(Keynote) Hybrid Inorganic-Organic Ion-Exchange Membranes for Electrochemical Applications: Electrical Response and Conductivity Mechanism

Abstract

Electrochemical energy conversion and storage (EECS) systems are of major interest for both industry and scientific community due to their high energy conversion efficiency and easy scalability [1]. Two families of EECS systems are attracting particular attention: ion-exchange membrane fuel cells 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, RFBs (with a particular reference to all-vanadium redox flow batteries, VRFBs) 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.Electrolyte membranes (EMs) are found at the core of both ion-exchange membrane fuel cells and VRFBs. 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 for both ion-exchange membrane fuel cells and VRFBs are based on perfluorinated ionomers such as Nafion™ and other similar macromolecules [2]. These systems exhibit an outstanding proton conductivity and a very high 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 large permeability to vanadium species [3, 4]. 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 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 so rise to a hybrid inorganic-organic membrane [5]; (ii) the fabrication of the EM with a different macromolecule (e.g., a sulfonated polyaromatic system such as sulfonated polyether ether ketone, SPEEK [6]) and (iii) the doping of the EM with an ion-conducting medium (e.g., phosphoric acid [7], or a proton-conducting ionic liquid [8]). All of these strategies operate by modulating the physicochemical properties of the EMs, with a particular reference to the details of the phase segregation at the nano/mesoscale.This work overviews the study of the interplay between the physicochemical properties of hybrid ion-exchange membranes (with a particular reference to the chemical composition, thermoanalytical properties, morphology and structure), and the electrical response as determined by means of advanced investigation techniques such as Broadband Electrical Spectroscopy (BES). Results allow to: (i) elucidate the interactions between the different phases and components within each EM; and (ii) clarify the conductivity mechanism. On these bases, it is possible to identify the most promising avenues of research to devise EMs for application in ion-exchange membrane fuel cells and RFBs exhibiting a performance and a cyclability beyond the state of the art. Acknowledgement We acknowledge the support from: (a) the European Union’s Horizon 2020 research and innovation programme under grant agreement 881603; (b) the project “Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells” funded by EIT Raw Materials; and (c) the project “HELPER” funded by the University of Padova; and (d) the BIRD 2020 program of UNIPD.