Anion Exchange Membranes: Correlation between Physicochemical Properties and Anion Conductivity By Broadband Electrical Spectroscopy
Authors: Di Noto Vito , Vezzù Keti , Negro Enrico , Bertasi Federico , Nawn Graeme , Herring Andrew M 
Published: February 2020

In recent years, anion exchange membrane fuel cells (AEMFCs) have been extensively studied owing to significant advantages over their proton exchange membrane fuel cell (PEMFC) counterparts [1]. The possibility to adopt electrocatalysts that do not comprise of precious metals as well as diminished poisoning effects are among the most relevant reasons for which AEMFCs are believed to be advantageous. However, AEMFCs do suffer from some drawbacks, specifically concerned with the anion exchange membrane (AEM) which is responsible for the selective migration of OH- anions from the cathode to the anode, and is one of the most critical components of the entire AEMFC. In particular, with respect to the proton exchange membranes used in PEMFCs, AEMs typically exhibit a lower ionic conductivity and an inferior chemical stability, the latter typically associated with the degradation of anion-exchange functionalities. For these reasons, it is very important to elucidate the details of the complex interplay between the nanostructure and the ion conductivity mechanism of the AEMs.
Over the last 30 years it has been demonstrated that conductivity in ion-conducting materials occurs via a number of different processes including: (a) the migration of ions between coordination sites [2-5]; and (b) the diffusion of conformational states of the host matrix (segmental motion). [2-5]. In ion-conducting membranes the long-range charge migration is often correlated with the dielectric relaxation modes of the polymeric chains; the latter are typically associated with the fluctuation of: a) the main backbone chain bearing permanent dipole moments; b) side chains; or c) functional groups involved in ion-dipole interactions. The key technique to investigate the interplay between structure and conductivity of ion-conducting materials is Broadband Electrical Spectroscopy (BES).
Here we present several case studies of AEMs paying particular attention to their thermal stability and the thermomechanical properties. BES is then adopted to study the electrical response of each material in terms of polarizations and relaxation phenomena. The results allow us to: (a) suggest a comprehensive model capable to rationalize the long-range charge transfer mechanism in AEMs; and (b) clarify how the chemical composition and nanostructure of the materials is influencing the coordination of mobile species.

Acknowledgements

The authors thank the StrategicProject “From materials for Membrane electrode Assemblies to electric Energy conversion and SToRAge devices” (MAESTRA) of the University of Padova for funding this activity.

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