Upcycling Low Linear Density Polyethylene Waste into Turbostratic Graphene for High Mass Loading Supercapacitors

Plastic pollution is triggering a global environmental crisis, prompting research efforts in recycling and upcycling to address the challenge. Upcycling plastic into graphene and porous carbon creates options to manage waste streams by converting the plastic into materials that can be used as electrodes in energy storage devices. Linear low-density polyethylene (LLDPE) is difficult to upcycle because it tends to decompose into light gases and hydrocarbons during thermal processing which limits how easily it can be converted into useful solid carbon materials. In this work, LLDPE was upcycled into a high quality turbostratic graphene using a pre-treatment step to oxidatively crosslink the polymer with the assistance of solid additives (KCl and K₂CO₃) that improve crosslinking by increasing the effective surface area of the polymer melt during processing. After this pretreatment step, the crosslinked polymer could then be carbonized and catalytically graphenized between 400-950 °C without complete decomposition of the material. The LLDPE derived graphene (LLDPE-G) obtained from this process has a Brunauer–Emmett–Teller (BET) specific surface area, up to 1800 m²g^-1 and average Raman I_D/I_G and I_2D/I_G ratios of 0.85 and 0.57, respectively, indicating high quality graphene. When used as an electrode material in symmetric supercapacitors, LLDPE-G possesses an outstanding specific capacitance up to 175 Fg^-1 at a mass loading of 20 mgcm^-2, which is two times the commercial requirement, yielding an excellent areal capacitance of 3.5 Fcm^-2. Moreover, LLDPE-G exhibits exceptional cycling stability with a capacitance retention of 95.8% after 100,000 cycles at a current density of 4.0 Ag^-1. Additionally, the KCl and K₂CO₃ were recycled and reused over 3 complete cycles to make new LLDPE-G with the material quality and electrocapacitive performance retained and verified after each cycle. Our approach creates new opportunities for upcycling not only waste LLDPE but also other varieties of PE to high value graphene materials for electrochemical energy storage device applications.