Investigation of electrical properties of bipolar membranes at steady state and with transient methods

Bipolar membranes (BPM) are made of conjoined cation and anion exchange films. Under applied reverse polarisation these BPM manifest the property of dissociating water into hydrogen and hydroxide ions. The electrochemical behaviour of the two commercially available BPM was investigated in a symmetric electrolyte system (NaCl-NaCl) and an asyntmetric acid-base system (NaOH-HCl) over a large domain of current densities up to 2 kA m(-2). Studies were performed using galvano-potentiometry (i/V), impedance spectroscopy and transient chronopotentiometric measurements. The temperature dependence of i/V curves enabled determining the activation energy related to the overall electrochemical process. The experimental results lead to suggest a model according to which the water splitting reaction occurring in the region of the BPM junction is very fast and remains reversible even under the imposition of large current densities. On account of the large exchange current which characterises this water dissociation mechanism in the presence of catalysts, the membrane overpotential appears to be unaffected by the reaction rate. The value of the overall resistance, if corrected for leakage current effects, is constant over a large domain of current densities. This resistance reflects essentially the properties of the hydrogen and/or hydroxide ion transport process across the cation and anion exchange films, respectively. The progressive establishment of a counter-ion steady state concentration independent of the current in both these films and depletion of salt in the junction brings about the deviation of i/V curves from linearity at low current densities. (C) 2001 Elsevier Science Ltd. All rights reserved.