Heating, cooling, and ventilation consume a major share of the energy used in buildings, and the demand is rising as cities experience more extreme temperatures and increasingly strict requirements for indoor comfort. A new study reports a multifunctional membrane designed to address several of these challenges at once. The material combines biochar made from food waste, graphene, and a phase change material in a paper-based structure that can store thermal energy, accelerate heat transfer, and allow moisture to pass through. The researchers say the technology could eventually support energy recovery ventilation, thermally responsive building envelopes, and other systems designed to reduce the energy required to maintain comfortable indoor conditions.
Published in Biochar, the study describes an engineered biochar-based phase change composite integrated into a commercial paper membrane. Unlike conventional insulation, which primarily slows heat flow, the new membrane is intended to actively absorb and release heat as indoor and outdoor temperatures change. Its operation depends on a phase change material, or PCM, which stores energy as it melts and releases that energy when it solidifies. This process allows the material to function as a compact thermal buffer, potentially reducing sudden temperature fluctuations and helping ventilation systems recover energy that would otherwise be lost as warm or cool air leaves a building.
The approach targets two persistent problems associated with phase change materials. Many PCMs can store substantial amounts of latent heat, but they typically have poor thermal conductivity, meaning that heat moves into and out of them too slowly for efficient practical use. They may also leak when they melt, particularly when the PCM is incorporated into a porous or flexible support. To overcome these limitations, the research team used biochar as a highly porous host material. Biochar is a carbon-rich solid produced by heating organic material under conditions with limited oxygen. In this work, mixed food waste was carbonized at 400 degrees Celsius and then chemically activated with potassium hydroxide at temperatures between 600 and 800 degrees Celsius.
Activation transforms the carbonized waste into a network of interconnected pores. These pores increase the material’s internal surface area and provide locations where the PCM can be held through capillary forces and interactions between the carbon surfaces and the phase change compound. The researchers then introduced a small quantity of graphene, a form of carbon consisting of atomically thin sheets. Graphene is known for its high thermal conductivity, and its inclusion was intended to create more efficient pathways for heat to travel through the composite. The resulting material was impregnated with docosane, a paraffin-like PCM selected for its ability to melt and solidify within a temperature range relevant to building thermal management.
Among the materials tested, biochar activated at 700 degrees Celsius delivered particularly strong structural characteristics. It reached a specific surface area of 323.1 square meters per gram, a measurement that reflects the total internal surface available within the porous carbon network. The sample also had a mesopore proportion of 82.8 percent. Mesopores, generally defined as pores between two and 50 nanometers wide, are especially important in composite PCM systems because they can provide enough volume for material storage while also helping restrict movement of the liquid phase. The researchers found that this engineered structure offered a balance between PCM loading, leakage resistance, and access to the internal surfaces needed for thermal interactions.
Graphene significantly enhanced the thermal behavior of the biochar-based composites. Compared with systems made from pristine engineered biochar, graphene-containing versions increased latent heat storage by as much as 72 percent. Latent heat is the energy absorbed or released during a phase transition without a corresponding change in temperature, making it a central measure of a PCM’s thermal storage capacity. The optimized formulation, identified as FK7G/C22, reached a phase change enthalpy of 93.1 joules per gram and underwent its principal transition at approximately 48.4 degrees Celsius. That temperature is high enough to be relevant to many building and ventilation environments while remaining within the operating range of common low-temperature thermal management systems.
Durability was another critical test. A material intended for daily building operation could experience thousands of melting and freezing events over its service life, so a high initial storage capacity would have limited value if the structure degraded quickly. The research team subjected the composite to 1,000 heating and cooling cycles. The material retained strong thermal performance during these repeated transitions, suggesting that the porous biochar framework and graphene reinforcement helped maintain the confinement of docosane. The researchers attribute the reduced leakage partly to capillary forces within the pores and partly to interfacial interactions between the PCM and the engineered carbon surfaces. These mechanisms can immobilize the liquid PCM sufficiently to preserve the composite’s shape and function during cycling.
To evaluate whether the composite could operate in a practical membrane, the team bonded the optimized material to a commercial paper substrate. The resulting hybrid membrane combined the flexibility and permeability of paper with the thermal storage behavior of the biochar-graphene-PCM composite. Its thermal conductivity was 96.1 percent higher than that of a reference pristine paper membrane, indicating that heat could move through the material far more rapidly. At the same time, the membrane retained 80.2 percent of the latent heat capacity measured in the bulk composite. This result demonstrates the central engineering challenge of membrane integration: adding a thermally active material without blocking the pores or reducing the amount of PCM available for energy storage.
The membrane also preserved the moisture-transfer properties required for ventilation applications. Water vapor permeability is essential in energy recovery ventilation because a membrane must exchange heat while allowing controlled moisture transport between outgoing and incoming air streams. The hybrid membrane exhibited an equivalent air layer thickness of 0.71. This value is below the critical threshold of 1.0 used in the reported assessment, and the researchers stated that the membrane met relevant performance requirements under ISO 12572. Maintaining vapor permeability while increasing thermal conductivity is significant because a membrane that stores heat effectively but traps moisture could contribute to condensation, mold growth, or reduced indoor air quality.
The study also presents the material as part of a broader circular strategy for construction and energy technologies. Food waste is widely available and can be converted into biochar rather than being discarded or treated solely as an environmental liability. Replacing a portion of conventional carbon-based materials with waste-derived carbon could reduce dependence on virgin resources, although the overall environmental benefit will depend on the energy used for carbonization, chemical activation, graphene production, and composite fabrication. The researchers emphasize that further work is needed to assess production energy demand, long-term performance under realistic humidity and temperature conditions, fire safety, scalability, and techno-economic feasibility. If those challenges can be addressed, the paper-based membrane could become a platform for next-generation energy recovery ventilation, responsive building facades, smart membranes, and systems designed to stabilize indoor thermal comfort while reducing the energy burden of heating and cooling.
Subject of Research: Engineered biochar-graphene hybrid paper membranes for thermal energy storage, heat transfer, moisture management, and ventilation energy recovery.
Article Title: Engineered biochar-graphene hybrid paper membranes for long-term thermal energy storage and ventilation energy recovery
News Publication Date: 21-Aug-2026
Web References: Biochar journal: https://link.springer.com/journal/42773 ; DOI: https://doi.org/10.1007/s42773-026-00646-4
References: 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). DOI: 10.1007/s42773-026-00646-4.
Image Credits: Dimberu G. Atinafu, Harn Wei Kua, Yujin Kang and Sumin Kim
Keywords
Biochar, graphene, phase change materials, thermal energy storage, food waste, energy recovery ventilation, paper membranes, porous carbon, building energy efficiency, moisture management, sustainable materials
