No abstract

The closed topology and tubular structure of carbon nanotubes [1±3] make them unique among different carbon forms and provide pathways for chemical studies. A number of investigations [4±8] have been carried out to find applications of nanotubes in catalysis, hydrogen storage, intercalation, etc. Since carbon-electrode-based fuel cells have been experimented with for decades, it is of importance to learn the electrodic performance of these new carbon structures. We report here results of the electrocatalytic reduction of dissolved oxygen (important H 2 ±O 2 fuel cell reaction), using microelectrodes constructed from multiwalled nanotubes. In parallel, ab initio calculations were performed for oxygen deposited on the lattice and defect sites [9] of nanotube surfaces to determine the charge transfer during oxygen reduction and compared with similar reactions on planar graphite. The microelectrodes were constructed in the following way (see Fig. 1). Multiwalled nanotubes (10 mg) prepared by the electric arc discharge process [3] and liquid paraffin (4 mL) were intimately mixed, placed in the narrow cylindrical slot of a Perspex holder and then packed by smooth vibration. The assembly was cured at 50 C for 30 min.From the inner side of the Perspex, contact to a copper lead was made through conducting paint. Carbon paste electrodes (based on commercially available graphite powder) were prepared similarly. Carbon nanotube electrodes were prepared earlier by similar techniques to probe bioelectrochemical reactions.[8]The need for oxygen reduction at catalytic surfaces has been recognized in fuel cells, batteries, and many other electrodic applications.[10±16] Hence, oxygen reduction at nanotube surfaces is of great interest. Electrochemical reduction of dissolved oxygen is carried out in aqueous acidic (H 2 SO 4 ) and neutral media (1 M KNO 3 ). The solution is first degassed by bubbling nitrogen gas for about 15± 30 min in order to record the background current±voltage curves. Under these conditions, no cyclic voltammetric peak in the potential range 0 to ±0.8 V were observed. The same solutions were then saturated with oxygen by bubbling oxygen gas for 15 min. The cyclic voltammetric curve showed a well-defined peak at E pc = ±0.31 V vs. SCE (saturated calomel electrode) in H 2 SO 4 solution (pH 2) at the carbon nanotube electrodes. At the carbon paste electrodes only an ill-defined peak is seen at E pc = ±0.48 V. In the KNO 3 medium (pH 6.2), the reduction of dissolved oxygen is observed at E pc = ±0.51 V at the carbon nanotube electrode. This peak is shifted at the carbon paste electrode by about 30 mV. The shift of the peaks, corresponding to the reaction on the nanotube electrodes, is a strong indication of the electrocatalysis on this electrode (see discussion below). The shift may be considered as an overpotential, which indicates a more facile reaction occurring at the nanotubes compared to other carbons. The electrochemical reduction of oxygen is a function of pH of the medium [10,11] as proton participation...