Upcycling Plastic Waste into Graphite Using Graphenic Additives for Energy Storage: Yield, Graphitic Quality, and Interaction Mechanisms via Experimentation and Molecular Dynamics

Abstract

This research presents pioneering work on transforming a variety of waste plastic into synthetic graphite of high quality and purity. Six recycled plastics in various forms were obtained−including reprocessed polypropylene, high-density polyethylene flakes, shredded polyethylene films, reprocessed polyethylene (all obtained from Pennsylvania Recycling Markets Center), polystyrene foams, and polyethylene terephthalate bottles (both sourced from a local recycling bin). The waste plastics were carbonized in sealed tubing reactors. The study shows that this versatile process can be used on a mix of waste plastics in a variety of recycled forms to obtain a uniform graphitic carbon phase, hence addressing the challenges of separation and transportation faced by the plastic recycling industry. The conversion yield to elemental carbon for recycled plastics was improved by up to 250% by using graphene oxide (GO) additives. Five different grades of GO and graphene were used to gain insights into the mechanisms of interaction between plastics and GO during pyrolysis. The effect of GO additives on carbonization was analyzed using thermogravimetric analysis/differential scanning calorimetry and ReaxFF-based reactive molecular dynamics simulations. The obtained cokes were graphitized at 2500 °C and the graphitic quality of the synthetic graphites was analyzed using X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The plasticwaste-derived synthetic graphites exhibit remarkable graphitic quality with crystallite sizes comparable with a model graphitizable material−anthracene coke. The thin, flake-like morphology and nanostructure featuring well-stacked contiguous lamellae make these graphitic carbons highly promising candidates for energy storage applications. Based on our experiments and atomistic-scale simulations, we propose interaction mechanisms between the plastic polymers and the graphenic additives that explain the chemical conversion pathways for GO-assisted waste plastic carbonization and graphitization.