The role of ionic conductivity and interface in electrical resistance, ion transport and transmembrane redox reactions through polyaniline membranes

Three different physico-chemical properties of HCl-doped polyaniline (PANI) membranes, separating two aqueous solutions are compared. The first one is electrical conductivity measured with electrochemical impedance spectroscopy (EIS). The second one is H+ ion permeation through the membrane under pH gradient, measured with pH electrodes, and the third is electron/ion coupled counter transport in a transmembrane redox reaction, measured with redox electrodes. Electrophysical properties of undoped membrane are not described by the Ohm's law. The non-linear dependence is due to a limited content of mobile electrons and is similar to the Child's law known for vacuum diodes and dielectrics. Although the impedance of the doped membrane separating aqueous solutions is much less than that for the undoped membrane, it is determined by ions and not by electrons. For doped membranes the ratio of ion diffusion coefficient and charge drift mobility, determined in the external electrical field, is close to the Einstein relationship, meaning that the same ions play the key role in both processes. Thickness dependent behaviour is used to differentiate the interfacial properties from those of the bulk properties of the membrane. Interpolation of properties to zero membrane thickness shows that the interfacial charge transport plays an important role in doped membrane impedance and influences ion and redox transport rates. The relative role of interface versus volume increases with acid doping, which makes the bulk membrane volume more permeable for ions.