Dual-regulated walnut green husk-derived porous carbon with enhanced diffusion accessibility for high-performance...

Biomass waste-derived porous carbons have been widely explored as cathode materials for aqueous zinc-ion energy storage, but their rate capability is often limited by sluggish hydrated Zn²⁺ transport within poorly accessible pores. Herein, walnut green husk, an agricultural biomass by-product, was converted into high-performance, diffusion-accessible hierarchical porous carbon through a dual-regulation strategy involving basic magnesium carbonate templating and potassium citrate activation. The optimized walnut green husk-derived porous carbon (WGC-Mg3/PC3) exhibits a high specific surface area of 1451.93 m² g^-1 and a large total pore volume of 0.8298 cm³ g^-1. It also shows an enlarged mesopore volume of 0.3107 cm³ g^-1 and an optimized micropore-volume fraction of ∼62%, forming a balanced micro/mesoporous architecture for micropore-dominated storage and mesopore-assisted ion transport. When used as the cathode in a zinc-ion hybrid capacitor, WGC-Mg3/PC3 delivers 170 mAh g^-1 at 0.1 A g^-1, retains 51.7 mAh g^-1 at 30 A g^-1, and achieves 135.7 Wh kg^-1 at 80 W kg^-1. The device also maintains 98% capacity retention after 10,000 cycles at 5 A g^-1. Kinetic analyses and ex situ characterizations indicate that the enhanced performance is associated with accessible ion-transport pathways, diffusion-involved Zn²⁺ storage kinetics, reversible interfacial precipitation/dissolution of Zn-containing basic sulfate species, and possible Zn-O interfacial interactions. This work demonstrates the conversion of walnut green husk into high-performance, diffusion-accessible carbon cathodes and shows that balancing micropore-derived storage sites with mesopore-assisted transport pathways helps alleviate hydrated Zn²⁺ diffusion limitations in porous carbon cathodes.