Waste‐Specific Upcycling of Agro‐Wastes Into Nano‐Carbon Plates and Their Reinforcing Role in Low‐Carbon Bast Fiber Automotive Composites

As the automotive sector seeks lower‐carbon alternatives to glass fiber–reinforced polymers, biomass‐derived carbons are increasingly explored as sustainable reinforcements. However, biomass precursors are often treated as generic carbon sources, leaving the relationships among waste origin, carbon structure, composite performance, and environmental impact insufficiently understood. Here, lemon peel, orange peel, walnut shell, and pistachio shell were converted under identical thermal–chemical conditions into defect‐rich nano‐carbon plates, enabling a controlled comparison of precursor‐dependent structure–property–environmental relationships. Citrus‐derived materials exhibited more wrinkled and oxygen‐rich morphologies, whereas nutshell‐derived materials, particularly those from pistachio shell, showed more carbon‐rich surfaces and higher C/O ratios. At 1 wt.% loading, the nano‐carbon plates increased PP tensile strength by up to approximately 34% while largely preserving ductility. In MAPP‐compatibilized composites containing 40 wt.% flax fiber, the pistachio‐derived system achieved tensile and flexural strengths of approximately 52 and 75 MPa, respectively. Cradle‐to‐gate LCA revealed strong feedstock‐ and scale‐dependent differences. The pistachio‐derived hybrid showed a GWP of 2.65 kg CO 2 eq with virgin PP, slightly below the 30GF‐PP benchmark of 2.78 kg CO 2 eq, while recycled PP reduced the GWP to 1.61 kg CO 2 eq, corresponding to a 42.1% reduction. This study establishes a feedstock‐sensitive upcycling strategy for realistic circular composite design.