Y. Gao, N.T. Huynh, K.-J. Kim, C. Wang, V.H. Pham, C. Matranga
National Energy Technology Laboratory,
United States
Keywords: plastic waste, polyethylene, upcycling, hybrid graphitic porous carbon, zinc-ion hybrid capacitors
Summary:
Polyethylene (PE) waste is a challenge to upcycle into useful materials because this plastic tends to decompose into volatile compounds when heated at relatively low temperatures. In this work, we report a chemical process that addresses this challenge by converting mixtures of linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE), and high-density polyethylene (HDPE) waste into a hybrid graphitic porous carbon (HGPC) that can be used as a zinc-ion hybrid capacitor cathode. The process uses a low temperature thermal oxidation pre-treatment step, with assistance of an inert solid additive (KCl) to increase the effective surface area of the PE melt, to functionalize, cross-link, and stabilize the PE waste followed by carbonization and catalytic graphitization steps at higher temperatures with a potassium carbonate (K2CO3) catalyst. The PE waste derived HGPC (PW-HGPC) has a hybrid structure composed of graphene-like carbon nanosheets grown on the surface of carbon particles, high porosity with the Brunauer–Emmett–Teller (BET) specific surface area up to 1,763 m2g-1, and good graphitic degree with average Raman I2D/IG ratios of 0.53. When used as a cathode material for zinc-ion hybrid capacitors, this PW-HGPC exhibits an excellent specific capacity up to 126.7 mAhg-1 at high mass loading of 10 mgcm-2. Moreover, PW-HGPC exhibits remarkable cycling stability with capacity retention of >94% after 10,000 cycles at a current density of 2.0 A g-1. Additionally, the KCl used as an inert surface area enhancer was recycled and reused over five cycles with the material quality and electrocapacitive performance of PW-HGPC retained and verified. Our method provides a new solution for upcycling PE wastes into high value-added carbon materials, not only for zinc-ion hybrid capacitors but also for other electrochemical energy storage device applications that utilize porous carbons as electrodes.