Conductivity and Relaxation Phenomena in Proton and Anionic Exchange Membranes by Broadband Electric Spectroscopy

Ionically conducting materials (ICM) are of great importance for the fabrication of portable batteries for electronic devices such as computers, tools, video and still cameras, and for the development of fuel cell and battery-powered electric vehicles, dye-sensitized solar cells, supercapacitors and sensors [1]. It has been suggested that conductivity in ICMs occurs via a number of different processes. The predominant conductivity processes are attributed to: a) the charge migration of ions between coordination sites in the host materials; [2-5] and b) the increase of conductivity due to relaxation phenomena involving the dynamics of the host materials [2-5]. Ions “hopping” to new chemical environments can lead to successful charge migration only if ion-occupying domains relax via reorganizational processes [2-5], which generally are coupled with relaxation events associated with the host matrix. Here, it will be described in a concise fashion, the instruments used to comprehensively study the electric response of ionic conductors. To provide the reader with the basic tools necessary for understanding broadband electric spectroscopy [6-8], the first part will review the general phenomena and basic theory behind each type of electric response that materials may exhibit when they are subjected to static or dynamic electric fields. This will be achieved by focusing on the practical use of equations, while referring to specialized texts for detailed explanations of the equations. Then, an overviews of the application of BES in the study of the charge transfer mechanisms pristine and hybrid inorganic-organic proton-conducting and anion-conducting membranes and the models adopted for the interpretation of conductivity mechanisms are described and a unified conductivity mechanism is proposed. References [1] Polymer Electrolytes: Fundamentals and Applications; Sequeira, C.; Santos, D., Eds.; Woodhead Publishing Limited: Oxford, 2010. [2] Di Noto, V. J. Phys. Chem. B, 104 (2000) 10116. [3] Di Noto, V.; Vittadello, M.; Lavina, S.; Fauri, M.; Biscazzo, S. J. Phys. Chem. B, 105 (2001) 4584. [4] Di Noto, V. J. Phys. Chem. B, 106 (2002) 11139. [5] Di Noto, V.; Vittadello, M.; Greenbaum, S. G.; Suarez, S.; Kano, K.; Furukawa, T. J. Phys. Chem. B, 108 (2004) 18832. [6] Di Noto, V.; Giffin, G. A.; Vezzù, K.; Piga, M.; Lavina S. Broadband Dielectric Spectroscopy: A Powerful Tool for the Determination of Charge Transfer Mechanisms in Ion Conductors, in Solid State Proton Conductors: Properties and Applications in Fuel Cells, P. Knauth, M. L. Di Vona, Eds., John Wiley & Sons Weinheim, Germany, 2012. [7] Runt, J. P.; Fitzgerald, J. J. Dielectric Spectroscopy of Polymeric Materials: Fundamentals and Applications; American Chemical Society: Washington, D.C., 1997. [8] Schoenhals, A.; Kremer, F. Broadband Dielectric Spectroscopy; Springer-Verlag: Berlin, 2003. Acknowledgements The authors thank the Strategic Project “From materials for Membrane electrode Assemblies to electric Energy conversion and SToRAge devices” (MAESTRA) of the University of Padova for funding this activity.