Characterization of a Cation-Exchange/Polyaniline Composite Membrane

Abstract

A composite membrane was prepared by chemical polymerization of a thin layer of polyaniline (PANI) in the presence of high oxidant concentration on a single face of a sulfonated cation-exchange membrane (Neosepta CMX from Tokuyama Soda Inc.). The CMX−PANI composite membrane was characterized as a function of the polymerization time by cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), and ion-exchange capacity (IEC) measurements. CV and XPS results demonstrate that, under our experimental conditions, the maximum PANI loading is attained after 1 h of polymerization. This is accompanied by a decrease in membrane ion-exchange capacity due to the presence of the cationic PANI acting as counterions for the sulfonate groups of the membrane. The optimal membrane permselectivity for protons against bivalent metallic cations (Zn2+ and Cu2+) was also found at a polymerization time of 1 h. In fact, at longer polymerization times, XPS data indicate the occurrence of PANI degradation. UV−visible spectroscopy confirmed the presence of benzoquinone, the major degradation product of PANI, in the solution used for the oxidation of aniline into polyaniline. Correlating XPS and IEC results with permselectivity of the composite membrane let us believe that although some small amount of PANI is still present at the surface and within the membrane, it is not sufficient to maintain good permselectivity properties. To create an effective blocking barrier to the bivalent cations, the PANI layer formed at the surface of the CMX membrane needs to be sufficiently thick and uniform.