Upcycled coffee grounds and beeswax-derived all-bio-based PCMs for dual-function solar harvesting and thermal storage

The development of shape-stabilized phase change materials (PCMs) for solar-thermal energy storage faces a trade-off: while conventional petroleum or mineral-derived systems achieve high energy density, they raise concerns about resource depletion and environmental impact. Here, we address this challenge by applying a circular economy approach, transforming waste coffee grounds and beeswax (BW) into a fully bio-based composite PCM that combines sustainable attributes with performance characteristics relevant for practical applications. A hierarchically porous carbon aerogel, prepared by carbonization of coffee waste, serves as a structural support for vacuum-impregnated BW and demonstrates a PCM loading capacity of 78.8 %. The composite exhibits a phase change enthalpy of 135.3 J/g at 48.7°C and maintains 98.2 % of its capacity after 500 thermal cycles. Under thermal stress conditions (70°C, 4 h), the material shows no detectable leakage. Additionally, the biochar framework enables dual functionality: efficient solar absorption and heat storage, yielding an 89.0 % solar-thermal conversion efficiency under a light intensity of 100 mW/cm 2 . The real-time temperature profile shows that phase transition aligns with variations in solar input, as evidenced by a stable 48°C plateau during energy storage. By marrying waste valorization with leak-proof thermal storage, this work establishes a promising and scalable approach for developing next-generation bio-based energy materials. The coffee ground-derived composites combine renewable feedstock, straightforward preparation, and long-term cyclic durability, suggesting their potential as candidate materials for solar thermal harvesting and storage applications.