Abstract
The development of sustainable materials for electrochemical ion separation remains a critical challenge in environmental remediation. Full-polymer electrochemical deionization (ECDI) systems, comprising of entirely redox-active polymer electrodes, offer a promising membrane-free platform for ion-selective separation due to their structural tunability and redox reversibility. However, there is a lack of fundamental understanding on how dopant chemistry and applied potentials govern the ion exchange/selectivity behavior and underlying mechanisms. Herein, we present a mechanistic investigation of a full-polymer ECDI system employing polypyrrole (PPy) electrodes with varied dopants. Through an electrochemical quartz crystal microbalance (EQCM), we identify distinct potential windows where cation or anion exchange dominates, governed by the synergy between dopant structure and polaron/bipolaron formation in the polymer backbone. Building on these insights, we introduce a facile electrochemical activation strategy, cyclic voltammetric deep reduction (CVDR), to induce controllable dopant displacement and create additional exchange vacancies within the PPy-SS_CVDR (SS: styrene sulfonate) electrode. When integrated into a symmetric full-polymer ECDI cell, the CVDR-activated PPy electrodes achieve an efficient removal of monovalent and divalent oxyanions (NO3-, SO42-, CO32-), demonstrating high removal capacity, improved diffusion kinetics, and reduced energy consumption. Overall, this work demonstrates the promising value of EQCM for elucidating the potential-dependent ion exchange mechanism and highlights the potential of electrochemically tunable, membrane-free architectures for the selective and regenerable removal of large oxyanions in the complex aqueous environment.