Membrane potential and impedance studies of polyaniline composite membranes: Effects of membrane morphology

Electronic and ionic transport characteristics of PANI composite porous membranes are presented in this study. Microporous cellulose ester membranes were modified by depositing polyaniline (PANI) either as a surface layer or as a layer on the pore walls of the base membrane. Various in situ chemical oxidative polymerization techniques were used that yielded distinctive PANI deposition sites in the base membranes. PANI intercalation levels in the composite membranes were evaluated using gravimetric method whereas the deposition site was characterized by scanning electron microscopy (SEM). Among the employed polymerization techniques, diaphragmatic polymerization of aniline using a two-compartment cell deposited PANI inside the membrane bulk with PANI content as high as 50% (w/w). Other techniques including solution-phase and vapor-phase polymerizations deposited PANI on the membrane surface with little in-bulk deposition. Electrochemical characterization of the membranes was conducted by electrochemical impedance spectroscopy (EIS) and membrane potential measurements in a free-standing membrane configuration. The charge transport processes showed a strong dependence on PANI deposition site in the base membrane along with PANI intercalation content. The transport processes in the membranes were modeled using equivalent circuit modeling technique. The composite membranes that had PANI layer at the surface showed highly resistive and diffusionally controlled charge transport. In contrast, in-pore PANI deposited membranes showed several orders of magnitude lower resistance and higher capacitance, both, were attributed to the redox transitions of PANI during the charge transport process. Membrane potential studies showed an anion-selective transport coupled with the proton conduction through the composite membranes. This study indicates the possible development of an electroactive porous membrane where the selective transport can be achieved through the electrochemical and electrostatic interactions of permeated species with in-pore deposited PANI layer.