Chronopotentiometry for the advanced current–voltage characterisation of bipolar membranes

Compared to steady-state current–voltage curves, chronopotentiometric measurements allow us to distinguish the contributions to the overall electric potential difference across a bipolar membrane. In this paper, the characteristic values of the electric potential difference across the bipolar membrane at different times are correlated to the corresponding concentration profiles in the bipolar membrane layers and the ion-transport processes are identified. For over-limiting current densities (i.e. current densities above the limiting current density), it is possible to distinguish the reversible and irreversible contributions to the steady-state electric potential difference. The irreversible contribution is attributed to the energy required to overcome the electric resistance whereas the reversible contribution corresponds to the electrochemical potential due to concentration gradients in the membrane layers. Further, the ohmic resistance of the membrane in equilibrium with the surrounding solution has been compared to the resistance in the transport state. For low current densities, the equilibrium resistance is lower than the transport resistance stemming from internal concentration polarisation. In contrast, the large numbers of hydroxide ions and protons produced at high current densities result in a reduced ohmic transport resistance due to their high ionic mobility. This reduced resistance is not enough to stop the increase of the irreversible contribution with higher current densities. With the possibility to split the steady-state potential into its contributions, bipolar membrane chronopotentiometry is a useful tool to identify transport limitations and to improve bipolar membranes for a reduced overall electric potential.