Carbon oxidation and its influence on self-discharge in aqueous electrochemical capacitors

The link between carbon oxidation and self-discharge is established for carbon-electrode, acidic-aqueous-electrolyte electrochemical capacitors. Using cyclic voltammetry, open-circuit self-discharge measurements, X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) a positive correlation is established between self-discharge and the rate of carbon oxidation. During oxidation, the Spectracarb 2225 carbon fabric is oxidized to CO2, and develops C-O and CO surface functionalities. The data show that C-O groups form initially, predominately during electrode charging; the CO groups form most significantly during the self-discharge. With repeated charge/self-discharge cycling, a decrease in self-discharge is observed, likely due to the depletion of active sites for carbon oxidation. Hence, eliminating these active surface sites, either through electrochemical oxidation or chemical oxidation, greatly reduces the self-discharge experienced by the positive carbon electrodes in aqueous electrochemical capacitors. The Spectracarb 2225 carbon exhibits ca. 50% less self-discharge when the carbon is fully oxidized (i.e., the surface sites are depleted) through electrochemical cycling versus the as-received carbon. We also show, for the first time, that surface site depletion results in longer plateau lengths and lower slopes on log-time self-discharge profiles; this is a useful diagnostic for identifying surface-site limited, activation-controlled reactions.