Upcycling biomass into hierarchical N-doped carbons for fast and stable water deionization

Capacitive deionization (CDI) offers an energy-efficient and scalable route for water desalination, but its practical deployment remains constrained by suboptimal electrode materials. Here, we report a dual-activation strategy that transforms waste orange peel into high-performance nitrogen-doped porous carbon (KBC0.5) via melamine templating and KOH etching. The resulting biochar exhibits a hierarchically integrated micro/mesoporous architecture with an ultrahigh surface area (2350 m 2 g −1 ) and abundant pyridinic and graphitic nitrogen sites. These structural features synergistically promote ion diffusion, charge transfer, and electrosorption capacity. As a CDI electrode, KBC0.5 delivers a high salt adsorption capacity of 18.3 mg g −1 at 1.2 V and a specific capacitance of 168.5 F g −1 , outperforming commercial YP-50F in both rate and capacity. It also exhibits excellent cycling stability (62.5 % SAC retention over 30 cycles), superior charge efficiency (up to 50 %), and a low specific energy consumption of 0.86–1.12 kWh kg −1 across a broad voltage window. This work highlights a sustainable pathway for converting biomass into advanced CDI materials, offering design insights into pore hierarchy, surface chemistry, and energy-responsiveness for next-generation water purification technologies. ● A dual-activation route converts orange peel into N-doped porous carbon (KBC0.5). ● KBC0.5 exhibits ultrahigh surface area (2350 m 2 g −1 ) and hierarchical micro/mesopores. ● As a CDI electrode, KBC0.5 achieves 18.3 mg g −1 salt adsorption at 1.2 V. ● KBC0.5 outperforms commercial YP-50F in both ion removal rate and capacity. ● This study provides a scalable biomass valorization strategy for sustainable water treatment.