Conducting Polymers in Electrolyte and Separator Engineering for Zn-Ion Hybrid Capacitors

Conducting polymers (CPs) have emerged as multifunctional components that modulate electrolyte and separator design for Zn-ion hybrid capacitors, with selected Zn-based battery and alkali-ion systems used as mechanistic references rather than direct performance evidence. Their π-conjugated backbones, tunable doping states, and reversible redox chemistry enable coupled ion transport, interfacial charge regulation, and mechanically compliant ion-conducting environments. However, CP integration also requires careful control of electronic leakage, self-discharge, short-circuit risk, and redox-state-dependent swelling, particularly when CP domains are introduced into separator architectures. This review examines how CP-containing electrolytes and separator-related architectures can regulate ion transport and interfacial charge distribution, provided that continuous electronic pathways are suppressed. In aqueous, quasi-solid gel, solid polymer, and redox-active electrolytes, CP-containing domains are examined in terms of ionic conductivity, Zn²⁺ transport, solvation regulation, and polarization mitigation. In separator-related architectures, spatially confined and electronically interrupted CP domains are discussed with emphasis on ion-flux homogenization, dendrite suppression, electron-blocking requirements, and dimensional stability. Comparative analyses separate directly demonstrated Zn-ion hybrid capacitor behavior from mechanisms inferred from related ion-storage systems. Finally, future directions are discussed for quantitative benchmarking, scalable CP processing, and architecture-controlled CP domains for coordinated electrolyte and separator design.