Can conductivity measurements serve as a tool for assessing pseudocapacitance processes occurring on carbon electrodes?

The dependence of the electronic conductivity of carbon electrodes on their potential while being immersed in acidic solutions was studied. The precursors for the carbons studied were Kapton polymer, cellulose, and cellulose that was carbonized in the presence of ammonium chloride (at 1000 °C), followed by a mild oxidation process (CO₂ at 900 °C) for various periods of time, in order to obtain carbons of large specific surface areas. Unlike the case of neutral electrolyte solutions, which show a parabolic dependence of the carbons' conductivity on the electrodes' potential, the dependence of the conductivity on the potential of carbon electrodes immersed in HCl solutions exhibits inverted parabolic-type dependence. Carbons of different surface chemistry were prepared in order to determine the main cause for this unusual behavior. It was found that when nitrogen heteroatoms are present in the carbons' structure, pseudofaradaic reactions occur, causing this phenomenon. When the nitrogen-containing functional groups on the carbon surface undergo redox reactions, the conjugated system adjacent to these nitrogen atoms changes and the carbon system loses highly conductive electronic arrangements. Therefore, upon the anodic or cathodic polarization of nitrogen-containing carbon electrodes, the nitrogen-containing groups are oxidized or reduced, respectively, and the electronic conductivity of these electrodes decreases accordingly.