Ionic Side-Chain Engineering in Conjugated Polyelectrolytes for High-Performance Pseudocapacitors

Abstract

Conjugated polyelectrolytes (CPEs) are promising materials for pseudocapacitor electrodes due to their redox-active backbones and tunable ionic functionalities. Here, we investigate the impact of ionic pendant groups on electrochemical performance by comparing anionic CPE-K (sulfonate-functionalized) and cationic CPE-Br (quaternary ammonium-functionalized), both featuring identical conjugated backbones. The opposite charge polarity influences how polarons are stabilized: notably, CPE-K undergoes self-doping due to its anionic sulfonate groups, while CPE-Br remains neutral. In their pristine states, CPE-Br exhibits a higher specific capacitance (95 F g^-1 vs. 84 F g^-1), more distinct redox features, and better rate capability than CPE-K. This is attributed to its cationic nature, which prevents self-doping and enhances anion penetration from the electrolyte. Electrochemical impedance spectroscopy further corroborates these findings, revealing significantly lower charge-transfer resistance and a smaller Warburg factor for CPE-Br, indicative of faster ion diffusion compared to CPE-K. Upon forming composites with single-walled carbon nanotubes (SWCNTs), both materials exhibit enhanced performance; notably, the 2:1 CPE-Br/SWCNT composite outperforms its CPE-K counterpart, achieving a high specific capacitance of 291 F g^-1 at 1 A g^-1 and retaining 254 F g^-1 at 5 A g^-1. Furthermore, it demonstrates excellent cycling stability over 1000 cycles with ~99% coulombic efficiency, underscoring its robust charge storage reversibility and long-term durability. These findings highlight the importance of pendant group design in optimizing ionic transport and redox behavior in CPE-based pseudocapacitor systems.