Electrochemical Activation of Glassy Carbon: Spectroscopic Ellipsometry of Surface Phase Formation

The surface layer formed on glassy carbon (GC) electrodes during electrochemical activation was studied for the first time by in situ spectroscopic ellipsometry in 1M for two different activation procedures. The activation procedure consisted either of a certain number of potential cycles between −0.5 and 1.86 V (SCE) with a scan rate of 150 mV/s or of applying an anodic potential of 1.86 V (SCE) for a certain time. The spectroscopic ellipsometry results can be well described by assuming a porous hydrated surface film, consisting of a mixture of GC and electrolyte on top of the GC substrate. The film thickness increases monotonically with the duration of the activation process. Cycling of the electrode yielded films with constant void fraction (electrolyte) of about 80% while constant potential oxidation gave films with increasing void fraction between 0% and 70%. Upon removal from the electrolyte the surface film collapses which is evidenced by a significant decrease in thickness and void fraction. On the basis of our ellipsometry results a diffusion coefficient of can be determined for the intrinsic redox processes in the activated surface layer. A charge density of 250 Ah/dm³and 210 Ah/kg can be calculated for the active layer on activated glassy carbon electrodes, which is of importance for potential battery or capacitor applications.