Sustainable upcycling of polypropylene-based masks into high-performance carbon materials for supercapacitors via molten salt carbonization and air activation

Abstract

Carbonization is a promising method for upcycling polypropylene (PP)-based plastic waste into high-value carbon materials, yet direct pyrolysis typically suffers from low carbon yield due to significant volatile losses. In this study, we propose a sustainable and integrated approach utilizing thiourea-promoted molten salt carbonization to enhance the carbon yield from PP-based waste masks. The incorporation of thiourea stabilizes the carbon structure by reducing excessive fragmentation of the carbon backbone, significantly increasing the carbon yield from 7.3 % to 21.6 %. Simultaneously, thiourea introduces sulfur and nitrogen co-doping, enhancing conductivity and electrochemical performance. Furthermore, air activation is employed to optimize the pore structure, increase surface area, and introduce oxygen-containing functional groups, greatly improving ion diffusion and electrolyte wettability. The resulting air-activated sulfur- and nitrogen-doped carbon materials exhibit exceptional electrochemical performance, with a specific capacitance of 345.6 F g⁻¹ at 1 A g⁻¹ and outstanding cycling stability, retaining 93.3 % capacitance after 20,000 cycles. The corresponding symmetric supercapacitor demonstrates an energy density of 40.1 Wh kg⁻¹ at a power density of 400 W kg⁻¹, alongside excellent cycling stability. This integrated strategy provides a sustainable pathway for converting PP-based waste plastics into high-performance carbon materials, with significant potential for applications in energy storage and environmental remediation.