Tailorable Dual-Redox Polymer with Molecular Flexibility for Enhanced Electrochemical Desalination and Water Purification

The global water crisis demands sustainable desalination innovations, with capacitive deionization (CDI) emerging as an energy-efficient electrochemical alternative. While organic materials demonstrate pseudocapacitive ion capture through ion coordination for CDI electrodes, their effectiveness remains constrained by molecular chain packing and deficient redox-active sites. This work introduces a novel biocompatible dual-redox polymer (PNDBI) with high molecular flexibility to address these limitations. In-situ characterization and theoretical analyses unveil that the concerted interaction between C═O and C═N bifunctional groups enhances Na⁺ capture. Concurrently, the polymer's pliable backbone and narrow HOMO-LUMO gap ensure active site accessibility and facilitate electron mobility, which endow the PNDBI polymer with substantial pseudocapacitive capacitance and remarkable stability for 4Na⁺ capture. The CDI device employing the PNDBI electrode demonstrates outstanding desalination performance, achieving a remarkable salt removal capacity of 112.1 mg g^-1 and a rapid removal rate of 3.7 mg g^-1 min^-1. Impressively, the CDI device exhibits excellent electrochemical regeneration stability, retaining 92.0% efficiency over 200 cycles, placing it among the state-of-the-art CDI devices reported. Beyond desalination, the PNDBI-based CDI device showcases significant multifunctionality, enabling efficient water purification through the removal of hard water ions and cationic dyes, thereby offering a versatile and sustainable solution for environmental remediation.