• dielectric spectroscopy
• diffusion coefficient
• electrical response
• electrode polarization
• hopping
• ionic conductivity
• ionic conductor
• permittivity

Iontronics is an emerging research field that has drawn increasing interest in last few years. The gist of iontronics is the combination of electronics and ionics for the fabrication of different types of devices, such as batteries, fuel cells, supercapacitors, ion-gated transistors and many others. In this respect, a complete understanding of the charge transport mechanisms in ion-conducting materials would be beneficial since it may well pave the way for the synthesis of new materials with tailored properties of ion conduction, providing in turn a boost in the technological applications. In the last decade many efforts have been made to unravel the underlying charge transport mechanisms of several ion-conducting materials, such as ionomers, polymerized ionic liquids and polymer electrolytes. Nonetheless, a complete understanding of the ion dynamics in disordered materials has not been achieved, yet.
In this thesis I have targeted the investigation of the ion transport properties of a polymer-based ionic conductor, which has been synthesized through the chemical reaction of low molecular weight polyethylene oxide (PEO) with metallic sodium, leading to the transformation of the PEO alcoholic end-groups into alcoxide end-groups along with sodium cations. Although such a PEO-based ionic conductor, referred to as Na-PEO, has been effectively used as the gate material in a thermally driven field effect transistor based on a nanowire, exploiting the ionic thermoelectric effect, an investigation of its basic physical properties is still missing to the best of my knowledge.
The charge transport properties, which are the most interesting ones as far as ion and thermoelectric ionic gating applications are concerned, have been effectively investigated by means of Dielectric Spectroscopy, while Fast Differential Scanning Calorimetry has been applied in order to understand the nature of a phase transition near room temperature, which has turned out to be a transition from the liquid to the crystalline state. Dielectric Spectroscopy has been applied to the study of Na-PEO properties using two different kinds of dielectric cell, which have been designed with the help of Finite Element Method (FEM) simulations. On the one hand, the use of dielectric cells with a conventional parallel plate geometry has made it possible to measure the electric response of Na-PEO in both the liquid and the crystalline state in a wide temperature range from 80 °C to below -100 °C. On the other hand, the use of specifically designed and fabricated interdigitated electrodes (IDEs) as less conventional dielectric cell allowed to better examine the electrode polarization phenomenon. Furthermore, the experimental set-up that was especially prepared for measuring with IDEs turned out to be most suitable for conducting measurements on varying temperature on a hygroscopic material like Na-PEO for long periods of time and with very good thermal precision and stability, owing to the fact that the sample was kept under vacuum without the need to use nitrogen gas for the thermal control.
Na-PEO permittivity and conductivity spectra as a function of frequency and temperature have been first analyzed using model independent methods, which have allowed to measure the temperature dependence of some bulk properties of the material, like the dielectric constant and the electric conductivity. These quantities have been used for verifying the time-temperature superposition of Na-PEO spectra and their accordance with some of the most widely accepted models for ion diffusion in disordered materials. Similarly to what has been reported in the literature for a number of ionically conducting disordered materials, the charge transport has turned out to be non-ohmic, showing instead a dispersion in the high-frequency region. Furthermore, special attention has been paid to the analysis of the electrode polarization effect, which is an ubiquitous phenomenon affecting the low frequency region of dielectric spectra of ionic conductors. Although in the past this phenomenon, essentially related to the setting up of a space charge region near the electrodes, has been often regarded as an unwanted effect that prevented from measuring the bulk properties at low frequencies, in the last few years it has drawn renewed interest owing to the fact that a better understanding of this interfacial phenomenon may well give access to quantities, like the self-diffusion coefficient and the Debye screening length, that are related to the microscopic dynamics of ions and might be useful for devices designing, as well.
The thesis is structured in three chapters. The main phenomena occurring in the analysis of the electric response of ionic conductors are presented in the first chapter. The second chapter is instead devoted to the description of the experimental methods used for the investigation of the charge transport properties of Na-PEO. Finally, the main results obtained from the analysis of permittivity and conductivity spectra of Na-PEO are outlined and discussed in the third chapter.