The influence of protons on the impedance of polyaniline films

The impedance of polyaniline (PAni) films at gold electrodes in aqueous sulphate solutions is studied in dependence on the pH of the solution. The existence of an "impedance pit" in acidic solutions is found to be characteristic for the oxidised PAni film; both impedance components (R as well as 1/omega C) are nearly independent of the electrode potential in this case. At infinite frequency the electrolyte resistance R-El and a second term, the film resistance R-F, contribute to the total resistance R (infinity). Both contributions depend on the pH of the solution. The film resistance R-F exhibit different values for the oxidised and the reduced state of the PAni, R-F is very low for oxidised PAni in acidic solution (pH < 1). It is found that the electrical conductivity of reduced PAni depends on the pH of the solution as well, indicating that protons participate in the conduction of reduced PAni. The frequency dependence of R and C of PAni films is shown for different electrode potentials. The influence of the proton concentration of the solution on the electrical properties of PAni is discussed. Both the initial oxidation of the PAni an acidic solution and the decrease of the pH of the solution in contact with oxidised PAni are accompanied by a strong increase of the capacitance, obviously caused by protonation equilibria and changes of the double layer. (C) 1997 Elsevier Science Ltd.