Electroanalytical characterization of electrochemical capacitor systems

Herein we report on an in-situ electroanalytical technique to investigate electrode-electrolyte behaviour during cycling in a conventional symmetrical electrochemical capacitor system based on an activated carbon (AC) electrodes with 1 M tetraethylammonium tetrafluoroborate (TEABF₄) in acetonitrile (ACN) as the electrolyte. The electrochemical cell is based on a Swagelok T-junction, with the perpendicular port being used for a Ag/Ag⁺ (0.01 M AgNO₃ in ACN) reference electrode. Situated between the opposing AC electrodes, between two separators, is a Pt sensing electrode, the potential of which is either monitored or controlled to explore either electrolyte fluxes during cycling, or the formation of soluble species in the electrolyte, respectively. It was deduced that electrolyte ions are strongly associated with the AC electrodes rather than between the electrodes even without an applied potential. Furthermore, aging of the electrochemical system with cycling (cyclic voltammetry) is also explored. Even with identical electrodes it is noted that the individual electrodes behaved differently as a result of anion versus cation association. Finally, at cell voltages approaching 2 V, oxidation of the positive electrode is observed, liberating species into the electrolyte. The implications of these observations have been discussed.