AC-IMPEDANCE SPECTRA OF BIPOLAR MEMBRANES - AN EXPERIMENTAL-STUDY

A bipolar membrane (BM) is composed of one cation and one anion ion-ex change layers joined together in series. In order to obtain the AC ele ctrical impedance of a BM, a small sinusoidal current perturbation was superimposed to the DC current, and the resulting frequency-dependent impedance spectra were recorded under different conditions of electri cal polarisation and temperature for five BMs. The experimental spectr a were measured in three current ranges: below the limiting current re gion, at the onset of the overlimiting region and in the electric fiel d enhanced water dissociation region. This allows for a better underst anding of the contributions of the salt and water ions to the measured impedance spectra. Measurements of the impedance of the forward biase d membrane were also carried out. Although the experimental impedance spectra appear to be in qualitative agreement with previous theoretica l models incorporating the effect of the electric field enhanced water dissociation, a quantitative analysis of the results is not still pos sible due to the high number of parameters involved. (C) 1998 Elsevier Science B.V, All rights reserved.