Diffusion behaviour of the BF4– anion in polyaniline fibrils. Influence of the physical state of polyaniline

Abstract

The diffusion coefficients (D) of BF₄^– in polyaniline at various electrode potentials have been estimated by electrochemical potential-step experiments assuming a cylindrical diffusion system. The D values were constant in the electrode potential region from 3.9 to 3.6 V and also from 3.2 to 2.7 V. However, they were much smaller than those obtained in the potential region from 3.9 to 3.6 V. The diffusion coefficients were independent of the direction of the potential step (doping or undoping). The observed current–time transient in the potential region from 3.9 to 3.6 V was in good agreement with the theoretical one, but a deviation was observed for the potential region from 3.6 to 3.2 V. The concentration profiles of three physical states of polyaniline with the electrode potential were obtained from absorbance changes of polyaniline during the doping or undoping processes of BF₄^–. By comparing these profiles with results obtained from the potential-step experiments, it can be seen that the deviation of the observed current–time transient from the theoretical one in the potential region between 3.2 and 3.6 V is due to a change in the physical state of polyaniline.