On the need for simultaneous electrochemical testing of positive and negative electrodes in carbon supercapacitors

Abstract

The lack of uniformity in the experimental conditions used for testing carbon supercapacitors by different authors highlights the need for standardized procedures to evaluate their performance. This study demonstrates the usefulness of a three-electrode arrangement in the investigation of electrochemical processes on both electrodes of double-layer carbon/carbon supercapacitors in a T-type sandwich-designed cell by performing simultaneous electrochemical measurements of positive and negative electrodes in six different acetonitrile-containing electrolytes. The testing employed electrochemical impedance spectroscopy, cyclic voltammetry, and constant current charging/discharging measurements. The results demonstrated that the impedance measurements were highly sensitive to the experimental arrangement and that the impedance data were distorted due to geometrical and electrical asymmetry within the cell. Such asymmetries did not affect cyclic voltammetry and constant current charging/discharging measurements, and the results proved valuable in the simultaneous investigations of both electrodes in supercapacitors. The study detected different behavior of anion and cation adsorption in the porous structure of carbon electrodes. While quaternary ammonium cations exhibit constant and even slightly increasing specific capacitances during successive cycling, specific capacitances of BF4− and PF6− anions exponentially decrease. Irreversible adsorption of anions inside the positive electrode explained such behavior. Overall, the results show the viability of a three-electrode arrangement in the investigation of electrochemical processes on both electrodes in supercapacitors and batteries. The results confirm that tetraethylammonium tetrafluoroborate is a superior electrolyte for carbon/carbon supercapacitors, while lithium tetrafluoroborate does not have adequate properties in acetonitrile for use in electrochemical power sources.