The impact of membrane orientation on ion flux in bipolar membranes

Bipolar membranes (BPMs) are asymmetric, layered ion exchange membranes that are increasingly being explored for use in electrochemical devices. This study aims to investigate the effect that the direction of a diffusive driving force across a salt solution concentration gradient has on the flux through a BPM due to its asymmetric nature. BPMs were fabricated using PiperION poly(aryl piperidinium) A40 as the anion exchange layer (AEL) and Nafion 212 as the cation exchange layer (CEL) with no interfacial junction catalyst. The permeability of sodium chloride through the BPM membranes was measured across a 0.5molL-1 concentration differential for both orientations of the membrane, e.g. AEL facing the 0.5molL-1 NaCl solution versus the CEL facing the 0.5molL-1 NaCl solution. A flux differential of (76.3 ± 4.8)% was measured for the BPM depending on the direction of the driving force. A model based on Fick’s law and the Donnan equilibrium was developed and used to show that the flux differential results from changes in the ionic environment at the AEL–CEL junction due to differences in the ion diffusion coefficients and fixed charge concentrations of the two layers. [Display omitted] •The diffusive ion flux through a bipolar membrane depends on the orientation of the membrane relative to the concentration potential.•A model to describe the ion concentration profile and flux through a bipolar membrane was developed.•The flux differential is caused by differences in the fixed charge concentration, as well as differences in the ionic diffusion coefficients between the two ion exchange layers.•A flux differential of 76.3% was observed for a bipolar membrane made from Nafion 212 and PiperION A40 in a 0.5molL-1 sodium chloride concentration cell.