Decoding transport selectivity of ions in polymer membranes by In-situ impedance spectroscopy

ABSTRACT Depending upon charge and chemical affinity, interplay between resistances of membrane (Rmem) and membrane–solution interface (RHT) may lead to preferential transport of an ion. Here, intermittent in-situ electrochemical impedance spectroscopy (EIS) was done during electrodriven transport (radiotracer based) to analyze the transport selectivity of Cs+ over Na+ in different membranes. Preference of the membranes for Cs+ was reflected in the time-dependent Nyquist plots itself. Bode plot analysis also indicated dominant Cs+ transport in terms of phase and frequency shift in crown ether (DB21C7) based membrane. In Cs+ selective polymer incluion membranes, irrespective of carrier, RHT contributed majorly to overall resistance. However, time dependence of RHT/Rmem was carrier as well as ion dependent. Interestingly, for nonselective ionic carrier, RHT/Rmem was majorly close to 1 and a reverese transport order than previous membranes were obtained. A higher Na+ transport (than Cs+) was also obtained for DB21C7 loaded Nafion, where, due to ion templating effect, Rmem was the governing factor. EIS spectral nature of a mixed feed solution follows that of the most preferred ion, thus suggesting that EIS can be used to study prospective real-life systems and can be used as a significant tool in designing ion-selective membranes.