Heating, cooling, and ventilation account for a substantial share of energy use in buildings, creating growing demand for materials that can regulate temperature while maintaining a comfortable indoor environment. Now, researchers have developed a multifunctional membrane that combines food waste derived biochar, graphene, and a phase change material to simultaneously store thermal energy, improve heat transfer, and manage moisture.
The study, published in Biochar, presents an engineered biochar based phase change composite integrated directly into a paper membrane for potential use in energy recovery ventilation and other building thermal management systems.
"Our goal was to create a material that does more than simply store heat. By engineering waste derived biochar and integrating it with graphene and a phase change material, we were able to combine thermal energy storage, heat transfer, structural stability, and moisture management within a single membrane system," said corresponding author Professor Sumin Kim of Yonsei University.
Phase change materials can absorb heat when they melt and release it when they solidify, making them attractive for thermal energy storage. However, conventional phase change materials often suffer from low thermal conductivity and leakage during melting. These limitations can reduce their effectiveness in practical energy systems.
To overcome these challenges, the researchers produced biochar from mixed food waste carbonized at 400 °C. The biochar was then activated with potassium hydroxide at temperatures ranging from 600 to 800 °C and combined with a small amount of graphene. The resulting porous carbon structure was impregnated with docosane, a phase change material capable of storing and releasing heat.
The biochar engineered at an activation temperature of 700 °C showed particularly strong performance. It had a surface area of 323.1 square meters per gram and a mesopore proportion of 82.8%, providing abundant space for the phase change material while helping prevent liquid leakage.
Graphene integration further improved the material's thermal performance. Compared with composites based on pristine engineered biochar, the graphene engineered systems increased latent heat storage by as much as 72.0%. The optimized FK7G/C22 composite reached a phase change enthalpy of 93.1 J/g at approximately 48.4 °C and maintained strong thermal performance during long term cycling tests.
The researchers subjected the composites to 1,000 heating and cooling cycles, demonstrating their potential durability for repeated thermal energy storage applications. The engineered porous structure also helped confine the molten phase change material through capillary forces and interfacial interactions.
The team then bonded the optimized composite to a commercial paper membrane to evaluate its potential for ventilation applications. The resulting membrane exhibited thermal conductivity 96.1% higher than that of the reference pristine paper membrane while retaining 80.2% of the latent heat of the bulk composite.
Importantly, improving heat transfer did not come at the expense of moisture management. The membrane showed high water vapor permeability, with an equivalent air layer thickness of 0.71, below the critical value of 1.0, and met relevant ISO 12572 performance requirements.
By converting food waste into a functional carbon material and reducing reliance on conventional carbon materials, the strategy also supports circular approaches to material production.
The researchers suggest that the technology could contribute to next generation energy recovery ventilation, smart building membranes, building facades, and thermal comfort systems. Future studies will need to examine production energy requirements and techno economic performance before large scale implementation.
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Journal Reference: Atinafu, D.G., Kua, H.W., Kang, Y. et al. Engineered biochar-graphene hybrid paper membranes for long-term thermal energy storage and ventilation energy recovery. Biochar 8, 132 (2026).
https://doi.org/10.1007/s42773-026-00646-4
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
