(Keynote) Electrical Response and Conductivity Mechanism in Ion-Exchange Membranes

Abstract

Electrochemical energy conversion and storage systems are very important in today’s worldwide efforts to decarbonize the energy sector. Indeed, these systems exhibit a very high energy conversion efficiency, are easy to scale-up and are not affected by geographical constrains [1]. Two important examples of these systems are ion-exchange membrane fuel cells (IEMFCs) and redox flow batteries (RFBs). On one hand, ion-exchange membrane fuel cells are particularly suited for light-duty vehicles and small-scale stationary systems (e.g., auxiliary power units) owing to their high energy and power densities. On the other hand, redox flow batteries are ideally suited to the large-scale storage of energy for the power grid as they are characterized by a very high turnover efficiency and an outstanding cyclability [2].Electrolyte membranes (EMs) are present at the core of both IEMFCs and RFBs. The role of such EMs is to: (i) keep separated the electroactive species involved in the redox reactions taking place at the electrodes and, at the same time, (ii) ensure a facile and selective migration of ions to prevent the polarization of the device and allow for the establishment of large current densities. As of today, the most widely adopted EMs are based on perfluorinated ionomers such as Nafion™, SPEEK and other similar macromolecules [3, 4]. These systems exhibit a high conductivity, chemical and electrochemical stability; however, they also suffer from important drawbacks such as: (i) a dramatic drop in conductivity at T > 80°C and in dry conditions; (ii) poor mechanical properties; and (iii) a high permeability to active species (e.g., vanadium) [5]. Consequently, it is often necessary to: (i) add humidification modules to ion-exchange membrane fuel cells, thus raising the bulk and cost of the device; or (ii) especially in the case of vanadium redox flow batteries (VRFBs), use thick EMs that curtail the current density.These drawbacks can be addressed by implementing several different strategies, that include: (i) the introduction of a filler in the EM, giving rise to a hybrid inorganic-organic membrane [6]; (ii) the fabrication of EMs with different macromolecules (e.g., polybenzimidazole, PBI) [6-11]. All of these strategies operate by modulating the physicochemical properties of the EMs, with a particular reference to the phase segregation at the nano/mesoscale [3]. This work overviews the study of the interplay between the physicochemical properties (i.e., the chemical composition, thermoanalytical properties, morphology and structure) and the electrical response of hybrid ion-exchange membranes by means of advanced investigation techniques such as Broadband Electrical Spectroscopy (BES).The results allow to elucidate the interplay among chemical and physical properties of the different phases within each EM and their conductivity mechanism. Indeed, the understanding of the relationship taking place between structure and ion conduction mechanisms is crucial in order to trigger the development of new high-performing proton and anion conducting membranes for applications in fuel cells and redox flow batteries.