One-pot in situ synthesis of expandable graphite-encapsulated paraffin composites for thermal energy storage

To mitigate the intermittency of renewable energy generation and achieve net-zero greenhouse gas emissions, a cyclically stable and energy-efficient thermal energy storage system (TESS) is increasingly required. To prevent leakage and enhance thermal conductivity and stability, TESS is typically fabricated by impregnating energy storage materials (ESMs) into porous conductive additives. However, the impregnation-based encapsulation process is both time-consuming (several to tens of hours) and energy-intensive, and the incorporation of additives reduces the proportion of ESMs, resulting in decreased energy capacity. In this study, we present an ultrafast (several minutes) and energy-efficient one-pot encapsulation method for in situ encapsulation of paraffin wax (PW) within the pores of expanded graphite (EG) via microwave irradiation of expandable graphite (Eg) in molten PW. The resulting binary PW/EG composites exhibit shape stability and no leakage even at 92 wt% PW loading. The PW/EG composite with 10 wt% EG demonstrates a thermal conductivity of 3.7 W/mK (16.2 times higher than that of pure PW), a thermal release/absorption efficiency of around 99.5 %, and almost 100 % capacity retention after thermal cycling. Additionally, we demonstrate that integrating PW/EG TESS with thermoelectric (TE) modules can increase TE power generation by more than fivefold, highlighting their potential for energy storage and conversion as well as mitigating waste heat source intermittency.