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 (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.

KEYWORDS:

• impedance spectroscopy
• selective transport
• polymer inclusion membrane
• crown ether
• Bode plot

Author contribution

S. C. formulated the initial project and conceived the experiments presented herein. A. M. M. and A. K. carried out the experiments. All subsequent data analysis and manuscript writing are by A. M. M. and S.C.

Novelty statement

In view of the continuous efforts to understand and improve the ion-selectivity, the novelties of this work are in the in-situ electrochemical impedance spectroscopy-based analysis of the transport parameters (here resistances of membrane and membrane-solution interface) leading to specific ion selectivity during electrodriven transport. Detailed comparison between different carriers and membrane morphology makes the analysis more general and establishes the applicability of EIS as an efficient characterization tool to design ion-selective membranes.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/01496395.2023.2219377.