Chronopotentiometry for the advanced current-voltage characterisation of bipolar membranes

Compared to steady-state current-voltage curves, chronopotentiometric measu rements allow us to distinguish the contributions to the overall electric p otential difference across a bipolar membrane. In this paper, the character istic values of the electric potential difference across the bipolar membra ne at different times are correlated to the corresponding concentration pro files in the bipolar membrane layers and the ion-transport processes are id entified. For over-limiting current densities (i.e. current densities above the limiting current density), it is possible to distinguish the reversibl e and irreversible contributions to the steady-state electric potential dif ference. The irreversible contribution is attributed to the energy required to overcome the electric resistance whereas the reversible contribution co rresponds to the electrochemical potential due to concentration gradients i n the membrane layers. Further, the ohmic resistance of the membrane in equ ilibrium with the surrounding solution has been compared to the resistance in the transport state. For low current densities, the equilibrium resistan ce is lower than the transport resistance stemming from internal concentrat ion polarisation. In contrast, the large numbers of hydroxide ions and prot ons 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 c urrent densities. With the possibility to split the steady-state potential into its contributions, bipolar membrane chronopotentiometry is a useful to ol to identify transport limitations and to improve bipolar membranes for a reduced overall electric potential. (C) 2001 Elsevier Science B.V. All rig hts reserved.