Food waste converted into biochar membranes for energy storage

Researchers have developed a multifunctional membrane that incorporates biochar derived from food waste, graphene, and a phase change material to enhance thermal energy storage and heat transfer while managing moisture. The study, published in Biochar, details the integration of this engineered biochar-based phase change composite into a paper membrane, aiming for applications in energy recovery ventilation and building thermal management systems.

"Our goal was to create a material that does more than simply store heat," said Professor Sumin Kim of Yonsei University. By combining waste-derived biochar with graphene and a phase change material, the team achieved a membrane system that integrates thermal energy storage, heat transfer, structural stability, and moisture management.

Phase change materials are valued for their ability to absorb and release heat during phase transitions, but they often face challenges such as low thermal conductivity and leakage. To address these issues, the researchers produced biochar from mixed food waste carbonized at 400 °C and activated it with potassium hydroxide at 600 to 800 °C, incorporating a small amount of graphene. This resulted in a porous carbon structure impregnated with docosane, a phase change material.

The biochar activated at 700 °C demonstrated strong performance, with a surface area of 323.1 square meters per gram and a mesopore proportion of 82.8%, providing ample space for the phase change material and preventing leakage. Graphene integration further enhanced thermal performance, increasing latent heat storage by up to 72.0% compared to composites based on pristine biochar. The optimized composite reached a phase change enthalpy of 93.1 J/g at approximately 48.4 °C and maintained performance over 1,000 heating and cooling cycles.

The team bonded the composite to a commercial paper membrane to assess its potential for ventilation applications. The resulting membrane showed a 96.1% increase in thermal conductivity over the reference pristine paper membrane while retaining 80.2% of the latent heat of the bulk composite. It also maintained high water vapor permeability, meeting ISO 12572 performance requirements.

This approach not only converts food waste into a functional carbon material but also supports circular material production strategies. The researchers suggest that this technology could advance energy recovery ventilation, smart building membranes, and thermal comfort systems. Future studies will explore production energy requirements and techno-economic performance for large-scale implementation.