Energy storage technologies, which combine high specific energy and power with sustainability and low costs, are fundamental drivers for an energy sustainable society. Conductive polymers in highly stable and non-toxic ionic liquid (IL) electrolytes have been proposed as possible systems that combine these advantages. Recently, it has been demonstrated that poly(3,4-ethylenedioxythiophene) (PEDOT) can undergo a redox reaction like in a battery and at the same time perform the characteristics of a capacitor by being doped/charged (de-doped/discharged) by anions from the IL. The storable amount of charges depends on the generated doping/de-doping sites in the polymer, which is a function of the electrode potential. These so called “hybrid-battery-capacitor” system combines the faradaic and capacitive properties in a single electrode [1,2], providing the possibility to improve cell voltages and energy densities.

High surface area materials such as three-dimensional conductive reticulated vitreous carbon (RVC) substrate, provide large surface area for electro-polymerisation of a thin film nano/micro-porous PEDOT active material (“footprint-area” [3]). The resulting composite has large capacitance supported by the rigid carbon substrate which helps to stabilise the typical swelling/contraction process observed in conducting polymers during the electrochemical charge/discharge process.

This study focuses on the synthesis and characterisations of thin electro-active PEDOT films on RVC substrates by electro-polymerisation in IL. The polymer synthesis produced a uniform film on the RVC with a high number of doping/de-doping sites in Lewis neutral 1-ethyl-3-methylimidazolium chloride aluminum chloride (EMImCl-AlCl₃) IL with AlCl₄^- as doping/de-doping anion.

The interaction of the conductive polymer films with the ionic liquid electrolyte are characterised by combined in-operando atomic force microscopy (AFM) and electrochemical quartz crystal microbalance measurements (EQCM), distinguishing the predominant battery or capacitor behaviour per doping/de-doping site and the correlated changes of the polymer structure as function of the electrode potential.

[1] Chemical Reviews 110 (2010) 4724–4771.

[2] Chemical Society Reviews 44 (2015) 1777–1790.

[3] Advanced Materials 26 (2014) 2440–2445.

Graphical abstract: (A-C) PEDOT electro-polymerised on RVC and (D) schematic illustration of the three-dimensional RVC-PEDOT composite, which is doped (charged) and de-doped (discharged) with AlCl₄^- anions of a chloroaluminate ionic liquid.