Electrochemical and mass measurements during small voltage amplitude perturbations of conducting polyaniline films

The electrochemistry of conducting polyaniline (PANI) films has been studied in sulfuric acid solutions using a.c. impedance and cyclic voltammetry (CV), coupled with the quartz crystal microbalance (QCMB) technique. In normal full potential range slow sweep CV experiments, the fact that the mass-to-charge ratio increases markedly with potential in the positive scan, but is essentially constant during the negative scan, may be related to slow water penetration into the film during its oxidation. When only small potential amplitudes of ca. 25 mV or less are employed, the CV and QCMB measurements reveal that additional film sites can be initially oxidized, but not reduced, unless the potential is made more negative. This leads to an apparent loss in film capacitance when only narrow potential ranges are cycled. These results, together with increased hysteresis in the mass response with increasing sweep rate as well as the known asymmetries of the PANI CV response, suggest that given enough time, water can penetrate the film, hydrating some of the reduced PANI sites and facilitating their oxidation but not their reduction. The subsequent steady-state but lowered film capacitance must reflect the reversible oxidation/reduction of film sites which are not susceptible to delayed water transport and/or the stabilizing effects due to site hydration. (C) 1998 Elsevier Science S.A.