Microwave-Directed Growth of Cup-Stacked Carbon Nanotubes from Mixed Plastics for Metal-Free Cathodic Application

Microwave irradiation has been shown to enhance the purity and structural quality of carbon nanotubes, and this study demonstrates that microwave-assisted pyrolysis of mixed waste plastics (polypropylene, polyethylene, and polystyrene) could lead to the formation of a distinctive cup-stacked carbon nanotube (CSCNT) architecture. Unlike conventional thermal pyrolysis, the microwave field inherently induces localized thermal gradients and nonequilibrium carbon dynamics that promote edge-selective nucleation and anisotropic elongation, yielding the CSCNTs in a graphitic purity of 99.35% without any purification. This unique architecture, featuring open graphitic edges and interlayer anisotropy, delivers a superior electrochemical performance as a metal-free cathode, achieving a specific capacitance of 256 F/g at 0.5 A/g. Even at a high current density of 5 A/g, the capacitance remained 28% higher than that of conventional MWCNTs, indicating excellent rate capability. CSCNTs also had high stabilities over 4000 cycles with 86.4% capacitance retention and 99.55% Coulombic efficiency. Mechanistic investigations reveal microwave-induced nonequilibrium carbon dynamics, which are governed by localized dipole polarization, thermal gradients, and heterogeneous stress distributions. These resulted in the directional carbon rearrangement that is responsible for the cup-stacked morphology. Based on the findings, an energy-efficient pathway to convert waste plastics into high-performance electrodes was established, underscoring the potential to address both plastic waste and sustainable energy material synthesis.