Enhancing fire survivability of biochar with medium-entropy oxide coatings

Biochar derived from low-value biomass offers a sustainable platform for lightweight structural and thermal insulation materials; however, its susceptibility to oxidative degradation under prolonged fire exposure limits practical implementation. Here, we report an entropy-engineered ceramic coating strategy to substantially enhance the fire survivability of biochar. Three-metal-element FeMnZn medium-entropy oxide (3ME-MEO) and five-metal-element FeMnZnAlZr MEO (5ME-MEO) coatings were synthesized via a low-temperature, scalable route and uniformly deposited onto porous biochar. Microscopic and spectroscopic analyses reveal homogeneous multielement distributions from the micro- to nanoscale, while the increased compositional complexity of MEO induces greater structural disorder without detectable macroscopic elemental segregation. Under direct ethanol-flame exposure at ~ 1000 °C, pristine biochar suffers severe oxidative degradation, whereas the entropy-engineered coatings effectively preserve the carbon framework. After 20 min, the mass loss decreases from 38.8% for pristine biochar to 23.5% for biochar/3ME-MEO and 18.5% for biochar/5ME-MEO after correcting for coating-derived mass loss. The MEO coatings remain spatially homogeneous after prolonged burning and provide sustained protection against thermal-oxidative attack. Hotplate and modified UL-94-type tests further reveal comparable surface-temperature evolution among the samples despite markedly different degrees of structural degradation. These findings indicate that enhanced fire survivability primarily arises from the protective barrier effect of the MEO coatings, which suppresses oxidative consumption of the carbon framework and mitigates direct flame attack, rather than from thermal insulation.