Ion transport and water dissociation in bipolar ion exchange membranes

Abstract

This paper treats ion transport and water dissociation in “bipolar membranes”, consisting of juxtaposed cation and anion exchangers. Bipolar membranes are the close ion analogues of the semiconductor p—n junction and show similar, but not identical, current rectification behavior. The major difference between these two systems is that bipolar membranes contain a total of four mobile species. We attempt a simplified treatment which includes the flux of all four ions and find that, though the current is on the average carried by only two ions, which two they are depends on the voltage range in question. The flux of minority carriers, though relatively unimportant at small applied voltages, becomes crucial at high voltages, and at very high voltages the process of water splitting dominates. When the fluxes of all four ions are taken into consideration it is possible to predict qualitatively the experimentally observed current—voltage curves over the entire voltage range. We discuss the importance of symmetry and the restrictions of studying an idealized system. Suggestions for further work are included.