Experimental and theoretical investigations of steady and transient states in systems of ion exchange bipolar membranes

Abstract

The electrochemical characteristics of one commercial bipolar ion exchange membrane and of two home-made bipolar membranes are investigated over a range of current densities up to 2 kA m −2. Studies are performed using galvano-potentiometry ( i/ V) and impedance spectrometry methods. The temperature dependence of i/ V curves enables the determination of the activation energies related to the overall electrochemical process of H + and OH − production by water dissociation at the membrane junction. The physical analysis of the experimental data is made on the basis of a neutral layer model for the membrane junction. The theoretical treatment leads first to establish a thermodynamic framework insuring the validity of the criteria used in the interpretation of the results in terms of the model. Application of current electrochemical kinetic concepts at steady state involves the idea that, in the presence of an efficient catalyst, a quasi-reversible state of the water dissociation reaction may be achieved at the junction. A theoretical approach is developed for treating the data obtained with transient measurements in absence of co-ion transport. This study reveals the intrinsic roles played in the overall process of respectively: (a) the H + and OH − ion transport; (b) the electrical double layers at the membrane junction boundaries; and (c) the chemical mechanism of water dissociation.