From agricultural waste to high-energy supercapacitors: carbon dots-enhanced hierarchical porous carbon derived from walnut shells

Walnut shells serve as a sustainable precursor for the synthesis of carbon dots incorporates porous carbon. By systematically investigating the relationship between alkali-carbon ratios and electrochemical performance, this study highlights the energy storage potential of walnut shell-derived carbon materials. • Porous carbon was obtained by treating natural walnut shells with convenient methods. • Carbon dots was successfully anchored on the surface of the carbon material. • PC-2 achieved a specific capacitance as high as 389 F g −1 at 2 mV s −1 . • After 5000 cycles, PC-2//PC-2 maintains 106 % cycle efficiency. Converting agricultural waste into high-value energy materials is key to sustainable development. Herein, a synergistic carbonization-activation method was used to fabricate hierarchical porous carbon (HPC) embedded with carbon dots (CDs) from walnut shells, addressing low surface activity in conventional biomass-derived carbons. With an optimized KOH-to-carbon ratio (2:1), the resulting PC-2 has a 3D porous network, with CDs anchored on pore walls. It exhibits a maximum specific capacitance of 389 F g −1 at 2 mV s −1 , which is benefited from CDs-facilitated fast electron transfer and hierarchical pores-enabled efficient ion diffusion. Assembled into a symmetrical supercapacitor, PC-2 delivers 18.6 Wh kg −1 at 23.3 W kg −1 and 106% performance retention after 5000 cycles. This scalable, eco-friendly route offers insights for designing CDs-integrated structures for next-generation energy storage.