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Upcycling waste plastics (e.g., polyethylene (PE)) into value‐added carbon products is regarded as a promising approach to address the increasingly serious waste plastic pollution and simultaneously achieve carbon neutrality. However, developing new carbonization technology routes to promote the oxidation of PE at low temperature and construct the stable cross‐linking network remains challenging. Here, a facile carbon‐grown‐on‐carbon strategy is proposed using carbon black (CB) to convert waste PE into core/shell carbon nanoparticles (CN) in high yields at low temperature. The yield of CN remarkably increases when the heating temperature decreases or the dosage of CB increases. The obtained CN displays turbostratic structure and closely aggregated granular morphology with a size of ≈80 nm. It is found that, prior to the oxidation and carbonization of PE, CB forms a 3D network architecture in the PE matrix. More importantly, CB not only catalyzes the partial oxidation of PE to form PE macromolecular radicals and introduce oxygen‐containing groups at low temperature in the early stage, but also favors for the construction of a stable cross‐linking network in the latter stage. This work offers a facile sustainable strategy for chemical upcycling of PE into value‐added carbon products without post‐treatments or usage of metallic catalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • Cube-like Mn-MOF is synthesized from discarded poly(ethylene terephthalate) bottles. • MnO/C nanoparticle is easily tailored from cube to sphere, octahedron or polyhedron. • Evaporator owns good water transport, light absorption and solar-thermal conversion. • Evaporator shows the evaporation rate of 2.38 kg m−2 h−1 with efficiency of 98.4 %. • Evaporator recovers low-grade heat from evaporation for thermoelectric generation. The integration of freshwater and electricity cogeneration through photothermal effect is regarded as a new and promising strategy for high-efficiency solar energy utilization. However, the central issue is the design of low-cost, flexible photothermal materials to efficiently utilize solar energy. Herein, flexible MnO/C nanoparticle-based solar evaporators are constructed to realize simultaneously interfacial solar evaporation and thermoelectricity generation. The MnO/C nanoparticle is facilely fabricated by pyrolysis of Mn-MOF, which is synthesized from discarded poly(ethylene terephthalate) bottles in one-pot solvothermal method. By altering pyrolysis temperature of Mn-MOF from 400 °C to 700 °C, the morphology of MnO/C nanoparticle is readily tailored from cube to sphere, octahedron and polyhedron. Owing to the fast water transport, strong sunlight absorptivity, high solar-to-heat conversion efficiency, and low evaporation enthalpy, the hybrid solar evaporator presents the high evaporation rate of 2.38 kg m−2 h−1 and solar-to-vapor conversion efficiency of 98.4% under 1 Sun irradiation, surpassing many advanced photothermal materials. More importantly, integrated with a thermoelectric module, the bifunctional solar evaporator recovers low-grade heat from water evaporation to perform thermoelectric conversion, achieving the maximum output voltage of 330 mV under 3 Sun irradiation. This work highlights the potential of flexible and durable solar evaporators for freshwater and power cogeneration, not merely contributing to achieving carbon neutrality and upcycling of waste plastics, but also opening opportunities for giant electricity supply from sustainable solar energy. [ABSTRACT FROM AUTHOR]