Novel three-dimensional electrodes: Electrochemical properties of carbon nanotube ensembles

We report on the electrochemical study of three carbon nanotube ensemble electrodes: single-walled nanotube paper, as-produced multiwalled nanotube towers, and heat-treated multiwalled nanotube towers. Cyclic voltammetry was used in 0.10 M KCl containing 5.0 mM K4Fe(CN)(6) to obtain information on both the capacitive background and electron transfer rate from the faradaic reaction of the redox species. The capacitance gives insight into the effective surface area (including both the exterior surface and the interior surface within the filth) as well as the "pseudocapacitance" due to faradaic reactions of surface bonded oxides. We found that a large portion of the carbon nanotube surface of the as-produced multiwalled nanotube tower electrode was blocked by amorphous carbon. It can be largely removed by prolonged heat treatment. Among the three types of samples, the single-walled nanotube paper electrode presented the largest volume specific capacitance, consistent with its highest carbon nanotube packing density (i.e., largest effective carbon nanotube surface area). The redox reaction of Fe(CN)(6)(3-)/Fe(CN)(6)(4-) was found to occur not only at the outer surface of the carbon nanotube film but also at the interior surface of the nanotube ensemble electrodes. This indicates that the carbon nanotube ensemble behaves as a three-dimensional electrode. The apparent electron transfer rate (as indicated by the redox peak separation) was found to correlate to both the area of the exposed side-walls (with graphite basal-plane-like properties) and the density of graphite edge-plane-like defects.