Step Potential Electrochemical Spectroscopy (SPECS) Technique to Investigate the Ageing of Carbon Electrodes in Organic-Based Electrochemical Capacitors

Abstract

Current research on electrode degradation in electrochemical capacitors (ECs) mainly focuses on the application of new materials and the establishment of their stability on the basis of various standards and qualitative tests. However, there exists limited research that focuses on the elucidation and determination of the degradation mechanisms and pathways of carbon electrodes. The proposed study uses an innovative approach with the use of the Step Potential Electrochemical Spectroscopy (SPECS) technique, which allows one to quantify the contribution of different charge-storing mechanisms, and successfully track any changes that take place after accelerated ageing, including structural changes of the electrode in the organic medium. Additionally, the role of oxygen content on the degradation of ECs was explored.In this report carbon material, namely binder-free Activated Carbon Cloth (ACC) Kynol® 507-20 was prepared in a furnace under a nitrogen atmosphere at different temperatures, namely 120, 500, and 1000ᵒC. This is so that the effect of the presence of different oxygen functional groups on the surface of carbon could be investigated. The material was selected as a self-standing electrode to eliminate any influence the binder would have on the electrode porosity. Subsequently, these materials were prepared as electrode materials and assembled in a symmetric two-electrode setup with 1M TEABF4/AN (tetraethylammonium tetrafluoroborate in Acetonitrile solvent). In each cell, the potential range of both electrodes was first determined in a 2-electrode Swagelok® cell with RE using galvanostatic charge/discharge (1 A g-1). The further procedure involved 3 CV cycles (1 mV s-1) at a predetermined potential for each electrode in a 3-electrode set-up, followed by SPECS measurement where 5-minute potentiostatic hold with 10 mV potential steps was applied. The obtained current vs. time plots were used to calculate the corresponding capacitance, diffusion and residual parameters. Floating tests at 2.7 V were then carried out for each system. According to the international standard (IEC 62391-1), system failure is reported when the initial capacitance drops below 80% of its initial value; thus floating experiments were halted after that value was reached for each system or when the resistance of the cell doubled in value. The electrochemical investigation of the individual aged electrodes was then repeated in the same manner.Our study allowed for a thorough examination of individual electrodes prior to and after floating experiments, which helped to elucidate the degradation causes and pathways in organic medium.