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

Depending upon charge and chemical affinity, interplay between resistances of membrane (R_mem) and membrane–solution interface (R_HT) 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, R_HT contributed majorly to overall resistance. However, time dependence of R_HT/R_mem was carrier as well as ion dependent. Interestingly, for nonselective ionic carrier, R_HT/R_mem 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, R_mem 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.