Interaction of 3D Conductive Polymer-Reticulated Vitreous Carbon Composites in Ionic Liquid As Energy Storage Material

Abstract

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. Figure 1