The role of PEDOT:PSS in (super)capacitors: A review

Supercapacitors are energy storage devices that, in contrast to classical capacitors, are able to deliver larger amounts of energy keeping a fast charge/discharge rates. They can be considered as the meeting point between batteries and classical capacitors. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is one of the most used conductive polymers (CPs) due to its high thermal stability, low electronic resistance and its ease of application. The role of PEDOT:PSS in supercapacitors where it substitutes the liquid electrolyte is a very interesting approach. Not only it results in a better performing but also a safer option than classical electrolytic capacitors. Despite their wide use in this type of devices, the charge storage mechanism of a PEDOT:PSS layer is still not fully understood. When they were conceived, CPs were automatically classified as pseudocapacitors in terms of their capacitive properties. However, recent analysis of the characteristics of PEDOT:PSS has challenged the origin of the capacitive properties. The mixed ionic-electronic conductivity as a result of the two phases present in PEDOT:PSS (PEDOT rich regions and PSS rich regions) translates into the formation of multiple capacitors in the nanometric scale. These contribute to the total capacitance and they resemble the capacitive mechanism of electrochemical double layers capacitors (EDLC). The combination of PEDOT:PSS with the enlarged surface area of a valve metal such as aluminium gives rise to a solid-state polymer capacitor with low equivalent series resistance (ESR), high capacitance and safer operation conditions than the liquid counterparts. This review covers the recent literature on the main two groups of supercapacitors, namely EDLC and pseudocapacitors, and positions PEDOT:PSS according to the latest findings. Additionally, it presents the challenges of achieving the optimal combination of PEDOT:PSS and aluminium towards better solid-state polymer capacitors at the nanoscale.