The electrical double layer of high surface porous carbon electrode

Summary A new technique has been developed for the investigation of double layers on solid electrodes enabling the direct measurement of ion adsorption and the detection of the electrocapillary maximum. The method employs high surface area porous electrodes. The capacities of these electrodes were found to be similar to those of mercury without specific adsorption: This leads to the conclusion that the smallest pores are wetted by the solution and are active in the electroadsorption process. Hysteresis and initial irreversible changes of the various double layer parameters were found and were attributed to irreversibility in the electrooxidation and reduction of the carbon electrode surface investigated. The elecrocapillary maximum was found after obtaining a complete electrocapillary-like curve, e.g. a graph of dimensional changes of the electrode versus potential. The dimensional changes upon adsorption are known to be proportional to variations of the surface tension of the solid. The pH of the sodium chloride solution which served as the electrolyte in this investigation, increased on negative charging of the electrode and decreased on positive charging. This effect was attributed to the decomposition of water to OH− and H2 or to H+ and O2 respectively. These products are assumed to be both adsorbed and dissolved in the solution, resulting in the pH changes observed. The adsorption of the cation and the anion was directly obtained by following the concentration changes of the solution and was also calculated thermodynamically. In spite of the hysteresis observed, the calculated values fitted the experimental ones within a limited range of potential of 0.4 V.