Food-Waste Biochar Turned Graphene Hybrid Stores Heat and Manages Moisture

Researchers have transformed mixed food waste into an engineered biochar-graphene hybrid that stores heat through thousands of melting and freezing cycles and, when bonded to a paper membrane, manages both heat and moisture in ventilation systems. The study, published as original research in the journal Biochar, describes composites built from food-waste biochar activated with potassium hydroxide, coated with small amounts of graphene, and loaded with the paraffin docosane as the phase-change material. The best formulation retained up to 90.2% of its latent heat after 1000 heating-cooling cycles and delivered a 72.0% higher latent heat than comparable composites made with unmodified biochar, addressing two long-standing weaknesses of organic phase-change materials: leakage when they melt and sluggish heat transfer.

Phase-change materials store and release large amounts of heat at nearly constant temperature as they melt and crystallize, which makes them attractive for buffering the temperature swings that ventilation systems must handle. Organic paraffins such as docosane have high phase-change enthalpies, but in their pure form they leak as they melt, conduct heat poorly, and can lose storage capacity over repeated cycles. Previous approaches have confined paraffins inside porous carbons, including graphene and activated carbon, but the authors note that these routes can suffer from insufficient shape stabilization, limited storage density, high production costs, and energy-intensive fabrication. Biochar, by contrast, offers high porosity and a surface chemistry rich in hydroxyl, carboxyl, and amine groups that support multiple bonding mechanisms with guest molecules, though biochar-based composites have historically shown insufficient energy storage density and heat transfer.

The team, led by Dimberu G. Atinafu and Sumin Kim, began with mixed food waste pyrolyzed at 400 degrees Celsius for one hour. The resulting biochar was mixed with potassium hydroxide at a 1:0.5 ratio and carbonized at 600, 700, or 800 degrees Celsius, producing samples labeled FK6, FK7, and FK8. Each was then coated with graphene through a one-step dip-coating process in which 5.0 grams of biochar was added to 50 milliliters of a 0.1 weight percent graphene suspension in ethanol, giving a theoretical graphene loading of roughly 1 weight percent relative to the biochar mass. Ultrasonication for one hour promoted even dispersion, and vacuum-assisted drying at 80 degrees Celsius overnight yielded the graphitized hybrids FK6G, FK7G, and FK8G.

Structural analysis showed why the activation temperature mattered so much. Nitrogen sorption measurements revealed type I/II isotherms with micropores between 1.5 and 2.0 nanometers and mesopores between 2.0 and 10.7 nanometers. The 700-degree sample, FK7G, exhibited the largest nitrogen sorption capacity, a mesopore proportion of 82.8%, an average pore diameter of 3.83 nanometers, and a BET specific surface area of 323.1 square meters per gram with a total pore volume of 0.582 cubic centimeters per gram. Raising the activation temperature to 800 degrees reduced the surface area to 314.7 square meters per gram, a 2.6% decrease, presumably because of thermal alteration of the graphene. Raman spectra showed intensity ratios of the disordered D-band to the graphitic G-band between 0.44 and 0.81, below 1.0, indicating high crystallinity, while the 2D-band analysis suggested bilayer to few-layer graphene character.

The researchers encapsulated docosane, a paraffin with a fusion enthalpy of 237.6 joules per gram, into these scaffolds by vacuum impregnation at 70 degrees Celsius, followed by vacuum separation of excess liquid paraffin. Differential scanning calorimetry showed that the graphitized composites melted between 47.6 and 48.4 degrees Celsius and crystallized between 37.5 and 38.3 degrees Celsius. Their melting enthalpies reached 78.6, 93.1, and 79.6 joules per gram for FK6G/C22, FK7G/C22, and FK8G/C22 respectively, representing increases of 12.6% to 72.0% over the corresponding ungraphitized biochar composites. The paraffin adsorption rate roughly doubled with biochar engineering, from 19.2% to 39.2%, and encapsulation efficiency improved by up to 72.4%. Notably, the phase-change enthalpy of FK7G/C22 was nearly identical to that of a composite made with pristine graphene, 95.6 joules per gram, suggesting the food-waste material can substitute for mined graphite-derived graphene at a fraction of the dosage.

Durability testing was central to the study. The composites endured 1000 heating-cooling cycles between 22 and 60 degrees Celsius in an accelerated thermal cycler, a regime the authors equate to roughly three years of energy-recovery ventilation operation on the assumption of one complete thermal cycle per day. After cycling, FK6G/C22 retained 93.1% of its latent heat and FK7G/C22 retained 90.2%. In leakage tests at 70 degrees Celsius, above the melting point of pure docosane, FK7G/C22 lost only 13.2 plus or minus 0.82% of its mass after 180 minutes, compared with 17.4 plus or minus 0.77% for the unmodified biochar system. Thermogravimetric analysis showed the composites remained stable up to about 159 degrees Celsius, with degradation of the paraffin beginning near 242 to 265 degrees Celsius depending on the formulation.

Heat transport improved markedly as well. Thermal diffusivity, measured with a xenon flash lamp, increased by nearly 188.3% when FK7G was introduced compared with pristine docosane, which the authors attribute to an effective heat-transfer network formed by the graphitic scaffold. The team also evaluated a heat storage factor combining density, enthalpy, and the square root of diffusivity; FK7G/C22 scored 38.1% higher than the unmodified biochar composite, while the unmodified composite scored below both the pristine and engineered versions. This trade-off analysis indicated that in phase-transition heat storage, the overall heat storage rate depends more on energy storage density than on thermal transport alone, a nuance the authors say should guide optimization of composite phase-change materials.

The final step integrated the best composite into a commercial paper membrane, a material class recognized as a green energy-recovery medium by ASHRAE. The coated membrane, designated FK7GC22-PM, was evaluated with a gravimetric dry-cup test following ISO 12572 and ASTM E96 protocols, with silica gel as the desiccant, a chamber at 23 plus or minus 0.5 degrees Celsius and 50% relative humidity, and weights recorded over five days. The composite membrane showed a water-vapor resistance factor of 179.24 and an equivalent air-layer thickness of 0.71 meters, below the critical threshold of 1.0 that defines a permeable membrane, meaning moisture could still diffuse through the system despite the added composite layer. In differential scanning calorimetry, the membrane retained 80.2% of the bulk composite’s enthalpy, with a melting enthalpy of 74.7 plus or minus 0.19 joules per gram at 46.7 degrees Celsius, and thermal conductivity measured by a plate heat-flow method reached 66.1 milliwatts per meter-kelvin, 96.1% higher than the pristine paper membrane’s 33.7 milliwatts per meter-kelvin.

The authors frame the work as a first application of this biochar-graphene toolbox to membrane systems and emphasize that the design addresses the high material cost and dosage requirements of graphene derived from non-renewable graphite ore by using economical biochar as the substitute matrix. Because direct bonding of the heat-storing composite into the paper membrane layer bypasses the auxiliary devices or increased ventilation rates that conventional paper-membrane energy-recovery systems rely on, the approach could reduce the energy penalty of ventilation in buildings. Fourier-transform infrared spectroscopy confirmed the composite integrated with the cellulose membrane without significant alteration of the paraffin’s chemical structure, preserving the latent heat density needed for practical storage.

Limitations remain explicit in the paper. The biochar-supported phase-change field is still described as being in an early, largely experimental stage, and the membrane performance was characterized under controlled laboratory conditions rather than in installed ventilation units. The authors state that detailed techno-economic analysis and assessments of synthesis energy consumption remain important aspects for future investigation. Even so, the combination of food-waste valorization, a 96.1% conductivity gain over the reference membrane, 90.2% enthalpy retention across 1000 cycles, and compliance with ISO 12572 permeability criteria offers a concrete, measured demonstration that engineered biowaste carbon can carry phase-change heat storage into flexible, moisture-permeable ventilation materials intended for human thermal comfort applications.

Subject of Research: Engineered food-waste biochar-graphene composites with docosane for thermal energy storage and ventilation membranes

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

Article References: Atinafu, D. G., Kua, H. W., Kang, Y., & Kim, S. (2026). Engineered biochar-graphene hybrid paper membranes for long-term thermal energy storage and ventilation energy recovery. Biochar, 8(1), Article 132. https://doi.org/10.1007/s42773-026-00646-4

Image Credits: AI Generated

DOI: 10.1007/s42773-026-00646-4

Keywords: biochar, graphene, phase-change materials, thermal energy storage, ventilation energy recovery, food waste, docosane, paper membranes, moisture management, latent heat, hygrothermal performance, Engineered