Comparative Study of Carbon and Conducting Polymer-Based Electrochemical Capacitors Utilizing Potassium Iodide Redox Electrolyte
Authors: Lubera Justyna Magdalena , Skunik-Nuckowska Magdalena , Raczka Patryk , Kulesza Pawel J. 
Published: May 2020

A key challenge for the existing electrochemical charge storage systems, enabling versatile and advanced applications in modern electronics, is related with the improvement of their performance in terms of gravimetric and volumetric energy and power, durability, exploitation costs and safety. However, neither batteries, nor electrochemical capacitors (ECs) technologies do not meet all these demands. Hence, the concept of hybrid capacitor has been developed allowing to combine the main advantages of batteries (high energy) and ECs (high power, long lifespan). Among various strategy of development of such hybrid devices is the incorporation of electrochemically active redox species in addition or instead of a conventional double-layer supporting electrolyte. To date, it has been widely reported that the electron transitions between redox centers present in the electrolyte at various oxidation states result in the increased capacity and energy in comparison to the simple electrical double-layer-type systems. By the fact that these processes are fast and highly reversible, the rate of charging/discharging remains on a level expected for high power systems. In this respect, it is important to refer to works describing electrocatalytic properties of certain systems (Pt, PEDOT, nanostructured carbons) toward electron transfers within the iodine/iodide redox couple with respect to dye sensitized solar cell applications [1, 2].
It is worth to emphasize that most of reported active electrolyte-based hybrid ECs rely on carbonaceous materials enabling low electrodes resistance, well-developed surface area and providing high number of active sites for the electrochemical reaction. In the present work, we propose an alternative electrode material based on a conducting polymer (poly(3,4-ethylenedioxythiophene (PEDOT)) to support the redox processes of an iodide-based active electrolyte, i.e. one of the most widely considered so far. The performance evaluation was based on the comparison to the to the system fabricated from the conventional high surface area activated carbon electrodes (AC). The structure, morphology and porosity of investigated materials were analysed by comparative physicochemical characterization (SEM, XRD, Raman, UV/Vis and N2 adsorption/desorption techniques). The electrochemical studies revealed that despite a low specific surface area of a PEDOT (S
BET=40 m2 g-1), the gravimetric cell capacitance is similar to obtained for AC-based cell (S
BET=2600 m2 g-1). Interestingly, despite the lower potential window of PEDOT system (1.2 V vs. 1.5 V for AC), the volumetric capacitance and energy is even much higher due to the high packing density of PEDOT electrodes. A better capacitance/energy responses were also observed upon complete parameters normalization, i.e. including the mass of electrolyte present in the pores and void spaces of the electrodes. It should be noted that such approach, although of a great importance in a viewpoint of realistic industrial application, and is often neglected by researchers.
The performance of both systems is also discussed in terms of stability and the effect of continuous operation (under galvanostatic cycling or potentiostatic ageing) on the self-discharge rate. As a result, a mechanism of parasitic charge loss under open circuit is described as a dual redox-shuttle effect and has been mainly attributed to the side reactions taking place at the negative electrode.

Acknowledgement

Financial support was provided by the National Centre for Research and Development (NCBR, Poland) under Techmatstrateg Grant no. 347431/14/NCBR/2018
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