Investigation of proton conductivity-free volume relationships of polymer composite electrolytes by using positron annihilation lifetime spectroscopy technique (PALS)

This thesis investigates the effects of different sulfonation degrees (DS) in SPEEK (sulfonated polyether ether ketone) matrices on proton conductivity and dielectric behaviors, evaluated in conjunction with free volume fractions measured using Positron Annihilation Lifetime Spectroscopy (PALS). The structural, thermal, and morphological properties of the electrolytes were characterized using FTIR, TGA, and SEM analyses, followed by determination of free volume fractions via PALS. These data provide critical insights into the conductivity mechanisms of the electrolytes. PALS results for SPEEK-65 and SPEEK-80 electrolytes reveal differences in the free volume fractions within their respective matrices. Specifically, SPEEK-65's lower free volume fraction indicates a lower DS and consequently less void space. In contrast, SPEEK-80's higher free volume fraction suggests greater voids, indicative of a higher DS. Proton conductivity and dielectric properties were measured over wide temperature (300-370 K) and frequency (20Hz- 1MHz) ranges and correlated with PALS outcomes. The impact of higher free volume fractions on the proton conduction mechanism and hence the electrical performance of the electrolyte has been elucidated. This relationship stems from the structural changes induced by the sulfonation process within the polymer matrix. In conclusion, PALS results enable a detailed analysis of the structural and electrical properties of SPEEK electrolytes, serving as a significant guide for optimizing these materials for applications in energy storage, fuel cell technologies, and other fields. This study highlights PALS as a powerful tool for understanding complex conductivity and dielectric behaviors, laying a crucial foundation for future research.