Revealing the effect of oxidative doping on the capacitive properties of PEDOT at the molecular level: From electronic structure to chain structure

Poly(3,4-ethylenedioxythiophene) (PEDOT) has become a key electrode material for supercapacitors due to its high conductivity, stability, and flexibility. Its charge storage is associated with redox reactions, and oxidation doping is a critical factor to determine the redox reaction. However, the mechanism by which oxidation doping affects PEDOT's capacitive performance lacks a thorough systematic explanation. This study controlled the oxidation doping characteristics of PEDOT by introducing chloromethyl (-MeCl) and hydroxymethyl (-MeOH) groups for surface modification. For the first time, the combination of density functional theory and experiment proves that the oxidative doping level of PEDOT is determined by both oxidation and doping processes. The electron-withdrawing effect of -MeCl lowers the highest occupied molecular orbital (HOMO) energy level of PEDOT and reduces the electron density on its conjugated backbone, making it more difficult to be oxidized. Conversely, the electron-donating effect of -MeOH promotes the oxidation of PEDOT. The steric hindrance effect of the side chain increases the interlayer spacing of PEDOT's molecular chains, hindering its doping. Under the combined effects of oxidation and doping, the oxidation doping level and carrier concentration of PEDOT decrease obviously after introducing -MeCl, while the decrease was not obvious after the introduction of -MeOH. The introduction of side groups, particularly -MeCl, significantly increases the interlayer spacing and π-π stacking distance of PEDOT, leading to a significant decrease in carrier mobility. Although increasing the interlayer spacing of PEDOT hinders its doping, it simultaneously exposes more active sites. Therefore, the redox reaction can be fully carried out at low current density, which obviously improves the capacitance of PEDOT. As the current density increases, the capacitance of PEDOT decreases most rapidly after the introduction of -MeCl. This is because the lowest carrier mobility hinders the redox reaction. -MeCl increases the oxidation potential of PEDOT, thereby enhancing its cycling stability, with a capacitance retention exceeding 99% after 2000 cycles. The introduction of -MeOH, which exhibits lower steric hindrance, exposed more active sites without significantly reducing the oxidation doping level and carrier mobility of PEDOT. Consequently, it significantly boosts the capacitance and energy density of PEDOT without noticeable degradation in cycling stability, delivering the best overall performance. This study systematically reveals for the first time the mechanism by which oxidative doping affects the capacitive properties of PEDOT. It is proved that surface modification of PEDOT is an effective method to improve its capacitance performance. This has important theoretical value for promoting the performance breakthrough of conductive polymer electrode materials.