| Graphitic carbons are ubiquitous in electrochemical applications because of their high conductivity and chemical inertness under harsh electrochemical conditions. For pseudocapacitors, the graphitic carbon often acts merely as a supporting, conductive substrate or additive for a pseudocapacitive transition metal oxide to achieve a high energy and power density. However, for thick metal oxide films the power can still be limited by ionic diffusion through the pores, low conductivity in the metal oxide itself, or slow electron transfer across the heterogeneous interface. Nanostructures overcome the first two of these power limitations because intrinsic nano-scale diffusion lengths make ionic diffusion and metal oxide resistance negligible. However, nano-scale thickness of the metal oxide does not improve the poor electron transfer at the graphitic carbon / metal oxide interface. Harsh oxidation (activation) of the graphitic carbon before depositing the metal oxide layer has been shown to improve the electrochemical performance of pseudocapacitors in bulk due to the electron transfer enhancing properties of the induced defects; however, little is known about the contributions of any single defect type because of the difficulty in precisely controlling the oxidation. In this paper an individual single-walled carbon nanotube (SWNT) - Manganese Oxide (MnO 2) nanostructure is used to study the graphitic carbon / metal oxide interface. An individual SWNT gives precise control of sidewall defects, and provides an opportunity to study a pristine and single-defect interface. This paper reports only on a pristine SWNT-MnO2 interface. First, a pulsed deposition method is demonstrated; it consists of a single nucleation pulse and multiple deposition pulses that give a conformal coating of MnO 2 on the pristine sidewall of the SWNT. These films have a minimum thickness of 10-40 nm, dependent on the duration of the nucleation pulse (10-100 ms), and they show a linear dependence in MnO2 growth with respect to the deposition pulse duration. It is found that deposition pulses between 0.6-0.7 V (vs. Pt) give the optimum cyclability and electrochemical performance. Also, MnO2 thicknesses between 150-350 nm give the fastest diffusion for current magnitudes above the background cable capacitance of the system. These optimized films are grown on both a SWNT (1 mum in length) and a Cr electrode (2 mum x 1 mum in area), and cyclic voltammograms (CVs) are taken at multiple scan rates. An equivalent circuit model is presented to fit the linear sweep voltammogram of the system, and a three parameter fit gives values for the interface resistance RCT , the pseudocapacitance C, and a leakage resistance RLeak. The pseudocapacitance from the fit matches well with the capacitance calculated from an independent method -- a good indication that the equivalent circuit model makes physical sense. Finally, the correctness of the model is discussed as well as ideas for future experiments. |
