Life-Cycle-Sustainable Capacitive Deionization Enabled by Organic Molecular Engineering for Selective Ammonium Capture

Selective ammonium (NH₄ ⁺) recovery from complex wastewater streams remains a pivotal challenge in sustainable nitrogen management. Capacitive deionization (CDI) offers an energy-efficient electrochemical route for ion separation, yet its practical application is constrained by the inherent trade-off between NH₄ ⁺ adsorption capacity and selectivity. Herein, we circumvent this fundamental limitation through the rational molecular engineering of an organic electroactive material, 1,4-bis(1H-imidazo[4,5-b]phenazin-2-yl)benzene (IPB), which features extended electron delocalization and abundant C═N moieties for NH₄ ⁺-specific redox-coupled capture. The resulting IPB-based CDI prototype delivers a high NH₄ ⁺ adsorption capacity of 267.23 mg g^-1 and a fast rate of 62.51 mg g^-1 min^-1, while sustaining remarkable selectivity in complex multi-ion matrices, thereby effectively surpassing the conventional capacity-selectivity trade-off. Notably, CDI adsorption reduces wastewater NH₄ ⁺ levels to below 3 mg L^-1 and achieves 82.3% direct recovery as liquid fertilizer during CDI desorption, as demonstrated by isotope tracing experiments. The CDI process exhibits a low carbon footprint of only 4.66 × 10^-2 t CO₂-equivalent per tonne of NH₄ ⁺ treated, underscoring its superior life-cycle sustainability. Therefore, this work proposes a molecularly engineered organic electrode material for high-performance wastewater remediation and circular nutrient recovery, paving the way toward carbon-neutral electrochemical water treatment technologies.