Current-voltage curves of bipolar membranes

Abstract

Bipolar membranes consist of a layered ion-exchange structure composed of a cation selective membrane joined to an anion selective membrane. They are analogous to semiconductor p-n devices as both of them present current-voltage curves exhibiting similar rectification properties. In this article, we present some current-voltage curves obtained for different-bipolar membranes ate several temperatures. The results can be interpreted in terms of a simple model for ion transport and field-enhanced water dissociation previously developed. The mechanism responsible for water splitting is assumed to be a catalytic proton transfer reaction between the charged groups and the water at the membrane interface. The effects of temperature are taken into account by introducing an Arrhenius-type relationship for the dependence of the forward rate constant of the reaction on temperature. Finally, comparison between theory and experiments provides reasonable values for the parameters introduced in the theoretical model. The analysis aims at developing a better physical understanding of a process in which chemical reactions and transport phenomena are coupled in such a way that the potential technological applications depend strongly on this coupling.

Modeling of Forward and Reverse Bias Conditions in Bipolar Membranes

“…Despite the complex phenomena which result from the coupling of ion transport and chemical reactions, a rather simple approach explained reasonably well the experimental trends observed in the I-V curves: high conductivity under forward bias conditions and high impedance first, and then field enhanced water dissociation for high enough applied voltages under reverse bias conditions. Comparison between theory and experiments [7,14] provided reasonable values for the parameters introduced in the model. Also, the temperature effects on the I -V curves were satisfactorily accounted for [7,14].…”

“…Comparison between theory and experiments [7,14] provided reasonable values for the parameters introduced in the model. Also, the temperature effects on the I -V curves were satisfactorily accounted for [7,14]. However, other effects not explicitly included in the model (see points ii)-v) above) can also play an important role in the bipolar membrane characterization.…”

Electrochemical mechanism of ion current rectification of polyelectrolyte gel diodes

“…These results imply that the conservation of the number of counterions plays an essential role in suppressing electric currents in reverse applied voltages. Our results are effective for relatively small applied voltages (for example, the experiments in refs 8,9), where the dissociations of water molecules by electric fields at the interface between the two gels are negligible 14,15 (in practice, the onset of water dissociations is sensitive to the chemical properties of polyelectrolyte gels 16 ).…”

“…The ionic groups of polyelectrolyte gels are essentially immobile (and thus are treated as uniform fixed charges), and, in contrast, their counterions freely diffuse in the entire volume of the system; our model thus extends the treatments of these ions that have been developed in soft matter physics [17][18][19][20][21][22][23] to electrochemical systems. Analogous models are used to treat electrolyzers [13][14][15]24,25 and fuel cells [26][27][28] that use polymer electrolyte membranes. We treat the cases in which counterions are not the reactants or products of electrochemical reactions.…”

“…Nanochannels of asymmetric shapes were shown to rectify ion currents that are generated by Ag/AgCl electrodes [3][4][5][6] . Polyelectrolyte gel diodes (PGDs) that are the double layers of two oppositely charged polyelectrolyte gels (that are swollen in aqueous solutions), sandwiched by two symmetric metal electrodes, are another type of ionic diodes that rectify ion currents [7][8][9][10][11][12][13][14][15][16] . These diodes are designed to operate with a physical mechanism that is analogous to conventional semiconductor diodes-the asymmetry in the permeability of ions across the interfaces between the two oppositely charged gels [8][9][10][11][12] .…”