Electroanalytical Characterization of Electrochemical Capacitor Systems

Abstract

This study aims to investigate the behaviour of the electrode materials during charging and discharging processes using an indicator electrode. Generally, the capacitive behaviour of the electrochemical capacitors is associated with the electrical double layer at the surface of the electrode, and in some materials pseudo-capacitance, which results from redox processes. More importantly, it is possible that some oxidized or reduced species which are soluble in electrolyte diffuse or migrate from one electrode to another electrode. Hence, an indicator electrode can detect these ions movement during the charging and discharging processes. In this study, each electrochemical cell has been made of two symmetrical activated carbon (AC) electrodes with the same mass of active material on each electrode. The AC electrochemical cells have been tested in the non-aqueous electrolyte (1 M TEABF4) and aqueous electrolyte (0.5 M K2SO4) using cycling voltammetry (CV) at 25 mV/s sweep rate between 0 and 2.3 V and 0 to 2 V respectively. It was followed by the step potential electrochemical spectroscopy (SPECS) experiment with a 10 mV potential step and 30 s of equilibration time. The SPECS method is based on applying a series of equal magnitude potential steps on a working electrode, with sufficient rest time to allow for equilibrium to be established for each step throughout an applied potential window. This slow scan rate allows the separation of the charge storage mechanisms, such as electrical double layer and diffusion-limited processes [1]. At the same time, the open circuit potential (OCP) of both electrodes have been measured with respect to the Ag/AgNO3 reference electrode during the CV and SPECS experiments. This technique allows one to see how an individual electrode behaves in the overall cell performance. Moreover, the electroanalytical characterisation of the AC electrochemical cell has been investigated using the indicator electrode. In this method, the platinum mesh was located in the middle of the two separated papers between two electrodes and the Pt mesh potential was clamped at -± 0.5 V with respect to the Ag/AgNO3 reference electrode during the CV and SPECS experiments. This new technique enables one to detect the migration of oxidised or reduced species from each of the electrodes during charging/discharging processes. In an ideal capacitor, this indicator electrode should have zero current; however, it has been observed that the indicator current changes at both positive and negative potentials during charge storage mechanism. This indicates the emitting and migration of redox species between the two electrodes. Also, it can be seen that there are different effects of aqueous and non-aqueous electrolyte on the behaviour of the redox migration. Reference: [1]. Marveh Forghani and Scott W. Donne, Journal of the Electrochemical Society 2018 165: A664-A673.