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    Correlation of proton conductivity and free volume in sulfonated polyether ether ketone electrolytes: a positron annihilation lifetime spectroscopy study
    (Elsevier Ltd, 2025) Lahmuni, Muhammet; Yılmazoğlu, Mesut; Abacı, Ufuk; Çoban, Ozan; Yumak Yahşi, Ayse; Tav, Cumali; Yahşi, Uğur
    Proton-conducting polymers play a pivotal role in clean energy technologies and various industrial applications, with a significant emphasis on enhancing energy efficiency and minimizing environmental impact. Sulfonated polyether ether ketone (SPEEK), which is renowned for its proton conductivity, has emerged as a key material in electrochemical processes, notably in proton exchange membrane (PEM) fuel cells. This study investigated the proton conductivity and dielectric behavior of SPEEK electrolytes at varying degree of sulfonation (DS) of 65% and 80%, correlating these properties with free volume profiles determined by positron annihilation lifetime spectroscopy (PALS). The SPEEK-65 and SPEEK-80 electrolytes were prepared via a controlled sulfonation process and characterized by FTIR, TGA, and SEM analyses. Proton conductivity and dielectric measurements were conducted at temperatures ranging from 300 to 370 K and frequencies ranging from 20 Hz to 1 MHz. The results revealed that SPEEK-80 exhibited a maximum proton conductivity of 3.4 × 10?2 S/m at 300 K and 1 MHz, which was significantly greater than the 4.38 × 10?3 S/m observed for SPEEK-65 under the same conditions. PALS analysis demonstrated a notable increase in free volume with increasing DS, with SPEEK-80 showing a higher o-Ps lifetime and intensity, indicating larger free volume sizes and fractions. These findings underscore the critical interplay between DS, free volume, and proton conductivity, offering insights into optimizing SPEEK-based electrolytes for advanced electrochemical applications.
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    Free volume impact on ionic conductivity of PVdF/GO/PVP solid polymer electrolytes via positron annihilation approach
    (Elsevier Ltd, 2025) Yılmazoğlu, Mesut; Okkay, Hikmet; Abacı, Ufuk; Tekay, Emre; Çoban, Ozan; Veziroğlu, Sümeyye; Yumak Yahşi, Ayşe; Tav, Cumali; Yahşi, Uğur
    This study reports the effects of free volume (FV) profiles on the ionic conductivities of PVdF/GO/PVP ternary polymer electrolytes using positron annihilation lifetime spectroscopy (PALS). The electrolytes were characterized by various tests such as FTIR, XRD, TGA-DTG, SEM, contact angle and DMA. FV profiles were evaluated by o-Ps lifetime (τ₃), intensity (I3) and FV fractions (fυ). PVdF exhibits a proton conductivity of 2.1 × 10⁻5 S/m at 80 °C. However, the introduction of GO leads to a decrease in conductivity, with PVdF/GO showing 1.7 × 10⁻5 S/m at 80 °C. The presence of PVP in PVdF/GO/PVP10 and PVdF/GO/PVP30 creates new FV spaces via hydrogen bonds and intermolecular interactions, expanding hydrophobic areas and increasing I₃ values. PVP's high mobility and positive charge density reduce the τ₃ values. In contrast, I₃ and fυ values decrease in PVdF/GO/PVP50, accompanied by a significant drop in τ₃ values and the proton conductivity and dielectric constant peak at 6.1 × 10⁻2 S/m and 77.38, respectively. High PVP concentration may enhance interactions within the polymer matrix, forming a dense structure that, despite reduced FV, maintains or enhances proton mobility through alternative conduction pathways and increased polarization. This study emphasizes the balance of FV and dielectric behavior for efficient electrochemical processes.
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    Graphene oxide-induced free volume effects in proton conductive chitosan/polyvinyl alcohol (CS/PVA)-based composite electrolytes: a positron annihilation lifetime study
    (Springer New York, 2025) Yılmazoğlu, Mesut; Okkay, Hikmet; Abacı, Ufuk; Tekay, Emre; Çoban, Ozan; Veziroğlu, Sümeyye; Yumak Yahşi, Ayşe; Tav, Cumali; Yahşi, Uğur
    In this study, chitosan/polyvinyl alcohol (CS/PVA)-based composite electrolytes containing ionic liquid (IL) and graphene oxide (GO) as conductivity-enhancing additives were prepared and characterized in detail in terms of free volume (FV) parameters, mechanical strength, and proton conductivity. FV parameters were determined by positron annihilation lifetime spectroscopy (PALS) and correlated with dynamic mechanical analysis (DMA), ionic conductivity, and dielectric measurements. According to PALS analyses, the CS-5 sample (4 wt% GO) exhibited the highest FV fraction (2.45% at 40 °C) with a corresponding ortho-positronium lifetime (τ₃) of 2.12 ns, and FV hole radius (R) of 2.96 nm. This microstructural expansion was directly associated with the maximum proton conductivity of 1.64 × 10⁻3 S/m at 1 MHz, demonstrating the role of GO in facilitating continuous proton pathways. Moreover, CS-5 achieved superior structural reinforcement, with a high-temperature storage modulus of 12.38 MPa, confirming that GO simultaneously enhances both conduction and mechanical stability. The strong correlation between FV fraction and proton conductivity experimentally validates the FV-based conduction theory, offering mechanistic evidence for the design of next-generation polymer electrolytes. These results highlight that tuning FV distribution through GO optimization provides a reliable strategy for advancing solid-state biopolymer electrolytes in energy storage and environmental applications.