Engineered biochar-graphene hybrid paper membranes for long-term thermal energy storage and ventilation energy recovery

Sustainable development underpins strategies for mitigating environmental challenges through the design of resource-efficient materials and energy-conscious systems. Phase-change materials (PCMs) show substantial potential for addressing challenges in thermal energy storage and regulation. Yet, their performance is often limited by seepage, structural instability, and reduced energy-storage density when phase change occurs far from heat sources. Although PCMs supported by confining agents can suppress leakage and enhance thermal conductivity, fabricating composites from eco-friendly materials with multifunctional capabilities, including moisture management, remains critical for thermal buffering and human thermal comfort. Herein, an engineered biochar-based phase-change composite integrated with a paper membrane is developed to advance both latent heat storage and moisture management within a membrane system while simultaneously ensuring leakage resistance and enhanced heat transfer in pristine PCMs by creating an interconnected network and surface capillary forces. Biochar was produced from food waste subjected to slow carbonization at 400 °C and subsequent activation with KOH at varying temperatures (600–800 °C), further graphitization via ultrasonication, and vacuum assembly with docosane (C22) for heat storage and moisture management. The pore characteristics, chemical compatibility, microstructure, thermal performance, thermal stability, moisture permeability, and heat-transfer behavior were comprehensively characterized. The engineered biochar obtained at 700 °C (FK7G) exhibited a pronounced BET surface area (323.1 m 2 g −1 ) with a high mesopore proportion (82.8%), resulting in up to 90.2% enthalpy retention after 1000 thermal cycles. In addition, the engineered composite system demonstrated up to 72.0% increase in latent heat compared with the pristine biochar composite. Benefiting from the graphitic nature of the engineered biohybrid, FK7G/C22 effectively bonded with a paper membrane for moisture management and met ISO 12572 and ASTM performance criteria, showing high water–vapor permeability with an equivalent air-layer thickness ( S d ) of 0.71, below the critical threshold of 1.0. Furthermore, the composite demonstrated enhanced thermal conductivity (96.1% higher than the reference pristine membrane) and latent heat retention (80.2%) at 46.7 °C, effectively managing both heat and moisture variations. This graphitized engineered biohybrid provides sustainable and scalable insights into bio-based phase-change composites that enhance human thermal comfort through advanced thermal management, hygrothermal control, and a reduced carbon footprint.