A. Gharpure, M. Kowalik, R. Rajagopalan, R. Vander Wal
Penn State University,
United States
Keywords: plastic waste, upcycling, graphite, energy storage
Summary:
In the United States, approximately 37 million tons of plastic are used every year according to the McKinsey estimates. Packing and food-service uses contribute 16 million tons which is single use disposable plastic. High density polyethylene (HDPE, used in milk jugs), low density polyethylene (LDPE, used in plastic bags, container, films and wraps), polypropylene (PP, used in yogurt containers and bottle caps) and polyethylene terephthalate (PET, used in soft drink bottles) are four main resin types making up approximately 85% of the single-use plastic. More than 70% of the single use plastic waste is sent to landfills. This presents a huge opportunity to upcycle the plastic waste into high value carbon materials. Concurrently a big surge in demand for carbon materials is expected from energy storage and transportation applications. As the current flake graphite market is small, even modest electric vehicle (EV) adoption rates would have big impact on graphite demand and price. Present electric vehicles need ~70 kg graphite. An EV adoption of 1% of the new car market will potentially increase graphite demand by 10%. Flake graphite production would need to double by 2025 just to meet Lithium-ion battery (LiB) manufacturing capacity currently under construction. When heated, plastics typically crack into light gases through chain unzipping and β-bond scission. This translates into low carbon yield and non-graphitizable residual carbon. We propose an innovative approach of employing GO as templating agent and closed reactor carbonization under autogenic pressure to increase yield and graphitic quality of carbons. Oxygen functional groups on GO would provide the necessary stabilization while the sp2 framework would serve as a template guiding reconstruction of polymer chains into graphitic material. The latter postulated templating effect could realize significant cost and energy savings by enabling graphitic structure at lower temperatures. Notably, a portion of this graphitic material could be used to derive the GO for templated graphitization of fresh feedstock, thus contributing to circular process and upcycling plastic economics. Upcycling overabundant plastic waste into graphitic carbons would support the renewable energy transition while reducing pollution, cost and CO2 emissions. Four commercially recycled plastics and their composites with GO have been used to evaluate comparative yield and graphitic quality of the obtained carbon materials. Pure plastics have extremely low carbon yields as they go through chain unzipping and β-bond scission to produce light gases. Therefore, they require sealed reactor during carbonization to obtain measurable carbon yield. GO provides excellent stabilization through the radical sites formed by the leaving groups. The key advantage of GO as additive is that it nets a substantial increase in carbon yields, nearly 300% in some plastics. There is a trade-off between carbon yield and graphitic quality in GO/plastic composites. After graphitization, graphitic quality of GO composites is slightly lower than all pure plastics (except HDPE) potentially because of crosslinking by reactive radical sites on GO which is responsible for the yield increase. Although comparatively of lower graphitic quality, GO/plastic composites possess lattice parameters comparable to graphitized anthracene coke.