Dopant-designed conducting polymers for constructing a high-performance, electrochemical deionization system achieving low energy consumption and long cycle life

Electrochemical deionization (ECDI) systems are attractive desalination devices that achieve relatively high salt adsorption capacity (SAC) with acceptable energy consumption, which can replace or be a promising complement to other desalination techniques. However, most ECDI systems are only suitable for brackish water desalination, and the energy consumption is significantly increased with the salt concentration of brine water without considering their short cycle life. Here a new concept that an ECDI cell constructed with both electrodes of the same conducting polymer (polypyrrole, PPy) shows the high SAC of Faradaic desalination in the brackish water of a wide concentration range and low cell voltages of ion-capturing/ion-releasing, leading to low energy consumption and excellent cycle life, is demonstrated. Anionic dopants of different sizes make PPy films exhibit distinct ion exchange abilities. Trapping large dopants, 4-methylbenzene-sulfonic acid (p-TS), enables the cationcapturing characteristics of the PPy-p-TS electrode, while the free movement of small dopants, perchlorate (ClO4), facilitates the anion exchange capability of the PPy-ClO4 electrode. Moreover, the operation parameters, including charging/discharging time ratio, mass loading ratio of PPy-p-TS/PPy-ClO4, and overall mass loading on two electrodes, are optimized to achieve excellent desalination performance, long cycle life, and the dual function of salt-removal and salt-concentrating. Finally, a three-dimensional ECDI Ragone plot is proposed to judge the performance of all ECDI systems. This work shows the design guidelines for constructing ECDI systems with high desalination capacity, fast deionization rate, low energy consumption, and long cycle life.