Comparison of bipolar membranes by means of chronopotentiometry

Chronopotentiometry is used as a tool to investigate the transport processes in and the energy requirements of different bipolar membranes under water splitting conditions. The bipolar membranes studied are the BP-1 from Tokuyama Corporation, Japan, the MB-3, the AQ-6 from Aqualytics, and a modified sample of the WSI bipolar membrane. In accordance with the predictions from phenomenological transport equations, the switch-on transition is faster for highly selective membranes, however, also the interface structure (roughness, effective contact area) has to be considered to interpret the results unambiguously. In general, the switch-off behaviour shows a faster relaxation for the less selective membranes. Some bipolar membranes show a remarkable phenomenon: a second charging and discharging time is observed, indicating asymmetric transport behaviour of the membrane layers. For bipolar membranes with firmly attached layers, the chronopotentiometric curves allow to separate the theoretically necessary electrical energy (the reversible contribution) from the energy lost by dissipation (the irreversible contribution). The reversible part is recorded at current switch-off after steady state operation. The ratio of the two contributions indicates that in the series of MB-3, AQ-6 and BP-1 the membranes show an increasing energy efficiency. This is also the order of increasing selectivity, as indicated by the limiting current density. For the WSI membrane, the transport processes and also the chronopotentiometric response curves are different because the layers are not permanently attached. Our results indicate that chronopotentiometric series can be used for detailed comparison of bipolar membranes.