Charge selective electrochemistry at high-surface area carbon fibers

Carbon fibers (DuPont E120) subjected to high anodic current or potential u ndergo fracture whereby the capacitance can increase dramatically by 2 or 3 orders of magnitude. The capacitance, assumed to correspond to an increase in the surface area, increased from 2 to 8 to 1000-5000 mu F/cm(2), as nor malized to the original fiber area. The calculated area-to-volume ratio is greater than 10(6) cm(2)/cm(3), based on an observed diameter of 20 mu m fo r a 2-cm-length fractured fiber. Interestingly, the shape and size of the c yclic voltammetric (CV) waves also changed dramatically, depending on the s olution pH and the nature of the electroactive species. The pH dependence i s related to the pK(a) of the carbon-oxygen groups, such as carboxylic acid s, which can be deprotonated when pH > pK(a) to produce a negatively charge d surface. Thus, negatively charged species such as ferro-/ferricyanide sho w small CV waves whereas, dopamine, which is positively charged, produces l arge peak-type waves. These observations support a model in which the penet ration into the interior and its micropores by electroactive species is con trolled by its charge and solution pH, which governs the charge on the carb on surface.