Fig. 1: Structural, electronic, and electrochemical characteristics of CPs.

a The chemical structures of representative CPs, which contain π-conjugated alternating single and double bonds along the polymer backbone. b A typical example of the conduction mechanism arising from inherent conjugation and doping, as illustrated by the electronic band and chemical structures of polythiophene under (i) p-type doping and (ii) n-type doping. Notably, the doping concept for CPs differs from that used in semiconductor physics, as CP conductivity arises from redox reactions along the polymeric backbone (reproduced with permission¹ under a CC-BY license, copyright 2019, MDPI). In the CPs, doping generates positive or negative charge-carriers (often referred to as polarons or bipolarons), which are delocalized along the polymer chains, thereby facilitating electronic conductivity. The charges introduced into the polymer backbone by these redox reactions are balanced by the dopant counterions, which are supplied by an oxidizing or reducing agent. c A schematic diagram showing two ion transport mechanisms in CPs, including liquid-like ion transport (lower left), which involves the motion of polymer segments and depends on the segmental relaxation rate, and solid-like ion transport (lower right), which relies on ions jumping over energy barriers in the polymer matrix and occurs within a timescale in which polymer movements are effectively frozen (reproduced with permission⁴, copyright 2020, ACS). d A schematic diagram of the electrochemical processes for CP electrodes in contact with an electrolyte, along with the corresponding cyclic voltammograms, illustrating (i) capacitive, (ii) faradaic, and (iii) volumetric doping processes (reproduced with permission⁵ under a CCA 3.0 Unported license, copyright 2019, Wiley). Notably, the faradaic and volumetric doping processes rely on the ionic conductivity of the CP. A radar plot e comparing the key properties of major electrode materials in HCs and a schematic diagram f showing the potential functions of CPs in addressing the cycle life and safety issues of HCs.