Advanced investigation of ion exchange membranes and their interfaces with liquid electrolyte solutions by electrochemical impedance spectroscopy

Introduction/Purpose: Ion exchange membranes (IEMs) have consolidated applications in energy conversion and storage systems, like fuel cells and battery separators. Moreover, in the perspective to address the global need for non-carbon-based and renewable energies, salinity-gradient power (SGP) harvesting by reverse electrodialysis (RED) is attracting significant interest in recent years [1]. For all these applications, one of the major concerns is the depth knowledge of the membranes and interfaces electric properties. Electrochemical Impedance Spectroscopy (EIS) is a powerful technique for measuring the electrical properties in the bulk and interfacial regions of solid and liquid materials in which coupled electrical processes occur at different rates [2]. Methods: A systematic EIS investigation on the effect of solution composition (the main mono- and multivalent ions present in seawater were investigated), concentration, temperature and flow rate, on the electrical resistance of anion- and cation- exchange membranes (AEMs and CEMs) and their interfaces, was carried out. Results: The ionic resistance of IEMs is influenced by the ionic strength of the contacting solutions because of the effect on the water volume fraction and membrane microstructure [3]. Moreover, the ions concentration profiles inside the membranes and at their interfaces are influenced by the ionic composition of the solutions through counter-ion exchange phenomena, as well as, co-ion diffusion in the case of a non-ideally permselective membrane [4]. The results highlighted a critical role of the Mg2+ on the ionic resistances of the CEMs while the resistances of the AEMs were less sensitive to multivalent ions. Conclusions: EIS investigation gave important insights about the effect of multivalent ions and electrolyte solution concentration on the transport properties of IEMs for energy conversion applications. The findings might assure one more step ahead in decision making for solution pretreatment and/or membrane design for for RED applications. Selected references: 1.Logan, B.E.; Elimelech, M. Membrane-based processes for sustainable power generation using water, Nature, 488 (2012) 313-319. 2.Barsoukov, E.; Macdonald, J. R. Impedance Spectroscopy. Theory, Experiment, and. Applications, Second Edition. John Wiley & Sons, New Jersey, 2005. 3.Fontananova, E.; Zhang, W.; Nicotera, I.; Simari, C.; van Baak, W.; Di Profio, G.; Curcio, E.; Drioli, E. Probing membrane and interface properties in concentrated electrolyte solutions, J. Membr. Sci. 459 (2014) 177-189. 4.Fontananova, E.; Messana, D.; Tufa, R. A.; Nicotera, I.; Kosma, V.; Curcio, E.; van Baak, W.; Drioli, E.; Di Profio, G. Effect of solution concentration and composition on the electrochemical properties of ion exchange membranes for energy conversion, J. Power Sources 340 (2017) 282-293. 5.Tufa, R.A.; Pawlowski, S.; Veerman, J.; Bouzek, K.; Fontananova, E.; Di Profio, G.; Velizarov, S.; Crespo, J.G.; Nijmeijer, K.; Curcio, E. Progress and prospects in reverse electrodialysis for salinity gradient energy conversion and storage, Appl. Energ. 225 (2018) 290-331.