal conductivity of 1.05 W/m·K, demonstrating potential for lunar power transmission. Furthermore, metal hydride composites with h-BN were fabricated for thermochemical energy storage, though alternative characterization methods are needed due to LFA limitations. These findings provide valuable insights into the scalable development of multifunctional h-BN composites for extreme environments.