Development of Functional Materials For Electrochemical Systems, Particularly Fuel Cells

The main objective of present thesis is to develop novel and inexpensive solid polymeric electrolytes capable of conducting protons at intermediate temperatures and anhydrous conditions. Such materials are interesting in view of potential applications for a variety of emerging electrochemical technologies such as high-temperature-polymer-electrolyte-membrane fuel cells (HT-PEMFC). Polymer electrolytes are prepared based on proper acid-base interactions between host polymers and guest molecules as an effective route to distribute modifier within the matrix structure. In anhydrous condition protons hop from site to site in a dense hydrogen bonding network formed among amphoteric jumping sites (Grotthhus mechanism). According to amphoteric nature, these jumping sites can be simply divided into two main categories: amine based- and phosphonic based sites. Solid electrolytes of both categories are prepared. All samples are systematically characterized as to morphological, thermal and structural properties. IR spectroscopy and X-ray diffraction (XRD) are applied to reveal any considerable structural interactions between functional groups. By means of thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), important information is collected on thermal stabilities and glass transition temperatures (Tg) of composite membranes. Proton conductivity is investigated by means of electrochemical impedance spectroscopy (EIS), which allows determination of proton conductivity and the activation energy of proton transfer in anhydrous state. An extended investigation is carried out in order to establish a correlation between proton conductivity and the content of modifiers in prepared materials. Additionally, compositional information of material is obtained from solid state nuclear magnetic resonance (SSNMR). Based on preparation and processing conditions, following important values are achieved: I. All composite membranes were thermally stable within intermediate temperature range (100-200 °C), II. Proton conductivities in excess of 0.01 Ω-1 cm-1 at 190 °C and anhydrous condition are obtained, III. For the first time, positive effect of phosphonic condensation reaction on proton conductivity is reported and IV. Phosphonic groups and aromatic amines are recommended as the most efficient proton conductive functional groups for intermediate temperatures applications.