Sodium Ion Diffusion in Nafion® Ion Exchange Membranes

Abstract

Development of membrane chlor-alkali cells during this past decade represents a major advancement for commercial electrochemical technology. Chemical engineering applications of separation processes such as this involve diffusional mass transfer that can be treated as a rate process. The chlor-alkali application includes sodium, chloride, and hydroxyl ions plus water as mobile species in caustic soda and brine solutions and also between immiscible membrane and solution phases. These immiscible phases are brought into contact to allow selective transfer of sodium and water from brine to the caustic solutions. Previous data reported on NaOH diffusion through Nation® membranes, Dnboh, are difficult to interpret because not only do they involve hydroxide and sodium ion fluxes to give average diffusion coefficients, but also unknown gradients of electrolyte and water concentration are present in the membrane phase. This present work greatly simplifies these problems encountered by isolating and measuring the precise sodium ion self-diffusion coefficient, DNa+, with radio-tracer techniques in various du Pont Nation® and EDA modified membranes. It then relates the DNa+ to equivalent weight, surface treatment, and fabric backing in these membranes. These data for DNa+ are very important in chlor-alkali cells because the sodium ion is the major current carrier; therefore, its value can be related to the relative activation energy and voltage drop among similar membranes. © 1980, The Electrochemical Society, Inc. All rights reserved.