Percolation effects in metal-cation-exchange membrane nanocomposites

Abstract

121 Modification of ion exchangers (granules, mem branes, fibers) by an inorganic component (metal, salt, oxide) is used for imparting new properties to the initial matrix or improving the existing ones [1–3]. If a dis perse material in introduced into an ion exchanger, the forming metal–ion exchanger composites have as many as two sorts of spatially separated active sites: metal particles and ionogenic groups of the matrix [3– 5]. The bifunctionality of such materials allows one to use them as catalysts of chemical and electrochemical reactions, electrode sensors, and highly reactive mate rials. The electronic conduction of nanocomposites, which determines their electrochemical activity, is a function of the metal content [6, 7]. In this work, by chemical deposition of a metal (copper, silver) into heterogeneous sulfo cation exchange membrane MK 40, nanocomposites with bulk distribution of metal particles were obtained. Multiple metal redeposition gives rise to a percolation threshold of specific electronic conduction of the composite. The number of deposition cycles necessary for the emergence of the percolation threshold is determined by the nature of the metal. It was shown for the first time that, in the course of the formation of a percolation cluster of metal particles in the ion exchange membrane, the boundary potential of the membrane, which characterizes its ion exchange properties, undergoes a step change. The purpose of this work was to study the depen dence of the electronic conduction and the equilib rium boundary potential of nanocomposites on the content of the metal component. Nanocomposites with different metal (Cu0, Ag0) contents were synthesized by cyclic alternation of ion exchange saturation and chemical reduction of metal ions according to a published procedure [5]. The matrix was heterogeneous styrene–divinylbenzene sulfo cation exchange membrane MK 40. Figure 1 presents scanning electron microscopy images of nanocomposites Cu0 · MK 40 (panel (a)) and Ag0 · MK 40 (panel (b)) after three cycles of metal deposi PHYSICAL CHEMISTRY