F.G. Wilhelm

Cation permeable membranes from blends of sulfonated poly (ether ether ketone) and poly (ether sulfone)

Sulfonated poly(aryl ether ether ketone), S-PEEK, is blended with non-sulfonated poly(ether sulfone) (PES) to adjust the properties of ion permeable and ion selective membranes. In this study, membranes are prepared from blends with (i) a S-PEEK content between 10 and 100 wt.% using one S-PEEK batch with a fixed degree of sulfonation and (ii) from batches of S-PEEK with a different degree of sulfonation, but with a fixed S-PEEK content in the blend. The transparent membranes are permeable for ions with selective transport of cations over anions. At contents of S-PEEK below 40%, phenomena related to a percolation threshold of the ion exchange functionalities are observed; the measured ion exchange capacity (IEC) indicates that not all functional groups are accessible in these blends. The transport properties of membranes with a S-PEEK content in the range of 50?80 wt.% are comparable to those known for commercial ion exchange membranes. In this range, a trade-off between resistance and selectivity with increasing IEC is observed. Both, the ion conductivity and the co-ion transport number increase with increasing IEC. This is mainly caused by the increased water content with increased IEC and the number of water molecules per fixed charge.

Optimisation strategies for the preparation of bipolar membranes with reduced salt ion leakage in acid–base electrodialysis

The salt ion fluxes across commercial bipolar membranes (BPMs) result in the salt contamination of the produced acids or bases especially at increased product concentrations. Often, bipolar membrane electrodialysis can only be applied when these fluxes are reduced. Here, a model is presented to predict the salt impurities using the limiting current density measured for a single bipolar membrane. The model is extended to relate the limiting current density to the experimentally determined properties of the separate membrane layers. A direct dependence has been found for the salt ion fluxes across the bipolar membrane on the square of the solution concentration and the effective salt diffusion coefficient. Further, the salt ion transport is inversely dependent on the fixed charge density and the thickness of the layers. The latter is not trivial ? the thickness in general does not play a role in the selectivity of separate anion or cation exchange membranes. The dependence of the salt ion transport on the membrane layer properties has been verified experimentally by characterising membranes prepared from commercially available anion exchange membranes and tailor-made cation-permeable layers. The presented model has proven to be both, simple and accurate enough to guide bipolar membrane development towards increased selectivity.

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.

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.

Current-voltage behaviour of bipolar membranes in concentrated salt solutions investigated with chronopotentiometry

Chronopotentiometry is used as a tool to obtain detailed information on the transport behaviour of the bipolar membrane BP-1 in solutions of high sodium chloride concentration above the limiting current density. We discuss critically the interpretation of the observed transition times. The occurrence of two such polarization times for low to moderate current densities is explained by the membrane asymmetry: the two membrane layers of opposite charge in general have different transport properties such as co-ion concentration and diffusion coefficient. The reversible and irreversible contributions to the transmembrane potential can be distinguished which allows the bipolar membrane energy requirements to be addressed. The experiments indicate that the increased voltage drop across bipolar membranes observed with higher solution concentrations can be explained on the basis of stronger concentration gradients in the membrane layers. The gradients become stronger with increased current density, but here the ohmic resistance under steady state transport conditions (the transport resistance) contributes to the increasing electrical potential. The transport resistance decreases with increasing current density due to the ion-exchange of the salt counter ions with the water splitting products. The experiments show that bipolar membranes should be operated at low current densities and low concentrations to minimize energy requirements. These findings are in contrast to the high current densities required to reduce impurities in the produced acid and base.

The role of the salt electrolyte on the electrical conductive properties of a polymeric bipolar membrane

We have studied the contribution of the salt electrolyte to the electrical conductive characteristics of a bipolar membrane. We present first a critical analysis of previous theoretical approaches, and discuss the limits of validity. Experimental current-voltage curves of several commercial bipolar membranes in the low current density region are also shown. Special attention is paid to the measuring procedure and the data reproducibility. The effects of temperature, concentration of the salt electrolyte and flow rate are also considered. The comparison of theory and experiment shows the crucial role of the salt ions on the electrical conductive properties of the membrane, even when the water splitting phenomenon occurs.