Comparative Study of Carbon and Conducting Polymer-Based Hybrid Electrochemical Capacitors Using Potassium Iodide Redox Electrolyte
Authors: Skunik-Nuckowska Magdalena , Lubera Justyna , Rączka Patryk , Dyjak Sławomir , Kulesza Pawel J. 
Published: July 2021

A key challenge for the existing electrochemical energy 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, safety and exploitation costs. 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 centres 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 should be fast and highly reversible, the rate of charging/discharging remains on a level expected for high power systems.
It is worth to emphasize that most of reported active electrolyte-based hybrid ECs rely on carbonaceous materials enabling low electrodes resistance, providing a well-developed surface area and a 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 the iodide-based active electrolyte. The performance evaluation is based on the comparison to the the system fabricated from the conventional high surface area activated carbon electrodes (AC). The structure, morphology and porosity of investigated materials are analysed with various physicochemical characterization methods (SEM, XRD, Raman, UV/Vis and N2 physisorption) and electroanalytical techniques. The electrochemical studies revealed that despite a low specific surface area of a PEDOT (S
BET equal to 40 m2 g-1), the gravimetric cell capacitance is similar to obtained for AC-based cell (S
BET=2600 m2 g-1). Interestingly, despite a lower potential window of PEDOT system (1.2 V vs. 1.5 V for the AC), the volumetric capacitance and energy is even 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 the 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 coulombic efficiency and the self-discharge rate, where the latter has been found to be significantly lower for PEDOT-type cell, with respect to the AC counterpart. The presence and the amount of undesired reaction by-products, i.e. iodates, is analysed ex-situ using electrochemical sensing methods. As a result, a mechanism of parasitic charge loss under open circuit, leading to self-discharge, is described and named as a “dual redox-shuttle effect”, mainly attributed to the 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.