Dopant Engineering of Conductive Polymer for Developing High-Efficiency, Membrane-Free, Full-Polymer Electrochemical Deionization Systems
Authors: Huang Hung-Yi , Tu Yi-Heng , Yang Yu-Hsiang , Hu Chi-Chang 
Published: December 2024

Electrochemical deionization (ECDI) systems represent an enticing avenue in desalination technology, offering notable desalination capacity while maintaining energy efficiency, thus presenting themselves as potential replacements for existing desalination methods. Traditionally, ECDI systems rely on electric double-layer (EDL) materials to eliminate salts via electrode-induced ion adsorption under external electric fields. However, the practical utility of this conventional approach is hindered by the limitations of the EDL mechanism, constraining desalination capability. Consequently, there has been a notable shift towards exploring pseudocapacitive-type or battery-type faradaic materials, which offer promising alternatives. This novel approach entails the capture of ions within the solution through electrode charge transfer, effectively reducing energy consumption and bolstering desalination performance. Furthermore, the unique characteristic of the "memory effect" in faradaic materials, as proposed by Hu et al., has revolutionized resource recovery and water purification, eliminating the need for membranes and further elevating the appeal of such materials within the field [1]. Among these faradaic materials, polypyrrole (PPy) is recognized for its affordability, stability, and straightforward synthesis, rendering it an excellent choice for anion insertion applications. Prior research indicates that PPy demonstrates distinct behaviors depending on dopant variations. Large-size dopants confer cation exchange capabilities to PPy, whereas small-size dopants imbue it with anion exchange characteristics. Despite these qualities, conducting polymers have predominantly served as either anion or cation-removal electrode materials in ECDI systems [2].
As a result, the study aims to design a low-voltage, membrane-free, ECDI system with high salt-removal capacity and excellent cycling stability through the utilization of an identical conducting polymer (i.e., PPy) doped with anions of different sizes for the positive (4-methylbenzenesulfonic acid, p-TS) and negative (ClO4
−) electrodes. Essentially, PPy is tailored to possess either cation or anion-exchange abilities to serve as both positive and negative electrode materials in the ECDI system. According to the results of XPS and CV measurements, the schematic diagram of the salt removal/release mechanisms on the two PPy electrodes is shown in Figure 1 [3]. The positive electrode (PPy-p-TS) exhibits the cation-capturing ability due to the trapping of bulky p-TS dopants with high electronegativity from the sulfonic acid groups during the electro-polymerization. Therefore, the reduction of PPy polarons (i.e., the + symbols) results in the presence of negative charges within the PPy-p-TS matrix, electrostatically attractive to cations. On the other hand, the negative (PPy-ClO4) electrode exhibits the anion-removing ability due to the positive charges generated on the oxidized PPy, which are charge-compensated by the small anion dopants (i.e., ClO4
− during the electro-polymerization step, which is replaced with Cl− during the desalination step). Moreover, to apply the unique "memory effect" in the ECDI system, the salt-removal/salt-concentration test was conducted in a full cell and the conductivity profile is shown in Figure 2. The PPy-p-TS//PPy-ClO4 system can transfer the total amount of 342 mg g−1 of NaCl in only 5 cycles with only 0.1902 kWh/kg-NaCl. With the characteristics of dual function and membrane-free, this full-polymer ECDI system has become an attractive and competitive technology in the water purification field.
Furthermore, the similar redox couples between positive and negative electrode materials result in closely aligned redox potentials for ion capturing/releasing, leading to significantly reduced cell voltages during both deionization and concentration processes. Through optimization of mass loading ratios, the positive and negative electrodes in this ECDI system operate within their respective desirable potential windows, demonstrating substantial desalination capabilities of up to 83.4 mg g−1 in a 60-minute single-pass operation. Additionally, the adjustments in the charging/discharging time ratio and mass loading of active materials yield a low energy consumption (EC) of 0.2225 kWh/kg-NaCl. Therefore, in comparison with the other ECDI systems in Figure 3, this full-polymer ECDI system lies in the ideal performance area with great SAC and SAR performance. Moreover, this system shows ultra-low EC compared to other systems and industrial RO plants. Meanwhile, this system demonstrates its great stability with a SAC retention of 88 % after 250 cycles (66.7 h). Ultimately, according to the proposed design guidelines, we successfully demonstrate a superior desalination performance, high stability, and dual-function ECDI system without membrane as a potential future desalination system.

References

[1] Yang, Yu-Hsiang, et al. "A high-capacity hybrid desalination system using battery type and pseudocapacitive type electrodes." Desalination 545 (2023): 116160.
[2] Huang, Hung-Yi, et al. "Probing host–dopant interactions in conducting polymers for improved performance of electrochemical deionization." Journal of Materials Chemistry A (2024).
[3] Huang, Hung-Yi, et al. "Dopant-designed conducting polymers for constructing a high-performance, electrochemical deionization system achieving low energy consumption and long cycle life." Chemical Engineering Journal 457 (2023): 141373.

Figure 1