Co-upcycling of polyurethane and biomass for synthesizing functionalized N-doped hierarchical porous carbon for high-performance aqueous zinc-ion hybrid capacitors

Aqueous Zn-ion hybrid capacitors (ZIHCs) are promising next-generation energy storage devices that combine high energy density with high power density. However, current carbon cathodes with a suboptimal porous structure do not match the zinc anode, especially hindering the transportation and storage of (Zn[H₂O]₆)²⁺, resulting in low energy density and poor cycling durability. To address this challenge, nitrogen-doped hierarchical porous carbons (NHPCs) have emerged as a promising cathode material. Conventional synthesis of NHPCs relies on costly and environmentally harmful nitrogen sources (e.g., cyanides, amines), hindering sustainable production. Herein, functionalized carbonaceous materials were synthesized using biomass and nitrogen-rich plastic waste (polyurethane, PU) as precursors through carbonization followed by activation. The obtained N-3@AC-4 (with a biomass to PU ratio of 3:1 and activator to carbon precursor ratio of 4:1) achieved a remarkable specific capacitance of 430.6 F g⁻¹, at a current density of 0.1 A g⁻¹ in a typical three-electrode system. When configured into ZIHCs, N-3@AC-4 maintained a high specific capacitance of 356.9 F g⁻¹, at the same current density, coupled with a superior energy density of 126.89 Wh kg⁻¹, at a power density of 80 W kg⁻¹. Furthermore, the device demonstrated long cycling stability, with ~85% capacitance retention after 5,000 cycles. Overall, this study presented a sustainable and effective strategy for synthesizing advanced carbonaceous cathode materials for high-performance aqueous ZIHCs.