Characterization of the electrode/electrolyte interphase of the electronically conducting polymer, polypyrrole

Abstract

An investigation was made to characterize the structure of the interphase of polypyrrole in terms of semiconductor theory and the molecular structure of the polymer. This investigation was carried out by performing experiments associated with four major aspects of the interphase at polypyrrole electrodes. These included physical and chemical characterization of the surface, measurement of the electronic properties, measurement of the double layer capacitance, and evaluation of the electrocatalytic performance of polypyrrole electrodes. A solute adsorption technique was used to analyze the surface area, and indicated that polypyrrole contains 6.2x10^11 cm^-2 pores of 5.0 A radius. These pores contribute to an observed roughness factor of about 465. When these pores are excluded, polypyrrole has an observed roughness factor of about 100. Depth profiling by X-ray photoelectron spectroscopy indicated an orientation effect of the pyrrole rings at the surface, with the nitrogen ends of the pyrrole rings pointing outward. Hall effect experiments showed that polypyrrole has carrier densities which range from 1x10^19 to 3x10^20 cm^-3 and carrier mobilities on the order of 0.9 cm^2V^-1s^-1. The carrier densities vary with the identity of the counterion, while the carrier mobilities do not. This effect is interpreted in terms of the charge density located on the counterion. The space charge capacitance and double layer capacitance were obtained by impedance spectroscopy. A Mott-Schottky plot indicated that polypyrrole is a p-type semiconductor, and has a flat band potential of -0.23 V vs. NHE. The double layer capacitance was found to be 37 μFcm^-2 when the electrochemically active area (RF-100) was taken into account. Surface states were observed by impedance spectroscopy. These tend to increase in density as the potential is made less positive, reaching a maximum observed value of 8.1x10^10 cm^-2 at 0.34 V vs. NHE. A molecular mechanism is proposed by which these states exist and interact with the electrode. Electrode kinetic experiments indicated that the electrocatalytic activity of polypyrrole decreases as the pH is increased from 1 to 5. This is interpreted in terms of a deactivation process due to deprotonation of the nitrogens located at the surface of polypyrrole.